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FEATURE • Megger
MIT 1020/2
Guard Terminal
By Jeff Jowett
I
nsulation testers, or megohmmeters, are the instruments of choice
for checking and measuring electrical
insulation. Beyond the stated function of
providing a resistance reading of the insulation around wires, motor and transformer windings, and any other kind of
electrical equipment, the total value of an
insulation test is much greater. It is a
quick and simple way to get an idea of the
overall condition of electrical equipment,
the degree of wear, and where it is on its
life cycle. It is a convenient way to
make much broader assessments of
an electrical maintenance program.
Insulation resistance readings act
something like the odometer readings on a car, but in reverse. They
start high…off the scale of all but
the best insulation testers…at time
of manufacture, and then drift gradually lower through the accumulation
of wear, moisture, dirt, contamination, burn tracks and stress.
All megohmmeters employ a dc
test current, and therefore positive
and negative terminals to connect
the test leads across the specimen.
Accordingly, these are generally
marked + and -, or possibly L and E for
14
10-kV, MIT1020/2 in use
“Line” and “Earth”, or some similar notation. Many testers, especially handhelds and 1kV and below models, have
only these two terminals. But wait!
What’s that third terminal on many units,
especially benchtop (landscape style) and
higher voltage (above 1kV) models? First
of all, it is NOT a “Ground”. Typically
marked with a “G”, the “Guard” terminal
is sometimes mistaken for a safety
Electrical Products & Solutions • February 2011
MIT 510/2
ground. Using it as if it were a ground
and connecting it in parallel with the low
side of the test will do no real harm, but it
will short circuit all of the test current and
invalidate the readings.
The function of the Guard is to act as a
shunt circuit for parallel leakage paths in
the test item. With it, one or more parallel
current paths can be removed from the
measurement, thereby permitting a precise reading of the remaining path. Again,
the guard terminal can be compared to an
automobile function, where many drivers
never use the added gears on an automatic transmission and don’t consider
the difference. Similarly, a three-terminal insulation tester can be used in the
two-terminal configuration for its life
and no real harm is done. It just isn’t
being employed to full advantage.
Without the guard, the test item
is being measured as a complete unit. With the guard,
the test item can be sectionalized and multiple readings
can be taken and compared.
Let’s start with some common
examples. The most prevalent use is
for the elimination of surface leakage. An
Continued on page 16
insulation tester
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Continued from page 14
Figure 1
measures the small (generally nano-amp)
current that flows through insulation to
ground or other conductors and increases
as the insulating material deteriorates over
time. This is termed “leakage” current.
But in addition to leakage through the insulating material, there can also be current drawn across the surface of the
material. A frequent occurrence is that of
surface leakage across cable terminations.
With the negative terminal connected to
the conductor and the positive to a sheath
or braid, the test voltage gradient exists
between these two points and will pull
current across the cable surface as well as
through the insulating material. This is of
interest for two reasons: how dirty is the
cable and what is its insulation condition?
These two factors are closely inter-related
but, with the use of the guard, can be measured and evaluated separately.
Surface leakage is promoted by dirt,
moisture and contamination on the cable’s
exterior jacket and exposed surfaces. Re-
member to separate the consideration of
this current from that of conductor current. It is extremely small but enough to
cause considerable damage over time. The
ions in water, dirt, and contaminating
chemicals, and the semi-conductor effect
of carbon burn tracks are sufficient to accommodate surface flow. Over
time, this will contribute to the
deterioration of the cable, but
surface leakage is more easily
dealt with than leakage
through the body of the insulation. First, it must be
identified and evaluated. In
order to separate out the
surface leakage, a
bare copper conductor can be
wrapped around
the cable termination between the alligator clips
of the two test leads. A third
lead then connects this to the
MIT 520/2
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Electrical Products & Solutions • February 2011
guard terminal (Fig. 1). Current traveling
across the surface from one clip to the
other will be intercepted and shunted back
through the guard circuit. This circuit returns to the tester’s transformer, bypassing the measurement function. It is
effectively shunted out of the measurement. Accordingly, the tester measures
only the leakage getting through the insulating material, from conductor to sheath.
The reading will be higher than without
the guard, because a portion of the leakage
current has been diverted, and the magnitude of the difference indicates the degree
to which each element contributed to the
overall resistance. It could be that the insulation is actually in good shape, and a
cleaning of the termination and remeasurement will verify this. It will also eliminate a possible future breakdown, at least
until dirt and contamination have had time
Figure 2
to build up again. Without the guard terminal, this critical determination, which
can be made quickly by little more than
switching leads, would be left to guesswork.
