Fuses

[Broughton 2]

I'm bewildered by the number of different DC fuse types there are and whether or not some are better suited for one purpose or another. Can anyone help me to understand?

 

For example, when I bought my sailaway it was fitted with a 250amp ANL main fuse. For the life of me I cannot imagine why it should be so large a fuse and am thinking of replacing with a smaller one based on total capacity of all the fuses used in the various circuits, but am I missing something about ANL fuses that I ought know about?

 

Also, I recently bought a Sterling 40A battery charger which came with a 60A 32V fuse (which I think(?) is AUE type fuse). Why a 60A fuse for a 40A charger and what's with the 32V?

 

Apart from ANL and AUE fuses there are AMT, AEQ and goodness knows how many other types available. Why and what, if any, are the differences?

[Dylan]

I'm bewildered by the number of different DC fuse types there are and whether or not some are better suited for one purpose or another. Can anyone help me to understand?

 

For example, when I bought my sailaway it was fitted with a 250amp ANL main fuse. For the life of me I cannot imagine why it should be so large a fuse and am thinking of replacing with a smaller one based on total capacity of all the fuses used in the various circuits, but am I missing something about ANL fuses that I ought know about?

 

Also, I recently bought a Sterling 40A battery charger which came with a 60A 32V fuse (which I think(?) is AUE type fuse). Why a 60A fuse for a 40A charger and what's with the 32V?

 

Apart from ANL and AUE fuses there are AMT, AEQ and goodness knows how many other types available. Why and what, if any, are the differences?

It would be quite in order to have a 250Amp supply fuse for a large inverter or similar. The purpose of a fuse is to protect the cable in the specific part of the circuit. It is not a question of totting up the fuse ratings at the distribution board. In a building it is quite normal to have a smaller fuse feeding a distribution board, than the amperage of all the fuses in the board added together. This comes about from "Diversity Factor" on the individual circuits. Don't get too worried, as long as the individual fuses are correctly rated to protect the circuit that they supply.

You are correct about all the different fuses and ratings, such things should have been harmonised years ago, though to be fair attempts continue to be made.

[chris w]

I'm bewildered by the number of different DC fuse types there are and whether or not some are better suited for one purpose or another. Can anyone help me to understand?

 

For example, when I bought my sailaway it was fitted with a 250amp ANL main fuse. For the life of me I cannot imagine why it should be so large a fuse and am thinking of replacing with a smaller one based on total capacity of all the fuses used in the various circuits, but am I missing something about ANL fuses that I ought know about?

 

Also, I recently bought a Sterling 40A battery charger which came with a 60A 32V fuse (which I think(?) is AUE type fuse). Why a 60A fuse for a 40A charger and what's with the 32V?

 

Apart from ANL and AUE fuses there are AMT, AEQ and goodness knows how many other types available. Why and what, if any, are the differences?

 

Using a 250A main fuse is to allow you to add more appliances. So long as the fuse rating (250A in this case) is lower than the cable's current rating then all is well. I happen to have a 100A main fuse on my boat.

 

The reason the 40A Sterling charger has a 60A fuse is so that the fuse doesn't blow when you have a current surge at switch on. Again, providing the fuse rating (60A in this case) is lower than the cable's current rating then all is well.

 

Chris

[Broughton 2]

It would be quite in order to have a 250Amp supply fuse for a large inverter or similar. The purpose of a fuse is to protect the cable in the specific part of the circuit. It is not a question of totting up the fuse ratings at the distribution board. In a building it is quite normal to have a smaller fuse feeding a distribution board, than the amperage of all the fuses in the board added together. This comes about from "Diversity Factor" on the individual circuits. Don't get too worried, as long as the individual fuses are correctly rated to protect the circuit that they supply.

You are correct about all the different fuses and ratings, such things should have been harmonised years ago, though to be fair attempts continue to be made.

 

Thanks Dylan. Good point about fuses being there to protect cables rather than components. I made sure all cables are rated well in excess of the fuses used. I would still like to know if the various fuse types have operating differences that suit particular applications.

Alan

[steelaway]

Thanks Dylan. Good point about fuses being there to protect cables rather than components. I made sure all cables are rated well in excess of the fuses used. I would still like to know if the various fuse types have operating differences that suit particular applications.

Alan

Hi There

 

I know what you mean - to many different sizes and types.

Yes there are all special

There are also at least 3 types for each type!!

