Troubleshooting with Insulation Resistance Test Instruments

Troubleshooting with Insulation
Resistance Test Instruments

INTRODUCTION DC voltage to be applied during an insula-
tion resistance test.
A troubleshooting procedure consists of
systematic testing designed to identify In order to obtain meaningful insula-
and correct a problem. Technicians must tion resistance measurements, the techni-
gather information and utilize tests to cian should carefully examine the system
identify problems and develop appro- under test. The best results are achieved
priate solutions. Following established when the following conditions are met:
troubleshooting procedures ensures that
problems are identified and quickly cor- • The system or equipment is taken out
rected to minimize equipment and pro- of service and disconnected from all
duction downtime. Consistent practices other circuits, switches, capacitors,
also generate consistent measurements overcurrent protection devices, and
for more accurate comparison and track- circuit breakers. Ensure that the mea-
ing over time. surements are not affected by leakage
current through switches and overcur-
For accurate low-resistance measurements rent protective devices.
when using a DMM, insulation multimeter,
or megohmmeter, the resistance of the test • The temperature of the conductor is
leads is subtracted from the total resistance. above the dew point of the ambient air.
Typical test lead resistance is between 0.2 Ω When this is not the case, a moisture
and 0.5 Ω. coating will form on the insulation
surface, and, in some cases will be
TROUBLESHOOTING OF absorbed by the material.
ELECTRIC MOTORS
• The surface of the conductor is free of
Insulation resistance testing is performed hydrocarbons and other foreign matter
when troubleshooting electric motors that can become conductive in humid
and related equipment. IEEE Standard conditions.
43-2000, Recommended Practice for
Testing Insulation Resistance of Rotating • Applied voltage is not higher than the
Machines, recommends the insulation system capacity. When testing low-
test voltage to apply, based on winding voltage systems, too much voltage can
rating, and minimum acceptable values overstress or damage insulation.
for electric motor windings. The IEEE
also provides typical guidelines for • The system under test has been complete-
ly discharged to the ground. The ground-
ing discharge time should be about five
times the testing charge time.

21

22 INSULATION RESISTANCE TESTING

• The effect of temperature is consid- Although insulating materials in practical
ered. Since insulation resistance is use can be solid, liquid, or gas, resistance
inversely proportional to insulation measurements generally refer to solid
temperature (resistance goes down as insulation. Factors that affect insulation
temperature goes up), the recorded resistance measurements are the nonuni-
readings are altered by changes in the formity of the material, the time period the
temperature of the insulating mate- sample is energized, the magnitude and
rial. It is recommended that tests be polarity of the voltage, the time required
performed at a standard conductor for the charge to build, material decay, and
temperature of 68°F (20°C). specimen contour.
When comparing readings to 68°F
Several different types of insulation
base temperature, double the resistance resistance tests are performed when
for every 18°F (10°C) above 68°F or troubleshooting electric motors. These
halve the resistance for every 18°F be- tests include an insulation spot test, a
low 68°F in temperature. For example, dielectric absorption test, and an insula-
a 1 MΩ resistance at 104°F (40°C) tion step voltage test.
will translate to 4 MΩ resistance at
68°F (20°C). Insulation Spot Test

To measure the temperature of motor An insulation spot test is an insulation
windings, use a noncontact temperature resistance test that can be used to verify
probe connected to a digital multimeter the condition of the insulation over the
(DMM) or an infrared thermometer. life of the motor. See Figure 3-2.
See Figure 3-1.
A spot test should be performed at reg-
An insulation multimeter can be used ular intervals (every six months, for ex-
to check insulation integrity between ample) to track the changes in insulation
commutator segments prior to rewinding. integrity. The test should also be performed
after a motor is serviced. The test measure-
ment data can be recorded on a test graph
over time.

To perform an insulation spot test,
apply the following procedure:

1. Connect a megohmmeter to measure
the resistance of each winding lead
to ground. Record the readings after
60 sec.

2. Discharge the motor windings.

Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments 23

NONCONTACT TEMPERATURE MEASUREMENT

1 mV DISPLAY
FOR EACH °F OR °C

DMM

Siemens

NONCONTACT
TEMPERATURE
PROBE

NONCONTACT TEMPERATURE PROBE

INFRARED
THERMOMETER

Siemens

INFRARED THERMOMETER

Figure 3-1. To measure the temperature of motor windings, use a noncontact
temperature probe connected to a DMM or an infrared thermometer.

