15A circuit breaker- need additional fuse?

[srah1953]

I was looking at installing additional switched circuit breakers. A blue sea item caught my eye (http://www.bluesea.com/products/4352/6_Position_BelowDeck_Circuit_Breaker_Panel ). This is described as having a 15A circuit breaker rating. Does this mean if I was connecting an item through it which needs a 1 amp or 3 amp fuse, that I'd have to install such a fuse separate to the circuit breaker? Presuming this is the case, would I be better going for fused switch panel and putting the requisite sized fuse in the panel?
Thanks

 

[VicS]

Any circuit must be protected by a fuse or a circuit breaker rated no higher than the safe working current of the lightest wire in the circuit.

If you cannot fit appropriately rated circuit breakers then additional fuses, or a fused panel, are necessary

 

[William_H]

A fuse or circuit breaker is essentially there to protect wiring and other equipment from burning due to excess current. So the fuse or cb rating is chosen to reflect the size of the wire. The wire must be able to carry the current that will trip the CB or blow the fuse before the wire gets too hot. (and of course big enough to supply the requirements)
So if you were using that fuse box to feed equipment like lights or radio. The wire would need to be quite heavy to carry 15 amps. I don't have the actual size to hand but certainly not light weight wire. Now heavy wire is not a bad thing in that it will tend to be robust and more resistant to corrosion damage etc. However heavy wire is not always convenient for small fittings. While it might be desirable to protect electronics with a suitably rated fuse. Generally a fuse will not protect electronics from damage from an internal fault. it may protect the electronics from starting a fire, though I think this unlikely and often they have an internal fuse anyway. So I think it right to mostly be concerned about protecting wiring from fire.
On small boats however it seems to be habit to have a separate fuse or CB for each light or item on the basis that if one fuse fails you only lose that one item or service. My own belief is that this is not so important. Failure or operation of a fuse or CB is not common. Indeed more problem with fuse holders with corrosion than actual fuse. So my opinion is to not try to fuse individual items but rather use one or just a few fuses for many items. You still however need the size of wire capable of carrying the current that relates to the fuse size. Of course the fuse or CB must be capable of carrying the combined currents of all the services connected to it. It seems to me that with LED lighting and modern electronics total current drain is quite low.
So that fuse panel might be OK but if the wiring is light so dictates a lower fuse rating then you must change the cb or fuse or fit an additional fuse of lower rating in a holder in the wire from the panel. Or just buy a fuse panel where you choose the rating of the fuse. Yes circuit breakers are more desirable if you get a fault that operates the CB then the fault will need to be fixed anyway. So fuse replacement is only a small part of the fix. So yes to your last sentence. good luck olewill

 

[halcyon]

On small boats however it seems to be habit to have a separate fuse or CB for each light or item on the basis that if one fuse fails you only lose that one item or service. My own belief is that this is not so important. Failure or operation of a fuse or CB is not common. Indeed more problem with fuse holders with corrosion than actual fuse. So my opinion is to not try to fuse individual items but rather use one or just a few fuses for many items. You still however need the size of wire capable of carrying the current that relates to the fuse size. Of course the fuse or CB must be capable of carrying the combined currents of all the services connected to it. It seems to me that with LED lighting and modern electronics total current drain is quite low.
So that fuse panel might be OK but if the wiring is light so dictates a lower fuse rating then you must change the cb or fuse or fit an additional fuse of lower rating in a holder in the wire from the panel. Or just buy a fuse panel where you choose the rating of the fuse. Yes circuit breakers are more desirable if you get a fault that operates the CB then the fault will need to be fixed anyway. So fuse replacement is only a small part of the fix. So yes to your last sentence. good luck olewill

The other side of the picture with multi-use circuits on one CB's is the increase reliance on electronic aids. Fine you hang your chart plotter, gps, AID, log, depth, wind inst, etc on one fuse, load no problem, but one fault and you loose the lot, what is the back-up ? With independent CB's you loose one circuit, other systems normally back up the failure.

A secondary consideration is if a fault occurs at sea, one circuit of multiple circuits has caused the fault, it may be intermittent, you have to find which circuit has the fault before you can start looking for a fault.

You don't need to fit 15 amp, circuit breakers are available from 100 ma, depending on make.

Fuse problem has always been corrosion of fuse in holder, secondary problem was carrying fuses for the various loads, with multiple circuits on one fuse, it can be the fuse stock to cover blown fuses while finding which circuit has the fault.

Brian

 

[lw395]

Any circuit must be protected by a fuse or a circuit breaker rated no higher than the safe working current of the lightest wire in the circuit.

If you cannot fit appropriately rated circuit breakers then additional fuses, or a fused panel, are necessary

Well, not exactly.
The wire only has to carry 15A or whatever for long enough to trip the breaker.
A cable that is rated at an amp continuous would probably be fine with a 20A breaker.
But I'd prefer to avoid that kind of ratio.

What you really must avoid is a cable so long and thin that it will only draw less than the breaker current when shorted.

With instruments you want to think how things are interconnected and fused as one unpowered unit on a bus can sometimes stop the bus.
I've seen this on older equipment.
I like fuses, I am reassured by their lack of moving parts....

 

[halcyon]

I like fuses, I am reassured by their lack of moving parts....

I've avoid circuit breakers with moving parts, they are made.


Brian

 

[srah1953]

Thank you all. Much appreciated.

 

[rob2]

I guess my version of pragmatism is slightly different and encompasses most of the above. As said, it is good to have each appliance separately fused, but I want to switch all instruments on with a single switch at the panel - so a biggish circuit breaker for the group is a useful feature with in-line fuses on the feed to each instrument. Should the circuit breaker fail, then as a temporary fix you can short across it and rely solely on the fuses, but be aware that there is no protection should a feed wire short above the fuseholder without it.

Rob.