Electrical Safety

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ELECTRICAL SAFETY TESTS
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Category: Electrical Safety
Last Updated: Monday, 28 October 2013 14:34

Article courtesy of Eastwood Park

The following paragraphs and diagrams describe the electrical safety tests commonly available on
medical equipment safety testers. Please note that although HEI 95 and DB9801 are no longer current,
they are referred to in the text since many medical electronics departments have used them as a basis for
local acceptance testing and even routine testing protocols. Protocols based on both sets of guidance are
also available on many medical equipment safety testers.

6.1 NORMAL CONDITION AND SINGLE FAULT CONDITIONS
A basic principle behind the philosophy of electrical safety is that in the event of a single abnormal
external condition arising or of the failure of a single means of protection against a hazard, no safety
hazard should arise. Such conditions are called "single fault conditions" (SFCs) and include such
situations as the interruption of the protective earth conductor or of one supply conductor, the appearance
of an external voltage on an applied part, the failure of basic insulation or of temperature limiting devices.
Where a single fault condition is not applied, the equipment is said to be in "normal condition" (NC).
However, it is important to understand that even in this condition, the performance of certain tests may
compromise the means of protection against electric shock. For example, if earth leakage current is
measured in normal condition, the impedance of the measuring device in series with the protective earth
conductor means that there is no effective supplementary protection against electric shock.
Many electrical safety tests are carried out under various single fault conditions in order to verify that
there is no hazard even should these conditions occur in practice. It is often the case that single fault
conditions represent the worst case and will give the most adverse results. Clearly the safety of the
equipment under test may be compromised when such tests are performed. Personnel carrying out
electrical safety tests should be aware that the normal means for protection against electric shock are not
necessarily operative during testing and should therefore exercise due precautions for their own safety
and that of others. In particular the equipment under test should not be touched during the safety testing
procedure by any persons.

6.2 PROTECTIVE EARTH CONTINUITY
The resistance of the protective earth conductor is measured between the earth pin on the mains plug
and a protectively earthed point on the equipment enclosure (see figure 6). The reading should not
normally exceed 0.2Ω at any such point. The test is obviously only applicable to class I equipment.
In IEC60601, the test is conducted using a 50Hz current between 10A and 25A for a period of at least 5
seconds. Although this is a type test, some medical equipment safety testers mimic this method. Damage
to equipment can occur if high currents are passed to points that are not protectively earthed, for
example, functional earths. Great care should be taken when high current testers are used to ensure that
the probe is connected to a point that is intended to be protectively earthed.
HEI 95 and DB9801 Supplement 1 recommended that the test be carried out at a current of 1A or less for
the reason described above.
Where the instrument used does not do so automatically, the resistance of the test leads used should be
deducted from the reading.
If protective earth continuity is satisfactory then insulation tests can be performed.

Applicable to

Class I, all types

Limit:

0.2Ω

DB9801 recommended?:

Yes, at 1A or less.

HEI 95 recommended?:

Yes, at 1A or less.

Notes:

Ensure probe is on a protectively earthed point
Figure 8. Measurement of protective earth continuity.

6.3 INSULATION TESTS

IEC 60601-1 (second edition), clause 17, lays down specifications for electrical separation of parts of
medical electrical equipment compliance to which is essentially verified by inspection and measurement
of leakage currents. Further tests on insulation are detailed under clause 20, "dielectric strength". These
tests use AC sources to test equipment that has been pre-conditioned to specified levels of humidity. The
tests described in the standard are type tests and are not suitable for use as routine tests.
HEI 95 and DB9801 recommended that for class I equipment the insulation resistance be measured at
the mains plug between the live and neutral pins connected together and the earth pin. Whereas HEI 95
recommended using a 500V DC insulation tester, DB 9801 recommended the use of 350V DC as the test
voltage. In practice this last requirement could prove difficult and it was acknowledged in a footnote that a
500 V DC test voltage is unlikely to cause any harm. The value obtained should normally be in excess of
50MΩ but may be less in exceptional circumstances. For example, equipment containing mineral
insulated heaters may have an insulation resistance as low as 1MΩ with no fault present. The test should
be conducted with all fuses intact and equipment switched on where mechanical on/off switches are
present (see figure 9).

Applicable to

Class I, all types

Limits:

Not less than 50MΩ

DB9801
recommended?:

Yes

HEI 95
recommended?:

Yes

Notes:

Equipment containing mineral insulated heaters may give values down to 1MΩ.
Check equipment is switched on.
Figure 9. Measurement of insulation resistance for class I equipment

HEI 95 further recommended for class II equipment that the insulation resistance be measured between
all applied parts connected together and any accessible conductive parts of the equipment. The value
should not normally be less than 50MΩ (see figure 10). DB9801 Supplement 1 did not recommend any
form of insulation test be applied to class II equipment.

Applicable to

Class II, all types having applied parts

Limits:

not less than 50MΩ.

DB9801 recommended?:

No

HEI 95 recommended?:

Yes

Notes:

Move probe to find worst case.
Figure 10. Measurement of insulation resistance for class II equipment.

Satisfactory earth continuity and insulation test results indicate that it is safe to proceed to leakage current
tests.

6.4 LEAKAGE CURRENT MEASURING DEVICE
The leakage current measuring device recommended by IEC 60601-1 loads the leakage current source
with a resistive impedance of about 1 kΩ and has a half power point at about 1kHz. The recommended
measuring device was changed slightly in detail between the 1979 and 1989 editions of the standard but
remained functionally very similar. Figure 11 shows the arrangements for the measuring device. The
millivolt meter used should be true RMS reading and should have an input impedance greater than 1 MΩ.
In practice this is easily achievable with most good quality modern multimeters. The meter in the
arrangements shown measures 1mV for each µA of leakage current.

Figure 11. Arrangements for measurement of
leakage currents.

6.5 EARTH LEAKAGE CURRENT
For class I equipment, earth leakage current is measured as shown in figure 12. The current should be
measured with the mains polarity normal and reversed. HEI 95 and DB9801 Supplement 1 recommended
that the earth leakage current be measured in normal condition (NC) only. Many safety testers offer the
opportunity to perform the test under single fault condition, neutral conductor open circuit. This
arrangement normally gives a higher leakage current reading.
One of the most significant changes with regard to electrical safety in the 2005 edition of IEC 60601-1 is
an increase by a factor of 10 in the allowable earth leakage current to 5mA in normal condition and 10mA
under single fault condition. The rationale for this is that the earth leakage current is not, of itself,
hazardous.
Higher values of earth leakage currents, in line with local regulation and IEC 60364-7-710 (electrical
supplies for medical locations), are allowed for permanently installed equipment connected to a dedicated
supply circuit.

Applicable to

Class I equipment, all types

Limits:

0.5mA in NC, 1mA in SFC or 5mA and 10mA respectively for equipment designed to
IEC60601-1:2005.

DB9801
recommended?:

Yes, in normal condition only.

HEI 95
recommended?:

Yes, in normal condition only.

Notes:

Measure with mains normal and reversed. Ensure equipment is switched on.
Figure 12. Measurement of earth leakage current.

