Fuse in a spur

[securespark]

This from another forum:

"Yes, there is. It's part of the gas regs. Gas boilers must be protected by a 3A fuse. Has been the case for many years.

Most oil boilers call for 5A - my understanding is because oil boilers also have an oil pump."

 

[bernardgreen]

Could it be that in any fault the maximum heat produced by a 3 amp supply would be about 1.5 kW ( allowing that the fuse will not blow immediately at 6 amps ) and this is considered ( by the writers of regulations ) to be insufficient heat to melt copper gas pipes.

 

[winston1]

So, gas boilers must be protected by a 3amp fuse. This must be why the manufacturers fit them inside the boiler casing. Could it be this internal fuse also feeds the programmer and controls connected to the boiler?
There is certainly no need for another external fuse apart from to protect the cable. Outside the UK these boilers are usually plugged into a Schuko socket which are protected by a 16 amp MCB.

 

[securespark]

Why does everyone keep banging on about boilers in other countries? We're talking about regulations in this country.

 

[Taylortwocities]

So, gas boilers must be protected by a 3amp fuse.

The WHOLE heating system must be protected by a 3amp fuse. this fuse is usually co-located with the SINGLE isolator for the WHOLE heating system.

If you care to look at a typical wiring diagram for such a system - lets say a honeywell S plan - you will spy a single 3A fused isolator. This is where the 230V (nominal) AC supply arrives.
This fused device feeds a lot of other things
Valves
pumps
programmer
thermostats
oh yes, and last but not least, it providdes power to the boiler.

Most of those devices do NOT have internal fuses. I have replaced several programmers when the installer has left a 13amp fuse in the FCU.
An overload fries the programmer, it doesn't blow an internal fuse.

 

[winston1]

There is a fuse inside the boiler casing. Programmers, pumps, etc are connected to the boiler. Does this internal fuse not protect those?

In other EU countries the very same boilers are connected to a circuit protected at 16 amps. We are part of the EU and regulations are supposed to be harmonised.

The external fuse is only to protect the cable. The cable is rated at more than 3 amps.

 

[Taylortwocities]

Programmers, pumps, etc are connected to the boiler.

Not as you suggest.
Programmers, valves etc control the boiler. So their control contacts turn the boiler on and off when heat is required.
Do look at the link I provided. Maybe you'll actually learn something.

 

[securespark]

Why do you keep banging on about boilers in other countries? We're talking about regulations in this country.

 

[EFLImpudence]

It is not the EU which is being stupid in this case, is it?

Put it another way (rather than how do you fit a 3A fuse in Europe); what is different about the supply to an identical boiler that Britain (apparently) demands that a 3A fuse must be fitted?

 

[eveares]

Thats the wrong fuse, you need a BS646 fuse.

Fun and games aside, lets not confuse and mislead the OP. He needs a 3A BS1362 fuse of course.

 

[stillp]

We are part of the EU and regulations are supposed to be harmonised.

National wiring codes and installation regulations are not harmonised.

 

[stillp]

should know what an external fuse is for and it is NOT to protect the boiler or its ancillary components. It is to protect the cable.

That's often said, by various people, but can you (or anyone) back it up with a regulation or other authoritative statement?

 

[EFLImpudence]

Isn't it apparent as Britain is the only place that uses them and "Ib ≤ In ≤ Iz" and 433.

Conversely, does any regulation state that an appliance shall be protected by the fixed wiring?

 

[stillp]

I'm (almost) sure I've seen FCUs (not BS 1363 types) used in other European countries. 433 etc is not really appropriate, as it is about protecting circuits of the installation, rather than protecting appliances that might be connected to that installation.
Some of the small products I was involved with simply didn't have the space inside for a fuse that could break the prospective fault current, so it seems quite reasonable that the manufacturers should limit that prospective fault current by specifying a more robust protective device be fitted in the circuit?

 

[DaveHerns]

Why would you want to use a 13a fuse if a 3a is sufficient?