Wire and Circuit Protection Standards and Repair Procedures fuses will fail under the end caps where the break in the element is out of sight. When some brands of circuit breakers trip, the reset lever remains in the “on” position and they give no visual clues as to their condition; this is especially true of the trip-free breakers required on boats. Fortunately it’s easy to test circuit breakers and fuses using your multimeter.
Testing Fuses
Fig. 4-13. “Ign. protected” label on a protected breaker. This is the only breaker type that should be used in an engine room or compartment where CNG or LPG are stored.
of the ABYC standards. There are exceptions to the rules for diesel-fueled boats, but on countless occasions I have seen jerricans of gasoline stored in diesel engine rooms. This circumvents the intent of the regulations, and one foolish move by the owner of these boats could cause a disaster. An improperly stored LPG (liquid propane gas) tank is also a time bomb.
Testing Fuses and Circuit Breakers One of the very first things you should check when a circuit of any type becomes inoperative is the condition of the fuses and circuit breakers, so it’s important that you know how to verify that they are working properly. You might be saying to yourself, “Hey, my boat has all glass bus-type fuses. All I need to do is look at a fuse and I can tell if it’s blown or not.” This is not necessarily true. Often these AGC
First, be sure the boat’s battery master switch is turned on. Now set your meter to the appropriate DC voltage and attach the black probe to the DC negative bus on the back of your distribution panel or to any good ground. Touch the probe of your red lead to the positive side of the fuse holder and take a reading of your battery voltage. If you don’t get a reading, you have a problem in the feed to the fuse from the positive bus bar. If you do get a reading of battery voltage, move your red probe to the terminal at the other end of the fuse and see if you get the same reading on your meter. If the reading is the same as the battery voltage, the fuse is working. If you don’t get a reading, the fuse is blown and needs to be replaced. If you get a reading but it’s lower than the battery voltage, you have a voltage drop that’s probably caused by a corroded fuse holder. Remove the fuse and clean it and the holder. Replace the fuse and measure again. The reading should be the same on both sides of the fuse.
Testing Circuit Breakers To check for electrical continuity through a circuit breaker, first switch the breaker on and make sure that the terminals are clean and tight. With one meter lead attached to a known good ground, take a reading on the positive side of the breaker the same way you did for a fuse. If you get an identical reading at the adjacent terminal on the breaker, the breaker is working fine and any problems you may be having with the circuit have nothing to do with the circuit protection. If you do not get an identical reading on both sides of the breaker, it’s faulty and will require replacement. Make sure to match the rating of the new breaker with the old one. Figures 4-14a and 4-14b on page 54 show this test. 53
