AndersGu
Well-Known Member
Do automatic circuit breakers work well in boats or do they have problems with rust etc? If they work well, can you recommend types/sources?
Circuit breakers vs ordinary fuses in boats?
- Thread starter AndersGu
- Start date
ccscott49
Well-Known Member
Merlin marine for the circuit breakers, they are far superior to fuses.
AndersGu
Well-Known Member
OK. I assume you mean the MEP range. Any idea what they cost? Could not find any prices on the Marlin website
William_H
Well-Known Member
Fuse holders often have very poor contact to the fuse ie not enough pressure and so are often a source of lectrical failure. A good circuit breaker should be baetter than a fuse holder. Fuses seldom actually blow in a fault situation but sometimes fail from vibration fatigue and old age. olewill
ccscott49
Well-Known Member
Screwed up, meant index marine and they are 12.75 pounds each.
whipper_snapper
Well-Known Member
As others have said, cb's work well in boats. However, one important thing to be aware of is that, unlike fuses, cb's have a maximum current that they can interrupt. This means that if massively overloaded they can weld closed and not protect your circuit. For this reason, if you are relying on cb's, you should have at least one conventional fuse in your main supply rated to blow below the maximum breaking capacity of your cb's.
AndersGu
Well-Known Member
Ah.. That is a very good point ideed. Perhaps I should opt for a watertight holder for those newfangled fuses found in modern cars, as this is the "master" fuse.
A
Anonymous
Guest
CBs are certainly a lot more convenient and probably safer as you cannot put a thicker than correct fuse wire or fuse into a CB! I wouldn't worry too much about the contacts welding on a boat as the maximum current will be limited by the shore supply's own protection and the length of comparatively high resistance shore power cable. Most marinas have individual breakers and often individual earth leakage breakers.
ccscott49
Well-Known Member
I think he wants these circuit breakers for his 12 volt system. that's what I was thinking anyway. Then of course he could have a sealed main fuse for the total system.
A
Anonymous
Guest
Ah, yes, I missed that.
Yes, that's quite right, you must have a good mains fuse but hopefully of a size that is never going to blow unless a real problem arises.
Our 12V CBs (fitted by Nauticat as standard) are made by Hager and are the same type used for mains. Normally you'd expect a specially chosen switch material for dc, as dc arcs and contacts can wear badly, but presumably Nauticat assume that the CBs will only be used for tripping in an overload, not for regular switching of the circuit. We have had no problems though that in itself isn't conclusive evidence that they are the best for the job!
Yes, that's quite right, you must have a good mains fuse but hopefully of a size that is never going to blow unless a real problem arises.
Our 12V CBs (fitted by Nauticat as standard) are made by Hager and are the same type used for mains. Normally you'd expect a specially chosen switch material for dc, as dc arcs and contacts can wear badly, but presumably Nauticat assume that the CBs will only be used for tripping in an overload, not for regular switching of the circuit. We have had no problems though that in itself isn't conclusive evidence that they are the best for the job!
RMA
Well-Known Member
My boat has had contact breakers with integral on/off switches in the main switch panel for 20 years and as far as I know none have ever failed electrically. I have had to replace a couple recently because the plastic bits that hold them in the panel have cracked. I have replaced them with identical units from ASAP Supplies (£11.98).
AndersGu
Well-Known Member
Yes. I am talking the "master" fuse from the battery. Battery is charged from a solar panel, and no consumer draws more than a couple of amps. In the end, I settled for a watertight holder that holds a 20A fuse.
whipper_snapper
Well-Known Member
Yes, sorry, when I said 'main supply' I meant main supply from battery. - not mains supply.
I agree a fuse should not be necessary to supplement a breaker on a mains shore power supply. But a battery with a couple of good thick wires shorting out can produce a phenomenal current - briefly!
I agree a fuse should not be necessary to supplement a breaker on a mains shore power supply. But a battery with a couple of good thick wires shorting out can produce a phenomenal current - briefly!
William_H
Well-Known Member
A fuse or circuit breaker is primarily there to protect wirng which will with sufficient current incandesce (glow red) and be a real fire danger. Now the fuse is a small piece of wire put into a safe container which will fuse (melt) at a lower current than that which will make the wiring a fire danger. So in design you start with the wire size you need for a particular application to minimise volt drop at the normal current then you size the fuse to suit the wire ie lower than the wire max safe current but more than the current needed for the service.
If you choose a circuit breaker normally hopefully the resistance of the wire will limit the current to that which the circuit breaker can handle. However this can also be function of the length of the wire from the battery to the short circuit via the circuit breaker and return. of course you don't know where the short will occur however in boats and cars we make some assumptions and take risks that some wires are not going to short to body (car or steel boat) or -ve. So we don't put fuses in the starter system or the wiring from the battery to the fuse. We hope that with good wire routing and support a short is unlikely and in the case of the stater system that the battery may not provide enough current to incandesce a heavy cable. wires in masts behind upholstery and furniture are a greater risk of short from damage.
So in a simular way we may decide that the max current of a circuit breaker is unlikely to be exceeded due to the resistance of the wire or that the wiring is unlikely to short to the negative wire at a place where the wiring path is so short that current will be that high. All these fgactors will dictate if you fit a main fuse to protect the cbs from too much current. I must say I have not seen it done but then that doesn't mean much.
Just one more comment. Modern cars have large current (60 amp) fuse that actually has wires bolted to the leads. So it is a fuse which can't be easily changed but it is a style which should be very reliable regarding contacts. If you think you need to be able to quickly replace the fuse you might consider fitting an ordinary fuse holder in paralell with spare fuses available and not fitted but I would think that an overkill. I suspect that on most small boats a CB is a replacement for a fuse. olewill
If you choose a circuit breaker normally hopefully the resistance of the wire will limit the current to that which the circuit breaker can handle. However this can also be function of the length of the wire from the battery to the short circuit via the circuit breaker and return. of course you don't know where the short will occur however in boats and cars we make some assumptions and take risks that some wires are not going to short to body (car or steel boat) or -ve. So we don't put fuses in the starter system or the wiring from the battery to the fuse. We hope that with good wire routing and support a short is unlikely and in the case of the stater system that the battery may not provide enough current to incandesce a heavy cable. wires in masts behind upholstery and furniture are a greater risk of short from damage.
So in a simular way we may decide that the max current of a circuit breaker is unlikely to be exceeded due to the resistance of the wire or that the wiring is unlikely to short to the negative wire at a place where the wiring path is so short that current will be that high. All these fgactors will dictate if you fit a main fuse to protect the cbs from too much current. I must say I have not seen it done but then that doesn't mean much.
Just one more comment. Modern cars have large current (60 amp) fuse that actually has wires bolted to the leads. So it is a fuse which can't be easily changed but it is a style which should be very reliable regarding contacts. If you think you need to be able to quickly replace the fuse you might consider fitting an ordinary fuse holder in paralell with spare fuses available and not fitted but I would think that an overkill. I suspect that on most small boats a CB is a replacement for a fuse. olewill
whipper_snapper
Well-Known Member
Quite. It is true that the V drop in smaller wires will limit the current. However, I would not want to rely on that for the main feed from the battery.
I have seen it done in a number of all breaker boats and did the same when I re-wired my own.
I have seen it done in a number of all breaker boats and did the same when I re-wired my own.
