Megger MIT1525, MIT1025 Data sheet

A Guide To Diagnostic

Insulation Testing Above 1 kV

TOLL FREE
CUSTOMER SERVICE NUMBER

1-866-254-0962

The word “Megger” is a registered trademark

WWW.MEGGER.COM/us

WHY A 10 KV INSULATION TESTER?

Megger invented insulation testing before the beginning of the 20th century and has continued to lead the market in
innovation and technological advancement. So, why did we develop a 10 kV model when all other suppliers stopped at
5 kV? The answer is in the IEEE standards. Megger developed a 10 kV unit to meet the new testing recommendations
outlined by the IEEE. Megger has offered a 10 kV insulation resistance tester since 2001.

In March 2000, The IEEE-SA Standards Board approved a revision to IEEE Std 43-1974. The “IEEE Recommended Practice
for Testing Insulation Resistance of Rotating Machinery,” Std 43-2000, emphasizes the need for upgrading current
practices to accommodate changes and improvements in insulating materials and the value of higher voltage testing
that reveals otherwise hidden flaws.

Following is a brief summary of the highlights of the standard:

n

Test voltages up to 10 kV are recommended for windings rated greater than 12 kV.

n

Both the Insulation Resistance test and the Polarization Index test are recommended.

n

Test results should be compared to historical values to identify changes.

n

In lieu of historical records, minimum acceptable values (based on the type of equipment) for both tests are

indicated.

n

Depending on the machine rating, the readings for one or both tests should exceed the minimum acceptable

values.

n

If the readings are below the minimum acceptable values, the winding is not recommended for an over voltage

test or for operation.

IEEE Std 43-2000 recommends a procedure for measuring insulation resistance of armature and field windings in
rotating machines rated 1 hp, 750 W or greater and applies to synchronous machines, induction machines, dc machines
and synchronous condensers. It does not apply to fractional horsepower machines. It also recommends the insulation
test voltage (based on winding rating) and minimum acceptable values of insulation resistance for ac and dc rotating
machine windings.

For more information on the IEEE Standard, please turn to page 23 in the booklet.

TABLE OF CONTENTS

Introduction ........................................................................2

What is Insulation? ............................................................2

What Causes insulation to Degrade? ...........................3

Electrical Stress ...........................................................3

Mechanical Stress .......................................................3

Chemical Attack .........................................................3

Thermal Stress ............................................................3

Environmental Contamination .................................3

How Can Predictive Maintenance Help Me? ...............3

The Benefit of New Technology ...................................4

How Insulation Resistance is Measured ...........................4

How an Insulation Resistance Tester Operates ...........4

Components of Test Current ........................................4

Capacitive Charging Current .....................................4

Absorption or Polarization Current ..........................4

Surface Leakage Current ...........................................5

Conduction Current ...................................................5

Connecting your Insulation Tester ...............................6

Selected Typical Connections .......................................6

Shielded Power Cable ................................................6

Circuit Breaker/Bushings ............................................6

Power Transformer ....................................................6

AC Generator .............................................................7

Insulation Resistance Tester Scale ................................7

Voltage Characteristics .................................................8

The Guard Terminal ......................................................9

Introduction ...............................................................9

How the Guard Terminal Works .............................10

Guard Terminal Performance ..................................10

The Guard Terminal as a Diagnostic Tool ..............11

Final Words ..............................................................11

Evaluation and Interpretation of Results .......................12

Interpretation of the Infinity Reading .......................12

Diagnostic High Voltage Insulation Tests ......................12

Spot Reading Test .......................................................12

Time vs. Resistance Test ..............................................14

Polarization Index Test ...............................................14

Step Voltage Test ........................................................16

Dielectric Discharge Test .............................................16

Different Problems/Different Tests ............................18

Appendices .......................................................................19

Potential Sources of Error/Ensuring Quality Test

Results ..........................................................................19

Test Leads .................................................................19

Making Measurements above 100 GΩ ...................19

Accuracy Statements ................................................19

Delivery of Stated Voltage ......................................19

Interference Rejection .............................................19

Rules on Testing and Comparing ............................20

CAT Rating ...................................................................20

CAT Rating Guidelines .............................................21

The Importance of a CAT Rating ............................21

Some CAT Rating Basic Statistics .............................21

Testing Insulation Resistance

of Rotating Machinery ................................................21

Effects of Temperature ...............................................23

Effects of Humidity .....................................................24

Ingress Protection ........................................................24

High Potential Testing ................................................26

Current (nA) Readings vs.

