Electrical Engineering Portal.com Megger Insulation Resistance Test

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electrical-engineering-portal.com http:/ htt p://elec /electri trical-engineering-po cal-engineering-po rta rtal.c l.co o m/megger- insulation- resistance-test#.UoVT resistance-test#.UoVTXiK XiKHa2g.f Ha2g.faceboo aceboo k

Megger In Ins sul ula at ion Res Resistance istance Test Test Edvard

Reme mbe r that that goo d insulation has has high resis tan tance ce ; poo r insulation, insulation, relatively relatively low resistance. The actual actual resis tan tance ce values can be hig her or lo wer values wer,, dep end ing upo n such factors as the the temp erature or moi sture content of the the insulation (resistance (resistan ce dec reases in tempe rat rature ure or moisture).

Good Ins Insula ulatio tion n Resistance Resistance? ?  As yo u know kno w goo go o d insulat ins ulatio ion n has high res ist ance; po o r ins ulat ulatio ion, n, relat ively low res r esis istt ance. The act a ctual ual resist anc ance e values values can be higher higher or lower, depe dependin nding g upon such f actors as t he temperature temperature or mois ture content of the insulation (resist ance decre decreases ases in temperature temperature or moist ure). With a little record- keep With keeping ing and and common common s ense, however, however, you can get a go od picture o f the insulation condition f rom values values t  t hat are o nly relative. relative. T he Megge Meggerr insulation t est er is a s mall all,, port abl able e instrument instrument t hat gives you a direct reading of insulation resistance in ohms or megohms. megohms . For For goo d insulation, the res ist anc ance e usually reads reads in t he megohm range. range . The Megger insu insulation lation t  t est er is es sentially a high-range high-range resist ance meter meter (o hm hmm meter) with a built- in directdirec t- curren currentt gene generato rato r. T his meter meter is of specia speciall const ruction with bot h current current and volt age coils, coils, enabling enabling true o hm hms s to be read directly directly,, independen independentt of the actual volt age applie applied. d. This method is non-destructive; that is, it does not cause deterioration of the insulation.

T he generato r can be handcranked or line-o perated to develop a high DC voltage which causes a small current through and over surf aces o f  the insulation being test ed (Fig. 2 ). This current (usually at an applied volt age of 500 volts or more) is measured by the ohmmeter, which has an indicating scale.

Figure 2 - Typic al Megg er tes t instrument hook-up to measure insulation re sistance.

Fig. 3 s hows a typical scale, which reads increasing resist ance values f rom left up to inf inity, or a res istance to o high to be measured.

What is “Good” Insulatio n? Every electric wire in your plant – whet her it’s in a moto r, generato r, cable, switch, transf ormer, etc. – is carefully covered with s ome f orm of electrical insulation. The wire itself is usually copper or aluminum, which is known to be a good conducto r of the electric current t hat powers yo ur equipment. T he insulation must be just the o pposit e f rom a conducto r: it s hould resist current and keep the current in its path along the conducto r. To understand insulation te sting you really don’t need to go into t he mathematics o f electricity, but one simple equatio n – o hm’s law – can be very helpf ul in appreciating many aspects . even if yo u’ve been exposed to this law befo re, it may be a goo d idea to review it in the light o f insulation tes ting.

Purpose of megger test T he purpose o f insulatio n around a conductor is much like that o f a pipe carrying water , and ohm’s law of  electricity can be more easily underst ood by a comparison with wat er f low. In Figure 1 we show this compariso n. Press ure on water f rom a pump causes f low along the pipe ( Fig. 1a). If the pipe were to spring a leak, you’d waste water and lose some water pressure. With electricity, voltage is like the pump pressure, causing electricity to f low along the copper wire ( Fig. 1b).  As in a wat er pipe, t here is so me resis tance t o f low, but it is much les s along t he wire t han it is t hro ugh t he insulation. Common sense tells us that the more voltage we have, the more current there’ll be.  Also , the lower the Figure 1 - Compariso n of water flow (a) with elec tric current (b) resistance of the wire, the more current f or t he same volt age. Actually, this is o hm’s law, which is expressed this way in equatio n form: e= IxR

where, e = volt age in volts I = current in amperes R = res istance in ohms Not e, however, that no insulation is perfect (t hat is, has inf inite resist ance) so s ome electricity does f low along the insulation o r t hrough it t o ground. Such a current may only be a millionth o f an ampere (one microampere) but it is the basis o f insulatio n test ing equipment. not e also t hat a higher voltage tends t o cause more current t hrough the insulation. T his small amount o f current would not , of course, harm goo d insulation but would be a problem if the insulatio n has deteriorat ed. now, to sum up our answer to the quest ion “what is ‘goo d’ insulation?” We have seen t hat, ess entially, “goo d” means a relatively high resist ance to current. Used t o describe an insulation material, “good” would also mean “the ability to keep a high resistance.” So, a suitable way of  measuring resistance can tell you how “goo d” the insulation is . Also , if you t ake measurements at regular  periods, you can check trends t oward its det erioration (more on t his later).

What Make s Insulat ion Go Bad? When your plant electrical system and equipment are new, the electrical insulation should be in top notch shape. Furthermore, manufacturers of wire, cable, mot ors , and so on have continually improved t heir  insulatio ns f or s ervices in industry. nevertheless, even to day, insulatio n is subject t o many ef f ects which can cause it to f ail – mechanical damage, vibration, excess ive heat o r cold, dirt, o il, corro sive vapors, mois ture f rom process es, o r just the humidity o n a muggy day. In various degrees, t hese enemies of insulatio n are at work as time goes on – combined with t he electrical st ress es that exist . As pin holes or cracks develop, mois ture and fo reign matt er penetrate t he surf aces of  the insulation, providing a low resist ance path f or leakage current. Once st arted, the dif f erent enemies tend t o aid each other, permitt ing excess ive current t hrough the insulation. Sometimes the dro p in insulat ion res ist ance is s udden, as when equipment is f loo ded. Usually, however, it dro ps gradually, giving plenty o f warning, if checked periodically. Such checks permit planned reconditioning bef ore s ervice f ailure. If there are no checks, a mot or with poo r insulatio n, f or example, may not o nly be dangerous t o t ouch when voltage is applied, but also be subject to burn out. What was go od insulation has become a partial conductor.  Advertisement Resource: Megger handbook

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