6.6 ENCLOSURE LEAKAGE CURRENT OR TOUCH CURRENT
Enclosure leakage current is measured between an exposed part of the equipment which is not intended
to be protectively earthed and true earth as shown in figure 13. The test is applicable to both class I and
class II equipment and should be performed with mains polarity both normal and reversed. HEI 95
recommended that the test be performed under the SFC protective earth open circuit for class I
equipment and under normal condition for class II equipment. DB9801 Supplement 1 recommended that
the test be carried out under normal condition only for both class I and class II equipment. Many safety
testers also allow the SFC's of interruption of live or neutral conductors to be selected. Points on class I
equipment which are likely not to be protectively earthed may include front panel fascias, handle
assemblies etc.
The term "enclosure leakage current" has been replaced in the new edition of the IEC 60601-1standard
by the term "touch current", bringing it into line with IEC 60950-1 for information technology equipment.
However, the limits for touch current are the same as the limits for enclosure leakage current under the
second edition of the standard, at 0.1 mA in normal condition and 0.5 mA under single fault condition.
In practice, if a piece of equipment has accessible conductive parts that are protectively earthed, then in
order to meet the new requirements for touch current, the earth leakage current would need to meet the
old limits. This is due to the fact that when the touch current is tested from a protectively earthed point
with the equipment protective earth conductor disconnected, the value will be the same as that achieved
for earth leakage current under normal condition.
Hence, where higher earth leakage currents are recorded for equipment designed to the new standard, it
is important to check the touch current under single fault condition, earth open circuit, from all accessible
conductive parts.

Applicable to

Class I and class II equipment, all types.

Limits:

0.1mA in NC, 0.5mA in SFC

DB9801
recommended?:

Yes, NC only

HEI 95 recommended?: Yes, class I SFC earth open circuit, class II NC.
Ensure equipment switched on. Normal and reverse mains. Move probe to find
worst case.

Notes:

Figure 13. Measurement of enclosure leakage current

6.7 PATIENT LEAKAGE CURRENT
Under IEC 60601-1, for class I and class II type B and BF equipment, the patient leakage current is
measured from all applied parts having the same function connected together and true earth (figure 14).
For type CF equipment the current is measured from each applied part in turn and the leakage current
leakage must not be exceeded at any one applied part (figure 15).
HEI 95 adhered to the same method, however, DB9801 Supplement 1 recommended that patient leakage
current be measured from each applied part in turn for all types of equipment, although the recommended
leakage current limits were not revised to take into account the changed test method for B and BF
equipment.
Great care must be taken when performing patient leakage current measurements that equipment outputs
are inactive. In particular, outputs of diathermy equipment and stimulators can be fatal and can damage
test equipment.

Applicable to

All classes, type B & BF equipment having applied parts.

Limits:

0.1mA in NC, 0.5mA in SFC.

DB9801 recommended?:

No

HEI 95 recommended?:

Yes, class I SFC earth open circuit, class II normal condition.

Notes:

Equipment on, but outputs inactive. Normal and reverse mains.

Figure 14. Measurement of patient leakage current with applied parts connected together

Applicable to

Class I and class II, type CF (B & BF for DB9801 only) equipment having applied
parts.

Limits:

0.01mA in NC, 0.05mA in SFC.

DB9801
recommended?:

Yes, all types, normal condition only.

HEI 95 recommended?: Yes, type CF only, class I SFC earth open circuit, class II normal condition.
quipment on, but outputs inactive. Normal and reverse mains. Limits are per
electrode.

Notes:

Figure 15. Measurement of patient leakage current for each applied part in turn

6.8 PATIENT AUXILIARY CURRENT
Patient auxiliary current is measured between any single patient connection and all other patient
connections of the same module or function connected together. Where all possible combinations are
tested together with all possible single fault conditions this yields an exceedingly large amount of data of
questionable value.

Applicable to

All classes and types of equipment having applied parts.

Limits:

Type B & BF - 0.1mA in NC, 0.5mA in SFC. Type CF - 0.01mA in NC, 0.05mA in
SFC.

DB9801
recommended?:

No.

HEI 95 recommended?: No.
Notes:

Ensure outputs are inactive. Normal and reverse mains.
Figure 16. Measurement of patient auxiliary current.

6.9 MAINS ON APPLIED PARTS (PATIENT LEAKAGE)
By applying mains voltage to the applied parts, the leakage current that would flow from an external
source into the patient circuits can be measured. The measuring arrangement is illustrated in figure 18.
Although the safety tester normally places a current limiting resistor in series with the measuring device
for the performance of this test, a shock hazard still exists. Therefore, great care should be taken if the
test is carried out in order to avoid the hazard presented by applying mains voltage to the applied parts.
Careful consideration should be given as to the necessity or usefulness of performing this test on a
routine basis when weighed against the associated hazard and the possibility of causing problems with
equipment. The purpose of the test under IEC 60601-1 is to ensure that there is no danger of electric
shock to a patient who for some unspecified reason is raised to a potential above earth due to the
connection of the applied parts of the equipment under test. The standard requires that the leakage
current limits specified are not exceeded. There is no guarantee that equipment performance will not be
adversely affected by the performance of the test. In particular, caution should be exercised in the case of
sensitive physiological measurement equipment. In short, the test is a "type test".
Most medical equipment safety testers refer to this test as "mains on applied parts", although this is not
universal. One manufacturer refers to the test simply as "Patient leakage - F-type". In all cases there
should be a hazard indication visible where the test is selected.

Applicable to

Class I & class II, types BF & CF having applied parts.

Limit:

Type BF - 5mA; type CF - 0.05mA per electrode.

DB9801 recommended?:

No.

HEI 95 recommended?:

No

Ensure outputs are inactive. Normal and reverse mains. Caution required,
especially on physiological measurement equipment.

Notes:

Figure 17. Mains on applied parts measurement arrangement

6.10 LEAKAGE CURRENT SUMMARY
The following table summarises the leakage current limits (in mA) specified by IEC60601-1 (second
edition) for the most commonly performed tests. Most equipment currently in use in hospitals today is
likely to have been designed to conform to this standard, but note that the allowable values of earth
leakage current have been increased in the third edition of the standard as discussed above.
The values stated are for d.c. or a.c. (r.m.s), although later amendments of the standard included
separate limits for the d.c. element of patient leakage and patient auxiliary currents at one tenth of the
values listed below. These have not been included in the table since, in practice, it is rare that there is a
problem solely with d.c. leakage where that is not evidenced by a problem with combined a.c and d.c.
leakage.

Type B
NC

Leakage current
Earth
Earth for fixed equipment
Enclosure
Patient
Mains on applied part
Patient auxiliary

Type BF

SFC

NC

Type CF

SFC

NC

SFC

0.5

1

0.5

1

0.5

1

5

10

5

10

5

10

0.1

0.5

0.1

0.5

0.1

0.5

0.1

0.5

0.1

0.5

0.01

0.05

-

-

-

5

-

0.05

0.1

0.5

0.1

0.5

0.01

0.05

* For class II type CF equipment HEI95 recommends a limit for enclosure leakage current of 0.01mA as
per the 1979 edition of BS 5724.
Table 2. Leakage current limits summary.

6.11 COMPARISON OF HEI 95 AND DB 9801 SUPPLEMENT 1 RECOMMENDATIONS
Test

HEI 95

DB9801 Supplement 1

Earth continuity

Use test current of 1A or less Limit
0.2ohm

Use test current of 1A or less Limit
0.2ohm

Insulation for Class 1 equipment

Measure between L and N connected
together and E using 500v DC tester.
Limit > 50MΩ. Investigate lower
values

Measure between L and N connected
together and E using 350v DC tester.
Limit > 20MΩ. Investigate lower
values

Insulation for Class II equipment

Measure between applied parts and
accessible conductive parts of the
equipment. Limit > 50MΩ.
Investigate lower values

No recommendation.

