Snags with submain, extending meter tails

Hi,
I've been discussing this on the IET forum and offline too. This is complicated - if anyone has a thought but doesn't want to commit to it in public, feel free to email me (address is valid).
I'm partly posting this here as the engineers over at the IET have been very helpful, but are good at pointing out all sorts of interesting snags. They did produce some interesting guidance but not really a definative answer.
So I'm here for a fresh dose of common sense and perhaps someone's done this before...
As previously mentioned, I need my consumer unit 14m away from the supply+meter. Moving the meter is out of the question.
------ So, my proposal has always been, which seems pretty standard is to:
Run 25mm2 3 core armoured cable from near the meter to new CU. The third core will be parallelled with the armour to function as the main earth (circuit CPC *and* earthing conductor). My calculations for disconnection times, R1+R2 and volt drops look perfectly OK, relative to the worst case final circuits.
In line with EDFs rules, I must provide overcurrent protection and means of isolation with 3m of the meter, so I was going to use a 100A DP switch fuse. Seems normal so far.
-----
However, some of the folk at the IET pointed out some theoretical snags, including:
Where does the MET (main earth terminal) and main bonding go?: next to the meter or next to the CU at the other end. Conclusion was the MET is better next to the CU, as during a fault on the distribution circuit, voltages on the "earth" conductors all round the house may easily hit 100+V for upto 5 seconds, so it's better the water pipes etc come up with them in the spirit of the equipotential zone that the house is supposed to be.
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.
Having read the whole of chapter 54 in the IEE Wiring Regs (17th) (twice!) and 2 guide books (ECA guide and NICEIC Earthing Snags) and been all over google, it appears that there's nothing wrong with my cable - it's CPC oversized if anything.
But there's nothing prescriptive about multiple main bonding.
*** So should I bond gas and water to the means of earth back at the meter as well, directly to EDF's earth terminal? BTW - this is a TN-S setup, Ze of 0.19 Ohms
I came up with another way round: put a Type S 100mA DP RCCD in place of the main switch in the switch fuse enclosure, and thus reduce the disconnection time of the distribution circuit to 0.15s which avoids all these issues. Unconventional with a TN-S system, but harmless at worst...
*** Overkill? Or not?
Random thoughts gratefully received...
Cheers
Tim
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very
this
of
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?

spirit
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.

difference
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.

the
disconnection
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.
From the supply head, install a 100amp RCD DP Switch after the meter. Bring your SWA to the point where you want it. Terminate in an appropriate box to allow new tails to your consumer unit. 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.
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.
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.
Good luck with it.
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Hi,
Thanks for taking the time to post :)
BigWallop coughed up some electrons that declared:

<snip>
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>

Good - I'm glad you agree - I feel comfortable with that as it keeps the equipotential zone inside the house good.

OK - thanks. <snip>

I agree I'm worrying too much. But when you're out of standard territory, you always worry that you're not doing enough.

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 InPA and 600A fault current, which is a fairly good bet most of the time by my figures.

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 :)

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??

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.

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.

Again, massive thanks. I'll be taking pictures of this.
Cheers
Tim
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<<<snipped>>>
Putting anything else in line will cause problems. The head fuse will take care of any phase short. When you consider that a short on the SWA will not have adverse effect on anything passed the new CU, then it's fine the way it is. The RCD will protect the rest, if anything does happen between your consumer unit and the supply. If anything else were needed on that end, would it not be there already? If you are so worried, then install another cutout fuse after the meter and before the RCD. Now it's getting into silly zone again. :-)

BS88-2
All you have to provide after the supply head is a double pole switch to isolate your new SWA, at the correct rating for the supply of course. The cutout fuse on the head is enough protection from fault or they would have to install further protection to the supply head already. Nothing else is needed. All you are doing is extending the supply cable. Nothing else.

Why? The supply is already protected by the RCD DP Switch and the cutout fuse at the head. The supply is taken care of. DONE!!! Finished !!!!

box
This is the bit. I would advise you to make provision for a separate box to terminate the SWA and allow connection of tails to the CU from there. An ABS box with three CEFCO terminal blocks inside, allows you to extend to another set of tails in future. That way you have flexibility to extend the final circuit to other installations, without touching the consumer at all.
Example: You want to install a separate CU for external lighting at a later date. A box will allow you to take other tails off without disrupting everything. But that's my opinion.

it,
earth.
a
Yep. Just protect the new supply cable, not the whole lot from there. The rest is going to have its own safety and protection built into it.

put
want
problems.
less
the
of
for
Yep. Again. Just protect the new supply and leave the rest to look after itself.

just
They have to. It has to be the same as the existing head. It can't be done any differently. And it has to be as close to the final circuits as possible for the sake of protection Etc. Just like it is now.

