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

significant when you consider circuit breaker

ratings, because if the ASCC is greater than the

ampere interrupting capacity (AIC) rating for a

breaker or fuse, the device may not function

when a short circuit occurs. Instead, in the case

of the breaker, the contacts can be welded

closed, and in the case of a fuse, the fuse element

can arc.

ABYC Standard E-11 establishes AIC limits

for AC and DC breakers and fuses, as shown in

the ABYC table below. Simply put, whatever

readings the SureTest gives you on this test

sequence, you need to be sure they are below

the AIC ratings shown in the table. Typical AIC

ratings for circuit breakers used in AC applica-

tions will exceed this value, in my experience.

For example, on docks I’ve checked, the ASCC

readings were well below the AIC ratings on the

breakers used due to the voltage drop induced

by the wire gauge and length of wire runs typi-

cal of marina and dock wiring schemes. Com-

mon ASCC readings on docks I’ve checked fall

in the 1,000 to 2,000 amp range.

GFCI and GFP Testing

One more useful SureTest function is testing

GFCI and GFP devices. (GFPs are also

54 electrical systems troubleshooting

known as RCDs—residual current devices,

and EPDs—equipment protecting devices.)

The ABYC describes GFCI devices as those

intended to protect people from shock haz-

ards associated with AC faults, and GFP

devices as those intended to protect equip-

ment from damage. The big difference in this

application between a SureTest and a $5 LED

circuit tester is that a SureTest will measure

both the trip rate (in milliamps, mA) and the

trip time for the GFCI or GFP. This informa-

tion can warn you of the device’s imminent

failure. Considering the statistically high fail-

ure rate of GFCIs, particularly in marine

applications (more on GFP testing in a

minute), the ability to perform these tests is

of paramount importance.

During the test, the instrument creates a

small but steadily increasing current imbal-

ance between the hot and neutral conductors.

The GFCI should respond by tripping

between 4 mA and 7 mA. Most GFCIs that

I’ve checked trip at 7 mA if all is well. Note

that the nominal rating of GFCIs in North

America is 5 mA, but as with all breaker-type

devices, the nominal rating and the actual trip

rate are always different values.

NOTES: 1. The main circuit breaker shall be considered to be the first circuit breaker connected to a source of AC power.
All subsequent breakers, including sub-main breakers connected in series with a main circuit breaker, shall be consid-
ered to be branch circuit breakers.

2. A fuse in series with, and ahead of, a circuit breaker may be required by the circuit breaker manufacturer to achieve
   the interrupting capacity in TABLE V-B.

The circuit breaker interrupting capacity for systems over 50 V from ABYC Standard E-11. (Courtesy ABYC)