Isolating two electrical circuits to maintain one unit

[mike.peppert]

On the advice of this group I'm about to make some changes to my thermal store and boiler configuration, which will also need some changes to the electrical configuration, (see thread "Need help setting up thermal store" started last december for all the details); I'm after some advice as to how best to complete this.

The current electrical setup is this: A mains circuit feeds a switched FCU which then provides power towards the wiring centre. From the wiring centre the time switch, pump, boiler, cylinder stat, room stat and zone valves all hang off in a fairly standard fashion. i.e. This FCU, (lets call it "A"), isolates all of the items above.

There is a second switched FCU which provides power to my solar controller, (Resol BS), and then onto the solar pumping station. i.e. This FCU, ("B"), isolates power to the solar controller and the solar pumping station.

So far so good.

What I want to achieve is that a currently unused relay closure on the Resol BS will be used to fire the boiler, (which in turn will run the pump). The simplest way of achieving this would be to provide a live from the time clock / cylinder stat, (via the wiring centre), into the Resol BS and out again back to the wiring centre. The problem I see with this is that isolating FCU "B" would then potentially leave a live on one of the terminals within the Resol if it is opened up for maintenance. Is it sufficient to attach a label to the Resol to the effect that "This unit may need isolating in two places to render it safe for maintenance"?, [I'm sure I've seen such a notice elsewhere in my travels].

NB. The 'obvious' answer is to power everything from one FCU, but I don't favour this plan as I don't really want to solar controller depowered during boiler and heating maintenance.

Any ideas? Mike

[David Hansen]

On Sun, 15 Feb 2009 13:44:17 -0800 (PST) someone who may be [e-mail] wrote this:-

"may" is not good. As a minimum a notice should indicate precisely how to isolate it. "This equipment is fed from multiple supplies. Isolate at isolator A beside this unit and isolator B at the boiler before working on it." is the sort of thing.

TLC used to notices which with some extra notices would fit the bill, but they no longer do.

[cynic]

Is the resol arranged such that the "other live" parts can be enclosed in a separate section or effectively made inaccessible by the use of a fingerproof screen with appropriate warning labels. Many motor control centres and industrial panels employ this technique which allows maintenance on the major part of the panel without necessarily shutting down critical sections. Another method would be by the use of a separate means of isolating device such as a plug-in relay in another enclosure outside the resol unit but with its coil supply derived from the resol. The possibilities are endless! Its only the current blame culture and the jobsworths busily creating rules about problems which "might" cause a problem one day which leads us into the situation where we must try to prevent damage to idiots poking around in electrical stuff where they should not be. I am now retired but spent many years working on systems which contained live parts which were recognised as such and only touched using appropriate devices with care. Death or pain is natures way of improving the intelligence of the human race!

[David Hansen]

On Mon, 16 Feb 2009 03:58:18 -0800 (PST) someone who may be cynic wrote this:-

There is a reasonable and practical test in the Electricity at Work Regulations which allows one to judge cases on their merits. Signs simply alert skilled persons to the potential danger.

[Martin Bonner]

On Feb 16, 11:58=A0am, cynic wrote: [snip reasonable advice]

Yerbut - Presented with a neatly wired system, with a nice FCU feeding the Resol, I'd be a bit p**d off with the joker who wired it up if I found that switching off the FCU didn't leave the Resol safe.

I agree the OP should be able to do what he wants to do - but he does need to take /some/ action to at least indicate that the situation is unusual and that additional care is required.

[Dave Osborne]

Not sure I quite appreciate your setup, but the obvious solution is to derive the live for the unused relay closure on the Resol BS from "A" rather than "B". You might need an intermediate relay (which should be located adjacent to "B") to achieve this. Thus, "B" is only providing a volt-free contact closure, which is safe when "A" is isolated.