But they have to reinforce the supply to the new position for you. Free of charge. Your new installation is not fully up to standard with it being a sub-main supply. Them installing the supply to the new position is better for them in long run, as they know they have no comeback to worry about in the future.
You can still do all the digging when you install your temporary supply, so they come along and lift your stuff out and put their new stuff in. But it's a whole new supply head at the new position for you.
You do have to have your full installation complete so they can test and connect. Any failure on your side of the new consumer unit will mean you pay for their return visit. That's why you have to have everything complete and tested from your temporary supply, then call them in to reinforce it all for you.
A sub-main supply is more hassle to the supplier than it's worth, that's why they will gladly change it for you. I know, they do it for us all the time. :-) As long as we know that our side is on and working correctly, and that we've prepared everything for the guys to come in and install. Then it's straight forward for everyone.

Yes please. :-)
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Hi,
Thanks for your reply.
Which DNO do you do your work with? Wonder if their requirements differ from mine?
BigWallop coughed up some electrons that declared:
<snip>

That would be the common sense and technically sound theory.
It's when EDF/SEEBOARD tell you that they want something withing 3m of the origin, but fail to clarify what "somethign" is... I've heard of people who fit DP isolators and people who fit switch-fuse units.
As there is a disparity of opinion on this specific point, I've just emailed EDF Technical requesting their specific requirements (if any). They usually reply within a day. No disrespect intended ;-> But there's been a lot of different opinion on this specific point.

One thing I'm lost on, is why have an RCD (RCCB?) if we are satisfied that the circuit will disconnect in less than 5 seconds (by my calculations, it will disconnect in 3 seconds at worst) and if we are no longer concerned about touch voltages.

It's a good idea, though I'm personally not keen on multiple adjacent CUs. My leaning is to leave enough spare ways that it will never be aproblem :) Cheers
Tim
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from
Take your pick http://www.nationalgrid.com/uk/Electricity/AboutElectricity/DistributionCompanies / :-)
They all want to help out. Set up a temporary supply so you can work on the new build. Then get the completed work tested and re-connected with a new supply. The guys will be in and out in no time, if everything comes up to scratch on their first visit. If you've done the grunt work ahead of them, and chosen a route to the new position, then it's a breeze for them. That's what they live for. :-)

who
emailed
usually
Yeeeessss...... That "something" does need clarified, doesn't it? And within 3 mtrs of? Is that 3 mtrs of the head or the meter, or the new position of the CU? Are you taking the meter to the new position as well?

the
An RCD will break supply if anything does happen on the skin of the SWA. If you are making a bond across from the CU to the SWA, any fault will break supply back at the head and keep the new install a little safer. But you are correct, the circuit should disconnect quick enough with just the protection provided as is. I'm only thinking of the route the SWA is taking, and what might happen if the skin is damaged Etc. But no more than residual protection should be necessary.

board
box
An
extend
at
That's my old ways, and they're set like that. :-) Always allow for expansion. lol
But, on this type of installation, don't go over the top with it. Protect the sub-mains supply as best you can, with as little equipment as possible. There is always something that makes this possible, and it's always in a smaller box.
Head..........Meter...........Tails...........Protection.............SWA.... ..............JOBDONE.
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BigWallop coughed up some electrons that declared:

I almost get the feeling that I'm the first person to bother asking in any detail(!).
Actually, the previous reply from them said, and I quote:
"EDF policy = 3 Metres Max length between meter and consumer unit Double insulated 25mm copper tails 16mm earth wire"
I've asked them to specifically clarify that again for a submain.
In the meantime, I rang Hager regarding their 100A switch fuse (IU44-11), which is a BS1361 type. They confirmed that the 100A DP isolator may be swapped for one of their Type S DP RCDs, in other words I might order one of them up and adapt it depending on EDF's reply. Equally well, the fuse component could be removed if necessary. I need something to terminate my SWA into anyway and something to join it to the tails to the meter.
I've got enough information to proceed, knowing it can be tweaked if someone gets arsey.

I'm like that too - I'm using a 20way board which will have at least 5 ways spare (some modules are double width, I don't have 15 circuits)

Wise words... That's why I'm getting all the information up front. Nothing worse than finding out the SWA isn't up to spec or what it's terminated into isn;t either.