Earth leakage current

Measure in normal condition Limit < Measure in normal condition Limit <
0.5mA
0.5mA

Enclosure leakage current

Measure in SFC, earth open circuit
for Class-1, NC for Class-II Limit
<0.5 mA for Class1 <0.1 mA for class
II
Measure in NC only Limit < 0.1 mA

Patient leakage current

Measure from all applied parts
connected together for B & BF
equipment and from each applied part
in turn for type CF. Measure under
SFC, eart open circuit for Class 1, NC
for classII. Limits :
Class I, B& BF < 0.5 mA
Class II, B& BF < 0.1 mA Measure from each applied part in
turn, for all types of equipment
Class I, CF < 0.05 mA per Measure under NC only Limits
Type B & BF <0.1 mA per
electrode
electrode
Class II, CF < 0.01 mA per
Type CF < 0.01 per electrode
electrode

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TEST AND INSPECTION PROTOCOLS
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7.1 WHEN TO TEST
As discussed at paragraphs under 5.6 above, user organisations should design and implement electrical
safety inspection and test regimes on the basis of risk assessments.
In practice, most user organisations have found it necessary to carry out electrical inspection and safety
testing on medical electrical equipment on the following occasions.
a.

On newly acquired equipment prior to being accepted for use

b.

During routine planned preventative maintenance.

c.

After repairs have been carried out on equipment.
A patient should never be connected to a piece of equipment that has not been checked.
The testing regime used in the case of acceptance testing will be slightly different to that used on other
occasions particularly as regards checks on the condition of packaging, presence of relevant
documentation and accessories. However, it is useful to use the acceptance testing procedure to lay
down baseline data for comparison when the equipment is tested on future scheduled services and after
repairs.

7.2 EXAMPLE INSPECTION AND TEST PROTOCOL

Annex 3 contains a test record sheet that is used to record inspection and test results produced by a
simple electrical safety protocol. It is not intended to be in any way prescriptive, but is included here
simply to illustrate many of the important features of an effective protocol.
Details of the equipment under test are recorded at the top of the form including the device serial number
and a plant number ascribed by the user organisation. This ensures that the record can be linked to the
particular item of equipment. The class and type/s of the equipment under test are also recorded here to
ensure that appropriate test limits are applied.
The details of the test equipment used are also recorded at the top of the form together with the
calibration date. This information is important for traceability since test results can only be proved to be
accurate if it can be demonstrated that the test equipment was in calibration.
The visual inspection checklist provides a record that the relevant parts of the equipment have been
inspected. This is very important since, in practice, the visual inspection is likely to flag up problems far
more often than the electrical safety tests themselves. It is also important that a record of visual
inspection is kept. Where user organisations use electronic means to record data downloaded from
electrical safety testers, it is important to add information on visual inspection to the record.
The electrical safety tests that are used in this particular protocol are few in number and are the same
tests, derived from IEC60601-1, that were selected for HEI 95. The earth continuity test is obviously
important for all class I equipment. The insulation test is intended to look at the insulation between the
mains part and the earth of the equipment under test, and may be regarded as a pre-test to verify that it is
safe to apply mains power in order to measure leakage currents.
Earth leakage current here is only measured under normal condition (NC). Note that "normal" and
"reverse" here mean that the leakage current is measured with L1 and L2 the right way round and the
wrong way round. Both of these conditions are defined as "normal condition". This test will not usually
produce as high a reading as if the test is conducted with under single fault condition, neutral open circuit.
However, in most cases, if there is no problem with earth leakage current under normal condition, there is
unlikely to be one under the single fault condition.
Enclosure leakage and patient leakage currents are both recommended under this protocol to measured
under single fault condition, earth open circuit (EOC). The rationale behind this is that any problems are
likely to be evident under this condition and it is not improbable that the fault condition may arise when the
equipment is in use.
At the foot of the form, it is recorded whether the equipment has passed or failed in the light of the visual
inspection and the electrical safety test results. The date of the test and the identity of the person who
performed the test must also be recorded.

The comments field below the table is a useful feature of any recording system. It allows any observations
to be recorded, for example, of peculiarities of the equipment under test or concerns about test results.
The record should be referred to by the person performing the next test and inspection on the equipment
prior to carrying out the inspection and test.

GENERAL POINTS ON SAFETY
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Many electrical safety tests are performed under single fault conditions such that a means for protection
against electric shock has been removed. In the case of patient leakage current with mains on applied
parts, a hazard is actually introduced.

Even under normal condition, the equipment under test cannot be regarded as safe, since the
supplementary protection may have been compromised by the test arrangement. For these reasons no
equipment under test should be touched whilst tests are being undertaken, as parts of the equipment may
be hazardous live. For similar reasons, tests should be conducted on suitable non-conductive surfaces
and conductive objects should be kept well clear of the equipment.

The potential hazard is exacerbated by the use of automatic testers when running in automatic or semiautomatic modes since hazardous voltages may appear on the equipment under test at any time without
any warning. Where it is not possible to remove equipment to a workshop facility for testing, particular
care must be taken to ensure that there is no possibility of any other persons coming into contact with the
equipment under test.

Many categories of medical electrical equipment can produce outputs for treatment purposes that, if
applied incorrectly to a person can prove fatal, or at least cause serious injuries. Examples of these
categories include surgical diathermy machines, nerve and muscle stimulators, short-wave therapy units
and defibrillators. Persons who have not had specific training on such equipment sufficient to enable them
to avoid the hazards should not be allowed to perform electrical safety testing on it.

The tests applied in the course of routine safety testing can cause damage to equipment if carried out
incorrectly or inappropriately. Such damage may lead directly or indirectly to patient injuries or death if the
equipment is put back into service in this condition. It is clear that only maintenance personnel who are
sufficiently trained to avoid such occurrences arising should carry out electrical safety testing of medical
equipment.

EQUIPMENT STANDARDS, GUIDANCE

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AND LEGISLATION
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5.1 TYPE TESTS AND ROUTINE TESTS
Before discussing the documentation relevant to electrical safety of medical electrical equipment, it is
important to distinguish between "type tests" and routine tests.
Standards for the manufacture of equipment normally detail tests which are intended to be carried out on
a single representative sample of a piece of equipment for which certification of compliance with a
standard is being sought. Such tests are carried out by approved test houses under tightly specified
environmental conditions. These tests are called "type tests" and are not intended for routine use. Indeed,
repetition of many of the tests would certainly cause deterioration in performance and safety of the
equipment under test.
Routine tests have an entirely different purpose than that for type tests. Routine tests are intended to
provide good indicators to the safety of equipment without subjecting it to undue stress that would be
liable to cause deterioration.
In summary then, it should be understood that International Electrotechnical Commission (IEC) or British
Standards (BS) manufacturers' standards for medical electrical equipment referred to below are intended
only for type testing and should not be used for acceptance, in-service or routine testing of equipment.

However, any tests that are used for the latter purposes should ideally be consistent with the standards to
which the equipment has been manufactured. Routine tests and test limits may therefore be derived (in
modified form) from the standards, with the strict proviso that any such tests should not damage or even
stress the equipment under test.

5.2 HTM 8
In 1963, the Department of Health and Social Security published Hospital Technical Memorandum
number 8 called "safety code for electro-medical apparatus". The purpose of the document was to
establish adequate standards for the design and construction of electro-medical apparatus since no other
relevant national standard existed at the time. Although the document was produced essentially for the
guidance of manufacturers, biomedical departments in hospitals were quick to adopt tests from the
document for the basis of their own medical electrical equipment safety testing regimes. Although tests
detailed in the code were type tests, many of them could be fairly easily be repeated without adverse
effects on the equipment as routine tests. Performance of the electrical safety tests was made easier by
the development of specialised medical equipment safety testers, specifically, the Liverpool tester. The
HTM was withdrawn on publication of BS5724 part 1 (see below).

