infinity on the scale of the megger (fig. 7-25) is similar
to a horizontal figure eight. During a test, a reading at or
near infinity means either that the insulation is in
excellent shape or the test leads are not making contact
with the component being tested.
If the test leads are connected to each other while
the hand crank is turned, the pointer will deflect to zero,
indicating no resistance between the test leads. A zero
deflection in the above-mentioned test (fig. 7-26) can
mean that the conductor under test is touching the
sheath or conduit surrounding it. This deflection could
also be an indication that the insulation is worn or
broken somewhere close to the test point. Any reading
near the low end of the scale may mean faulty or wet
The megger serves well as an insulation tester
because of the high-test voltage it produces. The low
voltage of an ohmmeter may not produce enough
leakage current through poor insulation to cause the
meter to indicate a problem even when one exists. But
the relatively high voltage of the megger will likely
cause enough leakage current to reveal an insulation
problem by a lower than normal resistance indication on
the meter scale.
How low is the resistance of bad insulation? How
high must the insulation resistance reading be before
you can be sure the insulation is good?
Here are some general observations about how you
can interpret periodic insulation resistance tests, and
what you should do with the results.
1. Fair to high values
and well main-
values, but show-
ing a constant
3. Low but well
4. So low as to be
WHAT TO DO
No cause for concern.
Locate and remedy
cause and check
Condition is probably all
right, but the cause of the
low values should be
C l e a n , d r y o u t , o r
otherwise raise the values
before placing equipment
in service (test wet equip-
ment while drying it out).
5. Fair or high Make tests at frequent
values, previously intervals until the cause of
well maintained low values is located and
but showing sud- remedied or until the
values become steady at a
level that is lower but safe
for operation or until
values become so low that
it is unsafe to keep the
equipment in operation.
Short-Time or Spot-Reading Tests
Several test methods are commonly used. We will
discuss the short-time or spot-reading tests.
In this method, you simply connect the megger
across the insulation to be tested and operate it for a
short, specific time period (60 seconds usually is
recommended). As shown in figure 7-28, you have
picked a point (to take the reading) on a curve of
increasing resistance values; quite often the value will
be less for 30 seconds, more for 60 seconds. Bear in
mind also that temperature and humidity, as well as
condition of the insulation, affect your reading.
If the apparatus you are testing has low capacitance,
such as a short run of type NM cable (Romex), the spot-
reading test is all that is necessary; however, most
equipment is capacitive, so your first spot reading on
equipment in your work areawith no prior testscan
be only a rough guide as to how good or bad the
insulation is. For many years, maintenance personnel
have used the 1-megohm rule to establish the allowable
lower limit for insulation resistance. The rule may be
Figure 7-28.Typical curve of insulation resistance (in
megohms) with time.