Hi,
Thanks for taking the time to post
BigWallop coughed up some electrons that declared:
"Tim S" wrote in message
...
Hi,
snip
What type of installation is currently at the supply head? Is there a
separate earth already there? What type of protection has the supplier
provided in the form of switching?
It's a single phase 100A supply with a BS1361 cutout, TN-S 16mm2 earth,
Ze=0.19 (by my measurement) and well PME'd in the network (TN-C-S is an
optional upgrade in my area). No user switching available before current
CU - isolation by removal of cutout fuse only. I'm going to slip a Wylex DP
switch (REC type) into the meter cabinet when I book the cutout fuse to be
pulled as EDF/SEEBOARD don't offer the option of an isolator (I've asked).
Hopefully the bloke won't mind if I ask nicely - that'll avoid future
nonsense.
snip
You are best to make the consumer unit your final circuits safety bonding
and keep it separate from the your new supply arrangement. That can look
after itself if done properly. Linking to the earth protection should be
enough.
Good - I'm glad you agree - I feel comfortable with that as it keeps the
equipotential zone inside the house good.
However, that leaves the reverse problem at the other end, eg big fault,
pipes go up to 100+V, exposed conductive parts like the switch-fuse case
don't. Ditto pipes outside and in shower see the 100+V potential
difference
with respect to the floor slab.
That's why I would say that the consumer unit should be the protection for
the final installation. A simple link to the new supply should take of
any fault between the CU and the supply head.
OK - thanks.
snip
You are thinking over kill on this, in my opinion. You new supply should
be
just that. All you are really doing is extending the supply to a new
position closer to where you want it.
I agree I'm worrying too much. But when you're out of standard territory,
you always worry that you're not doing enough.
From the supply head, install a 100amp RCD DP Switch after the meter.
Bring
Question - only an RCD? That will sort out earth-fault disconnection in a
timely manner (0.15s which is good), but what about a L-N short circuit?
Ok, EDF's cutout will ultimately rupture, but it's EDF themselves who say
that their cutout fuse is only for the protection of the meter tails up to
3m. Funny and rather annoying rule but I suppose they have their reasons.
I read that as "user must provide overload protection". I know that a BS88-2
100A fuse will have no practical discrimination against their 100A BS1361,
but at least I can prove it's protecting my cable.
I was also considering one of the Hager DP 100A MCBs instead of a BS88-2 -
this will just about discriminate against the cutout fuse and Hager state
selectivity with their own final circuit MCBs (RCBOs to be confirmed) upto
In=50A and 600A fault current, which is a fairly good bet most of the time
by my figures.
your SWA to the point where you want it. Terminate in an appropriate box
to
allow new tails to your consumer unit.
I've got that bit sorted - using a Hager JK series industrial metal board
with enough space to terminate the SWA into directly. Also takes RCBOs
without being cramped
The new supply is then protected
by
its own RCD back at the original supply head. If a fault occurs along it,
the RCD is there to break it.
I'd also go for a 2 core SWA and use the braid to supply the safety earth.
No need to extend the earth bonding back to the meter. If you have an
earthing point at the supply head, use it to bond with the SWA and maybe a
link from the new CU, but then a fault in your final circuits may also
cause the RCD at the supply to trip out.
I think a 100mA Type S RCD should handle selectivity OK (it's designed to)
against final circuit 30mA jobbies, so I'm happy with that.
The original reason I discounted 2-core was that the copper equivalent CSA
of the armour is only about 12mm2 (for this new fangled XLPE cable) (ie
less than 16mm2). Perhaps that's moot with RCD protection??
Your final installation will be bonded to create equalisation across the
full final circuits. Your protection is supplied by what you want to put
in
the consumer unit. With some current installations, that is the only
protection that is provided, so yours is really no different.
I think you are trying to over engineer it all. When all you really want
is
to extend the supply to your new CU position. Once you have provided
protection for the supply cable, the rest looks after itself at the new
consumer unit. Bonding them together may actually give you more problems.
Yep - I'd been round and round this in my head. One of the IET folk
suggested (tongue in cheek I think, to make a point) that I could put in a
125mm2 earthing conductor, therefore the touch voltage would always be less
than 50V so intrinsically safe.
But, when you step back, it is rather silly. I think I'm coming round to the
idea of one MET and one point of bonding, next to the CU at the load end of
the submain, and as you say, use the original MET just as a termination for
the earthing conductor.
You could also ask the supplier to reinforce your supply directly to the
new
position for you. Once you've made your installation complete, with just
a temporary supply to keep you working.
Interesting - I've not heard of that. Do you mean they bring their own
earthing conductor round to the new position and guarantee it?
Reason I was avoiding moving the supply head (which would actually be the
obvious and elegant thing to do) is that the thieving wotsits want upto
£1000 to do it, and I get to dig all the trenches and fit a new meter box
for them. The submain is about 200 quid in parts and a 10 times less work.
Good luck with it.
Again, massive thanks. I'll be taking pictures of this.
Cheers
Tim