5.3 BS 5724 OR IEC 60601
In 1979, HTM 8 was superseded by the British Standard BS 5724 part 1. This document is a
comprehensive specification for safety of medical electrical equipment. Part 1 covers the general
requirements, i.e. requirements common to all medical electrical equipment regardless of function. A
series of part 2's detailing particular requirements for specific categories of medical electrical equipment
followed publication of part 1 (see Annex 1).
BS 5724 is a far more detailed document than HTM 8, which it replaced. Like the HTM, the tests
contained in the standard are type tests. Some guidance was given in the 1979 edition of the standard on
recommended testing during manufacture and/or installation. Unfortunately, some routine test regimes
based on BS 5724 tended to be too rigorous for such application and in some cases caused damage to
equipment.
BS 5724 part 1 was revised in 1989, making it identical to the International Electro-technical Commission
standard IEC 601-1: 1988. References to routine tests were made even less specific than in the previous
edition. The standard was subsequently re-numbered as IEC 60601-1.
Any manufacturer obtaining compliance of an item of their equipment to BS5724 or IEC 60601 will be in
possession of a uniquely numbered certificate issued by the test house verifying that fact. Compliance to
the standard is a commonly used route used by manufacturers to obtain CE marking (see paragraphs at
5.6.1 below).

5.3.1 THE THIRD EDITION OF IEC60601-1

The third edition of IEC60601-1 was introduced in December 2005. The standard has been renamed
"General requirements for basic safety and essential performance" to reflect the fact that inadequate
equipment performance may give rise to hazards. The new standard is stated to replace the second
edition, although it is recognised that, in practice, due to the references made to the general standard by
particular standards (part twos), there is likely to be a fairly long transitionary period for compliance by
equipment manufacturers.
There are some significant changes in the new standard, some of which are worth noting here.
The new standard states that the manufacturer must have in place a risk management process that
complies to the requirements of ISO 14971 in order to ensure that the equipment design process results
in equipment that is suitable for its intended purpose and that any risks associated with its use are
acceptable.
Certain changes in terminology and numbering systems have been introduced in order to make the
standard more compatible with other IEC standards, in particular IEC 60950-1 (Information technology
equipment).
Collateral standards for medical electrical systems (IEC 60601-1-1) and programmable electrical medical
systems (IEC 60601-1-4) have been incorporated into the body of the new standard as new clauses.

5.4 GUIDANCE FROM THE UK DEPARTMENT OF HEALTH
The Department of Health has, in the past, issued two stand alone documents giving detailed guidance
on acceptance testing or pre-use checks on medical devices. Although both of these documents have
been superseded, they are discussed briefly below because they have been used by many equipment
user organisations as the basis for acceptance testing regimes, and even for routine testing regimes.
Additionally, a number of manufacturers of medical equipment safety testers have incorporated protocols
derived from these guidance documents into their testers' firmware.
A comparison between the test recommendations of both documents is provided in annex 2 for
information.

5.4.1 HOSPITAL EQUIPMENT INFORMATION 95
In August 1981, the DHSS issued HEI 95 entitled "Code of practice for acceptance testing of medical
electrical equipment". The document was produced partly to address the problems that had arisen due to
the misapplication of type tests from BS 5724 by some NHS biomedical departments.
As indicated by the title of the document, the code of practice detailed inspection and test procedures to
be performed on newly acquired medical electrical equipment before it was put into service. Inspection
procedures were clearly explained and the standard acceptance test log sheet given in the appendix of
the document contained references to the explanatory text.

The electrical safety testing recommendations offered in HEI 95 provided a testing regime that was
effective whilst being considerably simpler than many test regimes that were developed from the
recommendations of BS 5724. The reason for this is that the recommended electrical safety tests are
generally applied under worst-case conditions.
Although designed as a code of practice for acceptance testing the document has been widely adopted
and used as the basis of routine test regimes by hospital biomedical departments.
The document was officially withdrawn in December 1999 on the publication by the Medical Devices
Agency of MDA DB9801 Supplement 1 (see below).

5.4.2 DB9801 SUPPLEMENT 1
In December 1999, the Medical Devices Agency (now the Medicines and Healthcare Products Regulatory
Agency or MHRA) published Device Bulletin 9801 Supplement 1 entitled "Checks and tests for newly
delivered medical devices". The document was a supplement to Device Bulletin 9801, "Medical device
and equipment management for hospital and community based organisations", which was published by
the Medical Devices Agency in January 1998. The supplement superseded HEI 95.
The document was intended to be applicable to all newly delivered medical devices, including nonelectrical equipment, before being placed into service. Delivery checks detailed included paperwork
checks, visual inspection procedures and functional checks. Electrical safety checks and tests as well as
calibration checks were also recommended.
DB9801 Supplement 1 emphasised that new equipment under test should not be subjected to currents or
voltages exceeding those experienced under normal operating conditions. Hence none of the
recommended tests involved shorting applied parts together or applying high voltages to electrodes. It
was also suggested that medical electrical equipment not having applied parts could be safety tested
satisfactorily using non-specialist portable appliance testers.
Specimen forms for recording the results of checks and tests were given in the document. Rationales for
the checks and tests prescribed were also given in the annexes of the document.
DB0801 and its supplements were replaced by DB2006(05) in November 2006 (see below).

5.4.3 DEVICE BULLETIN DB2006(05)
In November 2006, the MHRA published (on their website only) Device Bulletin DB2006(05) - "Managing
Medical Devices - Guidance for healthcare and social services organisations". The document updates
and replaces guidance previously given in DB9801 ("Medical device and Equipment Management for
hospital and community based organisations") and its supplements. Section 4 of the new guidance
addresses "Delivery of a new piece of equipment" and hence replaces guidance previously given in

DB9801 Supplement 1. Having said that, much of the basic philosophy behind, and recommendations
from, the latter document have been retained.
The guidance stresses the importance of acceptance checks as a means of improving efficiency and
reducing risk. It also emphasises the necessity of recording checks and test results in order to meet
health and safety requirements, possible litigation demands and to enable safe and effective future device
management.
Delivery checks relevant to medical electrical equipment on delivery are divided into administrative tasks
("paperwork/database") and visual inspection. Recommended administrative checks and tasks include:

device compatibility with purchase specification



inclusion and appropriateness of user and service instructions



inclusion of compliance and calibration certificates and test results



adding device details to equipment management records



check for special requirements such as need for decontamination before use
The guidance further recommends that functional checks, electrical safety tests and calibration checks
(where appropriate) should be carried out prior to the equipment being placed into service.
No specific detail is given on safety tests, other than to emphasise that "pre-use tests should not exceed
the bounds of normal use". In connection with this, it states that the tests described in IEC 60601-1 are
"type tests" and are therefore not suitable for pre-use or maintenance tests (see paragraphs at 5.1
above).
The guidance does, however, point out the legal requirements for electrical safety testing under the
Health and Safety at Work etc Act 1974 and the Electricity at Work Regulations 1989. The guidance
states that "Responsible organisations should ensure that they have implemented electrical safety testing
procedures to comply with this legislation". The legal requirements are further discussed in these notes
under paragraphs at 5.6 below.
The full text of DB2006(05) is available free of charge on the MHRA website at www.mhra.gov.uk

5.5 FUTURE GUIDANCE
The International Electrotechnical Commission has been preparing IEC 62353 Edition 1: "Medical
Electrical Equipment - Recurrent test and test after repair of medical equipment" for some years.
Publication of this document is expected in the near future.
Also in preparation by the Institute of Physics and Engineering in Medicine (IPEM) is a publication called
"Electrical Safety Testing: A Workbench Guide".

5.6 UK LEGISLATION
There are a number of items of legislation applicable in the UK that impact in a fairly direct way on
maintenance procedures for medical electrical devices. These are discussed briefly below.

