Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

other specified criteria, such as temperature.
For example, if you are testing cable insulation,
make sure the cable is completely disconnected
from the circuit it serves at both ends. For
ground leakage tests on a tool or appliance,
there are no special concerns to address. But for
motor circuits, transformer windings, and gen-
erator winding sets, it’s best to follow the advice
of the equipment manual to ensure that you
don’t do more harm than good. As for tempera-
ture concerns, some equipment is tested at nor-
mal operating temperatures. In other cases, the
equipment manufacturer’s test procedure may
require you to adjust the effective resistance
value up or down based on actual temperature.
Even with a maximum measurement range
of 2,000 M, it’s not uncommon for a
megohmmeter to max out during testing.

Different models indicate this in various ways;

on the Extech model 380360, off the scale is

indicated by a screen display of 1 followed by

blank spaces, a decimal point, and the symbol

M (see photo page 102). This simply means

you have excellent insulation—it’s not an error.

Value and Utility

Meggers can be extremely useful, but the

range of applications for most marine techni-

cians and boatowners is quite limited, and

most technicians do not use them very often.

Unless you work on mega- or superyacht-sized

vessels, or in an industry where leakage and

resistance checks are regularly required, a

clamp-on leak tester will supply most of your

needs.

testing for residual current 101

transformer

primary

winding lead

chassis ground

should be

electrically

isolated

Testing for leakage and good ground isolation on a transformer winding set. Notice the meter lead connections at both
the transformer and at the meter end. The stud on the motor is the ground. The meter’s other lead is clipped to one of the
primary winding leads.