24 INSULATION RESISTANCE TESTING

INSULATION SPOT TEST

DISCHARGE

MOTOR WINDINGS 2

TO GROUND
5k ,5W
RESISTOR

T8 TO GROUND

T5

T4
T1

CONNECT MEGOHMMETER TEST
TO MEASURE RESISTANCE
OF EACH WINDING TO
GROUND AND RECORD
READING ON GRAPH 1

REPEAT STEPS 1 AND 2 3 MEGOHMMETER
EVERY SIX MONTHS

Figure 3-2. An insulation spot test is a test that checks motor insulation over the life
of the motor.

Interpret the results of the test graph condition after being rewound. Record
to determine the condition of the insula- the lowest meter reading on an insulation
tion. See Figure 3-3. Point A represents spot test graph if all readings are above
the motor insulation condition when the the minimum acceptable resistance. Ser-
motor was placed in service. Point B vice the motor if a reading does not meet
represents the effects of aging, contamina- the minimum acceptable resistance. The
tion, etc., on the motor insulation. Point lowest reading is used because a motor is
C represents motor insulation failure. only as good as its weakest point.
Point D represents the motor insulation

Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments 25

INSULATION SPOT TEST GRAPH

1000

500 A MOTOR PLACED

IN SERVICE B EFFECTS OF AGING,
CONTAMINATION, ETC.

RESISTANCE (IN M ) 100 D CONDITION AFTER

BEING REWOUND
50

10

5

C MOTOR INSULATION

FAILURE

012345
YEAR

Figure 3-3. Insulation spot test measurement data is recorded on a test graph
over time.

Dielectric Absorption Test

A dielectric absorption test is an insula- Megohmmeters can be used for testing
tion resistance test that checks the absorp- defective motor winding insulation.
tion characteristics of wet or contaminated
insulation. The test is performed over a
10 min period. See Figure 3-4.

To perform a dielectric absorption test,
apply the following procedure:

1. Connect a megohmmeter to measure
the resistance of each winding lead
to ground. Record the lowest meter
reading on a dielectric absorption
test graph if all readings are above
the minimum acceptable resistance.
Record the readings every 10 sec for
the first minute and every minute
thereafter for 10 min.

2. Discharge the motor windings.

26 INSULATION RESISTANCE TESTING

DIELECTRIC ABSORPTION TEST

5k ,5W 2 DISCHARGE
RESISTOR
TO GROUND MOTOR
WINDINGS

T7 T8 T9 TO GROUND
T4 T5 T6
T1 T2 T3

TEST

CONNECT MEGOHMMETER
TO MEASURE RESISTANCE
OF EACH WINDING TO
GROUND AND RECORD MEGOHMMETER
READING ON GRAPH 1

Figure 3-4. A dielectric absorption test is a test that checks the absorption characteristics
of wet or contaminated insulation.

Interpret the results of the test graph to a 1 min measurement. The polarization
determine the condition of the insulation. index is an indication of the condition of
See Figure 3-5. The slope of the curve the insulation. A low polarization index
indicates the condition of the insulation. (typically 1.5 or lower) indicates exces-
Good insulation (curve A) indicates a sive moisture or contamination. Different
continual increase in resistance. Moist insulation classes will have different mini-
or cracked insulation (curve B) shows mum acceptable polarization index values.
a relatively constant resistance. Service See Figure 3-6. Insulation classes
the motor if a reading does not meet the are based on the continuous operating
minimum acceptable resistance. temperature of the application. For ex-
ample, Class A insulation has a maxi-
A polarization index is the ratio of the mum operating temperature of 212°F,
insulation resistance of a machine wind- Class B, 248°F, Class F, 293°F, and
ing calculated by using values obtained Class H, 329°F.
from a 10 min measurement divided by

Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments 27

DIELECTRIC ABSORPTION TEST GRAPH

RESISTANCE 1000 CURVE A
(IN M ) 500 (GOOD INSULATION)

100 CURVE B
50 (MOIST OR CRACKED
INSULATION)
10
5

1 2 3 4 5 6 7 8 9 10
TIME (IN MINUTES)

Figure 3-5. Dielectric absorption test measurement data is recorded on a test graph
over time.

