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Bob UK
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Posted on Friday, 09 January, 2015 - 08:50:   Edit PostDelete PostView Post/Check IP

Today I gave mate a hand with his boat wiring.
The boat has two systems one system is engine starter and heater plugs etc. The other system runs the rest. It has two alternators so the systems are completely separate.
He's chosen rewirable fuses.
For engine bit 2 fuses one for heater plugs and a small one for oil and temp gauges.
The other system has a main fuse, a knife switch and then a row of fuses supplying a row of circuit breakers

The reason that it's a mix of fuses and circuit breakers is that on the engine bit there is no fuse on the starter. If a circuit breaker is used then it must be fused.

The reason is that because there is big batteries in circuit if a short occurs the amps can go mad at over 2000 amps this will weld the breaker points together.

So to limit the current a fuse of higher value is fitted.

The engine heater plugs need 60amps. The rest is 5amps max. The 55 amp difference is too much for safety. So fuses.

The other system hasn't got high amps of a starter. The fuse is 100amps which supplies a row of fuses that are 10% higher than the breaker they are connected to.

He's made an excellent job of the design and installation and the system is very safe. Plus two alternators gives extra safety at sea. I could not resist playing with the knife switch, it's all brass with a red handle. Very high quality kit.

This why cars don't have lots of circuit breakers instead of fuses.

Split charging.
There are two main ways of split charging. On is a diode and the other is a relay that is connected to the generator warning light. The diode suffers generally 0.5v foward voltage drop, a relay hasn't a voltage drop. Both types have a problem that the alternator can't reference the voltage from two separate sources and poor charging results.

The best way is two alternators. Which can be individual set so that both batteries get a accurate charge.
However for a caravan a relay is fine because it's assumed that the cars battery is fully charged before the caravan is used for two weeks a year and that also the caravans battery is charged. If either or both batteries aren't fully charged then problems start-------!

Knife switches. These are still available. They were about the first electrical switches invented. They are so simple that it can't not work. The distribution board on this boat looks like something doctor Frankenstein made.
All it needs is a flux capacitor.

It sounds overkill but a boat half way across the Bay of Biscay in storm with electrical failure is seriously dangerous.

Next stage is the battery monitor thingy. Which he's making.

(Message approved by david_gore)

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