Boat Electricsby Gavin Sorrell

Fuses

Circuit Protection – Fuses

What happens if your boat has no protection for all the electrical circuits? An interesting exercise (not to be recommended) might be to connect a length of 2mm² cable directly between the positive and negative cables of a battery. The instantaneous result is that the cable will overheat, frying the insulation and resulting in the disintegration of the cable. It’s no coincidence that most boat fires are caused by electrical faults and the prime culprit is a short circuit. Remembering that every circuit in a boat must be protected by a fuse or circuit breaker, the first question to be answered is which of the two to choose?
There is no doubt that the cheapest option is to select fuses rather than circuit breakers. Fuses are very compact and have high accuracy of current rating, very often in smaller increments than available for circuit breakers.
The main disadvantages of fuses include susceptibility to corrosion which can cause voltage drops, and sensitivity to physical damage. In an emergency, many circuit breakers can be manually tripped, whereas removing a fuse in such conditions is often not practicable.
Perhaps the most significant disadvantage of fuses is trying to find a replacement when one of the damned things blows at the most inopportune time.
The subject of rating causes some confusion. It is commonly perceived that the fuse is there to protect the load only. In reality, the fuse is there to protect the complete circuit from the battery to the load. Therefore, the fuse must be rated to the load as well as the cable that it is protecting. The rating may not exceed the maximum current that the cable is able to carry. If you have a pump that normally draws up to 8A, requiring say a 15A fuse, but the weakest cable in the circuit is only rated 10A, than a 10A fuse should be used.
There will be a larger common cable back to the battery from the switch panel, which should also be protected, with a much larger fuse or circuit breaker. For maximum short circuit protection locate the fuse as close to the battery as possible.
Make sure that only the positive side of a DC circuit is protected, never the negative side, which should remain unbroken at all times.
There are many varieties of fuses to suit different applications. Fuses to protect sensitive electronics with little tolerance to over current situations will be fast blow type. Circuits with electric motor loads can experience up to six times normal run current during the start up phase. A slow blow fuse is therefore required, or the fuse will be blown every time the motor is started.
Fuses and matching holders are available rated up to 1000A and more, so there is no excuse not to protect high current circuits such as starter motors.
The most commonly used fast blow fuses found on boats for ratings up to 30A are the 3AG glass fuses and automotive blade fuses. A big advantage of the auto blade fuses is that they are widely available, even from filling stations.
For higher current ratings and heavier cables, Maxi blade and Midi fuses (slow acting, 30A-80A) Mega fuses (slow acting, 80A-300A) and lift truck fuses (eg: Bussman ANL, fast acting 80A-500A) are recommended.
I strongly recommend you use only high quality fuses and fuse holders.
The price of a fuse is negligible relative to the value of the load it is protecting, so it is silly to skimp and save a few cents by buying a fuse of dubious origin, in doing so compromising the protection of the expensive load. Brand names such as Bussman or Littlefuse are recommended
Bad contact between fuse and fuseholder can cause resistance and overheating. Make sure that there is good electrical contact between the fuse and the contacts on the holder, when first installing and during routine maintenance.
Circuit protection is the best line of defence against electrical overloads and possible expensive damage to your boat and equipment. Just remember that setting out in a boat without spare fuses is like leaving without fuel in your tank!
Next month: Circuit Breakers

*Gavin Sorrell works in collaboration with Aquavolt Electric Boat Parts.
Tel: 02 9417 8455 www.aquavolt.com.au