MINIMUM ACCEPTABLE For example, if the 1 min reading
POLARIZATION
INDEX VALUES of Class B insulation is 80 MΩ and the
10 min reading is 90 MΩ, the polarization
Insulation Index Value index is 1.125 (90 MΩ ÷ 80 MΩ = 1.125).
The polarization index is lower than the
Class A 1.5
minimum accepted values because the
Class B 2.0
insulation contains excessive moisture
Class F 2.0
or contamination.

Class H 2.0

Figure 3-6. Different insulation classes Insulation Step Voltage Test
will have different minimum acceptable
polarization index values. An insulation step voltage test is a test that
creates electrical stress on internal insula-
IEEE Standard 43-2000, Recommended tion cracks to reveal aging or damage not
Practice for Testing Insulation Resistance found during other motor insulation tests.
of Rotating Machinery, covers measurement This test is done by testing the insulation
of polarization index testing. at two or more voltages and comparing
the results. The insulation step voltage

28 INSULATION RESISTANCE TESTING

test is performed only after an insulation 3. Increase the megohmmeter setting
spot test. See Figure 3-7. by increments of 500 V starting at
1000 V and ending at the high-end
To perform an insulation step voltage voltage for a range of motors within
test, apply the following procedure: the system. Record each reading after
60 sec.
1. Set the megohmmeter to 500 V and
connect to measure the resistance of 4. Discharge the motor windings.
each winding lead to ground. Take
each resistance reading after 60 sec. Interpret the results of the test to
Record the lowest reading. determine the condition of the insula-
tion. See Figure 3-8. The resistance of
2. Place the meter lead on the winding good insulation that is thoroughly dry
that has the lowest reading.

INSULATION STEP VOLTAGE TEST

TO GROUND 4 DISCHARGE

5k ,5W THE MOTOR
RESISTOR WINDINGS

T3 TO GROUND

T2
T1

SET MEGOHMMETER
TO 500 V AND MEASURE

RESISTANCE OF EACH

WINDING TO GROUND 1

PLACE METER LEAD V 2500V
000V 500V

ON WINDING WITH 0M 00M G 0G IR
LOWEST READING M 00G PORT

2 INCREASE METER 00k T

0
T

kM

SETTING BY 500 V; ON/ FUNCTION UP SCROLL DOWN ENTER
OFF
RECORD READING TEST

ON GRAPH AFTER 3
60 SECONDS

Figure 3-7. An insulation step voltage test is a test that creates electrical stress on
internal insulation cracks to reveal aging or damage not found during other motor
insulation tests.

Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments 29

(curve A) remains approximately the same TROUBLESHOOTING CABLE
at different voltage levels. The resistance AND WIRE INSTALLATIONS
of deteriorated insulation (curve B)
decreases substantially at different volt- When troubleshooting cable and wire
age levels. installations, they should be disconnected
from panels and machinery to keep them
The larger the insulation surface, the isolated. The conductors should be tested
lower the insulation resistance. The greater against each other and against ground.
the supply voltage to the motor, the thicker See Figure 3-9.
the insulation requirements. A 50 HP,
230 V motor requires the same insulation Cables and wires should be tested
as a 1 HP, 230 V motor. and maintained on a three-year cycle at
a minimum. Insulation resistance tests
Caution: A megohmmeter uses very high should be performed more frequently for
voltages during insulation resistance test- those systems that show deterioration of
ing (up to 5000 V). Always follow recom- insulation material. When performing
mended procedures and safety rules. After insulation tests on wire and cable, apply
performing insulation resistance tests with the following procedure:
a megohmmeter, connect the device being
tested to ground through a 5 kW, 5 W resis- 1. Inspect exposed sections of cables and
tor if the megohmmeter does not include a wires for physical damage. Replace or
discharge function. repair sections that exhibit damage.

