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

Rob Hamadi wrote:

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?

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.

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.
--
Regards,
Stuart.