Megger MIT1025, MIT410/2 Installation Manual
The Complete Guide
to Electrical Insulation
Testing
A STITCH IN TIME 1
WWW.MEGGER.COM
“A Stitch In Time”
The Complete Guide to
Electrical Insulation Testing
Copyright 2006
A STITCH IN TIME 1
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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
MEGGER 1 KV INSULATION TESTERS ................................................................64
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WHAT IS “GOOD” INSULATION?
Every electric wire in your plant – whether it’s 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 don’t need to go into the
mathematics of electricity, but one simple equation – Ohm’s law – can be very
helpful in appreciating many aspects. Even if you’ve 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 Ohm’s 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, you’d 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 1–Comparison of water flow (a) with electric current (b).
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Common sense tells us that the more voltage we have, the more current
there’ll be. Also, the lower the resistance of the wire, the more current for
the same voltage.
Actually, this is Ohm’s 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.
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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 nondestructive; that is, it does not cause deterioration of the insulation.
Figure 2–Typical Megger test instrument hook-up to measure insulation resistance.
A STITCH IN TIME 5
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 3–Typical 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
preventive maintenance of electrical equipment, using record cards as shown
in Fig. 4.
Figure 4–Typical 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.
6 A STITCH IN TIME
Whether you test monthly, twice a year, or once a year depends upon the
type, location, and importance of the equipment. For example, a small
pump motor or a short control cable may be vital to a process in your plant.
Experience is the best teacher in setting up the scheduled periods for your
equipment.
You should make these periodic tests in the same way each time. That is,
with the same test connections and with the same test voltage applied for
the same length of time. Also you should make tests at about the same
temperature, or correct them to the same temperature. A record of the
relative humidity near the equipment at the time of the test is also helpful
in evaluating the reading and trend. Later sections cover temperature
correction and humidity effects.
In summary, here are some general observations about how you can interpret
periodic insulation resistance tests, and what you should do with the result:
Condition What To Do
(a) Fair to high values No cause for concern.
and well maintained.
(b) Fair to high values, Locate and remedy the cause and
but showing a check the downward trend.
constant tendency
towards lower values.
(c) Low but well maintained. Condition is probably all right, but
cause of low values should be
checked.
(d) So low as to be unsafe. Clean, dry out, or otherwise raise the
values before placing equipment in
service. (Test wet equipment while
drying out.)
(e) Fair or high values, Make tests at frequent intervals until
previously well the cause of low values is located
maintained but showing and remedied; or until the values
sudden lowering. have become steady at a lower
level but safe for operation; or until
values become so low that it is unsafe
to keep the equipment in operation.
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FACTORS AFFECTING INSULATION RESISTANCE READINGS
Remember that the measured resistance (of the insulation) will be
determined by the voltage applied and the resultant current (R = E/I).
There are a number of things that affect current, including temperature
of the insulation and humidity, as mentioned in the previous section.
Right now, let’s just consider the nature of current through insulation
and the effect of how long voltage is applied.
Current through and along insulation is made up partly of a relatively
steady current in leakage paths over the insulation surface. Electricity
also flows through the volume of the insulation. Actually, as shown in
Fig. 5, our total current comprises three components:
1. Capacitance Charging Current
Current that starts out high and drops after the insulation has been
charged to full voltage (much like water flow in a garden hose when
you first turn on the spigot).
2. Absorption Current
Also an initially high current which then drops (for reasons discussed
under the section Time-Resistance Method).
3. Conduction or Leakage Current
A small essentially steady current both through and over the
insulation.
As shown in Fig. 5, the total current is the sum of the three components
and it is this current that can be measured directly by a microammeter,
or in terms of megohms at a particular voltage by means of a Megger
instrument (ohmmeter). Because the total current depends upon the
time that the voltage is applied, you can see now why Ohm’s Law R = E/I
only holds, theoretically, at an infinite time (that is, you’d have to wait
forever before taking a reading).
In practice, as you will see in the test methods described below, you
read a value that is the apparent resistance – a useful value to diagnose
troubles, which is what you want to do.
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Figure 5–Curves showing components of current measured during DC testing of insulation.
Note also in Fig. 5 that the charging current disappears relatively rapidly
as the equipment under test becomes charged. Larger units with more
capacitance will take longer to be charged. This current also is the stored
energy initially discharged after your test, by short-circuiting and grounding
the insulation. ALWAYS TAKE THIS SAFETY MEASURE.