5.6.1 MEDICAL DEVICES DIRECTIVE
Since the Medical Devices Directive (Council Directive 93/42/EEC) became law in the UK in 1994, it has
been mandatory that all medical devices put on to the market are appropriately CE marked to indicate
compliance with the directive. An important component of the directive is a list of "essential requirements"
to which all medical devices must comply. Compliance with these requirements can be interpreted
essentially as meaning that the medical device is fit for purpose.
Depending on the risk class under which a particular medical device is classified, there are various means
by which a manufacturer is able to demonstrate conformity with the directive. For devices in the lowest
risk category (class I), self declaration is acceptable, whilst for medium and higher risk devices (classes
IIa, IIb and III), the assessment route is more rigorous and may include auditing of the manufacturers'
quality assurance system and independent type testing to a recognised standard (e.g. IEC 60601) of a
representative production sample by a "notified body". Each notified body may be identified by a unique
number that appears to the top right of the CE mark on medical devices.
In each member state a "Competent Authority" is authorised by that country's government to ensure that
the requirements of the directive are carried out. In the UK, the competent authority is the Secretary of
State for Health who has delegated day to day running of the competent authority to the Medicines and
Healthcare Products Regulatory Agency (MHRA). The Medical Devices Directive is enshrined into UK law
by the medical Devices Regulations 2002.
As far as the purchaser of equipment is concerned, all medical devices purchased within any EEC
member state should be appropriately CE marked. Conformity to the directive should be confirmed by the
equipment supplier by means of a "declaration of conformity" prior to purchase.

5.6.2 HEALTH AND SAFETY AT WORK ETC. ACT 1974
The Health and Safety at Work etc. Act 1974 (HASAWA) act may be regarded as the "catch all" act that
covers all aspects of health and safety in the workplace. It places responsibility on employers and
employees for the health, safety and welfare of all persons that may be affected by activities of an
employer (including NHS Trusts). The overarching nature of the act is illustrated by part 1, section 3,
paragraph 1 of the act that states:
"It shall be the duty of every employer to conduct his undertaking in such a way as to ensure, so far as is
reasonably practicable, that persons not in his employment who may be affected thereby are not thereby
exposed to risks to their health and safety".

There are many sets of Regulations that are made under the act that spell out in detail what must be done
to meet the requirements of the act. The Regulations are said to be "made under the Act", and nonconformity to any such regulations is therefore an offence under the HASAWA.

5.6.3 ELECTRICITY AT WORK REGULATIONS 1989
Particular requirements with regard to electrical equipment are imposed by the Electricity at Work
Regulations 1989. Some significant extracts from the regulations are quoted below. It should be noted
when reading them that the word "systems" refers to electrical installations and any equipment capable of
being made live by them.
Regulation 4(1)"All systems shall at all times be of such construction as to prevent, so far as is
reasonably practicable, danger".
Regulation 4(2)"As necessary to prevent danger, all systems shall be maintained so as to prevent, so far
as is reasonably practicable, such danger".
Regulation 4(3) "Every work activity, including operation, use and maintenance of a system and work
near a system, shall be carried out in such a manner as not to give rise, so far as is reasonably
practicable, to danger".
Regulation 16"No person shall be engaged in any work activity where knowledge or experience is
necessary to prevent danger or, where appropriate, injury, unless he possesses such knowledge or
experience, or is under such degree of supervision as may be appropriate having regard to the nature of
the work."
Although the Electricity at Work Regulations clearly put requirements on employers and employees with
regard to the necessity for maintaining electrical safety, the means by which this should be done are not
spelt out in the Regulations.

5.6.4 MANAGEMENT OF HEALTH AND SAFETY AT WORK REGULATIONS 1999
The Management of Health and Safety at Work Regulations 1999 set out the need for organisations to
develop formalised management systems for health and safety. These systems will form a part of the
organisations health and safety policy.
The policy should detail arrangements for effective planning, organisation, control, monitoring and review
of protective and preventative measures. Hence protocols for electrical safety inspection and testing of
medical equipment should be a part of this policy.
A major plank of the regulations is prescription of the use of risk assessments as a tool in managing
health and safety effectively. The prime obligation under health and safety legislation is to eliminate or
minimise risks to health and safety of anyone who may be affected by work activities. Safe systems of
work and effective preventative measures to achieve this can only be developed following effective risk
assessments.

CLASSES AND TYPES OF MEDICAL
ELECTRICAL EQUIPMENT
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All electrical equipment is categorised into classes according to the method of protection against electric
shock that is used. For mains powered electrical equipment there are usually two levels of protection
used, called "basic" and "supplementary" protection. The supplementary protection is intended to come
into play in the event of failure of the basic protection.

4.1 CLASS I EQUIPMENT
Class I equipment has a protective earth. The basic means of protection is the insulation between live
parts and exposed conductive parts such as the metal enclosure. In the event of a fault that would
otherwise cause an exposed conductive part to become live, the supplementary protection (i.e. the
protective earth) comes into effect. A large fault current flows from the mains part to earth via the
protective earth conductor, which causes a protective device (usually a fuse) in the mains circuit to
disconnect the equipment from the supply.
It is important to realise that not all equipment having an earth connection is necessarily class I. The earth
conductor may be for functional purposes only such as screening. In this case the size of the conductor
may not be large enough to safely carry a fault current that would flow in the event of a mains short to
earth for the length of time required for the fuse to disconnect the supply.

Class I medical electrical equipment should have fuses at the equipment end of the mains supply lead in
both the live and neutral conductors, so that the supplementary protection is operative when the
equipment is connected to an incorrectly wired socket outlet.
Further confusion can arise due to the use of plastic laminates for finishing equipment. A case that
appears to be plastic does not necessarily indicate that the equipment is not class I.
There is no agreed symbol in use to indicate that equipment is class I and it is not mandatory to state on
the equipment itself that it is class I. Where any doubt exists, reference should be made to equipment
manuals.
The symbols below may be seen on medical electrical equipment adjacent to terminals.

Figure 6. Symbols seen on earthed
equipment.

4.2 CLASS II EQUIPMENT
The method of protection against electric shock in the case of class II equipment is either double
insulation or reinforced insulation. In double insulated equipment the basic protection is afforded by the
first layer of insulation. If the basic protection fails then supplementary protection is provided by a second
layer of insulation preventing contact with live parts.
In practice, the basic insulation may be afforded by physical separation of live conductors from the
equipment enclosure, so that the basic insulation material is air. The enclosure material then forms the
supplementary insulation.
Reinforced insulation is defined in standards as being a single layer of insulation offering the same
degree of protection against electric shock as double insulation.
Class II medical electrical equipment should be fused at the equipment end of the supply lead in either
mains conductor or in both conductors if the equipment has a functional earth.
The symbol for class II equipment is two concentric squares illustrating double insulation as shown
below.

Figure 7. Symbol for class II equipment

4.3 CLASS III EQUIPMENT
Class III equipment is defined in some equipment standards as that in which protection against electric
shock relies on the fact that no voltages higher than safety extra low voltage (SELV) are present. SELV is
defined in turn in the relevant standard as a voltage not exceeding 25V ac or 60V dc.
In practice such equipment is either battery operated or supplied by a SELV transformer.
If battery operated equipment is capable of being operated when connected to the mains (for example, for
battery charging) then it must be safety tested as either class I or class II equipment. Similarly, equipment
powered from a SELV transformer should be tested in conjunction with the transformer as class I or class
II equipment as appropriate.
It is interesting to note that the current IEC standards relating to safety of medical electrical equipment do
not recognise Class III equipment since limitation of voltage is not deemed sufficient to ensure safety of
the patient. All medical electrical equipment that is capable of mains connection must be classified as
class I or class II. Medical electrical equipment having no mains connection is simply referred to as
"internally powered".

4.4 EQUIPMENT TYPES
As described above, the class of equipment defines the method of protection against electric shock. The
degree of protection for medical electrical equipment is defined by the type designation. The reason for
the existence of type designations is that different pieces of medical electrical equipment have different
areas of application and therefore different electrical safety requirements. For example, it would not be
necessary to make a particular piece medical electrical equipment safe enough for direct cardiac
connection if there is no possibility of this situation arising.
Table 1 shows the symbols and definitions for each type classification of medical electrical equipment.
Type Symbol

B

Definition
Equipment providing a particular degree of protection against electric
shock, particularly regarding allowable leakage currents and reliability of
the protective earth connection (if present).

