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

result is audible noise, or static. In video or

satellite TV systems, the audio may be affected

and screen “static” may also occur. In com-

puter or navigation equipment, the noise may

cause operational problems with the gear.

The NMEA’s installation standard recom-

mends connecting a networking cable shield

only at one end of the shield. The standard

calls for the shield to be connected at the

talker end of networked devices (i.e., the data

delivery side, as opposed to the listener end,

which might simply be a display screen.)

Comparing ABYC and NMEA Grounding Standards

In the field, no troubleshooter is in a position

to determine how equipment is designed

internally, at least as far as its grounding system

124 electrical systems troubleshooting

AC plug

signal ground earth ground equipment chassis ground

AC plug

device 1 device 2

earth grounds linked

device 1 device 2

still linked here through common AC ground (earth)

linked through cable shield

loop broken here—cable sheath not linked @ both ends

Single-Point Ground System

Continuous Loop

This typical equipment installation (top) has an unintentional ground loop that will very likely pick up interference. The

ground loop above can be easily eliminated with the simple reconfiguration shown (bottom) and still be in compliance

with NMEA recommendations.