Resistance (MΩ) Readings .......................................26

Burn Capability ............................................................26

Drying Out Electrical Equipment ................................26

Test Item Discharge .....................................................27

Charging Time for Large Equipment .........................28

Motor Driven Insulation Testers .................................28

Megger Insulation Testers ...............................................29

MIT510/2 and MIT520/2

5-kV Insulation Testers ................................................29

MJ15 and BM15

5-kV Insulation Testers ................................................29

S1-552/2 and S1-554/2

5-kV Insulation Testers ................................................29

MIT1020/2

10-kV Insulation Tester ...............................................30

S1-1052/2 and S1-1054/2

10-kV Insulation Testers ..............................................30

A GUIDE TO DIAGNOSTIC INSULATION TESTING ABOVE 1 KV 3

INTRODUCTION

Electrical insulation degrades over a period of time
because of various stresses, which are imposed upon it
during its normal working life. The insulation has been
designed to withstand these stresses for a period of
years, which would be regarded as the working life of
that insulation. This often runs into decades.

Abnormal stresses can bring about an increase in this
natural aging process that can severely shorten the
working life of the insulation. For this reason it is good
practice to perform regular testing to identify whether
increased aging is taking place and, if possible, to
identify whether the effects may be reversible or not.

The purpose of diagnostic insulation testing is:

n

To identify increased aging.

n

To identify the cause of this aging.

n

To identify, if possible, the most appropriate actions

to correct the situation.

In its simplest form, diagnostic testing takes the form of
a “Spot Test.” Most electrical maintenance professionals
have made spot tests where a voltage is applied to the
insulation and a resistance is measured. The diagnosis
in this case is limited to “the insulation is good” or “the
insulation is bad.” But having made this diagnosis what
do we do about it? It’s a bit like going to the doctor with
a bad cough and the doctor simply telling you, “You’ve
got a bad cough.” You wouldn’t be happy to come away
with only that information. You expect the doctor to
examine you, carry out a few tests, and tell you why you
have a bad cough and what to do about it to cure the
cough.

In insulation testing, a spot test on its own is the
equivalent of the doctor telling you that you are well
or you are sick. It’s minimal information. This is the sort
of test that is typically applied to low-voltage circuits
where the cost of a failure is low and equipment can be
replaced easily and inexpensively. Since the equipment
being tested is low voltage equipment, these tests are
typically performed using a 500 or 1000 V test voltage
and will be familiar to all electrical maintenance
personnel.

However, if the doctor records the results of his
examination and compares them with those from
previous visits, then a trend might be apparent which
could lead to medication being prescribed. Similarly,
if insulation resistance readings are recorded and
compared with previously obtained readings, it may be
possible to see a trend and to prescribe remedial actions
if such are called for.

Diagnostic insulation testing at voltages above 1 kV is an
area that is less familiar to many electrical maintenance
personnel. The purpose of this booklet, therefore, is to:

n

Acquaint the reader with making diagnostic

insulation resistance tests.

n

Provide guidelines for evaluating the results of these

diagnostic insulation resistance tests.

n

Introduce the benefits of multi-voltage testing at

higher voltages.

A series of appendices are included at the end of
the booklet to provide the reader with additional
information related to diagnostic insulation testing.

This booklet is based on the principles established in
the booklet “A Stitch in Time… The Complete Guide to
Electrical insulation Testing” first published in 1966 by
the James G. Biddle Company.

WHAT IS INSULATION?