BF

As type B but with isolated or floating (F - type) applied part or parts.

CF

Equipment providing a higher degree of protection against electric shock
than type BF, particularly with regard to allowable leakage currents, and
having floating applied parts.
Table 1. Medical electrical equipment types

All medical electrical equipment should be marked by the manufacturer with one of the type symbols
above.

CURRENT THINKING ON TESTING
PROTECTIVE EARTHING
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Last Updated: Sunday, 06 October 2013 12:38

John Backes, Product Manager at Rigel Medical, part of the Seaward Group, considers the merits of 25A
and 200mA test currents for testing protective earthing conductors in electromedical devices.
Debate in the medical industry on the most appropriate test current for checking the integrity of the
protective earthing conductor of medical devices has been around for many years.
Historically, some have traditionally favoured a higher test current of 25A or 10A originating from the IEC
60601-1 requirements on the premise that it will best detect any damaged conductors present. In addition,
when analogue instruments were widely used for low resistance measurement, it was often necessary to
use high test currents to produce sufficient voltage drop across the sample to generate the necessary
needle deflection.
With modern electronics and digital technology, this is no longer necessary and more recently, given the
growth in hand held test instruments, others have come to favour a lower test current of 1A or less as a
means of eliminating any risk of damage to the equipment under test.

In reality, the different test currents both have their merits. Various International Standards and Code of
Practices for In-service Testing and Inspection of Medical Electrical Equipment recommend a variety of
test currents from 25A down to 200mA. However, for routine testing and testing after repair of nonmedical appliances and testing of fixed installations, the majority of European standards now specify a
test current of 200mA.

PROTECTIVE EARTHING CONDUCTORS
Protective earthing conductors are designed to prevent electric shock by allowing the passage of electric
current under fault conditions. In Class I electrical equipment the protective earthing conductor resistance
needs to be of sufficiently low value to prevent the voltage on external metal parts rising to a level where
the shock potential presents a hazard to life.
A variety of national and international standards define a maximum acceptable level of resistance of a
protective earthing conductor and the precautions associated with medical equipment are significantly
greater than those associated with industrial commercial and electrical products.
These standards not only lay down the maximum resistance values but also define the test current, the
open circuit voltage and the duration of that test. Depending on the time at which the tests have been
conducted, different criteria will apply at the design stage, the conformance testing stage, manufacturing
and in service testing.
With any item of electromedical equipment it is likely that the protective earthing conductor will comprise
various lengths of flexible cable linking the equipment to the point of electrical supply. It is also possible
that various types of switching mechanism may exist including relays and electrical switches.
Any measurement of a protective earthing conductor will therefore encounter both bulk and contact forms
of electrical resistance. Both these types of resistance can have implications on the use of different test
methods with varying currents, voltages and time durations.
Figure 1 shows the different types of resistance making up the total measured resistance.

Bulk resistance is the material along the conductors' path. This will tend to be constant although it will be
affected by temperature and in certain cases by physical pressure.
Contact resistance, however, is a variable resistance that occurs at the interface between two conducting
surfaces. Contact resistance is made up of constriction resistance and film resistance and will be
dependent on the contact force between the two surfaces in contact.
Careful inspection of the contact interface between two conducting materials will show that surfaces that
may appear flat and uniform to the naked eye will invariably comprise a series of rough peaks and valleys
when viewed under a microscope.
In reality, the two mating surfaces will therefore only make contact with each other where the surface
peaks (asperites) meet and the actual surface area of this real contact area is typically much smaller than
may be apparent.
In these circumstances constriction resistance occurs as the electrical current is channelled through small
point contacts that occur at these peak points or interfaces. Layers of oxide and dirt that are formed on
the material's surface also create film resistance. These oxides have higher resistance than the
conducting material on either side of the junction.
Constriction resistance could be reduced by increasing the force applied between the two surfaces as
shown in Figure 2 below.

Film resistance is typically overcome by cleaning the surfaces between the two contacts although this is
not always practical and oxidation might occur again immediately after a connection has been cleaned.
Figure 3 shows the effects on the total resistance by reducing the constriction resistance. Unlike film
resistance, constriction resistance and therefore the total resistance is reduced by increasing the force
applied between the two surfaces. Bulk resistance is assumed as constant.

Test done at our laboratory demonstrate the effects of film resistance in relation to the level of current
passing through the contacts.

Figure 4 shows the effect of film resistance in relation to the test current in a connection within a typical
IEC lead. At each stage of the test, the test current was increased and the total resistance was measured.
As the test current is rising (shown in blue - rising), the film resistance is being reduced as a result. In this
test, the film resistance was completely eliminated at a test current of 8 Ampere and ones this point was
reached, the test current was reduced in steps (Shown in red - Falling).

Tests demonstrated that once film resistance was cleared in an existing connection; film resistance no
longer effected the total resistance measurement.
During our tests, bulk resistance and constriction resistance were kept as a constant.
The impact of these different types of resistance can therefore have significant impact on the results
obtained from varying levels of test current. It follows, therefore that the level of test current will affect the
measurement when film resistance is considered.

HIGH CURRENT TESTING
The perceived benefit of the relatively high 25A test current is that it will be capable of overcoming the
implications of film resistance.
However, and conversely, excessively high levels of test current will cause temperature rise throughout
the protective earthing conductor path. If applied long enough will have a significant impact on the
resistance value measures.
In the event of a damaged protective earthing conductor, where most strands are broken, a high current
test may also detect the damage by 'fusing' the cable.
Fusing occurs due to the heating effect of the test current - the current flows, generating heat and the wire
melts apart resulting in an open circuit. The fusing action is produced by a temperature rise in the cable
and it therefore takes a finite time for the cable to fuse.

The temperature rise and hence the ability to fuse a damaged cable depends upon the test current and
the test duration. In protective fuses this is referred to as the I2t rating. The higher the current or the
longer the test duration the higher the probability of fusing the damaged cable.
The probability of the test fusing a cable with broken strands will therefore depend on:
a.

how many strands are broken

b.

the magnitude of the test current

c.

the duration of the test
However, tests carried out on a stranded 1.5mm² - 48 x 0.22mm² cable, using a 25Amps AC constant
current learned that 95% of the strands needed to be broken to fuse the lead in 30 seconds. In practise
however, earth continuity tests are carried out in shorter time duration, typically 2 to 5 seconds during
routine maintenance, making the likelihood of fusing at 25 Ampere unlikely.
The purpose of the earth continuity test is to ensure that accessible conductive parts, which rely upon
protective earthing as a means of protection against electric shock, are connected to the protective earth
of the supply.
There may also be accessible conductive parts which are connected to protective earth for functional
reasons such as signal screening and these earth paths may not be designed to carry high currents.
Passing a high test current through them may therefore result in damage to the equipment under test.

200MA TESTING
A 200mA test current is rapidly becoming the European standard for inservice testing and testing after
repair. In particular, those test instruments that comply with the requirements of the VDE 0751 (German
standard) and the imminent IEC 62353 (Standard for in-service and routine testing of Medical Electronic
Equipment) are capable of making accurate resistance measurements using a 200mA test current.
The use of a lower test current such as 200mA also reduces or eliminates the risk of damage to the
equipment under test caused by passing high test currents through paths to ground that are not intended
to provide protective earthing.
One of the reasons often provided for the use of a higher test current is that the resistance values being
measured are in the order of 0.1 ohms and, in principle a higher test current will aid the measurement
process. However, this particular argument loses some of its merits, with modern advances in test
technology enabling very accurate resistance measurements to be made using low test currents.
Recently new test technology has been pioneered in the form of a newpatented low energy, high current
test that overcomes the previous contact resistance problems that inhibited the wider application of
protective earth testing using 1A or 200mA test currents.

