RCDs - sourcing of.

[Rob Hamadi]

I don't *think* that this is an entirely stupid question, so here goes. Background: running power out to shed, 50m of 4mm SWA, spurred off the ring main. House earth is TN-S, planning on using a 100mA time delay RCD FCU at the kitchen end and a two way CU with a 30mA RCD & 6A & 16A MCBs. So far, so good.

Problem: I've tried the usual suspects (Screwfix, TLC, Toolstation etc) and while the garage CU is widely available, I can't find a 100mA time delay RCD FCU. I could get an enclosure and a two module RCD (TLC do an MK one for some infeasibly large amount of money) but it's not the ideal form factor for the location in the kitchen - Ideally I just want to drop the RCD FCU into the box currently occupied by a fused switch.

Any ideas, or am I looking for a product that nobody makes?

TIA

-- Rob

[Lurch]

On Sun, 03 Jun 2007 21:05:32 -0000, Rob Hamadi mused:

Why?

[Owain]

I think you are looking for a product no-one makes, as 100mA t/d RCDs are really only used in split-load TT installations. Most applications for which an FCU could be used would probably need 30mA protection anyway, if they needed protection.

(a) Why do you need to RCD the cable?

(b) Why a 16A MCB in the shed, the spur should be limited to 13A by the fuse in the FCU anyway. Or were you going to use an unfused spur, in which case you can't use a 16A+6A MCBs anyway

(c) you could use a 30mA RCD FCU, no CU in the shed, a 3A FCU in the shed off the socket circuit for the lights, and if lights failing with power tools spinning down is the issue, a batter back up emergency light.

(d) better to run a new circuit from the CU if at all possible.

Owain

[John Rumm]

To answer your direct question:

TLC will supply if you ask them, although they don't have it in the online cat for some reason. RS carry the MK6400S as a stock item.

However to echo Lurch's question: Why do you want a RCD at the head end at all with a TN-S supply?

The usual reason for using a time delayed RCD on the head end is when you are unable to meet the earth fault disconnection time as a result of having a high fault loop impedance (typically TT installs). With a TN-S supply this is unlikely to be the case.

[Lurch]

On Mon, 04 Jun 2007 00:29:32 +0100, John Rumm mused:

Also I'm wondering if the ring main is RCD protected. If so, all this discrimination could be subject to random acts of something or other.

[Rob Hamadi]

I've got a very old CU with no RCD. Replacing it with a new split load CU is a job for some time in the future, at which time I'd plan on running the shed off the non-RCD side.

-- Rob

[Rob Hamadi]

Seemed like it couldn't hurt, belt and braces and all that.

'Cos 16A is what comes in the stantard Garage CUs. Now that you mention it, it does seem a bit redundant...

I planned on an RCD in the shed because I was a bit concerned about the extra impedance of the 50m run if the only RCD was in the house. If the impedance is not a worry then your solution is not only simpler, but much cheaper as well. I had planned on using a (no- maintained) emergency light as well.

Indeed, and I will, when I get around to fitting a decent CU. Thanks.

-- Rob

[Owain]

Ah, *ting-a-ling* 50m run, yes, impedance might be an issue. As may exporting the house earth.

Use FCU in house, RCD FCU (or an RCD protected socket) in the shed, and a 3A FCU in the shed for lights?

Owain

[Lurch]

On Mon, 04 Jun 2007 06:24:10 -0000, Rob Hamadi mused:

I think I'd drop the RCD from the house end for the shed. It's going to be redundant come the board change anyway. Also, as Owain has pointed out, the 16A MCB would be redundant but it's worth having for the future if it's going to be on its own circuit eventually.

[Rob Hamadi]

That's a plan then. Cheapo Volex garage CU in the shed, FCU in the house (at least until I swap the main CU).

Thanks chaps.

-- Rob