Wiring Immersion Heater

[Kevin]

The plumber doing some work on the cylinder commented on wing the immersion heater through a double pole switch rather than the single pole spur switch. He mentioned the heating from the high load of the heater but I don't see why a double pole switch will handle the heat effect better. Doesn't a double pole just switch the neutral as well? The immersion heater circuit is a spur off of its own mcb on the consumer unit then via a single pole switch in the kitchen and a second single pole switch in the airing cupboard. The immersion heater is only ever used when the boiler is out of commission for any reason.

Kevin

[Tim Watts]

Normal switched fused spur is fine.

[spuorgelgoog]

Fused connection unit (spur switch) should also have a double pole switch anyway. However they're rated at 13 amp max and contain a cartridge fuse. The fuse contacts especially can be vulnerable to poor contact and heating from a sustained 3 kW load.

A 20 amp switch would be more robust.

Owain

[John Rumm]

Either are fine in most cases (yes the DP does just switch the N in addition).

There is a small advantage to DP switching if the immersion is on a circuit that shares a RCD with other circuits, since a fault heater could cause nuisance trips via N to E leakage, so being able to fully isolate it is handy.

[Brian Gaff]

Of course it is, for many y years the cheap timer on ours worked and it was just a single contact on the live side. I think the newer clock has a double pole relay in it though, for whatever reason. Brian

[DerbyBorn]

I have had to replace melted fused spur units.

I guess it is vital to go for best quality.

>

[Andrew Gabriel]

Absolutely, and good tight connections.

If it's on a circuit which is protected by 16A MCB, don't use a fused spur, just use a 20A switch. In this case, the 13A fuse is doing nothing except getting hot and possibly contributing to earlier failure...

[harryagain]

All electrical equipment must be capable of isolation and that must be adjacent to it for repairs and maintenance. Isolation means cutting off Live/phase(s) and Neutral So a plug and socket would be fine. Or a double pole switch. But not a single pole switch. So your electric cooker, immersion heater, instantaneous electric shower, all fixed equipment, has a double pole switch. (Or should) It has been so for fifty years to my knowledge.

In days of yore, single pole switches were normal on sockets and "spur switches" Now you can get ones with double pole switches

This is why some fans need a three pole switch. L+N+control wire.

[ARW]

I am not sure that it has ever been a requirement to switch the neutral on a TN system and that that "requirement" is a load of bollocks made up by people talking themselves into believing it after misreading the regs.

[The Natural Philosopher]

Its fairly common practice and in SOME of the regs that SOME kit must be totally isolatable.

That includes all bathroom kit and usually cookers and immersion heaters.

Not sure the latter is a regulation tho.

[ARW]

The manufacturers of the equipment often say you need "Omnipolar switching" - but you would not switch the earth would you?

The main switch of the CU is an isolator suitable for all isolation purposes. This is local isolation that the OP is talking about. If you are allowed to work on a circuit with a single pole MCB as the isolation why can you not work on an appliance that is locally isolated via a single pole device?

If a cooker switch need changing where is the isolation for the cooker switch? On a TN system you would use the MCB for the cooker circuit - and nothing else.

Anyway - I have proof that I am correct.

harry said "All electrical equipment must be capable of isolation and that must be adjacent to it for repairs and maintenance. Isolation means cutting off Live/phase(s) and Neutral So a plug and socket would be fine. Or a double pole switch. But not a single pole switch.".

I rest my case;-)

[Dave Liquorice]

+1 Hager are a decent make I believe. Didn't stop one of their switched fused connection units going into meltdown when feeding one of the storage heaters. All the storage heaters had SFCU's, note past tense they all have 20A DP switches now.

[John Rumm]

16th edition wording[1]:

"Except as required by Regulation 476-01-03, in TN-S or TN-C-S systems the neutral conductor need not be isolated or switched, where the neutral conductor can reliably be regarded as being at earth potential. For supplies which arc provided in accordance with the Electricity Safety, Quality and Continuity Regulations 2002, the supply neutral conductor (PEN or N) is considered to be connected with earth by a suitably low resistance."

There are additional requirements for isolation for items requiring mechanical maintenance.

[1] cos I have that to hand in a PDF and can't be arsed to get the book down from the shelf and scan it!

[The Natural Philosopher]

safer to have it left in place

At the CU you can GUARANTEE - because its attached to the bus - that its the live you are disconnecting..

Do you trust the sparky to have not swapped live and neutral down on a spur?

Never said Harry was right, just that you weren't 100% right either.

[harryagain]

The cooker switch next to the cooker is double pole. So the cooker can be tested and repaired.

The reason for isoation is simple. It is for repairs and maintenance. How can you perform an insulation test if the neutral is still connected (to earth)? Also in certain (unsual) fault cases the neutral might become live.

[John Rumm]

There are separate and specific requirements for cooker switches however that don't apply to immersion heaters.

As Adam highlighted, in a domestic environment you could turn off the main switch of the CU for the purpose of isolating an immersion if absolutely necessary.

You disconnect the cable at the connection point or in the CU, same as you would for any other circuit.

Neutral is always a live wire, not just under fault conditions.

[ARW]

Guarantee my arse. Here is one I took earllier.

eg

No, but you prove dead after switching off

My case is watertight and the photo is extra proof.

[ARW]

Disconnect the appliance neutral somewhere. Of course with lighting circuit there are not usually lots of double pole switches used.

[Tim Watts]

So not only is the 2nd RCD polarity reversed, it's being fed by the 1st (right) RCD?

[harryagain]

Wired up wrong (reverse polarity) by someone.