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

Determining the VOP Value

For this example, we are using the AEMC
Fault Mapper’s method to determine VOP,
but the steps will be similar for other brands:

1 Cut a length of duplex conductor at least
60 feet (18.5 m) long. Measure the length
accurately with a tape measure.

2 Connect the meter’s red lead to one of the
two pieces of wire and the black lead to
the other (strip back the insulation on
both leads first). (Don’t twist the conduc-
tors together at the far end or you’ll create
a short circuit.)

3 Turn on the Fault Mapper, push the meter’s
test button, and note the indicated length.

4 Use the scroll keys to adjust the VOP
value up and down until the test result
equals the actual length of the cable. Your
final VOP value will be appropriate for all
cables of the same gauge and type.

I’ve tested many cable types and gauges
and have found that all the cable commonly
installed in boats falls in the range of 60% to
65% VOP. If you’re off by a percentage point
or so, it’s not the end of the world, but it will
create a small inaccuracy in the meter’s calcu-
lation of distance to the fault. Table 3-1 gives
the VOP values for commonly used American
Wire Gauge (AWG) sizes of boat cable, along
with metric equivalents.

Using the TDR to Locate a Fault

Before you can begin testing, you must deter-
mine the size of the cable or wire you are test-
ing. All approved wire and cabling has its size
printed on its insulation jacket (in AWG in
North America; in mm^2 for the rest of the
world) as in the top photo.
Once you know the wire size, follow these
steps:

testing continuity and tracing circuits 41

AWG Metric Equivalent (mm^2 ) VOP (%)

88 63

10 5 64

12 3 6 2

14 2 61

16 1 61

Velocity of Propagation Values for

Typical DC Boat Wiring

TABLE

3-1

This wire’s size——4.0 mm^2 ——is printed on its insulation

jacket. This is roughly equivalent to 10 to 12 AWG.

fuses negative

positive

bus

bar

Attach the meter’s leads to the small fuse panel at the source of the test

circuit: the red lead goes to the DC positive source at the fuse, and the

black lead to the DC negative. (Anywhere on this bus will do, as it is a

common return for all the circuits on the panel.) The TDR screen displays

a VOP value of 61%, which corresponds to the circuit’s 14 AWG wire.

The meter’s internal battery status (fully charged) is also shown graphi-

cally at the bottom of the display.

1