You can see further from Fig. 5 that the absorption current decreases at a
relatively slow rate, depending upon the exact nature of the insulation. This
stored energy, too, must be released at the end of a test, and requires a
longer time than the capacitance charging current – about four times as long
as the voltage was applied.
With good insulation, the conduction or leakage current should build up to a
steady value that is constant for the applied voltage, as shown in Fig. 5. Any
increase of leakage current with time is a warning of trouble, as discussed in
the tests described in the following section.
A STITCH IN TIME 9
With a background now of how time affects the meaning of instrument
readings, let’s consider three common test methods: (1) short-time or spot
reading; (2) time-resistance; and (3) step or multi-voltage tests.
TYPES OF INSULATION RESISTANCE TESTS
Short-Time or Spot-Reading Test
In this method, you simply connect the Megger instrument across the
insulation to be tested and operate it for a short, specific time period
(60 seconds is usually recommended). As shown schematically in Fig. 6,
you’ve simply picked a point on a curve of increasing resistance values;
quite often the value would be less for 30 seconds, more for 60 seconds.
Bear in mind also that temperature and humidity, as well as condition of
your insulation affect your reading.
Figure 6–Typical curve of insulation resistance (in megohms) with time for the “short time” or
“spot-reading” test method.
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If the apparatus you are testing has very small capacitance, such as a
short run of house wiring, the spot reading test is all that is necessary.
However, most equipment is capacitive and so your very first spot reading
on equipment in your plant, with no prior tests, can be only a rough guide
as to how good or bad the insulation is. For many years, maintenance
professionals have used the one-megohm rule to establish the allowable
lower limit for insulation resistance. The rule may be stated:
Insulation resistance should be approximately one megohm for each 1,000
volts of operating voltage, with a minimum value of one megohm.
For example, a motor rated at 2,400 volts should have a minimum insulation
resistance of 2.4 megohms. In practice, megohm readings normally are
considerably above this minimum value in new equipment or when
insulation is in good condition.
By taking readings periodically and recording them, you have a better basis
of judging the actual insulation condition. Any persistent downward trend
is usually fair warning of trouble ahead, even though the readings may be
higher than the suggested minimum safe values. Equally true, as long as
your periodic readings are consistent, they may be ok, even though lower
than the recommended minimum values. The curves of Fig. 7 show typical
behavior of insulation resistance under varying plant operating conditions.
The curves were plotted from spot readings taken with a Megger instrument
over a period of months.
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Figure 7–Typical behavior of insulation resistance over a period of months under varying operating
conditions, (curves plotted from spot readings with a Megger instrument).
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Time-Resistance Method
This method is fairly independent of temperature and often can give
you conclusive information without records of past tests. It is based on
the absorption effect of good insulation compared to that of moist or
contaminated insulation. You simply take successive readings at specific times
and note the differences in readings (see curves, Fig. 8). Tests by this method
are sometimes referred to as absorption tests.
Note that good insulation shows a continual increase in resistance (less
current – see curve A) over a period of time (in the order of 5 to 10 minutes).
This is caused by the absorption current we spoke of earlier; good insulation
shows this charge effect over a time period much longer than the time
required to charge the capacitance of the insulation.
If the insulation contains much moisture or contaminants, the absorption
effect is masked by a high leakage current which stays at a fairly constant
value, keeping the resistance reading low (remember: R = E/I).
Figure 8–Typical curves showing dielectric absorption effect in a “time-resistance” test, made on
capacitive equipment such as a large motor winding.
The time-resistance test is of value also because it is independent of
equipment size. The increase in resistance for clean and dry insulation occurs
in the same manner whether a motor is large or small. You can, therefore,
compare several motors and establish standards for new ones, regardless of
their horsepower ratings.
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Fig. 9 shows how a 60-second test would appear for good and perhaps bad
insulation. When the insulation is in good shape, the 60-second reading is
higher than the 30-second reading.
Figure 9–Typical card plot of a time-resistance or double-reading test.
A further advantage of this double-reading test, as it is sometimes called,
is that it gives you a clearer picture, even when a spot reading says the
insulation looks fine.
For example, let’s say the spot reading on a synchronous motor was 10
megohms. Now, let’s assume that the double-reading check shows that the
insulation resistance holds steady at 10 megohms while you hold voltage up
to 60 seconds. This means there may be dirt or moisture in the windings that
bears watching. On the other hand, if the pointer shows a gradual increase
between the 30-second and the 60-second checks, then you’re reasonably
sure the windings are in good shape.
