Fuses for led lighting

[pophops]

I've just replaced the bulbs in my ceiling fittings with LEDs.

There are 12 fittings and each fitting had 2 x 10w bulbs which have been replaced with 2 x 1.2w LEDs.

The fittings are on 2 different circuits, each protected by an 8amp fuse.

In the light of Jenlyn's recent experiences, should I be reducing the size of the fuse in order to reduce the risk of fire? And if so what size of fuse would be appropriate?

[b0atman]

On the basis that you probably will not have all lights on at the same time why not try 1 amp.

[ditchcrawler]

On the basis that you probably will not have all lights on at the same time why not try 1 amp.

I think that would hold with all of them on. Its only a guess as he hasn't told us what wattage they are.

Edited December 31, 2014 by ditchcrawler

[GUMPY]

Fuses are there to protect the cable not the device at the end of it. So if your fuses were thw correct value before you changed the lights then just leave them. Unless of course you have introduced some very thin cable into the circuit.

[Sea Dog]

I think that would hold with all of them on. Its only a guess as he hasn't told us what wattage they are.

 

Well there's this bit you missed (sorry Ditchcrawler, it's easy done )....

 

There are 12 fittings and each fitting had 2 x 10w bulbs which have been replaced with 2 x 1.2w LEDs.

 

So the new total load is 28.8 watts (hence 2.4 amps at 12 volts) and, if the split into 2 circuits is even, 14.4 watts for each circuit so slightly over 1 amp each (again at 12 volts, or half that if you're 24v).

[BargeeSpud]

Fuses are there to protect the cable not the device at the end of it. So if your fuses were thw correct value before you changed the lights then just leave them. Unless of course you have introduced some very thin cable into the circuit.

 

Concur, I've had that drummed into me this last 8 weeks or so of the build!

[bottle]

Yes fuses/circuit breakers etc. are used to protect the cable but there is no harm in reducing the size (amps) of any of these.

 

Where there is harm and danger is in using a fuse/circuit breaker etc. that is has a larger rating than the cable rating but then we get into the discussion of ring circuits.

 

To Pophops, I would not and have not changed mine after changing from halogen to LED lighting.

[Chalky]

The "fuse is to protect the load" is there to protect the cable. It doesn't however stop you from fitting a smaller fuse since you're still protecting the cable.

[Tony Brooks]

 

Concur, I've had that drummed into me this last 8 weeks or so of the build!

 

Nah - you are both now proven to be wrong as shown by some of the responses to a recent thread on the topic of fuses, even one from a professional. It seems fuses are there to protect the appliance as well as the cables. (Says he with tongue very firmly in cheek). However the OP may have a point so in view of the other LED topic so it would not hurt to reduce the fuse size but I would want to know the total rating of the bulbs.

[BargeeSpud]

 

Nah - you are both now proven to be wrong as shown by some of the responses to a recent thread on the topic of fuses, even one from a professional. It seems fuses are there to protect the appliance as well as the cables. (Says he with tongue very firmly in cheek). However the OP may have a point so in view of the other LED topic so it would not hurt to reduce the fuse size but I would want to know the total rating of the bulbs.

 

Really Tony? If you're 100% sure about that, then are you saying that both Julian & I are just perpetuating another boaty urban myth?

 

To throw a spanner into your professional's opinion from the other thread, here's another professional opinion as quoted in response on a similar thread on the IET Forum -

 

"The prime purpose of a fuse is to protect the supply.

The size of fuse is dictated by the supply capacity and the load.

The load dictates the cable/flex size/capacity."

 

I interpret the supply to be the cable & the load to be the appliance, so I would say that both arguments are correct, in that whilst the fuse is there to protect the cable (supply), the appliance (load) dictates the cable size & both dictate the fuse size.

 

Confusing ain't it?

[b0atman]

The Fuse is to protect the Appliance the cable should be rated suitable for the Appliance .

An extra benefit of the fuse is that it will protect the cable in the event of a fault in said cable.

Because of volt drop note that larger size cable would tend to be fitted in boats.

[Tony Brooks]

 

Really Tony? If you're 100% sure about that, then are you saying that both Julian & I are just perpetuating another boaty urban myth?