INSULATION STEP VOLTAGE TEST GRAPH

1000

500

CURVE A
(DRY INSULATION)
100

RESISTANCE 50 CURVE B
(IN M ) (DETERIORATED
INSULATION)

10

5

12345
VOLTAGE (IN kV)

Figure 3-8. Insulation step voltage test measurement data is recorded on a test graph
over time.

30 INSULATION RESISTANCE TESTING

TROUBLESHOOTING CABLE AND WIRE INSTALLATIONS

METAL CONDUIT TEST

CONDUCTOR
INSULATION

CABLE CONDUCTORS
INSULATION

Figure 3-9. When troubleshooting cable and wire installations, the conductors should
be tested against each other and against ground.

2. Inspect cables and wires for proper Low-Voltage Cables
grounding, cable support, and termi-
nation. Terminate those sections that A cable length test should be performed
are not properly terminated. on newly installed low-voltage cables.
Knowing the actual length of low-voltage
3. If cables and wires are properly cables is required because low-voltage
terminated, verify that neutrals and cables, like all conductors, have resistance
grounds are properly terminated for that reduces the amount of power passing
operation of protective devices. through them. The greater the length of a
cable, the greater the power reduction (at-
4. Perform an insulation resistance tenuation) created by the cable. Because
test on each conductor in the cable. the signals are transmitted at low power,
Apply 1000 VDC for 1 min (one- any reduction in power caused by a cable
minute insulation resistance test) must be kept to a minimum.
to low voltage cables (1 kV or
less) and use a megohmmeter, or Cable length testers measure the length of
insulation multimeter (IMM) to cables and also indicate the distance to a
measure the insulation resistance. cable fault by accessing the end of a cable
(Use a hipot tester to perform a DC with two or more conductors.
hipot test on cables ranging from
1 kV to 69 kV.)

Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments 31

TROUBLESHOOTING boards are used to distribute utility power
ELECTRICAL SWITCHGEAR via busways to transformers, panelboards,
AND SWITCHBOARDS and motor starters. See Figure 3-10. An
assembly may be part of a load center,
Electrical switchgear and switchboards substation, or distribution board.
are freestanding assemblies of metal-
enclosed sections containing circuit Switchgear and related assemblies
breakers and/or fused disconnect switch- should be tested and maintained at a mini-
es. They also contain bus bars, cable mum of once a year. Inspections should be
termination points, backup protection performed more frequently if equipment
devices, and various forms of controls and is in an environment that has excessive
instrumentation. Switchgear or switch- dirt or moisture. Per plant procedures and
manufacturers recommendations, the best

SWITCHGEAR POWER DISTRIBUTION

OUTDOOR FEEDER BUSWAY

480 V FROM FEEDER BUSWAY
UTILITY 480 V

SWITCHGEAR/
SWITCHBOARD

PANELBOARD
TRANSFORMER

TRANSFORMER 120 V 480 V
480 V 480 V

120 V

PANELBOARD PANELBOARD 480 V
MOTOR STARTER

Figure 3-10. Switchgear or switchboards are used to distribute utility power via busways
to transformers, panelboards, and motor starters.

