A Stitch in Time...

A STITCH IN TIME

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The Complete
Guide to Electrical
Insulation Testing
A STITCH IN TIME

A Stitch In Time
The Complete Guide to
Electrical Insulation Testing
Copyright 2006 
A STITCH IN TIME
ConTEnTS
PAGE
WHAT IS Good InSUlATIon? ........................................................................ 3
WHAT MAkeS InSUlATIon Go BAd? ................................................................ 4
HoW InSUlATIon ReSISTAnCe IS MeASURed .................................................. 5
HoW To InTeRPReT ReSISTAnCe ReAdInGS .................................................... 6
FACToRS AFFeCTInG InSUlATIon ReSISTAnCe ReAdInGS .............................. 8
TyPeS oF InSUlATIon ReSISTAnCe TeSTS ....................................................... 10
TeST VolTAGe VS. eqUIPMenT RATInG .......................................................... 16
AC TeSTInG VS. dC ............................................................................................ 17
USe oF dC dIeleCTRIC TeST SeT ...................................................................... 18
TeSTS dURInG dRyInG oUT oF eqUIPMenT .................................................. 18
eFFeCT oF TeMPeRATURe on InSUlATIon ReSISTAnCe................................ 21
eFFeCTS oF HUMIdITy ...................................................................................... 23
PRePARATIon oF APPARATUS To TeST ........................................................... 24
SAFeTy PReCAUTIonS ...................................................................................... 26
ConneCTIonS FoR TeSTInG InSUlATIon ReSISTAnCe oF
eleCTRICAl eqUIPMenT .................................................................................. 27
AddITIonAl noTeS ABoUT USInG A MeGGeR InSUlATIon TeSTeR ........... 33
InTeRPReTATIon-MInIMUM VAlUeS .............................................................. 36
MInIMUM VAlUeS FoR InSUlATIon ReSISTAnCe .......................................... 38
TeSTS USInG MUlTI-VolTAGe MeGGeR InSUlATIon TeSTeRS ...................... 42
STeP-VolTAGe MeTHod .................................................................................. 48
USe oF A GUARd TeRMInAl ............................................................................ 50
BUSHInGS, PoTHeAdS And InSUlAToRS ....................................................... 54
oUTdooR oIl CIRCUIT BReAkeRS ................................................................... 57
SeTTInG UP A MAInTenAnCe PRoGRAM ....................................................... 60
HoW oFTen SHoUld yoU TeST? .................................................................... 60
MeGGeR 5 And 10 kV InSUlATIon TeSTeRS .................................................. 62
A STITCH IN TIME

WHAT IS Good InSulATIon?
every electric wire in your plant whether its in a motor, generator, cable,
switch, transformer, etc. is carefully covered with some form of electrical
insulation. The wire itself is usually copper or aluminum, which is known to
be a good conductor of the electric current that powers your equipment. The
insulation must be just the opposite from a conductor: it should resist current
and keep the current in its path along the conductor.
To understand insulation testing you really dont need to go into the
mathematics of electricity, but one simple equation ohms law can be very
helpful in appreciating many aspects. even if youve been exposed to this law
before, it may be a good idea to review it in the light of insulation testing.
The purpose of insulation around a conductor is much like that of a pipe
carrying water, and ohms law of electricity can be more easily understood by
a comparison with water flow. In Fig. 1 we show this comparison. Pressure on
water from a pump causes flow along the pipe (Fig. 1a). If the pipe were to
spring a leak, youd waste water and lose some water pressure.
With electricity, voltage is like the pump pressure, causing electricity to flow
along the copper wire (Fig. 1b). As in a water pipe, there is some resistance
to flow, but it is much less along the wire than it is through the insulation.
Figure 1Comparison of water flow (a) with electric current (b). 
A STITCH IN TIME
Common sense tells us that the more voltage we have, the more current
therell be. Also, the lower the resistance of the wire, the more current for
the same voltage.
Actually, this is ohms law, which is expressed this way in equation form:

e = I x R
where,
e = voltage in volts

I = current in amperes

R = resistance in ohms
note, however, that no insulation is perfect (that is, has infinite resistance) so
some electricity does flow along the insulation or through it to ground. Such
a current may only be a millionth of an ampere (one microampere) but it is
the basis of insulation testing equipment. note also that a higher voltage
tends to cause more current through the insulation. This small amount of
current would not, of course, harm good insulation but would be a problem
if the insulation has deteriorated.
now, to sum up our answer to the question what is good insulation?
We have seen that, essentially, good means a relatively high resistance
to current. Used to 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 good the insulation is. Also, if you
take measurements at regular periods, you can check trends toward its
deterioration (more on this later).
WHAT MAkES InSulATIon 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, motors, and so on have continually improved their insulations for
services in industry. nevertheless, even today, insulation is subject to many
effects which can cause it to fail mechanical damage, vibration, excessive
heat or cold, dirt, oil, corrosive vapors, moisture from processes, or just the
humidity on a muggy day.
In various degrees, these enemies of insulation are at work as time goes
on combined with the electrical stresses that exist. As pin holes or cracks
develop, moisture and foreign matter penetrate the surfaces of the
insulation, providing a low resistance path for leakage current.
A STITCH IN TIME

once started, the different enemies tend to aid each other, permitting
excessive current through the insulation.
Sometimes the drop in insulation resistance is sudden, as when equipment
is flooded. Usually, however, it drops gradually, giving plenty of warning,
if checked periodically. Such checks permit planned reconditioning before
service failure. If there are no checks, a motor with poor insulation, for
example, may not only be dangerous to touch when voltage is applied,
but also be subject to burn out. What was good insulation has become a
partial conductor.
HoW InSulATIon RESISTAnCE IS MEASuREd
you have seen that good insulation has high resistance; poor insulation,
relatively low resistance. The actual resistance values can be higher or lower,
depending upon such factors as the temperature or moisture content of the
insulation (resistance decreases in temperature or moisture). With a little
record-keeping and common sense, however, you can get a good picture of
the insulation condition from values that are only relative.
The Megger insulation tester is a small, portable instrument that gives you
a direct reading of insulation resistance in ohms or megohms. For good
insulation, the resistance usually reads in the megohm range.
The Megger insulation tester is essentially a high-range resistance meter
(ohmmeter) with a built-in direct-current generator. This meter is of special
construction with both current and voltage coils, enabling true ohms to be
read directly, independent of the actual voltage applied. This method is non-
destructive; that is, it does not cause deterioration of the insulation.
Figure 2Typical Megger test instrument hook-up to measure insulation resistance. 
A STITCH IN TIME
The generator can be hand-cranked or line-operated to develop a high
dC voltage which causes a small current through and over surfaces of the
insulation being tested (Fig. 2). This current (usually at an applied voltage
of 500 volts or more) is measured by the ohmmeter, which has an indicating
scale. Fig. 3 shows a typical scale, which reads increasing resistance values
from left up to infinity, or a resistance too high to be measured.
Figure 4Typical record of insulation resistance of a mill motor. Curve A shows test values as
measured; Curve B shows same values corrected to 20°C (see page 22), giving a definite downward
trend toward an unsafe condition. Reverse side of card (at right) is used to record the test data.
Figure 3Typical scale on the Megger insulation tester.
HoW To InTERPRET RESISTAnCE REAdInGS
As previously mentioned, insulation resistance readings should be considered
relative. They can be quite different for one motor or machine tested three
days in a row, yet not mean bad insulation. What really matters is the trend
in readings over a time period, showing lessening resistance and warning
of coming problems. Periodic testing is, therefore, your best approach to