As a result the new concept successfully conquers variations in measurement that can be caused by high
film resistance between the test probe and the electromedical equipment under test, for example, when
measuring continuity of tarnished parts in detachable IEC power cables.
Importantly, the new low current test technology enables valid earth continuity tests to be carried out
using battery powered testers, significantly increasing the portability and versatility of hand held safety
analysers and speeding up the testing process.

SUMMARY
Both 25A and 200mA are recommended internationally as a valid test current for in-service testing and
inspection of medical electrical equipment and both are of value to biomedical engineers and technicians.
However, a high test current doesn't necessarily detect a damaged protective earth path and does not
always give better accuracy. In addition, modern electronic technology means that low current testing can
now be applied more effectively than may have been the case in the past.
Whatever the test current, contact resistance is an ever present variable. However, a low energy, high
current pulse prior to a 200mA test can overcome such problems. Furthermore, low current 200mA earth
continuity testing has a further advantage in that it can be undertaken with battery power rather than
mains supply, enabling significant design and practical improvements to be incorporated in modern
electromedical safety testers.

CONCLUSION
Provided modern techniques are used to measure the ground resistance during routine testing and
contact resistance is properly addressed, for example by using a low energy high current pulse prior to
200mA test current, the lower test current is preferred for routine field maintenance as this would provide
you with the benefits of:


Increased safety of the operator



Reduced risk of damage to the in-service medical equipment (DUT)



Smaller test instruments to include valid ground bond measurements



Battery operated test equipment



Increased flexibility of the test engineer due to lightweight test equipment



Cost reduction due to reduced down time of medical equipment



More economical availability of test equipment

John Backes is active as UK representative in working group 14 (testing to the general standard) of IEC
subcommittee 62A: Common Aspects of Electrical Equipment in Medical Practise.
Rigel Medical, part of the Seaward Group is a market-leading manufacturer of portable biomedical test
equipment. The company has pioneered the introduction of a range of test instruments and technical
innovations to make the safety testing of medical devices faster and easier for contractors and service
engineers.

Seaward Electronic Ltd. is based at Bracken Hill, South West Industrial Estate, Peterlee, County Durham,
SR8 2SW
Tel. (0191) 586 3511 Fax. (0191) 586 0227
E mail: [email protected] Web : www.seaward.co.uk

HAZARDS OF MEDICAL ELECTRICAL

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EQUIPMENT
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Article courtesy of Eastwood Park

1 HAZARDS OF MEDICAL ELECTRICAL EQUIPMENT
Medical electrical equipment can present a range of hazards to the patient, the user, or to service
personnel. Many such hazards are common to many or all types of medical electrical equipment, whilst
others are peculiar to particular categories of equipment.
The hazard presented by electricity exists in all cases where medical electrical equipment is used, and
there is therefore both a moral and legal obligation to take measures to minimise the risk. Because there
is currently very little official guidance on precisely what measures should be in place in order to achieve
this in respect to medical equipment, user organisations have developed procedures based on their own

experience and risk assessments. The information in these notes is intended to assist in the development
of suitable procedures to this end.
Any test and inspection regime intended to minimise the electrical risks from medical electrical equipment
should take into account the likely degree of risk from electrical hazards compared to other hazards of
medical equipment. For this reason, various hazards associated with medical electrical equipment are
discussed briefly below.

1.1 MECHANICAL HAZARDS
All types of medical electrical equipment can present mechanical hazards. These can range from
insecure fittings of controls to loose fixings of wheels on equipment trolleys. The former may prevent a
piece of life supporting equipment from being operated properly, whilst the latter could cause serious
accidents in the clinical environment.
Such hazards may seem too obvious to warrant mentioning, but it is unfortunately all too common for
such mundane problems to be overlooked whilst problems of a more technical nature are addressed.

1.2 RISK OF FIRE OR EXPLOSION
All mains powered electrical equipment can present the risk of fire in the event of certain faults occurring
such as internal or external short circuits. In certain environments such fires may cause explosions.
Although the use of flammable anaesthetics is not common today, it should be recognised that many of
the medical gases currently in use, such oxygen or nitrous oxide, vigorously support combustion.
Wherever there is an elevated concentration of such gases, there is an increased risk of fire initiated by
electrical faults.

1.3 ABSENCE OF FUNCTION
Since many pieces of medical electrical equipment are life supporting or monitor vital functions, the
absence of function of such a piece of equipment would not be merely inconvenient, but could threaten
life.

1.4 EXCESSIVE OR INSUFFICIENT OUTPUT
In order to perform its desired function equipment must deliver its specified output. Too high an output, for
example, in the case of surgical diathermy units, would clearly be hazardous. Equally, too low an output
would result in inadequate therapy, which in turn may delay patient recovery, cause patient injury or even
death. This highlights the importance of correct calibration procedures.

1.5 INFECTION

Medical equipment that has been inadequately decontaminated after use may cause infection through the
transmission of microorganisms to any person who subsequently comes into contact with it. Clearly,
patients, nursing staff and service personnel are potentially at risk here.

1.6 MISUSE
Misuse of equipment is one of the most common causes of adverse incidents involving medical devices.
Such misuse may be a result of inadequate user training or of poor user instructions.

1.7 RISK OF EXPOSURE TO SPURIOUS ELECTRIC CURRENTS
All electrical equipment has the potential to expose people to the risk of spurious electric currents. In the
case of medical electrical equipment, the risk is potentially greater since patients are intentionally
connected to such equipment and may not benefit from the same natural protection factors that apply to
people in other circumstances. Whilst all of the hazards listed are important, the prevention of many of
them require methods peculiar to the particular type of equipment under consideration. For example, in
order to avoid the risk of excessive output of surgical diathermy units, knowledge of radio frequency
power measurement techniques is required. However, the electrical hazards are common to all types of
medical electrical equipment and can minimised by the use of safety testing and inspection regimes which
can be applied to all types of medical electrical equipment.


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LEAKAGE CURRENTS
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Last Updated: Tuesday, 22 October 2013 19:53

Article courtesy of Eastwood Park

Most safety testing regimes for medical electrical equipment involve the measurement of certain "leakage
currents", because the level of them can help to verify whether or not a piece of equipment is electrically
safe. In this section the various leakage currents that are commonly measurable with medical equipment
safety testers are described and their significance discussed. The precise methods of measurement along
with applicable safe limits are discussed later under paragraphs at 6.

3.1 CAUSES OF LEAKAGE CURRENTS

If any conductor is raised to a potential above that of earth, some current is bound to flow from that
conductor to earth. This is true even of conductors that are well insulated from earth, since there is no
such thing as perfect insulation or infinite impedance. The amount of current that flows depends on:
a.

the voltage on the conductor.

b.

the capacitive reactance between the conductor and earth.

c.

the resistance between the conductor and earth.
The currents that flow from or between conductors that are insulated from earth and from each other are
called leakage currents, and are normally small. However, since the amount of current required to
produce adverse physiological effects is also small, such currents must be limited by the design of
equipment to safe values.
For medical electrical equipment, several different leakage currents are defined according to the paths
that the currents take.

3.2 EARTH LEAKAGE CURRENT
Earth leakage current is the current that normally flows in the earth conductor of a protectively earthed
piece of equipment. In medical electrical equipment, very often, the mains is connected to a transformer
having an earthed screen. Most of the earth leakage current finds its way to earth via the impedance of
the insulation between the transformer primary and the inter-winding screen, since this is the point at
which the insulation impedance is at its lowest (see figure 2).

