Replacing a central heating clock

[Simon]

I will shortly be replacing a central heating controller clock. The entire house is covered by a single RCD at the consumer unit, so I guess good practice is to shut down the entire house rather than rely on the single pole MCB feeding the clock, and try not to touch neutral to anything earthed. What would a professional electrician do in this case?

I plan to do this job in daylight, as the lighting will either be off from the start (if I switch off the entire house) or may go off suddenly (if I only switch off the MCB and get careless with the wires). How would a proper electrician handle this situation? Do they carry powerful torches for use in the winter months? Is limiting myself to daylight hours the sign of a ill-prepared bogder?

Thanks

[Owain]

Either rely on the MCB, or turn off at the RCD, open the CU and disconnect the heating circuit L and N, then turn the power back on. If you do this (a) the main terminals of the RCD will be live even when the RCD is switched off, and (b) tape/secure the circuit wires safely out of the way so they can't unexpectedly touch anything and become live.

A torch is a useful accessory. It need not be particularly powerful; an LED head-torch would be sufficient.

A professional electrician would be more concerned about where the next cup of tea was coming from if the electric kettle wasn't working.

You may be lucky and find the controller can be replaced without changing the wiring. A lot of them slot onto a common backplate.

Do not worry about mimicing professional electricians. It's much more important to do the job properly.

Owain

[Rumble]

Well, the central heating should either have a 13A plug or be connected to a Fused Spur. If it's a plug, unplug it.

If the fused spur has got double pole switching, turn it off and tape it off with some insulating tape whilst you rewire the timer.

If the fused spur does not have double pole switching, change it for one that does, then turn it off and tape it off with some insulating tape whilst you rewire the timer.

A pro would turn the fused spur off and then grumble when the RCD tripped. He would then remove the neutral wire from the input side of the spur and tape it up whilst he rewired the timer.

[John Rumm]

Switched FCUs will usually have double pole switches since this is what is required for isolation.

[David Hansen]

On Thu, 08 Nov 2007 23:14:21 +0000 someone who may be Rumble wrote this:-

If it is fed from a power circuit.

Alternatively it may be fed by a dedicated circuit, though even then a local fuse may be necessary if some equipment is designed to be protected by a 3A fuse.

[Rumble]

I'm pretty sure that the switch in an FCU is for functional switching not isolation. See chapter 537 of the regs.

[Ed Sirett]

Switch the CH off at its (double pole) switch or remove its plug. After that the lights stay, the kettle is working and you only have the job to consider. Read the FAQs.

All time switches do the same job that is to provide connect the mains to certain wires at certain times of the day.

[Ed Sirett]

I have just looked up 537 and I don't see anything about the use of a FCU being unacceptable. The FCU has a double pole switch. In my book an unswitched socket or switched FCU is entirely acceptable as the maintenance (and emergency isolator) for a domestic CH system.

[Rumble]

I wasn't suggesting that FCU's were unacceptable. I was observing that it is inappropriate to refer to a switched FCU as an isolator.

Isolation has a specific meaning within the Regulations.

Double pole switching is *not* required (on a single phase supply) for isolation and FCUs do not comply with the majority of the other requirements of 537-02, viz:

FCU doesn't have minimum 3mm contact gap.

FCU doesn't have positive indication of contact position.

FCU doesn't have provision against unintentional reclosure.

FCU doesn't have a provision for locking off.

[John Rumm]

Does not have to have, but many do anyway (the DP ones at least).

They have a readily observable switch position, and possibly a neon.

That depends a bit, on many you can also open the fuse tray and put a padlock through it (by design), although obviously that does not guarantee someone will not close the switch and make the neutral connection.

See pdf page 7 for example (also note the comment on contact separation on page 2):