32 INSULATION RESISTANCE TESTING

results are achieved when the following Although standard insulation resis-
actions are applied: tance specifications can be applied when
troubleshooting most motors and other
• Inspect electrical sections of switch- electrical devices, certain situations re-
gear for damage. Repair or replace quire higher resistance specifications to
damaged sections. be applied. Most stated specifications list
the maximum amount of acceptable leak-
• Inspect all bus connections for high age current and not the actual resistance.
resistance by using an insulation mul- For example, a 3-wire handheld electri-
timeter. Repair or replace sections that cal appliance or tool must be insulated
are not in compliance. enough to allow no more than 0.75 mA
(0.00075 A) of leakage current to flow
• Inspect insulation material for physi- through the exposed parts to ground.
cal damage or contaminated surfaces.
Clean or repair any insulation that is Typical resistance values are one-third
not in compliance. higher than the 1 MΩ per 1 kV industry
standard. Medical equipment and electri-
• Perform insulation resistance tests on cal devices rated as “double-insulated”
each bus section, phase-to-phase, and have much higher insulation ratings
phase-to-ground by using a megohm- than ordinary equipment. Miscellaneous
meter or insulation multimeter. Typical equipment includes two-prong Cat-
switchgear insulation resistance values egory II power cord devices, three-prong
range from 50 MΩ to 20,000 MΩ. In- Category I power cord devices, and dry-
sulation resistance values that are less type transformers.
than the equipment manufacturer’s
recommended minimum resistance Insulation resistance values vary with tem-
value should be investigated for ad- perature and the amount of moisture in the
ditional problems. insulation. The temperature and humidity
should be recorded at the time the insulation
• Always perform tests on deenergized test readings are taken.
equipment.
Two-Prong Category II Power
TROUBLESHOOTING Cord Devices
MISCELLANEOUS
ELECTRICAL DEVICES A two-prong Category II power cord
device is a device that has only two con-
Miscellaneous equipment includes such ductors extending from it, one hot and one
equipment as portable electrical devices, neutral. Two-prong devices do not have
double-insulated tools, extension cords, a third ground (green wire) prong on the
transformers, and any electrical device power cord. Certain two-prong devices
that receives power through a separate are classified as double-insulated.
power conductor. Insulation resistance
testing is performed on such equipment A double-insulated device is an elec-
as part of regular maintenance programs trical product designed so that a single
as well as for standard OSHA and ground fault cannot cause a dangerous
NEC® compliance.

Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments 33

electrical shock through any exposed impedance (resistance) path from all
sections of the product that could come non-current-carrying parts to earth
in contact with an electrician. Double- ground. Equipment used in the medical
insulated devices include not only the field will have an even lower acceptable
standard insulation used on conductors maximum leakage current limit.
but also extra insulating material between
the energized parts of the device and the Three-Prong Category I Power
parts that can be contacted. Cord Devices

When troubleshooting these devices A three-prong Category I power cord de-
for insulation resistance, leakage current vice is a device that has three conductors
is measured from the exposed metal parts extending from it, one hot, one neutral,
to ground. See Figure 3-11. To test a two- and one ground. Any leakage current
prong Category II power cord device for will flow through the ground (green)
insulation resistance, apply the following conductor back to ground during normal
procedures: operation. The ground conductor prevents
the exposed metal parts of the electrical
1. If the device has no exposed metal, device from becoming energized to the
wrap metal foil on the exposed point of causing an electrical shock.
plastic parts (such as the handles). See Figure 3-12. To test a three-prong
The metal foil simulates a wet hand Category I power cord device for insu-
contacting the electrical device. lation resistance, apply the following
procedure:
2. Ensure that the electrical device
being tested is not plugged into a 1. Ensure that the electrical device be-
power source. ing tested is not plugged into a power
source.
3. Connect the ground test lead to
the metal foil wrap on the tool 2. Connect the ground test lead to a metal
under test. portion of the equipment under test,
such as a motor housing.
4. Connect the voltage test lead to the
neutral (larger) blade on the tool’s 3. Connect the voltage test lead to the
power cord. neutral (larger) blade on the tool’s
power cord.
5. Use the insulation resistance tester to
supply the test voltage. 4. Measure the amount of leakage current
in mA and record the readings.
6. Measure the amount of leakage cur-
rent in mA and record the readings. The more power a load requires, the larger
Typical specified maximum leakage the amount of current flow. For example, a
current for a double-insulated, two- 10 HP motor draws approximately 28 A when
prong Category II device is typically wired for 230 V. A 20 HP motor draws ap-
0.25 mA (0.00025 A). proximately 54 A when wired for 230 V.