Slow Blow - to resist a surge.

Fast Blow - that dosn't

Standard - somewhere in between

Always check the fuse carefuly before replacing it.

Although usually a fuse is there only to protect the cable from overheating and catching fire.

 

Alex

[Dylan]

Thanks Dylan. Good point about fuses being there to protect cables rather than components. I made sure all cables are rated well in excess of the fuses used. I would still like to know if the various fuse types have operating differences that suit particular applications.

Alan

Fusing can get quite involved and there is a lot more to it than meets the eye. Fusing for motor starting in industry can get quite complex for instance. Consequently there are many different grades of fuse or breaker these days. entire books have been written on such matters. Luckily at this level it tends to be simpler. The "megalink" fuses are good for the main supply fusng. To the horror of many I have used domestic 240volt style mcb's for my 12V domestic circuits on the boat, although the horrified ones could never give a proper reason why I shouldn't do same. The boat safety guy was fine about it and claimed quite a few people were doing the same. After much consideration I decided they would be fine, and are of a far better standard than some of the so called "DC" breakers that I have seen in chandlers etc. Thus far my decision has been sound.

[chris w]

To the horror of many I have used domestic 240volt style mcb's for my 12V domestic circuits on the boat, although the horrified ones could never give a proper reason why I shouldn't do same. The boat safety guy was fine about it and claimed quite a few people were doing the same. After much consideration I decided they would be fine, and are of a far better standard than some of the so called "DC" breakers that I have seen in chandlers etc. Thus far my decision has been sound.

 

 

Dylan

 

It is very dangerous to use AC MCB's in a DC circuit unless they are specifically designated for both uses. The arc discharge requirements are very different for AC and DC. There are different voltage and current ratings for AC and DC and the two are not interchangeable.

 

The fact the boat safety guy said it's OK is of no value IMHO. Firstly, he almost certainly won't be an electronics engineer and therefore wouldn't know either way and secondly he is mistaking consensus for wisdom.

 

 

Chris

[Keeping Up]

One problem is that many MCB's which would in reality be perfectly OK for DC, are not marked as such simply because the manufacturer has no interest in any market other than the 240v ac market.

 

I have no evidence either way to prove whether an AC-rated MCB would be satisfactory or not in the 12v DC boating environment, other than to say that I have never heard of an instance where one has proved unstisfactory. I have them in my boat, and they are still OK after 16 years, but they have never (afaik) been subjected to any particularly harsh treatment and anyway a sample size of one is not exactly signifcant! Then again, I'm not going to change them.

[Dylan]

DylanIt is very dangerous to use AC MCB's in a DC circuit unless they are specifically designated for both uses. The arc discharge requirements are very different for AC and DC. There are different voltage and current ratings for AC and DC and the two are not interchangeable.The fact the boat safety guy said it's OK is of no value IMHO. Firstly, he almost certainly won't be an electronics engineer and therefore wouldn't know either way and secondly he is mistaking consensus for wisdom.Chris

What you say is absolutely correct and I wouldn't argue with it. The thing that made me consider that they were OK: I picked a panel up in a well known chandlers "designed" for DC marine use, and the switches stated "AC only" !! (think I have seen this on here before). I have rated the circuits with the shortcomings in mind. In design, we always used to downgrade AC switches for DC use, for reasons you state.

One problem is that many MCB's which would in reality be perfectly OK for DC, are not marked as such simply because the manufacturer has no interest in any market other than the 240v ac market.I have no evidence either way to prove whether an AC-rated MCB would be satisfactory or not in the 12v DC boating environment, other than to say that I have never heard of an instance where one has proved unstisfactory. I have them in my boat, and they are still OK after 16 years, but they have never (afaik) been subjected to any particularly harsh treatment and anyway a sample size of one is not exactly signifcant! Then again, I'm not going to change them.

Good points. I have a good relationship with various manufacturers still, and will contact them regarding same.

[chris w]

What you say is absolutely correct and I wouldn't argue with it. The thing that made me consider that they were OK: I picked a panel up in a well known chandlers "designed" for DC marine use, and the switches stated "AC only" !!

 

I would query the "AC only" with the chandlery and see what they have to say because if the MCBs don't function correctly in an overcurent situation and the cables catch fire, they would be legally liable (not the manufacturer) as your contract of sale is with the chandlery.

