Fuses : Are circuit-breakers always acceptable replacements?

[Clifford]

Lots of equipment will say in the installation instructions "must be protected by a fuse". But does it have to be literally a fuse?

 

If the gear comes with a fuseholder and a fuse, then I guess I would use that - in case there was something special about it (speed of blowing or something) that makes it essential.

 

But that apart, can you always use a circuit breaker of the same capacity ?

 

Or are there times when 'fuse' must mean 'fuse' and nothing else? What about in the BSS?

Edited May 12, 2014 by Clifford

[Tony Brooks]

As long as the circuit breaker is DC rated (for 12/24V use) then there is virtually no difference between a fuse and circuit breaker. however circuit breakers come in different "blowing" (opening) curves and I suspect they are mores sensitive than a fuse so you may find a circuit breaker opens when a fuse does not blow. This simply means choosing a breaker with a more appropriate curve.

 

I don't know enough to go further but I am sure another member will be along to explain the curves. I think they are designate A, B & C.

[Guest]

Plenty of reading here:

 

http://en.wikipedia.org/wiki/Circuit_breaker

 

ps some people will say that 240V style MCB's should not be used at 12V dc. This is not necessarily accurate, but it is vital to check with the manufacturer before doing so. I used Hager MCB's for my 12Vdc system, after approval from their technical department (ten years ago now.)

[Robbo]

Lots of equipment will say in the installation instructions "must be protected by a fuse". But does it gave to be literally a fuse?

 

If the gear comes with a fuseholder and a fuse, then I guess I would use that - in case there was something special about it (speed of blowing or something) that makes it essential.

 

Fuses tend protect sensitive electronics, breakers wiring.

 

See here; http://www.bdfuses.com/breaker.php

[Chalky]

Fuses, like circuit breakers come in different ratings and with different blow characteristics. Fuses are thermal devices whereas most circuit breakers are magnetic. The effect is that a fuse can be running at significantly above its rated value for a long period and not blow whereas a circuit breaker can trip at the slightest hint of an overload. 3 quick fixes:-

 

1 - choose a breaker with an appropriate trip characteristic

2 - re-arrange the circuits between breakers so that there isn't an issue

3 - remember that the breaker is there to protect the cable not the load. In that case you might (and I repeat might) be able to up the breaker rating to one that doesn't nuisance trip provided that the breaker is capable of protecting the cable.

 

The significant differences between the AC and DC parts are the arc suppression and the material that the contacts are made from. For higher DC voltages these are significant issues but for 12V there shouldn't be a problem.

 

Like Catweasel I've used Hager breakers for all the circuits on the boat. the only exceptions are the 200A and 400A mega fuses that are the last line of defence to protect the batteries in case of a short circuit.

[Keeping Up]

 

Fuses tend protect sensitive electronics, ..

 

I've found the opposite to be the case, in that sensitive electronics usually protect the fuses.

 

BTW I also use Hager breakers, with a megafuse as the last line of defence.

[Clifford]

Thanks very much.

 

Hoping to avoid stuipid questions when I get an electrician in. But maybe 'internet expert' questions will be even more annoying.

 

I'm enjoying learning about this stuff, thanks again.

[Tony Brooks]

Can I add than NO fuse or circuit breaker in the wiring system should be used to protect any equipment. It is only there to protect the cables but may well also protect equipment.

 

If equipment needs protecting then it should have its own protection in addition to any cable protection and as Robbo points out it is often a fuse on circuit boards or mounted on/in the equipment.

[Clifford]

Thanks Tony.

 

Btw, thanks for all your course materials online. They were practically the first technical stuff I read when we first got a share in a narrowboat, long before we thought of getting a boat of our own. I refer to them a lot. I put off going on one of your courses too long - and you stopped doing them!

 

Very readable materials, I was reading them again this morning.

[by'eck]

Can I add than NO fuse or circuit breaker in the wiring system should be used to protect any equipment. It is only there to protect the cables but may well also protect equipment.

 

If equipment needs protecting then it should have its own protection in addition to any cable protection and as Robbo points out it is often a fuse on circuit boards or mounted on/in the equipment.

 

Nevertheless there are very many DC powered devices offered where the manufacturer specifies a fuse (or breaker) to protect their product. My view is that the supply cables should be well over rated to the max current draw of the device (when working normally) and the fuse value to be around 150% of such located at the supply end of the cable. In this case cabling protection will never be an issue.