When leakage current exceeds the
specified limit, a three-prong power
cord must be used. The ground (green)
wire is added to carry the leakage
current to ground by providing a low

34 INSULATION RESISTANCE TESTING

TWO-PRONG CATEGORY II POWER CORD DEVICES

01 2 3 45

MICROAMPERES 6 MEASURE AMOUNT OF LEAKAGE
DC
CURRENT AND RECORD READING.
(0.25 mA MAXIMUM LEAKAGE
CURRENT ALLOWED)

HIPOT 5 USE INSULATION
TESTER
RESISTANCE TESTER TO
STABILIZATION 15 KV OUTPUT SUPPLY TEST VOLTAGE
MEGOHMMETER
OUT IN

01 2 3 4 50 3 6 9 12 15 CAUTION
HIGH
0 1.5 3 4.5 6 7.5
VOLTAGE

MICROAMPERES KILOVOLTS
DC DC
120 V

VOLTAGE
CONTROL

CAL CAL CAL 40 60 AC ON OUTPUT ON
II I
II

EXT II 20 80 OFF OFF
INST II

MULTIPLIER MULTIPLIER 0 100

3 CONNECT GROUND VOLTAGE CONTROL
SET ON 100
TEST LEAD TO METAL
FOIL WRAP

DOUBLE-INSULATED
ELECTRICAL DEVICE
WITHOUT GROUND

PLASTIC HOUSED
TOOL REQUIRES
METAL FOIL WRAP 1 CATEGORY II
ELECTRICAL
DEVICE

TWO-PRONG
POWER CORD

DEVICE NOT PLUGGED 4 CONNECT VOLTAGE
INTO POWER SOURCE
TEST LEAD TO
NEUTRAL PRONG

2

Figure 3-11. A two-prong Category II power cord device has only two conductors
extending from the device, one hot and one neutral, and does not have a third ground
prong on the power cord.

Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments 35

THREE-PRONG CATEGORY I POWER CORD DEVICES

01 2 3 45

MICROAMPERES 4 MEASURE AMOUNT OF LEAKAGE
DC
CURRENT AND RECORD READING
(3.5 mA MAXIMUM LEAKAGE
CURRENT ALLOWED)

HIPOT
TESTER

STABILIZATION 15 KV OUTPUT
MEGOHMMETER
OUT IN

01 2 3 4 50 3 6 9 12 15 CAUTION
HIGH
0 1.5 3 4.5 6 7.5
VOLTAGE

MICROAMPERES KILOVOLTS
DC DC
120 V

VOLTAGE
CONTROL

CAL CAL CAL 40 60 AC ON OUTPUT ON
II I
II

EXT II 20 80 OFF OFF
INST II

MULTIPLIER MULTIPLIER 0 100

CATEGORY I VOLTAGE CONTROL SET ON 100
ELECTRICAL
2 CONNECT GROUND TEST LEAD
DEVICE
TO METAL PORTION OF
EQUIPMENT UNDER TEST

ELECTRICAL DEVICE
WITH GROUND PLUG

1 ELECTRICAL DEVICE

THREE-PRONG POWER
CORD NOT PLUGGED
INTO POWER SOURCE

3 CONNECT VOLTAGE

TEST LEAD TO
NEUTRAL PRONG

GROUND
PRONG

Figure 3-12. A three-prong Category I power cord device has three conductors
extending from the device, one hot, one neutral, and one ground (green wire).

36 INSULATION RESISTANCE TESTING

The typical specified maximum leak-
age current for a three-prong category I
device is typically 0.75 mA (0.00075 A)
for handheld electrical devices such as
disc grinders, and 3.5 mA (0.0035 A) for
non-handheld electrical devices such as
floor buffers, electric motors, small drill
presses, and air compressors.

A reference multimeter can be used to
take resistance measurements from 2 Ω to
20 MΩ, as well as more accurate voltage,
current, and temperature measurements
than a standard handheld meter.

General Electric Company a nonconducting material, such as varnish
or a high-temperature polymer, and then
Autotransformers are used in starting formed into a core that reduces electrical
rotating machinery such as synchronous losses. Dry-type transformer maintenance
and induction motors. and testing should be performed and
recorded, starting with the initial instal-
Dry-Type Transformers lation of the transformer.