Every electric wire in a facility, whether it’s in a motor,
generator, cable, switch, transformer, or whatever is
covered with some form of electrical insulation. While
the wire itself is a good conductor (usually made of
copper or aluminum) of the electric current that powers
electrical equipment, the insulation must resist current
and keep the current in its path along the conductor.
Understanding Ohm’s Law, which is expressed in
the following equation, is the key to understanding
insulation testing:

E = I x R

where,

E = voltage in volts

I = current in amperes

R = resistance in ohms

For a given resistance, the higher the voltage, the
greater the current. Alternatively, the lower the
resistance of the wire, the more current that flows for
the same voltage.

No insulation is perfect (has infinite resistance), so some
current does flow along the insulation or through it to
ground. Such a current may be insignificantly small for
most practical purposes but it is the basis of insulation
testing equipment.

So what is “good” insulation? “Good” means a relatively
high resistance to current flow. When used to describe
an insulation material, “good” also means “the ability
to maintain a high resistance.” Measuring resistance can
tell you how “good” the insulation is.

4 A GUIDE TO DIAGNOSTIC INSULATION TESTING ABOVE 1 KV

What Causes insulation to Degrade?

There are five basic causes for insulation degradation.
They interact with each other and cause a gradual spiral
of decline in insulation quality.

Electrical Stress

Insulation is designed for a particular application.
Overvoltages and undervoltages cause abnormal stresses
within the insulation, which can lead to cracking or
delamination of the insulation.

Mechanical Stress

Mechanical damage such as hitting a cable while digging
a trench is fairly obvious but mechanical stresses also
may occur from running a machine out of balance
or frequent stops and starts. The resulting vibration
from machine operation may cause defects within the
insulation.

Chemical Attack

While you would expect insulation to be affected by
corrosive vapors, dirt and oil can also operate to reduce
the effectiveness of insulation.

Thermal Stress

Running a piece of machinery in excessively hot or cold
conditions will cause over expansion or contraction of
the insulation which might result in cracks and failures.
However, thermal stresses are also incurred every time
a machine is started or stopped. Unless the machinery is
designed for intermittent use, every stop and start will
adversely affect the aging process of the insulation.

Environmental Contamination

Environmental contamination covers a multitude
of agents ranging from moisture from processes, to
humidity on a muggy day, and even to attack by rodents
that gnaw their way into the insulation.

Insulation begins to degrade as soon as it is put in
service. The insulation in any given application will
have been designed to provide good service over many
years under normal operating conditions. However,
abnormal conditions may have a damaging effect
which, if left unchecked, will speed up the rate of
degradation and will ultimately cause a failure in the
insulation. Insulation is deemed to have failed if it fails
to adequately prevent electrical current from flowing in
undesirable paths. This includes current flow across the
outer or inner surfaces of the insulation (surface leakage
current), through the body of the insulation (conduction
current) or for a variety of other reasons.

For example, pinholes or cracks can develop in the
insulation or moisture and foreign matter can penetrate
the surface(s). These contaminants readily ionize
under the effect of an applied voltage providing a
low resistance path for surface leakage current which
increases compared with dry uncontaminated surfaces.
Cleaning and drying the insulation, however, will easily
rectify the situation.

Other enemies of insulation may produce deterioration
that is not so easily cured. However, once insulation
degradation has started, the various initiators tend to
assist each other to increase the rate of decline.

How Can Predictive Maintenance Help Me?

While there are cases where the drop in insulation
resistance can be sudden, such as when equipment is
flooded, it usually drops gradually, giving plenty of
warning if tested periodically. These regular checks
permit planned reconditioning prior to service failure
and/or a shock condition.

Without a periodic testing program all failures will come
as a surprise, unplanned, inconvenient and quite possibly
very expensive in time and resources and, therefore,
money to rectify. For instance, take a small motor that
is used to pump material, which will solidify if allowed
to stand, around a processing plant. Unexpected failure
of this motor will cost tens maybe even hundreds
of thousands of dollars to rectify if downtime of
the plant is also calculated. However, if diagnostic
insulation testing had been included in the preventive
maintenance program it may have been possible to plan
maintenance or replacement of the failing motor at
a time when the line was inactive thereby minimizing
costs. Indeed, it may have been that the motor could
have been improved while it was still running.

