Electrical advice.

[Arthur]

This is from the Installation manual for WB 28i combi.

11.3 The following connection alternatives must be used: A 3 amp fused three-pin plug and unswitched shuttered socket outlet (both complying with the requirements of BS 1363)or a double pole isolator with a contact separation of 3mm in all poles and supplying the appliance and controls only.

Not sure what it means but will I get away with a 15amp fused switched spur? The combi comes with fuses (one is a spare) I think are 2 or 3 amp that fit inside the box.

Thanks.

Arthur

[Stefek Zaba]

It means a fused switched spur, or a 20A double-pole isolator switch ;-) Subject to me being wrong (hey, it happens a couple of times a decade, and I'm only running at 1 so far this millenium ;-) both those flavours of wiring accessory have the necessary 3mm contact separation for all UK-sold and 'therefore' BS-compliant instances.

Those fuses are to protect the combi. Any fuse you put in the FCU or plug protect the cable, not the appliance; and for the short length of flex typically used, even the 32A MCB back at t'consumer unit will provide short-circuit protection (if it's only control electronics, no possibility of an overload; if there's an integral pump, I s'pose it

*could* in theory produce an overload if it stalls, though I suspect the stalled motor coils would burn out long before the flex started to fizz!).

Stefek

[BigWallop]

Or, in other words, Arthur. No. A 15 Amps fuse is to heavy for the supply to the electrical control gear of the heating system, and the maximum recommended rating for the safety fuse in the isolator for the system should be just 3 Amps.

You can use a 2000 Amps rated Fused Connector Unit, just as long as you replace the fuse for one rated at 3 Amps, and not leave the 2000 Amps rated fuse in the connector unit. :-)

[Set Square]

It means that you must be able to totally isolate it - i.e. live *and* neutral - from the mains. With a 3-pin plug, you simply pull the plug - but make sure it only has a 3A fuse in it.

If you use a fused switched spur, it must be double pole - i.e. must switch both live and neutral (which not many do), the contact separation when switched off must be at least 3 mm (which not many are) and the fuse must be

3A *not* 15A.

Apart from that, what you are suggesting is fine!

[Christian McArdle]

Correction: I've never heard of a non-DP FCU. Indeed it is getting difficult to even find SP switched sockets, let alone FCUs.

Correction: I've never heard of a FCU without a 3mm contact gap.

Christian.

[Dave Plowman (News)]

I'd say this hasn't been updated. At one time pretty well all sockets were single pole switched. So if the socket was incorrectly wired (or the neutral was floating for some reason), switching it off didn't make things safe. But double pole switched sockets are common these days, if not the norm.

[John]

Accepted standards pedantically require the supply to a domestic CH circuit to be protected by a 3A fuse. Its not entirely technically justifiable but it keeps a semblance of safety in place for heating installers to operate to. I wish the pillock who I followed on one job had read the standard as I found a 30A junction box in the middle of a cooker cable which had a 0.5mm 3 core flex branching straight off the 6mm cooker cable and into the back of a time clock from where it served the rest of the heating system. "Isolation" for work required the cooker fuse to be withdrawn. :-( I have also come across an occasional switched spur unti which only switched the live conductor. I think most current ones are ok but its worth checking

[Ed Sirett]

Circulators have fairly pathetic induction motor coils I doubt that the stalled current is much greater than the running current. In fact it might well be the same with just the power factor varying as the impellor spins up.

If/When I get a chance and I have my tong-tester to hand I'll see what the difference is when I hold the pump stalled with big screwdriver.

The usual electrical failure mode of induction motors is earth faulting.

[Chip]

On Tue, 21 Jun 2005 18:23:50 +0100,it is alleged that Ed Sirett spake thusly in uk.d-i-y:

Keep us posted, I would like to discover what the stalled current is too, I had a neighbour's heating system that was repeatedly blowing 5 amp fuses in the FCU, and only when as the pump kicked in. [no electric valves, open flue conventional boiler with the boiler purely controlled via the HW side of the programmer and the cylinder and boiler stats, the type where the CH won't do anything if the HW is off].

They had recently moved a radiator, I postulated that it might be flux/solder residue/other nameless gunk infesting the pump. The fuse blowing was not a violent blow, just after a few seconds of humming, the power went off.

After attacking the pump gently with a mallet [yeah, I know ;-)], it held, and has been fine ever since [this was circa dec, so maybe 3 months of running the heating, now just running the DHW]