A dry-type transformer is a transformer An IMM set to measure resistance can
that uses a non-liquid material for in- be used to check for open circuits in coils,
sulation. Dry-type transformers can be short circuits between primary and sec-
constructed as either toroidal (doughnut- ondary coils, or coils shorted to the core
shaped) or laminated. Toroidal transform- without power applied to the transformer.
ers typically have copper wire wrapped See Figure 3-13.
around a cylindrical core so the magnetic
flux within the coil does not leak out, has Open Circuits in Coils. The resistance
good coil efficiency, and has little effect of each coil is checked with a DMM.
on other components. Laminated trans- The winding is open and the transformer
formers contain laminated-steel cores. is bad if any of the coils show an infinite
The steel laminations are insulated with resistance reading. Note that very low
resistance readings do not indicate a short,
but rather the resistance of the wire.

Short Circuits between Primary and
Secondary Coils. A check for short cir-
cuits should be made between the primary
and secondary coils of the transformer.
Checking for short circuits between the
primary and secondary coils should be
performed with a megohmmeter.

Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments 37

TESTING TRANSFORMERS

TRANSFORMER NORMAL
PRIMARY RESISTANCE

WIRES READING
INDICATES
GOOD COIL

SECONDARY IMM
WIRES

OPEN CIRCUITS IN COILS

INFINITE READING PRIMARY
WHEN CHECKING FOR WIRES
SHORT BETWEEN COILS
INDICATES NOT SHORTED TRANSFORMER
SECONDARY
TO EACH OTHER WIRES

IMM

SHORT CIRCUIT BETWEEN
PRIMARY AND SECONDARY COILS

PRIMARY TRANSFORMER INFINITE READING WHEN
WIRES CHECKING FOR SHORT
BETWEEN WINDING AND
IMM BODY INDICATES GOOD
TRANSFORMER

SECONDARY
WIRES

COILS SHORTED TO CORE

Figure 3-13. An IMM set to measure resistance can be used to check for open circuits
in coils. When testing for short circuits between primary and secondary coils, or for
coils shorted to the core without power applied to the transformer, use the insulation
test function of the IMM.

38 INSULATION RESISTANCE TESTING

Coils Shorted to Core. A resistance TRANSFORMER INSULATION
check is made from each transformer coil RESISTANCE VALUES
to the core of the transformer. All coils
should show an infinite resistance reading Transformer Minimum Minimum
to the core. The transformer should not be Coil Rating* Test Insulation
used if a resistance is shown between any Resistance‡
coil and the core. Voltage†

Transformer Maintenance. Trans- 0 - 600 1000 500
former manufacturers provide recom- 601 - 5000 2500 5000
mended maintenance procedures and 5001 - 15,000 5000 25,000
schedules for their equipment, but the
actual performance of a specific trans- * in VAC
former should determine the frequency † in VDC
of the maintenance and troubleshooting to ‡ in MΩ
be performed.
Figure 3-14. Typical transformer insulation
The majority of transformer failures resistance values range from 500 MΩ to
are caused by poor maintenance. Trans- 25,000 MΩ.
formers should be tested and maintenance
performed at least once annually. When When inspecting small (600 V or less)
performing insulation resistance tests on dry-type transformers, apply the follow-
dry-type transformers, apply the follow- ing procedures:
ing procedures:
1. Visually inspect for physical dam-
1. Visually inspect for physical dam- age: cracked insulation, defective
age: cracked insulation, defective wiring, tightness of connections, and
wiring, tightness of connections, and dirt/moisture on the coil.
dirt/moisture on the coil.
2. Verify proper core and equipment
2. Verify proper core and equipment grounding.
grounding.
3. Clean the unit prior to making any
3. Use a megohmmeter or insulation tests.
multimeter to perform insulation
resistance tests on winding to wind- 4. Use a megohmmeter or insulation
ing or windings to ground. Per- multimeter to perform insulation
form insulation resistance tests for resistance tests on winding to wind-
each winding. ing or windings to ground. Per-
form insulation resistance tests for
Typical transformer insulation re- each winding.
sistance values range from 500 MΩ to
25,000 MΩ. See Figure 3-14. Insulation To help prevent electrical problems that
resistance test readings for transformer cause equipment damage, neutral conductors
windings should be within 1% of adjacent should be the same size as, or larger than,
windings. hot conductors.