Time-resistance tests on large rotating electrical machinery – especially with
high operating voltage – require high insulation resistance ranges and a very
constant test voltage. A heavy-duty Megger test set, line-operated, serves
this need. Similarly, such an instrument is better adapted for large cables,
bushings, transformers and switchgear.
14 A STITCH IN TIME
Dielectric Absorption Ratio
TABLE I — Condition of Insulation Indicated by
The ratio of two time-resistance readings (such as a 60-second reading
divided by a 30-second reading) is called a dielectric absorption ratio. It is
useful in recording information about insulation. If the ratio is a 10-minute
reading divided by a 1-minute reading, the value is called the polarization
index.
With hand-cranked Megger instruments, it’s a lot easier for you to run the
test for only 60 seconds, taking your first reading at 30 seconds. If you have a
line-operated Megger instrument, you’ll get best results by running the test
10 minutes, taking readings at 1- and at 10-minutes, to get the polarization
index. Table I gives values of the ratios and corresponding relative
conditions of the insulation that they indicate.
Dielectric Absorption Ratios*
INSULATION
CONDITION
Dangerous
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Questionable
Good
Excellent
*These values must be considered tentative and relative—
subject to experience with the time-resistance method
over a period of time.
**In some cases, with motors, values approximately 20%
higher than shown here indicate a dry brittle winding which
will fail under shock conditions or during starts. For preventive maintenance, the motor winding should be cleaned,
treated, and dried to restore winding flexibility.
***These results would be satisfactory for equipment with very
low capacitance such as short runs of house wiring.
60/30-SECOND
RATIO
—
1.0 to 1.25
1.4 to 1.6
Above 1.6**
10/1-MINUTE RATIO
(POLARIZATION INDEX)
Less than 1
1.0 to 2***
2 to 4
Above 4**
TEST VOLTAGE VS. EQUIPMENT RATING
Commonly used DC test voltages for routine maintenance are as follows:
Equipment AC Rating DC Test Voltage
up to 100 volts 100 and 250 volts
440 to 550 volts 500 and 1,000 volts
2,400 volts 1,000 to 2,500 volts
or higher
4,160 volts and above 1,000 to 5,000 volts,
or higher
Test voltages used for proof testing of equipment are considerably higher
than those used for routine maintenance. Although there are no published
industry standards for DC maximum proof test voltages to be used with
rotating equipment, the schedule given below is customarily used. For
specific recommendations on your equipment, you should consult the
manufacturer of the equipment.
Proof Test Voltages for Rotating Equipment:
Factory AC Test = 2 x Nameplate Rating + 1000 volts
DC Proof Test on Installation = 0.8 x Factory AC Test x 1.6
DC Proof Test After Service = 0.6 x Factory AC Test x 1.6
Example:
Motor with 2,400 VAC nameplate rating–
Factory AC Test = 2(2,400) +1,000 = 5,800 VAC
Max. DC Test on Installation = 0.8(5,800)1.6 = 7,424 VDC
Max. DC Test After Service = 0.6(5,800)1.6 = 5,568 VDC
16 A STITCH IN TIME
AC TESTING VS. DC
Up to now, we’ve talked about testing with DC voltage, but you will hear
of AC testing and need to know the difference. Remember that we spoke
of the kinds of current produced in insulation by DC? (The initial surge of
charging current, the drop with time to absorption current, and then, after
more time, the steady conduction current.) We saw that in insulation testing,
the conduction or leakage current is the one that gives us the information
we need.
In contrast, testing with AC gives a charging current that is extremely
large compared to the other kinds; leakage current is relatively minor. AC
frequently is used for high-potential testing; voltage is increased to some
specified point to see if the insulation can stand that particular voltage. It
is a GO/NO-GO type of test and can cause deterioration of the insulation, in
contrast to the DC test which is basically non-destructive.
If an AC test voltage has been used and you want to use DC tests as
an alternative, you will need to increase the maximum DC test voltage
somewhat to obtain equivalent results.
In some cases, AC testing may be more suitable for proof testing of
equipment (that is, seeing that the equipment meets prescribed standards).
You run the voltage up to the selected value and the equipment either
passes or doesn’t pass the test. With the DC test, you get a more qualitative
picture; you can meter the leakage current as you increase the voltage and
obtain specific values of insulation resistance.