Another frequent use of the guard terminal is in the testing of transformer bushings. As these are often outdoors, the
buildup of dirt, moisture and contaminants
is paramount. In early morning there may
be a coating of dew. If a maintenance test
is performed on the insulating ceramic,
current can track through this surface
coating and substantially bring down the
reading. But the real concern is cracks or
pinholes in the ceramic that can be made
by lightning strokes or fault spikes. If
merely tested with a two-terminal configuration, the low reading caused by surface
tracking cannot be separated from the possibility of structural damage to the ceramic. The bushing may Continued on page 18
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Continued from page 16
be removed and returned to the shop only
to yield a satisfactorily high reading once
the dew has evaporated. Again, if the surface leakage is intercepted and “guarded
out”, a single test will reveal the actual
condition of the ceramic. The guarding
can be implemented with a simple bare
copper wire, but specialized devices, like
bushing guard springs, are also available
(Fig. 2).
By extension, it can be seen that the
guard terminal can perform numerous
other functions to sectionalize and refine
measurements wherever suspected parallel
leakage paths may occur. A cable can be
further sectionalized by guarding out various conductors while exploring the condition between the remainder. The
insulation between stator and rotor can be
measured while guarding out the case of a
motor, or the individual windings tested
to ground while guarding out the other
windings. In the same manner, a transformer’s primary and secondary can be
tested for leakage between them while
Figure 3
guarding ground (Fig. 3), or individual
winding to ground with the other windings guarded out. Any test item with parallel leakage paths can be considered a
three-terminal network of guard, line and
FOR FREE INFO, CIRCLE 30 ON READER SERVICE CARD
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Electrical Products & Solutions • February 2011
earth, and by switching the leads between
these elements, any one path can be singled out for measurement. This capability is narrowing down or isolating the
trouble to a particular Continued on page 20
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conductor, winding, or ground. For maintenance, it helps show where the greatest
needs are for cleaning, drying or replacement.
Finally, however, it is wise to be
aware that all guards are not the same.
Like other measurement functions, they
have associated specifications. This
could come as a surprise to people who
have worked with them for years, as it
is not a prominent spec and the ability
to evaluate the guard’s capabilities is not
commonly advertised or known. The
central issue is available test current. Insulation testers, although outputting high
voltage, are capable of only very limited
test current. This is what primarily separates them from “high-pots”. They are
FOR FREE INFO, CIRCLE 31 ON READER SERVICE CARD
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Electrical Products & Solutions • February 2011
not intended to break a test item down
and take it out of service; they are intended to make a measurement. The operator then makes the decision as to what
is to be done with the test item. Accordingly, test current is limited to only a
few milli-amps, the maximum amount
of current that insulation might accommodate while still being considered at
least nominally “good” insulation. Once
the tester’s current limit is being
reached, something has to give. A little
arithmetic and Ohm’s Law will show
that this translates into something around
1 to 5 MΩ, below which insulation is not
generally considered “good”.
An insulation tester worth its salt will
NOT drop test voltage below the selected
value, provided that the load being tested
is nominally “good” insulation. But at
“breakdown” levels of resistance, the
tester should not be contributing to further deterioration by applying high voltage. Accordingly, voltage will collapse.
The critical implication for the guard circuit is that it should not be competing successfully for the limited available current.
When designing a tester to reach the market on price rather than capability, one of
the easiest things to cut is guard performance. No one is likely to look for it, and
the tester can be specified on the basis of
its two-terminal performance, without the
effect of the guard. A poorly designed
guard circuit can load down the tester.
Surface leakage will often be ten or more
times that of the insulation. Voltage will
collapse around the guard circuit and the
readings will no longer be truly indicative.
Guard terminal error can be as much as
80%!
Thankfully, there’s a simple safeguard.
Don’t be satisfied with vague general
statements but look for a precise and rigorous definition of the guard error. The
percent accuracy should be stated while
guarding a specific surface leakage (in kΩ)
against a specifically stated test load (in
MΩ). These values should all represent
reasonable and realistic parameters to the
experienced technician. If no specification
is available, beware! If the guard terminal
error is limited to an acceptable accuracy…say on the order of 2%...the operator can use the tester with confidence and
the added capability will provide an extra
tool for the detailed assessment of electrical condition and troubleshooting. ❏