operator and all the boatowners in the
area—a potentially deadly situation exists.
(Note: Your previous residual current
readings and SureTest readings are clues to
this possibility. Take these clues very seri-
ously; you could save somebody’s life.)AC Leakage from Degraded Insulation
The second common cause of residual current
is the breakdown of insulation in motor and
generator windings and in shore-power isola-
tion transformers. Leakage can occur between98 electrical systems troubleshooting
adjacent windings and between the windings
and case grounds. It can also be influenced by
ambient temperature and humidity. However,
this breakdown can be difficult to detect
because the insulation may still retain mostof
its insulating properties, and the device may
continue to function normally even as the
leakage is occurring.
The effectiveness of insulation is a func-
tion of its resistance (impedance), measured in
ohms (). Materials that are used as insula-
tion of course have very high impedance—so
high, in fact, as to be off the scale of the ohm-
meter function on a standard DVOM, evenMAKING AN EXTENSION PROBETo make an extension probe, you’ll need the following supplies:
broomstick
crimp-on ring terminal
several small screws
square of copper flashing
extended multimeter lead
To begin, remove the pointed probe end common to most meter leads and attach the crimp-on ring terminal. The
terminal should have a hole in it for the small screw you’ll use to attach the copper plate.
You can get copper flashing at your local hardware store. (Home Depot sells 5-by-7-inch precut pieces that are
just right for this job.) The actual size isn’t critical, but bigger is better since it makes the lead more sensitive to finding
current fields.
Use small screws to attach the copper square to the lead and secure it to the end of the broomstick handle.extended lead black lead (plugs into
the grounding terminal
on the shore-power
pedestal)meter copper square
(goes in water)Here’s the extended probe rig I made.