 

To throw a spanner into your professional's opinion from the other thread, here's another professional opinion as quoted in response on a similar thread on the IET Forum -

 

"The prime purpose of a fuse is to protect the supply.

The size of fuse is dictated by the supply capacity and the load.

The load dictates the cable/flex size/capacity."

 

I interpret the supply to be the cable & the load to be the appliance, so I would say that both arguments are correct, in that whilst the fuse is there to protect the cable (supply), the appliance (load) dictates the cable size & both dictate the fuse size.

 

Confusing ain't it?

 

 

Did you notice the "tongue firmly in cheek" bit?

 

I agree that the safest way for "ordinary" people is to specify the fuse to protect the cable. The cable is also specified for the load BUT on multi-outlet circuits this must lead to the fuse being too large for individual loads.

 

I agree - confusing but it is vital that the message that goes out is that fuses must ALWAYS be small enough to protect the cable.

[blackrose]

My understanding on led lights (which may of course be wrong), is that they are extremely sensitive to voltage spikes and should be regulated. Voltage regulation is done either at the circuit or on the bulb itself. While none of this is really relevant when it comes to the minimal current drawn by the leds, since the cables are already protected then I really don't think that reducing the size of a fuse on the circuit will really accomplish anything. If on the other hand you're trying to protect the bulbs then it's the voltage regulation that's the important thing.

[alan_fincher]

There are 12 fittings and each fitting had 2 x 10w bulbs which have been replaced with 2 x 1.2w LEDs.

The fittings are on 2 different circuits, each protected by an 8amp fuse.

 

Just out of interest.....

 

If the above is all true, then you had 20 watts in each of 12 fittings. Even assuming that there was an even split, with 6 fittings on each circuit, then had you turned all the fittings on a circuit on, you would have been consuming 6 * 20 watts = 120 watts per circuit.

 

At a nominal 12 volts, that equates to 10 amps.

 

So either you never ever turned all the lights on on one circuit at the same time, or you were exploiting the fact that a fuse rated at 8 amps will actually allow rather more than 8 amps without blowing, and were close to the margins of the fuse failing.

 

But back to the question......

 

In my view, if it really is the case that some LED lights can go wrong, and even burst into flames, there is certainly an argument for fusing the circuit at a level where the fuse should happily take the combined current of all LEDs working correctly, but where it might blow if one of them unexpectedly starts drawing a far higher current. My guess is that for a fault bad enough to cause smoke or flames, there could be a very considerable increase in current over what you would have for a properly functioning unit. However with your 8 amp fuses, that increase may still well not be enough to blow them, so the current will continue to flow, making the effect more dramatic, and possibly increasing the likelihood of it creating far worse problems. Although I have not done so myself, I can see an argument for reducing the fuse value to one enough to run the lights, rather than what is required to protect the cable.

[ditchcrawler]

The only time all the halogen lights are switched on is when the boat is at Crick boat show so the boat looks bright, the owner soon discovers that they soon sop the batteries

[nicknorman]

A fuse can never (or rarely) protect the appliance. Who comes up with this BS!

 

Consider a washing machine or dishwasher. The fuse has to be rated to at least cope with the water heating bit. But for a good chunk of the wash, the heater is not powered, so the remaining electronics and motor now has a grossly over-rated fuse that will not stop it going on fire if a fault develops.

 

Ditto in the case of an LED lighting circuit where only one, or all, of the lights might be on at any one time.

 

If the fuse was to protect the appliance, appliances would never go on fire but in the real world they do. Why? Because it is mostly impossible to design a fuse to protect a faulty appliance.

[Tony Brooks]

[pophops]

 

 

So either you never ever turned all the lights on on one circuit at the same time, or you were exploiting the fact that a fuse rated at 8 amps will actually allow rather more than 8 amps without blowing, and were close to the margins of the fuse failing.

 

 

 

You're right about not using all the lights at once. I have always been in the habit of using only 2 or 3 lights at any one time in order to conserve power.

[by'eck]

A fuse can never (or rarely) protect the appliance. Who comes up with this BS!

