What Fuse or Circuit Breaker ?

[alec]

I am setting up a switch, as close as possible to the Services battery for the services main supply lead as recommended.

I will then need to insert a fuse or CB.

Three questions if I may :-

1. If I calculate the highest potential load in amps, how do I then calculate what size fuse I need ?

2. Should I fit a fuse or a CB ?

3. If I fit a CB what type of CB should I use ? . I gather there are two types. Thermal or Magnetic.

I am an 8.5m boat with only, lights, autohelm, instruments and radar.

Thanks in advance.

 

[lw395]

You need more than one fuse or breaker, because the max current drawn by auto pilot and radar is quite high, whereas the lighting wiring will burn out at a lower current if short circuited.
A fuse rating of twice the nominal max load is about right, it will then cope with the very short surges at switch on. If the nominal rating of the wiring is at least the fuse rating, it will be protected by the fuse. Wire is normally rated with mains voltages in mind, a drop of a few volts is allowable at mains, but not at 12V, so the wiring is normally much bigger for the same current at 12V.
I prefer fuses to breakers, on the grounds that they have no moving parts, and fail safe, whereas breakers are intricate bits of hardware which then get exposed to salt atmosphere and neglect and don't get tested. A breaker as well as a fuse might be a good idea on any motor (eg autopilot) that can be stalled.
I would also suggest getting a book like Calder's 12V bible as a good way of following one persons set of ideas, rather than mixing say my ideas with someone elses on the forum. There's more to it than can be put on here, hence Mr Calder selling books on the subject!
www.rswww.com sell fuses and breakers and most other things, they also function as a gateway to all sorts of data, like time to blow for fuses and all the other stuff thats in the makers datasheet.
Hope that helps.
ps try your local library for books.

 

[st599]

"A fuse rating of twice the nominal max load is about right, it will then cope with the very short surges at switch on. "

How big a power spike are you getting? The fuse is not there to protect the kit, it's there to protect the wiring, so it should be the next value above the required max and below the current carrying capacity of the wire (NB in it's installed location)

 

[pvb]

You're right to install a fuse in your main supply lead - it's a good idea. You should put the fuse as close as possible to the services battery. As far as the fuse rating is concerned, the idea of the fuse is to protect the wire, so the fuse should be appropriately rated for the size of your main supply lead. So, for example, if the main supply lead is capable of handling 50A, your fuse should be 50A or less. If your maximum electrical load is much less than the current capability of the main supply lead, you could derate the fuse accordingly if you wanted to.

Although I've used the term "fuse", you could equally well use a circuit breaker. Blue Sea make some reasonably priced thermal breakers. Thermal circuit breakers are more resistant to surge currents, so will be less prone to "nuisance trips" than magnetic breakers.

I'm assuming that your main supply lead goes to a switch panel where there are individual fuses/breakers for each circuit.

 

[Anonymous]

'Fusing' is a surprisingly technical subject. As another poster said, you main fuse is to stop your wiring from getting so hot that the insulation gets dangerously hot. The easiest and (IMO) best solution is a standard domestic circuit breaker -- the ones you buy for house wiring. These are invariably combined breakers. They have a thermal trip that trips on long, slow overloads (150% overload for several minutes, I think but you can look it up). They also contain a magnetic element which trips at some higher level (I can't remember what, but it is over 150%) but trips on milliseconds. Strictly speaking, the contacts are rated for ac mains but Nauticat have been using them for years, my family have been using them since the 1970s and in practice for normal domestic loads there is no problem. Much easier to reset when they trip, as well /forums/images/graemlins/smile.gif

 

[pvb]

The big advantage of proper marine breakers over domestic breakers is that marine breakers tend to be waterproof.

 

[Anonymous]

Agreed, but waterproof boxes with DIN rails are cheap and readily available, if needed.

 

[halcyon]

[ QUOTE ]
The big advantage of proper marine breakers over domestic breakers is that marine breakers tend to be waterproof.

[/ QUOTE ]

Which one are you thinking of ?

Brian

 

[pvb]

Maybe something like the Blue Sea 185-Series, which are IP67 rated.

 

[lw395]

That looks a little more expensive than a fuse!

 

[pvb]

[ QUOTE ]
That looks a little more expensive than a fuse!

[/ QUOTE ]About £30 - a MegaFuse and holder is around £25, so the breaker's not a lot more expensive (especially as it can be re-used).