Figure 2. Earth leakage current path

Under normal conditions, a person who is in contact with the earthed metal enclosure of the equipment
and with another earthed object would suffer no adverse effects even if a fairly large earth leakage current
were to flow. This is because the impedance to earth from the enclosure is much lower through the
protective earth conductor than it is through the person. However, if the protective earth conductor
becomes open circuited, then the situation changes. Now, if the impedance between the transformer
primary and the enclosure is of the same order of magnitude as the impedance between the enclosure
and earth through the person, a shock hazard exists.

It is a fundamental safety requirement that in the event of a single fault occurring, such as the earth
becoming open circuit, no hazard should exist. It is clear that in order for this to be the case in the above
example, the impedance between the mains part (the transformer primary and so on) and the enclosure
needs to be high. This would be evidenced when the equipment is in the normal condition by a low earth
leakage current. In other words, if the earth leakage current is low then the risk of electric shock in the
event of a fault is minimised.

3.3 ENCLOSURE LEAKAGE CURRENT OR TOUCH CURRENT
The terms "enclosure leakage current" and "touch current" should be taken to be synonymous. The
former term is used in the bulk of this text. The terms are further discussed in connection with the
electrical test methods underparagraphs 6.6. Enclosure leakage current is defined as the current that
flows from an exposed conductive part of the enclosure to earth through a conductor other than the
protective earth conductor.

If a protective earth conductor is connected to the enclosure, there is little point in attempting to measure
the enclosure leakage current from another protectively earthed point on the enclosure, since any
measuring device used is effectively shorted out by the low resistance of the protective earth. Equally,
there is little point in measuring the enclosure leakage current from a protectively earthed point on the
enclosure with the protective earth open circuit, since this would give the same reading as measurement
of earth leakage current as described above. For these reasons, it is usual when testing medical electrical
equipment to measure enclosure leakage current from points on the enclosure that are not intended to be
protectively earthed (see figure 3). On many pieces of equipment, no such points exist. This is not a
problem. The test is included in test regimes to cover the eventuality where such points do exist and to
ensure that no hazardous leakage currents will flow from them.

Figure 3. Enclosure leakage current path

3.4 PATIENT LEAKAGE CURRENT
Patient leakage current is the leakage current that flows through a patient connected to an applied part or
parts. It can either flow from the applied parts via the patient to earth or from an external source of high
potential via the patient and the applied parts to earth. Figures 4a and 4b illustrate the two scenarios.

Figure 4a. Patient leakage current path from equipment

Figure 4b. Patient leakage current path to equipment

3.5 PATIENT AUXILIARY CURRENT
The patient auxiliary current is defined as the current that normally flows between parts of the applied part
through the patient, which is not intended to produce a physiological effect (see figure 5).

Figure 5. Patient auxiliary current path

PHYSIOLOGICAL EFFECTS OF

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ELECTRICITY
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Category: Electrical Safety
Last Updated: Sunday, 06 October 2013 12:19

Article courtesy of Eastwood Park

2.1 ELECTROLYSIS
The movement of ions of opposite polarities in opposite directions through a medium is called electrolysis
and can be made to occur by passing DC current through body tissues or fluids. If a DC current is passed
through body tissues for a period of minutes, ulceration begins to occur. Such ulcers, while not normally
fatal, can be painful and take long periods to heal.

2.2 BURNS
When an electric current passes through any substance having electrical resistance, heat is produced.
The amount of heat depends on the power dissipated (I2R). Whether or not the heat produces a burn
depends on the current density.
Human tissue is capable of carrying electric current quite successfully. Skin normally has a fairly high
electrical resistance while the moist tissue underneath the skin has a much lower resistance. Electrical
burns often produce their most marked effects near to the skin, although it is fairly common for internal
electrical burns to be produced, which, if not fatal, can cause long lasting and painful injury.

2.3 MUSCLE CRAMPS
When an electrical stimulus is applied to a motor nerve or a muscle, the muscle does exactly what it is
designed to do in the presence of such a stimulus i.e. it contracts. The prolonged involuntary contraction
of muscles (tetanus) caused by an external electrical stimulus is responsible for the phenomenon where a
person who is holding an electrically live object can be unable to let go.

2.4 RESPIRATORY ARREST
The muscles between the ribs (intercostal muscles) need to repeatedly contract and relax in order to
facilitate breathing. Prolonged tetanus of these muscles can therefore prevent breathing.

2.5 CARDIAC ARREST
The heart is a muscular organ, which needs to be able to contract and relax repetitively in order to
perform its function as a pump for the blood. Tetanus of the heart musculature will prevent the pumping
process.

2.6 VENTRICULAR FIBRILLATION
The ventricles of the heart are the chambers responsible for pumping blood out of the heart. When the
heart is in ventricular fibrillation, the musculature of the ventricles undergoes irregular, uncoordinated
twitching resulting in no net blood flow. The condition proves fatal if not corrected in a very short space of
time.

Ventricular fibrillation can be triggered by very small electrical stimuli. A current as low as 70 mA flowing
from hand to hand across the chest, or 20µA directly through the heart may be sufficient. It is for this
reason that most deaths from electric shock are attributable to the occurrence of ventricular fibrillation.

2.7 EFFECT OF FREQUENCY ON NEURO-MUSCULAR STIMULATION
The amount of current required to stimulate muscles is dependent to some extent on frequency. Referring
to figure 1, it can be seen that the smallest current required to prevent the release of an electrically live
object occurs at a frequency of around 50 Hz. Above 10 kHz the neuro-muscular response to current
decreases almost exponentially.

Figure 1. Current required to prevent release of a live object.

2.8 NATURAL PROTECTION FACTORS
Many people have received electric shocks from mains potentials and above and lived to tell the tale. Part
of the reason for this is the existence of certain natural protection factors.
Ordinarily, a person subject to an unexpected electrical stimulus is protected to some extent by automatic
and intentional reflex actions. The automatic contraction of muscles on receiving an electrical stimulus
often acts to disconnect the person from the source of the stimulus. Intentional reactions of the person
receiving the shock normally serve the same purpose. It is important to realise that a patient in the clinical
environment who may have electrical equipment intentionally connected to them and may also be
anaesthetised is relatively unprotected by these mechanisms.
Normally, a person who is subject to an electric shock receives the shock through the skin, which has a
high electrical resistance compared to the moist body tissues below, and hence serves to reduce the
amount of current that would otherwise flow. Again, a patient does not necessarily enjoy the same degree
of protection. The resistance of the skin may intentionally have been lowered in order to allow good
connections of monitoring electrodes to be made or, in the case of a patient undergoing surgery, there
may be no skin present in the current path.

The absence of natural protection factors as described above highlights the need for stringent electrical
safety specifications for medical electrical equipment and for routine test and inspection regimes aimed at
verifying electrical safety.

BIBLIOGRAPHY (ARTICLES A-I)
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Category: Electrical Safety
Last Updated: Sunday, 06 October 2013 12:37

Reference has been made to the following documents in the preparation of these notes.
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BS 5724 Part 1: 1989/IEC601-1:1988 - British Standard - Medical electrical equipment - Part 1.
General requirements for safety

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IEC 60601-1 Third edition: 2005 - International standard - Medical electrical equipment - Part 1:
General requirements for basic safety and essential performance

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Health Equipment Information Number 95 (HEI95) August 1981: Code of practice for acceptance
testing of medical electrical equipment (withdrawn Dec 1999)

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MDA Device Bulletin DB9801 Supplement 1: December 1999 - Checks and tests for newly
delivered medical devices (replaced November 2006)

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MHRA Device Bulletin DB2006(05): Managing Medical Devices - Guidance for healthcare and
social services organisations

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HSE memorandum of guidance on the Electricity at Work Regulations 1989 (ISBN 0-11-8839632)

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IEE Code of Practice for In-service Inspection and Testing of Electrical Equipment (ISBN 085296-776-4)