As the size of your equipment increases, there are also marked economic
advantages in DC over AC testing. As the test voltage increases, both the
cost and weight of AC equipment go up much faster than with comparable
DC test equipment. This is because the AC test set must supply the charging
current which becomes and remains very high in the larger machines. As
explained previously, in DC testing, this current drops rapidly after the initial
charging period.
A STITCH IN TIME 17
In summary, DC test sets are employed almost exclusively for high-voltage
maintenance and field testing for the following reasons:
1. Lower cost
2. Lighter weight
3. Smaller size
4. Non-destructive
5. Better information, both in quality and quantity
USE OF DC DIELECTRIC TEST SET
The Megger instrument, reading directly in ohms and megohms of
insulation resistance, is your best bet for routine in-plant maintenance.
However, some plants, particularly with higher voltage ratings in equipment,
use another Megger product – the dielectric test set. So, you should be
aware of this instrument and its use in insulation resistance measurements.
The dielectric test set can be used to determine insulation resistance by the
same test methods as outlined for the Megger instrument; that is, the shorttime, time-resistance and step-voltage tests. It is designed for other uses, too,
but for insulation testing it provides: (1) an adjustable output voltage and
(2) a monitoring of the resultant current in micro-amperes. The Megger DC
Dielectric Test Sets are available with voltage outputs ranging from 5 kV up
to 160 kV.
The curves of Fig. 5 are plotted as current versus time, as are curves for
insulation measurements on typical equipment given near the end of this
manual. Megger supplies graph paper which makes it easy to plot megohms
of insulation resistance from your voltage and current readings.
TESTS DURING DRYING OUT OF EQUIPMENT
Wet electrical equipment is a common hazard faced by all maintenance
engineers. If the equipment is wet from fresh water, you go right ahead
drying it out. However, if you’ve got salt water, you must first wash away the
salt with fresh water. Otherwise, you’ll leave very corrosive deposits of salt
on metal and insulating surfaces as well as in crevices of the insulation. With
moisture, such deposits form a very good conductor of electricity. Also, you
should remove oil or grease from the insulation, using a suitable solvent.
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There are various ways to dry out electrical equipment, depending upon
its size and portability. You can use a blast of hot air, an oven, circulation
of current through conductors, or a combination of techniques. Local plant
conditions and facilities, together with information from the equipment
manufacturers, can serve as a guide to the best method for your particular
equipment.
In some cases, or with certain equipment, drying out may not be necessary.
You can check this by insulation resistance tests, if you have records of
previous tests on the apparatus. When drying out is required, such records
are also helpful to determine when the insulation is free of moisture.
NOTE: Wet equipment is susceptible to voltage breakdown. Therefore, you
should use a low-voltage Megger tester (100 or 250 VDC), at least
in the early stages of a drying-out run. If a low-voltage instrument
is not readily available, slow cranking of a 500-volt tester may be
substituted.
Many testers have an additional test range measuring in kilohms
(kW). This measurement is typically made at only a few volts, and
is the ideal initial measurement to be made on flooded equipment.
This range measures below the Megohm range, and can, therefore,
provide an actual measurement to use as a benchmark in monitoring
the drying process. If a kilohm measurement is obtained, insulation
has been thoroughly saturated, but may be reclaimable. Alternately
test and dry, watching the readings rise until they reach the Megohm
range, at which time higher voltage tests can be safely employed.
As an example of how important past readings are, let’s look at a 100-hp
motor that’s been flooded. After a clean-up, a spot reading with the Megger
tester shows 1.5 megohms. Offhand, you’d probably say this is ok. What’s
more, if past records showed the insulation resistance to run between 1 and
2 megohms, you’d be sure.
On the other hand, if past records showed the normal resistance values to
run 10 or 20 megohms, then you would know that water was still in the
motor windings.
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Figure 10-Typical drying curve where one-minute readings of insulation resistance are taken every
four hours.
The typical drying-out curve for a DC motor armature (Fig. 10) shows how
insulation resistance changes. During the first part of the run, the resistance
actually decreases because of the higher temperature. Then it rises at a
constant temperature as drying proceeds. Finally, it rises to a high value, as
room temperature (20°C) is reached.
Note that if you conduct insulation resistance tests during drying, and you
have readings of previous tests on the dry equipment, you’ll know when
you’ve reached the safe value for the unit. You may prefer to use a timeresistance test, taken periodically (say, once a shift), using the dielectric
absorption ratio or polarization index to follow dry-out progress (no need to
correct for temperature).
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