 

With any MCB, one of the knotty problems is quenching the arc when the current is suddenly interrupted. In AC circuits, the arc tends to self-dissipate because of the change of current direction every 10mS. In DC circuits, special designs are used to overcome this issue. Using an "AC only" MCB in a DC circuit may mean that the MCB doesn't break the circuit because the contacts weld together or, they do, but only this time and not the next time a fault occurs. The size of the arc is proportional to the speed of break (or dI/dt mathematically speaking).

 

IMHO, it's not worth the risk to use these. There are mini bimetallic-strip DC breakers readily available for marine use. I have a panel of 20 of them with currents up to 15A each. A bi-metallic strip will tend to break slower than a sprung/electromagnetic AC breaker and hence produce a smaller arc.

 

Chris

[Dylan]

I would query the "AC only" with the chandlery and see what they have to say because if the MCBs don't function correctly in an overcurent situation and the cables catch fire, they would be legally liable (not the manufacturer) as your contract of sale is with the chandlery.

 

With any MCB, one of the knotty problems is quenching the arc when the current is suddenly interrupted. In AC circuits, the arc tends to self-dissipate because of the change of current direction every 10mS. In DC circuits, special designs are used to overcome this issue. Using an "AC only" MCB in a DC circuit may mean that the MCB doesn't break the circuit because the contacts weld together or, they do, but only this time and not the next time a fault occurs. The size of the arc is proportional to the speed of break (or dI/dt mathematically speaking).

 

IMHO, it's not worth the risk to use these. There are mini bimetallic-strip DC breakers readily available for marine use. I have a panel of 20 of them with currents up to 15A each. A bi-metallic strip will tend to break slower than a sprung/electromagnetic AC breaker and hence produce a smaller arc.

 

Chris

Replies so far, some to follow:

 

Replies from:

 

Square D

Our C60 range of MCB's will continue to operate at an absolute minimum of 12 Vdc or 12Vac applications.

 

Regards

Technical Support

Schneider-electric

 

MEM:

Our range of MCB's are fine to operate at 12v DC the only characteristic that will change is the " In " value which on AC for a B type is 3-5 x In, and on DC this will increase to 4-7 x In.This is working on multiplying the AC " In " by 1.4 . Any further questions please do not hesitate to contact us.

 

MK:

Unfortunately our MCB's are only suitable for operation at 240 volts.

 

Might be useful info for others. MK surprises me but I suspect that they are much the same as the others, and simply aren't certified for DC. Not to worry mine are MEM so I am happy to stick with them.

Editedto remove accidental smiley that could be taken as offensive!

Edited January 24, 2008 by Dylan

[Keeping Up]

Interesting. Mine are Hager, have you asked them as well?

[Dylan]

Interesting. Mine are Hager, have you asked them as well?

Never thought about Hagar, will check it.

[Keeping Up]

Note the spelling - Hagar is Horrible

[Dylan]

Note the spelling - Hagar is Horrible

my typping is ruubish sory. hagEr it is

[Keeping Up]

I only picked it up so quickly because I'd Googled for Hagar a few minutes before and couldn't find anything to do with MCBs. Then I remembered the spelling and found their site - only to be told that I had to be a registered professional to view their technical specs. Once I was professional but not any more.

[Dylan]

I only picked it up so quickly because I'd Googled for Hagar a few minutes before and couldn't find anything to do with MCBs. Then I remembered the spelling and found their site - only to be told that I had to be a registered professional to view their technical specs. Once I was professional but not any more.

I know the feeling.............

[Yoda]

I only picked it up so quickly because I'd Googled for Hagar a few minutes before and couldn't find anything to do with MCBs. Then I remembered the spelling and found their site - only to be told that I had to be a registered professional to view their technical specs. Once I was professional but not any more.

 

Old sod!

[Dylan]

Note the spelling - Hagar is Horrible

There's more:

 

 

Dear Sir,

We would not recommend using the Crabtree MCB, it has only been tested down to 24v DC.

You are best to use a MCB designed for this type of use.

Regards

Ashley Robinson

Product Specialist Crabtree

 

A single pole 10A mcb part number MTN110 is suitable for your 12V DC supply. This is a type "B" characteristic mcb and would have a magnetic setting of 4.5-7 x In. on a DC supply, (normally 3-5 x In at 230V 50Hz.) regards Phil

Hager Technical Support UK

Hager Ltd

Hortonwood 50

TELFORD, Shropshire

TF1 7FT