after they have begun losing their effective-
ness. This is not surprising, since ohmmeters
are typically used to look for failures of con-
ductivity, not of insulation.
Enter the megohmmeter, commonly known
as a megger. The megger is used to measure high
resistance values in insulation in the mega,
or millions-of-ohms, range (1 megohm =
1,000,000 ohms). The model I use (an Extech
380360, which costs just under $200) can read
up to 2,000 M, or 2 billionohms. That is very
high resistance.
Caution: Be extremely careful when
using a megger; it can destroy circuitry and
cause personal injury or death.As with other
instruments we’ve covered, the megohmmeter
sends a signal through the circuit—only in this
case, it’s anywhere from 250 to 1,000 volts! (In
contrast, the typical DVOM set to the ohms
scale sends only about 1 or 2 volts through the
circuit.) This amount of voltage can do some
damage—not only to sensitive electronic con-
trol circuitry but to you.
Because of its powerful capabilities, the
use of a megger has some limitations and
precautions:
- Only use a megger on heavy-gauge wire
winding sets. - Only use a megger when the manufac-
turer of the equipment specifies the use of
one as a test method and provides instruc-
tions on how to isolate any circuitry asso-
ciated with the windings. - Only perform megohmmeter tests with
the equipment workshop manual in
hand, and follow the manufacturer’s test
procedure and specifications to the letter.The specification I use regularly is a mini-
mum of 1 megohm, which is based on gener-
ally accepted U.S. military and Transport
Canada guidelines for ships and large “rotating
machinery,” as they often refer to generators,
and is their minimum requirement.
A megohmmeter test is often timed, as it
takes a while for the windings to become “sat-
urated” as the high voltage is applied. In a
maintenance situation, technicians may use a
megger to periodically test for gradual deterio-
ration of insulation over specified time inter-
vals, so extended tracking and data logging is
sometimes necessary. For most recreational
boat applications, however, this sort of work is
quite uncommon, as the labor costs will typi-
cally outweigh the cost of simply replacing
components.
So when do I use a megger? If I get higher
than normal residual current readings from a
boat’s shore cord, for example, and I am able to
trace the leakage to a particular circuit, I use
the megger to further isolate the problem.
If the circuit feeds a refrigeration system, then
the compressor motor windings are suspect. If
excessive leakage is detected from a shore-
power-supplied battery charger, its transformer
windings may be the problem. If residual cur-
rent is only noticed when the AC system is
being supplied by an onboard generator, then
its windings may be at fault. All of these prob-
lems may be caused by insulation breakdown,
and a megger might be the best tool to confirmtesting for residual current 99Model 380360, Extech Instruments, http://www.extech.com
Model 1026, AEMC, http://www.aemc.comMEGOHMMETERS