If advanced insulation degradation goes undetected
there is an increase in the possibility of electrical shock
or even death for personnel; there is an increase in the
possibility of electrically induced fires; the useful life
of the electrical equipment can be reduced and/or the
facility can face unscheduled and expensive downtime.
Measuring insulation quality on a regular basis is a
crucial part of any maintenance program as it helps
predict and prevent electrical equipment breakdown.

This is particularly appropriate now when we consider
that large parts of the electrical network in the USA and
Europe were installed in the 1950s in a burst of postwar
investment. Some equipment is approaching the end of
its design life, while some has already exceeded it but is
still operating satisfactorily.

A GUIDE TO DIAGNOSTIC INSULATION TESTING ABOVE 1 KV 5

Since diagnostic testing is generally reserved for more
critical items we normally, but not always, find that
diagnostic testers have voltage outputs of 5 or 10 kV,
these voltages being more suitable for testing the assets
which themselves are usually medium voltage machines,
cables, transformers, etc.

The Benefit of New Technology

Insulation testers date back to the early 20th century
when Sidney Evershed and Ernest Vignoles developed
their first insulation tester (which developed in 1903
into the Megger

®

range of testers).

In the early days, most instruments were hand-cranked.
This limited their ability to carry out tests which took
an extended time to complete, and limited the voltage
stability to the operator’s ability to crank steadily.
Later, these same instruments were capable of having
an external motor drive added which helped with long
duration tests but did very little to improve the voltage
stability. However, the range of these instruments rarely
exceeded 1000 MΩ. The analog movements were very
heavy and actually served to damp out any transient
events.

The appearance of electronics and the development
of battery technology revolutionized the design of
insulation testers. Modern instruments are line or
battery-powered and produce very stable test voltages
under a wide variety of conditions. They are also able
to measure very small currents so that their insulation
resistance measuring range is extended several
thousandfold into the teraohm (TΩ) range. Some
can even replace the pencil, paper and stopwatch,
which were formerly used to manually collect results,
by recording data in memory for later download
and analysis. It is fortunate that these astonishing
enhancements were made since the manufacturers of
insulating material have been working hard also, with
the result that modern insulating materials now exhibit
much higher resistances than those in the early 20th
century.

Newer technology offers enhanced performance so
that established procedures can yield greater insights
and new methods can be made available. Modern
instruments deliver stable voltage over their full
resistance range, with microprocessor sensitivity in the
measuring circuit enabling measurements in the TΩ
range. The combination of stable voltage and enhanced
sensitivity enables the tester to measure the minuscule
amounts of current that are passed by quality insulation
in new, capital equipment. Accordingly, sophisticated
procedures that rely on precise measurement have been
developed and may be easily implemented.

Now that the insulation tester isn’t limited to values
associated with faulty or aged equipment, it can be
used to pinpoint the test item’s position anywhere
along its aging curve. The “infinity” indication that is a
delight to the repair technician represents a void to the
diagnostician. Some instruments have diagnostic tests
preprogrammed into their software and can run them
automatically, filling that void with valuable analytical
data.

HOW INSULATION RESISTANCE IS MEASURED

How an Insulation Resistance Tester Operates

The Megger® insulation tester is a portable instrument
that provides a direct reading of insulation resistance
in ohms, megohms, gigohms, or teraohms (depending
on the model chosen) regardless of the test voltage
selected. For good insulation, the resistance usually reads
in the megohm or higher range. The Megger insulation
tester is essentially a high-range resistance meter
(ohmmeter) with a built-in dc generator.

The instrument’s generator, which can be hand-cranked,
battery or line-operated, develops a high dc voltage
that causes several small currents through and over
surfaces of the insulation being tested. The total current
is measured by the ohmmeter, which has an analog
indicating scale, digital readout or both.

Components of Test Current

If we apply a test voltage across a piece of insulation,
then by measuring the resultant current and applying
Ohm’s Law (R=E/I), we can calculate the resistance of the
insulation. Unfortunately, more than one current flows,
which tends to complicate matters.

Capacitive Charging Current

We are all familiar with the current required to charge
the capacitance of the insulation being tested. This
current is initially large but relatively short lived,
dropping exponentially to a value close to zero as the
item under test is charged. Insulating material becomes
charged in the same way as a dielectric in a capacitor.

