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

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

planning on using a 100mA time
delay RCD FCU at the kitchen end

Why?
--
Regards,
Stuart.

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.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

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.

On Jun 4, 1:03 am, Lurch wrote:
On Mon, 04 Jun 2007 00:29:32 +0100, John Rumm
mused:

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.

Thanks for the info.

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.

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

On Jun 3, 11:06 pm, Owain wrote:

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

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

(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

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

(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.

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.

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

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

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

On Jun 4, 1:03 am, Lurch wrote:
On Mon, 04 Jun 2007 00:29:32 +0100, John Rumm
mused:

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.

Thanks for the info.

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.

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.

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.
--
Regards,
Stuart.

On Jun 4, 10:34 am, Lurch wrote:

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.
--
Regards,
Stuart.

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