The Value of Circuit Breakers and Fuses - SailNet Community

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Old 03-24-2003
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The Value of Circuit Breakers and Fuses


The combination of loose wiring, errant throttle cables, and a hot, vibrating engine invites disaster.
"I could have lost the boat or our lives. I'm lucky."

A new client of mine called the other day, disconcerted about a recent electrical fire. The cause was simple and preventable. A fuse or circuit breaker in the line could have saved $10,000 in damages.

What happened was that a small breaker panel had been installed just behind his shift/throttle control. After years of shifting and throttling, the control had worked loose, contacting the main wire that fed power to the panel, consequently creating a direct short between the heavy positive wire and the engine block via the shift and throttle cables. There was no switch, fuse or circuit breaker in the positive feeder wire, and, consequently, no way to interrupt the short circuit. What resulted was an orange and black cloud of smoke and fire that melted the plastic and wire. The client was on board and he struggled to contain the fire with an extinguisher. Luckily, the wires managed to burn themselves out, but not without causing significant damage. The autopilot and the vessel's sophisticated charging system were destroyed. Numerous circuits both AC and DC, along with five separate shift and throttle cables and the bow thruster motor were damaged.

Direct contacts between positive and ground are the most dangerous kinds of shorts in a DC electrical system due to the tremendous heat that can be generated. Short-circuit protection can be broken into two different categories: prevention and automatic circuit interruption.


The kinetic nature of boats means extra care needs to be taken when it comes to keeping the flow of electrons running smoothly.
Prevention can be easy and inexpensive. Any energized electrical parts should be kept covered by a boot, or plastic shield. Good practice dictates that one should not mount energized terminals or parts near freely moving mechanical parts. Because of their kinetic nature, boats have a lot of moving parts that can take on lives of their own involuntarily. Even a wrench in a locker with an uncovered battery could be considered a hazard. When the boat is in a chop or swell, it doesn't take much to have the wrench short against the terminals and start a fire. The battery, too, is potentially a moving part if it isn't strapped down.

Wires that aren't secured can flex and break or vibrate loose from their terminals. If a positive wire contacts any wire or boat part at ground potential, a short to ground results. Wires that run across sharp edges are at risk of having the insulation cut or chaffed. Remember that everything on a boat moves. An exposed energized conductor is just waiting for a direct path to ground.

Wires laying across hot surfaces such as the engine's exhaust system can take only minutes before their insulation melts off, allowing the conductor to come into direct contact with the engine body, almost certainly a ground. And wires in oily bilge water are at risk too. Either the insulation deteriorates, or becomes hard and brittle, cracking off with time, again creating a hazardous situation.

Before throwing our hands up in despair about the risks to DC wiring on board, we can take solace knowing that there are guardian angels living among our wires. They are called circuit-protection devices (CPDs) or circuit-interrupting devices, which include fuses and circuit breakers. When too much current is flowing, these devices are designed to open the circuit and stop the current flow. Marine stores supply many types of these fuses and breakers.


The best place to put a circuit-protection device is as close to the power source as possible.
Usually the best place to put a CPD is as close as possible to the power source. Affordable and compact, ignition-proof circuit breakers are now available, allowing protection of the current loads at or near the batteries themselves. If a short occurs in any of these wires, these marvelous breakers will cut off power at the source rendering the offending wires docile, without current. Inverters and large electric motors require heavy-duty fuses. The only circuits that are commonly installed without CPDs are starter motor circuits. The current demands of these motors are just too high to reliably fuse these wires. Starter motor circuits require heavy-duty switches to allow manual disconnection of these circuits.

"Don't wire a bilge pump that requires 10 Amps to a 30 Amp circuit breaker. A clogged pump could cause enough heat to start a fire without even tripping the breaker."

When choosing and installing fuses and breakers you need to make some well-informed decisions. For instance, too much amperage rating in a CPD is a potential source of problems. Don't wire a bilge pump that requires 10 Amps to a 30 Amp circuit breaker. A clogged pump could cause enough heat to start a fire without even tripping the breaker.

Another precaution is to use wire that is properly sized for the amperage draw at the distance it must flow from the source of power to the equipment and back. Undersized wire is a common problem with circuits. Another potential problem area is found in bad wire splices and connections. This leads to poor contact and heat-generating resistance, where wire is attached to equipment or panels, or in wire-to-wire splices. (For information on this particular topic see Keeping Wire Woes at Bay.)

Another problem occurs when owners upgrade to more powerful systems without changing wire size, component ratings, or installation methods. Are you thinking of upgrading to a high-output alternator? Here are some questions you should ask. What size and type batteries will you have? How large do the positive and negative wires on the alternator need to be? Have you chosen the right type of wire for the environment it will be in? Where, what type, and what amperage rating CPDs should you install in the positive wire? How should you connect the wires to the studs to ensure good contact even after hundreds of hours of vibration? How should you support the wires throughout their run so they don't vibrate loose? Will corrosion be an issue? Is it okay to run wires above the engine's exhaust? Are heavy-duty pulleys, mounts and belts required for the larger alternator?


There's a method to the madness. Consulting with an electrician if only to get more familiar with your boat's electrical system can pay big dividends down the road.
As an electrician, I am happy to come and inspect a boat's wiring, for which I charge my normal consulting rate. A well-trained electrician has a lot of knowledge and resources available to him/her and can often point out problems, perhaps areas where improvements in wiring could greatly improve safety.

Another great source of safety information on this topic can be found in numerous books on boat wiring. Most of these books have sections devoted to safe wiring practices. Nigel Calder's book Boatowner's Mechanical and Electrical Manual has a very clear description and interpretation of The ABYC Standards and Practices for Small Craft, the definitive text on boat safety. Charlie Wing's book, Boatowner's Illustrated Handbook of Wiring, has an excellent description of safe standards and practices for DC wiring.

Next time you find yourself crawling around the hidden passages of your boat, wondering if it is alright to just temporarily twist that wire around the battery post, run it over the engine, wrap it around some fuel lines, pass it through a hole with sharp fiberglass edges and wire nut it to your VHF pigtail, you'll know the answer. Just because an installation appears to work well, doesn't mean it's safe.

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