Ship's main fuse

[blackbeard]

That is to say, the one between the main battery switch (on my boat, the one marked "1, 1+2, 2, 0") and the panel which allows me to switch on power to various systems (such as navigation lights, electronics, interior lights and so forth).
The outputs to navigation lights, electronics etc are all fused. However, there is no ship's main fuse as described above, so that a fault in the wiring between the main battery switch and the panel could perhaps start a fire.
So far as I can see, the electrics are as the builder intended.
I'm minded to put a fuse of, say, 30 A somewhere just downstream of the main battery switch to guard against the possibility of a short circuit occurring. But is there something I have missed? Is there a reason why the builder doesn't fit such a fuse? do I really need one? what problems might it introduce? what might normal good practice be?

 

[William_H]

Fusies

Fusing is an interesting question. The fuse is intended to incandesce and part with an excess current. The fuse rating is chosen so that all normal loads will be carried without fusing but the fuse will blow before any wiring in the circuit itself gets too hot.(to start a fire) So the fuse must be related to the wire size it protects and the current to be carried.
Now theoretically you should have fuse right at the battery terminal. However as this will carry starting current this becomes impractical. Starter current is so high that cables are not usually heavy enough to carry all the possible current long term. In fact the battery can fail fairly quickly at the hundreds of amps we are concerned with. So we often don't fuse the starter circuit. However we do fuse the lower current items that use thin wire.
Now we have also to do a risk analysis on firstly the chances of a short circuity developing. If it is a fibreglass boat then it would need a short to the negative or engine metal parts. Secondly what is the fire risk if the wire did become red hot. Perhaps more difficult.
I think in practice your arrangement without a main fuse to the switch board is OK. Providing wiring is well attached and separated from the negatives so no chance of chafing and is fairly short.
good luck olewill

 

[NorthUp]

Southern hemispherical

electricity appears to be the same as up here then!

Further to the OP's 30 amp fuse- while this may be sufficient for your domestic load (assuming another battery for engine starting), your charging current will be much higher on engine restarting for a period of time, maybe up to 50 or 60 amps initially.
You dont need a fuse here.

 

[mitiempo]

ABYC requires fuses or circuit breakers within 7" of the battery, and within 7" of the main switch (40" if in a conduit). Fusing from battery to starter or from main switch to starter is exempt. In practice fusing a starter is not a great problem however as the duration the starter is used (10 seconds typical) does not cause a problem normally, especially on the starter of a under 50hp diesel. The original poster's 30 amp fuse from main switch to panel does sound a bit small however as for example the Blue Seas 8 breaker dc panel (their most popular) is rated for 100 amps. It is sold with 5- 15 amp breakers with 3 empty spots. Fusing is done not for expected load but for wire ampacity. Generally the fuse should be close to but less than the wire ampacity.

 

[alexrunic]

my boat did not have a fuse either, I have left the main feed to the engine un-fused but the feed up to the fuse board has got a fuse near the battery switch / batteries. It only cost a couple of quid to do.