 

Consider a washing machine or dishwasher. The fuse has to be rated to at least cope with the water heating bit. But for a good chunk of the wash, the heater is not powered, so the remaining electronics and motor now has a grossly over-rated fuse that will not stop it going on fire if a fault develops.

 

Ditto in the case of an LED lighting circuit where only one, or all, of the lights might be on at any one time.

 

If the fuse was to protect the appliance, appliances would never go on fire but in the real world they do. Why? Because it is mostly impossible to design a fuse to protect a faulty appliance.

 

Only true up to a point. There have been many occurences where correctly chosen fuses have saved expensive DC devices that were not faulty but simply overloaded. You should know better than to perpetuate this myth Nick. Your theory is indisputable but practice suggests you are generally wrong on this issue.

Edited January 1, 2015 by by'eck

[nicknorman]

Only true up to a point. There have been many occurences where correctly chosen fuses have saved expensive DC devices that were not faulty but simply overloaded. You should know better than to perpetuate this myth Nick. Your theory is indisputable but practice suggests you are generally wrong on this issue.

I take the middle path on this. Obviously there is no point in rating the fuse well above the max service demand just because the cabling can take it, but equally let's not kid ourselves that this will in some way remove the possibility of a fire. It might reduce it, but only very slightly.

[smileypete]

I take the middle path on this. Obviously there is no point in rating the fuse well above the max service demand just because the cabling can take it, but equally let's not kid ourselves that this will in some way remove the possibility of a fire. It might reduce it, but only very slightly.

 

OK Nick, say you have a replacement bilge pump which calls for a 5A fuse, but the existing wiring and breaker is 10A, what do you do?

 

cheers, Pete.

~smpt~

[nicknorman]

OK Nick, say you have a replacement bilge pump which calls for a 5A fuse, but the existing wiring and breaker is 10A, what do you do?

 

cheers, Pete.

~smpt~

Well apart from not having any breakers on my boat, if the appliance specifies a particular fuse size then that is what should be fitted, though it could of course be fitted at the pump, with the 10A breaker left in situ.

 

In the particular case of a motor, a common failure mode would be motor stalled and it will either be intrinsically designed not to overheat, or will require a fuse to prevent it. Most appliances are not just motors though. This is why I was careful not to use absolutes in my earlier posts.

Edited January 1, 2015 by nicknorman

[jenlyn]

 

OK Nick, say you have a replacement bilge pump which calls for a 5A fuse, but the existing wiring and breaker is 10A, what do you do?

 

cheers, Pete.

~smpt~

You put a 5 amp fuse in, to protect the component.

[by'eck]

I take the middle path on this. Obviously there is no point in rating the fuse well above the max service demand just because the cabling can take it, but equally let's not kid ourselves that this will in some way remove the possibility of a fire. It might reduce it, but only very slightly.

 

I don't need to kid myself. I was referring to real life situations. FWIW I have never had an overheated DC wiring issue (even on a 500 amp fused circuit) although I am not in any way suggesting the possibility should be ignored. If a single fuse can save both wiring and device it makes sense to choose the value accordingly.

 

If people want to promote the bad practice of having multiple devices on one DC feed (lighting circuits aside) then yes of course each spur needs an appropriate fuse to protect the individual device. Note that in such case at least two fuses are needed, one of which will be in a random remote location, there will be extra connections as well, all leading to potential unreliability and fault finding difficulties.

 

Other comments from folk who should know better, regarding having faith that a manufacturer will provide internal fusing to protect the device falls down on so many levels. Not least that it often doesn't make economic sense to provide such internally when a simple recommendation can be made to fit an external one.

[smileypete]

Well apart from not having any breakers on my boat, if the appliance specifies a particular fuse size then that is what should be fitted, though it could of course be fitted at the pump, with the 10A breaker left in situ.

 

In the particular case of a motor, a common failure mode would be motor stalled and it will either be intrinsically designed not to overheat, or will require a fuse to prevent it. Most appliances are not just motors though. This is why I was careful not to use absolutes in my earlier posts.

 

Yeah so it's not exactly BS then.

 

If the manufacturers instructions call for a fuse, then the role of the fuse is to protect the circuit, not just the cable, plus a bit of common sense about load relative to cable sizes.

 

cheers, Pete.

~smpt~