Absorption or Polarization Current

Absorption current is actually made up of up to three
components, which decay at a decreasing rate to a value
close to zero over a period of several minutes.

The first is caused by a general drift of free electrons
through the insulation under the effect of the electric
field.

6 A GUIDE TO DIAGNOSTIC INSULATION TESTING ABOVE 1 KV

The second is caused by molecular distortion whereby
the imposed electric field distorts the negative charge of
the electron shells circulating around the nucleus toward
the positive voltage.

The third is due to the alignment of polarized molecules
within the electric field applied. This alignment is fairly
random in a neutral state, but when an electric field is
applied, these polarized molecules line up with the field
to a greater or lesser extent.

Conduction Current

Conduction current is steady through the insulation
and is usually represented by a very high value resistor
in parallel with the capacitance of the insulation. It
is a component of the Leakage Current, which is the
current that would be measured when the insulation is
fully charged and full absorption has taken place. Note
that it includes surface leakage, which can be reduced
or eliminated by the use of the guard terminal (to be
discussed later).

The graph in Figure 2 shows the nature of each of the
components of current with respect to time.

Figure 1: Alignment of Polarized Molecules

The three currents are generally considered together as
a single current and are mainly affected by the type and
condition of the bonding material used in the insulation.
Although the absorption current approaches zero, the
process takes much, much longer than with capacitive
current.

Orientational polarization is increased in the presence
of absorbed moisture since contaminated materials
are more polarized. This increases the degree of
polarization. Depolymerization of the insulation also
leads to increased absorption current.

Not all materials possess all three components and,
indeed, material such as polyethylene exhibits little, if
any, polarization absorption.

Surface Leakage Current

The surface leakage current is present because the
surface of the insulation is contaminated with moisture
or salts. The current is constant with time and depends
on the degree of ionization present, which is itself
dependent on temperature. It is often ignored as a
separate current, being included with the conduction
current below as the total leakage current.

Figure 2: Components of Test Current

The total current is the sum of these components.
(Leakage current is shown as one current.) 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 insulation tester. Some instruments
offer the alternatives of displaying a measurement in
terms of current or as a resistance.

Because the total current depends upon the time
that the voltage is applied, Ohm’s Law (R = E/I) only
holds, theoretically, at an infinite time (that implies
waiting forever before taking a reading). It is also
highly dependent upon starting from a base level of
total discharge. The first step in any insulation test is,
therefore, to ensure that the insulation is completely
discharged.

A GUIDE TO DIAGNOSTIC INSULATION TESTING ABOVE 1 KV 7

Please note: 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 is stored energy and, for safety reasons,
must be discharged after the test. Fortunately,
the discharge of this energy takes place relatively
quickly. During testing, 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 much longer time to discharge than
the capacitance charging current.

Connecting your Insulation Tester

With modern insulating materials there is little, if
any, difference in the reading obtained, regardless of
which way the terminals are connected. However, on
older insulation, a little known phenomenon called
electroendosmosis causes the lower reading to be
obtained with the positive terminal connected to the
grounded side of the insulation being tested. If testing
an underground cable, the positive terminal would
normally be connected to the outside of the cable since
this will be grounded by contact with the soil, as shown
in Figure 3. Please note that you do not connect directly
to the insulation but rather to the cable’s neutral or
ground.

Selected Typical Connections

Shielded Power Cable

Connected to measure the insulation resistance between
one conductor and ground.

Figure 4: Connection to a Shielded Power Cable

Circuit Breaker/Bushings

Figure 3: Simplistic Connection to a Cable

8 A GUIDE TO DIAGNOSTIC INSULATION TESTING ABOVE 1 KV

Figure 5: Connection to a Circuit Breaker

Power Transformer

Figure 6: Connection to a Power Transformer

AC Generator

Keen observers will note that the hookup to measure
the circuit breaker bushing included the connection of
the third, or Guard, terminal. The use of this terminal is
explained in greater detail later in this booklet.