Head..........Meter...........Tails...........Protection.............SWA....
Yep - I think we're singing to the same sheet. It's only the "protection" that's attracting differing views.
BTW, meter stays where it is. As I mentioned, EDF charge a fortune to move their gear. 35 quid for a fuse pull is bad enough!
Cheers
Tim
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<<<snipped>>>
Head..........Meter...........Tails...........Protection.............SWA....
WHAT???????? They want paid to remove the fuse? Bugger off!!!!!! Pull it yourself and let them seal it when your finished with it. If they get stroppey. Tell them you had to pull it for emergency reasons.
God Almighty!!!! What are we paying our bills for? You do know that a KiloWattHour of supplied power is costed at 3 pence to the suppliers, and that the rest of the charge on your bill is for maintenance and administration. I'm sure you've paid enough already, for them to give you a new supply to the new CU position.
Honestly, man, you're being to soft with them. Tell them you are installing a temporary sub-main, and that you'd like the new final circuits tested and the supply re-enforced to the new position, please. They'll tell you what they need from you, and you tell them when you're ready for them to do the work.
As I have said before, though. Make sure your new install is up to scratch before calling them in to change it over. You will be charged for a failure and re-visit to the site. But with the way you are going about designing the sub-mains supply, I don't think you're going to make mistakes with your new install to the final circuits. The length you're talking about is a breeze for them. They should be in and out before the kettle boils for the tea and biscuits you offered them when they arrived. :-)
It's worth a phone call to ask their procedures on such matters.
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<snip>

Worse they may disconnect you and leave you disconnected (extreme cases admittedly, but it does happen)

Always a good idea

Ditto
--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)>
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BigWallop coughed up some electrons that declared:
<snip>

Did I mention the 1000 quid to move the meter...
Which is why it's not getting moved.

Not dealt with EDF before, have you?... ;->
Takes a week of bouncing around call centres who feign ignorance and pass you somewhere else. After that week, you find random internal numbers on the IET web forum. You call these until you get lucky and hit the technical dept.
Then you communicate with them via the technical department's email, that usually turns around in a day...
The fuse pull procedure is: Phone up, book pull, fax in your EIC (doubt they even read it). Bloke turns up AM and pulls fuse. Comes back PM to replace. If I have the isolator fitted in their box, he'll probably hang around for 10 minutes while I move the tails over. Nice thing with the Wylex REC2S isolator is that there's a separate cover for the load terminals so it's very safe to work on.
But in general nothing's easy with EDF.
Cheers
Tim
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Pull
and
you
what
the
designing
technical
they
That's where your bill payment goes. It goes to pay for a the call center in Istanbul. Cheaper to get the plane over there and book it directly, then. :-)
Who do you pay for the electricity supply? Go through them. Tell them you are moving everything to the new position, and let them deal with the new supply. I'm sure they know a bloke down't pub that'll do it for a tenner. As for the fuse. Do it yourself. They seal it once the new supply is complete and tested. Save yourself a few pennies on that, at least.
I don't believe the faffing about they make everyone go through now. I think I'll start up as an electricity supplier and steal all their customers from them. No joke.
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Tim S wrote:

I can't see the need for a RCD at the switchfuse position.

Can be handy if you decide you want a totally separate one for outside/garden circuits for example.
--
Cheers,

John.

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John Rumm coughed up some electrons that declared:

Me neither, but BigWallop's in that field so I won't discount anything just yet, with no disrespect to anyone else.
To be fair, this comes up on the IET forum from time to time and the answers are different every time, with the only common factor being the "3m rule".
When (if) I get a reply from EDF Technical, I'll post a summary here.
Ignoring them for the minute, I can see no need for anything beyond a DP isolator.
Their 100A BS1361 fuse is quite capable of disconnecting my submain in 5s - though it would not if the max permitted Ze of 0.8 Ohms was apparant (need 630A to meet 5s).
I used a science principle in my calcs - my measured Ze was 0.19 +/-5% +/-0.07 Ohm
That's the uncertainty in a Megger 1552 plus not knowing which pair of leads from 2 sets the LI test was calibrated with (0.04 Ohms between them).
So I'm working to the max Ze with the worst case uncertainty applied, which is 0.34. Even with this figure the disconnect times are fine. That gives a bit of latitude for their supply varying a bit over time.
So I don't know why the DNOs have this rule.
Unless they merely want an isolator at that position... Or the right to prat around with their cutout fuse whever the mood takes them, safe in the knowledge it won't make the installation unsafe...
When you get articles written by the ECA saying "stick an MCB in" you tend to assume there's a reason. Even the NICEIC Snags and Solutions books don't cover a simple supply extension.
What I suspect happens in reality is noone gives a hoot and just does something reasonable. Me, being me, wants to be able to justify stuff with confidence. Silly rabbit, me...