 

There have been several occasions on my various boats when expensive motors driving toilet pumps, bowthrusters etc. have been saved when drawing excessive current whilst stalled.

[Tony Brooks]

I agree but that is because the manufacturer is cutting costs but maybe not cutting prices. In my view a toilet macerator should be equipped with a thermal cut out to protect itself and me are. Likewise bow thrusters. It is just substandard design when such protection is not provided.

[by'eck]

I agree but that is because the manufacturer is cutting costs but maybe not cutting prices. In my view a toilet macerator should be equipped with a thermal cut out to protect itself and me are. Likewise bow thrusters. It is just substandard design when such protection is not provided.

 

Either way, the fuse or breaker is not just there to just protect the cable which is the point I was making in response to your statement to the contrary.

 

I would add that thermal and electronic overload protection is not foolproof and would rather have a quicker reacting fuse or breaker blow/trip than any possibility of stress on a device whilst waiting for built-in protection to work.

 

FWIW my bowthruster motor does have thermal protection but I was grateful that the manufacturer recommended 600 amp fuse blew first when the motor was partially stalled by a re-enforced plastic bag caught on its prop. Similar situation with Combi when shore power failed, overloading inverter to the tune of 160%. The 500 amp fuse blew saving the day.

[Tony Brooks]

Thanks Tony.

 

Btw, thanks for all your course materials online. They were practically the first technical stuff I read when we first got a share in a narrowboat, long before we thought of getting a boat of our own. I refer to them a lot. I put off going on one of your courses too long - and you stopped doing them!

 

Very readable materials, I was reading them again this morning.

 

Please be aware that at least one of the more frequently stolen diagrams is now incorrect on the website. All the circuit diagrams that a boater may use are correct as far as I know but at least one "internal" diagram is not. If you have/are printing them, out please email me for the correct diagram. The course books are correct.

[Guest]

Fuse discrimination is sometimes difficult to get right, especially where there is a mix of protective devices. A few weeks ago a cheapo Chinese power pack decided to expire at home. It was protected by a 5 amp cartridge fuse in the plug top, but when it expired, the 5 amp fuse held, and the 32A MCB in the consumer unit tripped, along with the RCD.

[John V]

Plug top fuses of he same value vary enormously depending on where they are manufactured. Mcb's come in varying "speed" types, the most common domestic one is a "B" but a "faster" "A" is around as is the much "slower" "C"

"C's" are normally used in motor circuits and can handle the very high starting currents but still will trip with a sustained overload. Other specialist types are around with different spec's for special jobs

[Chalky]

All fuses are thermal devices and the blow characteristics change with ambient temperature and what they're bolted to. They also change with the age of the device. Magnetic circuit breakers are a lot more predictable.

 

Fuse discrimination is sometimes difficult to get right, especially where there is a mix of protective devices. A few weeks ago a cheapo Chinese power pack decided to expire at home. It was protected by a 5 amp cartridge fuse in the plug top, but when it expired, the 5 amp fuse held, and the 32A MCB in the consumer unit tripped, along with the RCD.

 

I've heard similar stories of fuses carrying far more than their rated current.

[Keeping Up]

There was even a range of fuses, which I was instrumental in having banned around 1980, that would blow at around their rated current but the metallic vapour would then condense on the inside of the (ceramic) cartridge. This formed a metallic film that was better cooled than the original wire so they would then conduct again and would happily pass a much higher current than their original rating.

[Guest]

There was even a range of fuses, which I was instrumental in having banned around 1980, that would blow at around their rated current but the metallic vapour would then condense on the inside of the (ceramic) cartridge. This formed a metallic film that was better cooled than the original wire so they would then conduct again and would happily pass a much higher current than their original rating.

Amazing.

 

As an aside, I notice in Germany that there appears to be no fuse in their plug tops. I assume that they trust the MCB's enough to provide overload protection for the appliance as well as the cables within the circuit?

I have always assumed we put a smaller cartridge fuse in the plug top owing to our "clog iron" 32Amp fuse wire that we once used?

Edited May 13, 2014 by Guest

[John V]

Continental circuits (or at least the ones I have come across) are radial circuits fused at 16A as opposed to 32A ring mains, maybe they think that will help..... .......Personally I don't like the thought of a hand held hairdryer, with a bit of 2 core damp string as a cable, protected by a 16A device