Figure 7: Connection to an AC Generator

Insulation Resistance Tester Scale

Most modern insulation testers offer displays that
provide the operator with both a digital readout of the
result and some form of analog readout. Figure 8 is a
representation of the Megger MIT520/2 display.

nearly impossible to discern from the dancing digits of
an LCD. A few examples are listed here:

n

As the test voltage increases and the item under

test approaches breakdown, corona discharge
will cause the pointer to “jitter,” indicating to the
operator that the maximum voltage that the item
can withstand is being approached. This warning
happens in time to terminate the test before actual
breakdown, and possible damage, occurs.

n

To the experienced operator, the speed at which

the pointer travels imparts information on the
capacitance of the item under test. This is a useful
property in high-voltage cable testing, and relates
to the theoretical basis of the more sophisticated
dielectric discharge test that is described elsewhere
in this booklet.

n

If the pointer alternately rises and drops back, it

could indicate arcing in the item under test that is
too small to cause the automatic shutdown of the
tester. Such information helps direct the operator in
pinpointing a problem.

n

Observing a pointer as it slows to an apparent halt

(it may still be moving, but at a “speed” likened
to that of a clock hand) can be more agreeable to
taking a quick or spot reading than trying to decide
when a digital display has reasonably stabilized.
No digital display “freezes” on a precise number
without at least some fluctuation of the least
significant digit.

Figure 8: Megger MT520/2 Display

When an insulation tester is “hooked up” to the item to
be tested, and a test is started, several things occur. The
three different currents, capacitive charging, dielectric
absorption, and conduction/leakage are flowing. The sum
of these three currents will cause the instrument display
to vary with the reading increasing, initially quickly and
then more slowly with time.

With an analog display, the movement of the pointer
may provide information to an experienced operator.
Is the pointer traveling smoothly, or “stuttering?” Is
it rising steadily or intermittently dropping back? This
valuable supplementary information would be difficult or

This kind of detail is difficult or impossible for the eye to
extract from the scrolling digits on an electronic display.
But whereas pointer travel may be desirable, when it
stops, the operator is left to interpolate the reading
between the scale markings, introducing an element of
judgment, which can be a source of error. Digital models
present no such problem, as they inform the operator
exactly (within the unit’s accuracy specification) what
measurement has been taken. And remember, most will
give you a value of capacitance at the end of the test.

Most Megger insulation testers above 1 kV come with
an analog/digital display. One of the advantages of
this display is that the analog portion of the meter
will sway and oscillate, indicating to the operator that
the item under test has not yet reached a steady state
and is still under the influence of the absorption and
charging current. This indication means that the item
should be tested longer or that there is a problem. When
the analog portion of the display becomes steady, the
instrument displays the result in an unambiguous digital
direct reading form, with no multipliers or math to
perform.

A GUIDE TO DIAGNOSTIC INSULATION TESTING ABOVE 1 KV 9

Unlike the analog/digital display mentioned above, an
“average sensing” bar graph meter does not provide
a real-time indication of insulation resistance. Some
instruments offer a curved bar graph in place of a
genuine logarithmic arc, in which the low end of the
scale is expanded relative to the high end. The bar graph
takes readings over time, performs calculations and
then displays the results. The problem with this type
of meter is its principal of operation. If an event occurs
when the bar graph is not taking readings, it will be
missed and not shown on the display. Additionally, bar
graph simulations of pointer travel may not appear to
the eye the same as the familiar pointer travel and may
not replicate a mechanical movement to the expected
degree.

When doing insulation testing, the more the operator
knows about the results (during and after the test), the
better his/her decision on how to correct the problem, if
one exists. If something is missed during a test because
the instrument had a bar graph style meter, important
information could also be missed.

Voltage Characteristics

The output voltage of an insulation tester depends on
the resistance it is measuring. At low resistances, say tens
of ohms, the output voltage will be close to zero, maybe
a few volts. As the resistance load is increased so the
test voltage will increase until it reaches the requested
voltage. As the resistance increases further, the test
voltage will slowly increase until a steady value is
reached. This value will probably be slightly in excess of
the requested nominal voltage (e.g. 5104 V when 5000 V
was selected).

