Why A 3A Fuse Protecting Gas-Related Electrical Circuits?

[Dan Robinson]

Then the MI's will stipulate.

 

[D_Hailsham]

Is it written in Gas Regs, or just manufacturer's recommendations?

It's common sense, set down in writing for those who don't have any! After all, a 13A fuse wouldn't give much protection for a circuit drawing less than an amp!

 

[securespark]

Thanks. Why 3A, though?

 

[D_Hailsham]

Probably because they are the most appropriate "standard" fuse, readily available from many shops. Other size fuses are available - if you know where to buy them.

You must have come across table lamps with 60W bulbs fitted with a 13A fuse because the owner thought that was the correct fuse -"It's a 13A socket, so it must have a 13A fuse".

Now that low wattage LED lighting is more readily available, the experts will have to rethink plug fuses. Even a 500mA may be too large.

 

[ban-all-sheds]

It's common sense

Actually it's anything but, which is why this discussion is taking place.

a 13A fuse wouldn't give much protection for a circuit drawing less than an amp!

A circuit drawing less than an amp will draw less than an amp no mater what the fuse rating.

In terms of protection, a 3A fuse won't provide much more than a 13A one, because it will allow 5-6A to flow indefinitely, and there aren't many kW rated components on PCBs. It might allow 10A to flow for a couple of seconds, which is more than enough time for all the expensive blue smoke to make good its escape.

Fuses are meant to protect circuit cables from damage, and if the device supplied by the cable cannot create an overload (i.e. overcurrent occurring in a circuit which is electrically sound) then it only has to be able to protect against faults, and there isn't an installation-level fuse or MCB in the world which will work fast enough to prevent further damage to a PCB if something on it generates a fault. Which is why you find small-value fuses on PCBs themselves.

 

[ban-all-sheds]

You must have come across table lamps with 60W bulbs fitted with a 13A fuse because the owner thought that was the correct fuse -"It's a 13A socket, so it must have a 13A fuse".

A table lamp cannot overload the flex, the only overcurrent that can arise would be from a fault, so as long as the 13A fuse can protect the flex from a fault it is the correct fuse. The rating needed has nothing to do with the bulb, and everything to do with the flex.

Now that low wattage LED lighting is more readily available, the experts will have to rethink plug fuses. Even a 500mA may be too large.

No they won't, and no it won't.

 

[D_Hailsham]

A table lamp cannot overload the flex, the only overcurrent that can arise would be from a fault, so as long as the 13A fuse can protect the flex from a fault it is the correct fuse. The rating needed has nothing to do with the bulb, and everything to do with the flex.

How many table lamps are supplied with a cable capable of carrying a continuous current of 13A?

 

[bernardgreen]

Not all faults are zero ohms. A low resistance fault could allow a current high enough to cause local heating but not high enough to blow the fuse.

Which is best 1380 watts ( 6 amps required to eventually blow a 3 amp fuse ) or 460 watts ( 2 amps required to eventually blow a 1 amp fuse )

Or 5980 watts ( 26 amps through a 13 amp fuse ).

 

[ban-all-sheds]

How many table lamps are supplied with a cable capable of carrying a continuous current of 13A?

How many table lamps are supplied with, or later fitted with, 3kW bulbs?

 

[D_Hailsham]

How many table lamps are supplied with, or later fitted with, 3kW bulbs?

You are missing the point.

1. The cable supplied should be suitable for the recommended max wattage of the bulb; i.e it will not overheat in normal use;
   
2. The fuse used should blow, if a fault develops, before the cable overheats enough to cause damage, e.g a fire;
3. If the fuse rating is more than the safe rating of the cable and a fault develops, the cable may overheat and cause damage before the fuse blows.

So you do not find a 13A fuse fitted to a table lamp because it will not provide adequate protection to the cable, not because you don't find 3kW bulbs in table lamps. As you said, the fuse is there to protect the cable.

 

[ban-all-sheds]

No - I am not the one missing the point.

As there are no table lamps with 3kW bulbs in them, there are no table lamps which will generate a 13A current in the flex.

A table lamp cannot, it just cannot, overload the flex, therefore the flex has only to be suitable for the load of the table lamp.

As the table lamp cannot overload the flex, the fuse does not have to protect the flex against overloads, it only has to protect it against faults. So as long as a 13A fuse will do that to the flex then the flex does not have to be rated at 13A.

In short a table lamp does not have to have a flex rated at 13A in order for there to be a 13A fuse in the plug.

As you said, the fuse is there to protect the cable.

Yes, against faults, not overloads.

 

[ban-all-sheds]

Now there's a thing...

This appears to be the current understanding:

1. There are no requirements in the Wiring Regulations for gas appliance supplies to be fused at 3A.
   
   
2. There are no requirements in any gas regulations for gas appliance supplies to be fused at 3A.
   
   
3. The Wiring Regulations do not make it mandatory to abide by any manufacturer instructions.
   
   
4. The Gas Safety (Installation and Use) Regulations (I'm assuming it is they) do make it mandatory to abide by any manufacturer instructions.
   
   
5. Some gas appliance manufacturers require 3A fusing in the UK.
   
   
6. Some gas appliance manufacturers do not require 3A fusing outside the UK.
   
   
7. Nobody knows why #5 is justified, particularly when #6 applies to the same appliance sold elsewhere.
   
   
8. Notwithstanding #7, #4 means that observing #5 is not optional.

They don't. I'm sure I was assured that they do, but I was too trusting - just looked, and they do not.

 

[D_Hailsham]

So as long as a 13A fuse will do that to the flex then the flex does not have to be rated at 13A.

If that is correct, why are table lamps now fitted with 3A fuses as standard?

 

[EFLImpudence]

Just seen this on P&CH.

If that is correct, why are table lamps now fitted with 3A fuses as standard?

That is the point of the discussion.

The answer is probably "because the fitters (of the fuse don't understand" or "because plugs from manufacturers have either 13A or 3A fuses".
You could ask "Why are they not fitted with a lower rated fuse?".

The same applies to boilers and CH systems.

 

[FiremanT]

...which over-ride gas regulations

No they don't.

Is a sub 3A (plug) fuse readily available?