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Changing to LED - Change MCB?

SamKingfisher

By SamKingfisher
in Boat Building & Maintenance

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SamKingfisher Proficient

SamKingfisher

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I presume that now I have changed all my lighting from incandescent/halogen to LED I should, to be safe and correct, change the MCB to one that is rated for the new maximum amps drawn by the LEDs, i.e. 16amp MCB to 6amp MCB.

 

Or does it not matter, as a short will trip the 16amp MCB just the same as it would have before?

 

I helped clean up after an electrically caused boat fire in Exeter (pretty bad one) the other day and it's rather concentrated my mind.

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RobinJ Experienced

RobinJ

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I presume that now I have changed all my lighting from incandescent/halogen to LED I should, to be safe and correct, change the MCB to one that is rated for the new maximum amps drawn by the LEDs, i.e. 16amp MCB to 6amp MCB.

You are correct, the MCB should be rated at the maximum load, so if you have 10 12W bulbs, then minimum of 10A etc. If they all draw the same current its easier!

It is advisable to leave a margin, especially if your using motors etc., just for surge, but you want to limit the current that flows during failure to as little as possible!

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John V Veteran

John V

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No.

 

The fuse/CB is to protect the cable. Nothing whatsoever to do with what is on the end of it.

 

If the fuse/CB was previously correct for the cable, it still is.

 

Although I hesitate to argue with the master, I would point out that we don't know what size of cable has been used by the manufacturer of the light fittings if they have also been changed. Might it be safer to assume (Yes I know "an ass of u and me" )that they are a bit smaller and change the trip to the the loading now required?

 

Ok so I didn't check first and had to correct an error!

Edited by John V
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ditchcrawler Veteran II

ditchcrawlerDonate to Canal World

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Aye, surely if I was in a house, and using redundant shower cable to power an immersion, I'd fuse for the immersion not the cable? Hypothetically.

Yes because you would be protecting the bit of 3 core flex. But just think of a 3 KW electric fire plugged into a 13 Amp socket on a ring main with a 30A fuse.

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Guest

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No.

 

The fuse/CB is to protect the cable. Nothing whatsoever to do with what is on the end of it.

 

If the fuse/CB was previously correct for the cable, it still is.

Agreed. It can do no harm to lower the rating of the fuse/mcb, but I don't see the need. The (quality) led will be fused, as Bottle states. If we were fusing for fittings and not cable, we would need some sort of "variable" mcb to cover the amount of lights/devices that we had switched on at any one time.

 

Where applicable it's also best to stick to the MI (manufacturers instructions) for the equipment the fuse is protecting.

 

So if they say fit a fuse of XYZ, that's what to do, regardless of whether cable can carry a lot more.

 

cheers, Pete.

~smpt~

At one time I would have agreed 100%, but would trust my own judgement more these days, having seen some of the crap appliances coming into the country from overseas. It is a bloody electrical nightmare.

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bottle Veteran II

bottle

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Aye, surely if I was in a house, and using redundant shower cable to power an immersion, I'd fuse for the immersion not the cable? Hypothetically.

 

No.

 

The original 'fuse' would be retained to protect the shower supply cable and then a 'Switched Fused Spur' would be connected and fused for the ongoing cable to the immersion heater.

 

but

 

What you say is not wrong, just not normal practice.

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Biggles Veteran

Biggles

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You are correct, the MCB should be rated at the maximum load, so if you have 10 12W bulbs, then minimum of 10A etc. If they all draw the same current its easier!

It is advisable to leave a margin, especially if your using motors etc., just for surge, but you want to limit the current that flows during failure to as little as possible!

 

 

I have read no further than this, but WRONG. The protection is for the cable NOT the end resistance. Simple, so simple even I with my very limited understanding of ellecy thing know this to be the truth.

 

If it was ok for the previous load, it will be good for the new.

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Guest

Guest

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I have read no further than this, but WRONG. The protection is for the cable NOT the end resistance. Simple, so simple even I with my very limited understanding of ellecy thing know this to be the truth.

 

If it was ok for the previous load, it will be good for the new.

Can only agree. The example of the immersion heater is slightly different, in that it is a single fixed load, and the mcb can be the only fuse in this instance (or it could in the last edition of iee regs that I worked to.) In the case of lighting and ring main circuits, it is different because the load is variable. The only way that fusing can be achieved is to fuse the main conductors at their rated capacity with an mcb, then fuse each appliance where necessary. 13A plug tops carry a fuse (or should, though I have had ones off a famous auction site that didn't) and the low consumption (quality)LED's carry a fuse. The only other way is to fuse every light at the panel individually, then you would need a 36 way panel. No need to worry about the main conductors, providing they are fused within their rating.

 

That is my take on it; others may have a different opinion.

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smileypete Veteran II

smileypete

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Posted (edited)

At one time I would have agreed 100%, but would trust my own judgement more these days, having seen some of the crap appliances coming into the country from overseas. It is a bloody electrical nightmare.

I was thinking more of boat electrics, say if the instruction sheet for a bilge pump calls for a 3A fuse and the cable is good for 6A, what fuse would you fit?

 

cheers, Pete.

~smpt~

Edited by smileypete
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6A at the distribution board and a 3A locally to the pump. ;)

No reason why not. That said this suggested circuit only has the one appliance, so fusing at 3 amp at dis board would be no problem. It is a different ball game when there are several appliances on one circuit with varying loads, such as a lighting circuit or a ring main.

 

I was thinking more of boat electrics, say if the instruction sheet for a bilge pump calls for a 3A fuse and the cable is good for 6A, what fuse would you fit?

 

cheers, Pete.

~smpt~

See my other reply.

3 amp in this hypothetical case, since there is only one appliance on the circuit (similar to the immersion heater example). On my particular installation it would be easier to use a 6A MCB at the panel, then fuse pump locally with a 3 amp inline or similar, though every installation is different. My bilge pump is directly off the batteries via an inline fuse so this situation wouldn't arise anyway. The (hypothetical) larger than necessary cable would likely be for volt drop compensation.

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bottle Veteran II

bottle

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No reason why not. That said this suggested circuit only has the one appliance, so fusing at 3 amp at dis board would be no problem. It is a different ball game when there are several appliances on one circuit with varying loads, such as a lighting circuit or a ring main.

<<<<<<<<<<<<<<<<<<<<<<SNIP

 

I know. ;)

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RobinJ Experienced

RobinJ

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I have read no further than this, but WRONG. The protection is for the cable NOT the end resistance. Simple, so simple even I with my very limited understanding of ellecy thing know this to be the truth.

 

If it was ok for the previous load, it will be good for the new.

It occurs to me that both myself and Gibbo are right, just not necessarily for the same reason :blush:

In a normal situation. you would either have a standard fused ring main or a lighting circuit designed to feed a fixed number of items, standard cable and standard load.

Here we can assume that the cable is matched to the MCB, since it has worked up to now, but in a marine/automotive environment, fuses are in place not just to protect the cable, but the rest of the system, including fire (and I have seen it fail).

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