You should always ensure that an insulation tester is
provided with a “load graph” that indicates output
voltage characteristics against load resistance or,
alternatively, an integral voltmeter that actually
measures the terminal voltage during a test and displays
it continuously. By this means you can ensure that an
adequate voltage is produced over the resistance range
of interest.

A quality insulation tester will have a voltage
characteristic that exhibits a sharp rise in voltage up to a
level of resistance commensurate with good insulation.
A fast rise time ensures an effective measurement. The
voltage characteristic shown in Figure 9 represents a
good characteristic. In this example, the output voltage
will have reached 500 V at a load as low as 500 kΩ
and 1000 V by 1 MΩ. These values are legislated by
international standards for testing wiring in houses,
shops, etc. While this is hardly a typical use for typical
diagnostic insulation testers, it does provide a good
benchmark for the serious manufacturer. Similar figures
would apply at higher voltages. Voltage should rise
sharply up to anywhere from one to five megohms,
depending on the voltage selection, and maintain that
voltage at all higher resistances.

With lower quality insulation testers, voltage ramp is
far slower. The instruments typified by the poor curve
shown in Figure 10 do not produce the rated voltage
until much higher resistances have been reached. Thus
tests could produce results that provide pass levels of
insulation but have only been subjected to half the
desired test voltage.

Figure 9: Good Load Curve

10 A GUIDE TO DIAGNOSTIC INSULATION TESTING ABOVE 1 KV

Figure 10: Poor Load Curve

Figure 11: Use of the Guard Terminal on a Power Cable

The Guard Terminal

Introduction

When making an insulation test, we are often so
preoccupied with the resistance of the actual insulator
that we forget the resistance path on the outer
surface of the insulating material. This resistance path
can be very much part of our measurement and can
dramatically affect the results.

As a refresher, the total current that flows during an
insulation resistance test is made up of three main
components:

1. The charging current, which is charging up the
object’s capacitance.

circuit that diverts surface leakage current around
the measurement function. If parallel leakage paths
exist, a guard connection will eliminate those from the
measurement, and give a more precise reading of the
leakage between the remaining elements.

Surface leakage is essentially a resistance in parallel
with the true insulation resistance of the material being
tested. When making a two-terminal measurement, this
resistance path is very much part of the measurement
and can effect the readings dramatically. A three­terminal measurement, which includes the use of the
guard terminal, ignores the surface leakage. This can be
quite important when testing high voltage components
like Insulators, bushings and cables where high
resistance values are expected.

As an example, dirt and moisture on a transformer
bushing will promote surface leakage between the +
and – connections, thereby bringing down the reading
and possibly giving a false impression that the bushing is
defective. Connecting the guard to a bare wire wrapped
around the bushing will intercept this current and yield
a measurement based predominantly upon leakage
through defects in the ceramic.

2. An absorption current, which is the current that is
being drawn into the insulation by the polarizing of
the electrons; initially high but drops over time (at a
rate slower than the charging current).

3. The conduction or leakage current, which is the
small, steady state current that divides into two
parts:

a. The conduction path through the insulation.

b. The current flowing over the surface of the

insulation.

The current flowing over the surface is the component
of current that we do not want to measure if we want
to measure the insulation resistance of the material.
Surface leakage introduces errors into the measurement
of insulation resistance. Removing the surface leakage
from the measurement becomes more critical the higher
the expected insulation resistance values.

Some insulation testers have two terminals, others have
three. As these are dc testers, two of the terminals are
the + and -. The third (if present) is a guard. It does
not have to be used and many operators use insulation
testers satisfactorily without ever employing the guard.
However, it affords the operator an extra function for
diagnosis of equipment problems. The guard is a shunt

Figure 12: Guard Terminal Diagram

It is most important not to confuse the guard with a
ground. Connecting the guard and return lead to the
same element of the test item only shunts the current
that is supposed to be measured, and thereby short­circuits the measurement function. When selecting a
tester, consider:

n

The goals of testing (basic installation checks don’t

generally require a guard).

n

The electrical composition of the items to be

tested (motors and transformers can be tested for
leakage between windings, with ground leakage
eliminated).

A GUIDE TO DIAGNOSTIC INSULATION TESTING ABOVE 1 KV 11