Just for context, here's my final circuit list so far (I'm pretty happy with this):
2 x 32A ring circuits covering kitchen and utility area 1 x 32A ring for rest of ground floor 1 x 32A ring for first floor 1 x 10A lighting for left side of ground floor (inc kitchen/dining room) 1 x 10A lighting for right side plus upstairs[1] 1 x 40A cooker circuit (we use gas, but best have a circuit in place) 1 x 45A distribution circuit for boiler + backup thermal store heating (3 x 3kW)[2] 1 x 16A radial feeding two external 16A weatherproof commando sockets[3] 1 x 10A external lighting, including direct feed to shed lights[4] 1 x 32A (or 25A TBD) radial to shed sockets.
[1] This arrangement gives a good split of load.
[2] It's more practical to feed one big cable round to the heating cupboard and split with MCBs (as part of the control panel maybe) there. Only need one RCBO at the head end, less ways in the main CU too.
[3] I like commando sockets for external use. I'll make up a couple of long 16A - 13A socket trailing leads for those long summers in the garden with a laptop :) Naturally we'll have a convenient inside isolator.
[4] Why not submain to the shed? Well, my calculations show it's impossible to meet volt drop requirements on the lighting component without using excessively oversized cable. I have a need for a few outside lights so it seemed sensible to maintain separate circuits back the the main CU and lump all the lights together. Shed will have a couple of local DP isolators for safety.
I might use DP RCBOs for the outside circuits, or at least the sockets, which are 2 module wide, so at the extreme the above occupies 14 ways from a 20 way board.
So you see, I'm not too worried about expansion :)
Cheers
Tim
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Tim S wrote:

2m and 3m come up quite often as the limit before a switch fuse is required IME.

Ta
I can see the need for a fuse to offer fault protection, since a fault on the submain may otherwise not be cleared in time, and would result in taking out a fuse on the distributors side of the network.
The need for a RCD could only be driven by two possibilities, one being risk of direct contact, and the second being a loop impedance that is too high to reliably clear a fault. The chances of the former on a SWA submain are to all intents and purposes nil, and the later we know not to be true in this case.

(Don't think I have actually met a TN-S system yet that was as bad as the "notional" value).
A fuse is also probably better here in that it will discriminate with just about any MCB you care to use.

That is probably quite likely. They won't always be able to offer TN-S with a very low impedance.

Quick bung from one of the MCB manufacturers perhaps ;-)

Not really - I like to have all potential questions answered as well ;-)

<snip comments that seem fair enough>

One presumes the distance to the shed lights remains the same either way, so house circuit fed shed lighting is unlikely to dim less than a submain based installation (more in all probability!). This sounds like an example of complying with the letter rather than the spirit of the rule ;-)

I usually have a policy of making sure that anything outside does not share a RCD with something inside. Since the outside circuits are a more likely source of nuisance trips etc.
Here I have installed two split load CUs in the house[1] (preceded by one main switch in a separate two module enclosure)). Both TT, the second smaller CU is reserved for outside stuff. The 30mA RCD carries outside sockets and lights, the 100mA type S RCD directly feeding things like the garage and my workshop (via BS 88 cartridge fuse carries). The garage/workshop has another split load CU with inside lights on the main switch part, and sockets and outside lights on the 30mA RCD section.
[1] When we got here there was one CU with everything sharing a single 30mA trip RCD. Needless to say a horse farting would plunge the place into darkness.

That's ok then!
--
Cheers,

John.

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John Rumm coughed up some electrons that declared:

<snip>
Agreed, they'll generally discriminate for faults upto around 2-3kA + even for the biggest final circuit MCB.
Hager do an interesting range of MCBs, the HMF series, type C in double pole upto 100A which will discriminate with most of the standard Hager CU MCBs (and presumably RCBOs, need to check, not in the data sheet).
Discrimination is close though, with the max fault current limited to 600A in one common case. That's quite close. Chances are that a fault any distance down a final circuit would be OK, but close to the CU might be borderline. OTOH, despite Hager failing to quote a let-though energy figure for the 100A device, I suspect it would, looking at data for the lower current devices in the range.
One of those design choices that's not obvious, but a balance of compromises...

I should perhaps clarify that the shed submain would originate at the CU too, so distribution characterstics upto the CU are common to both solutions.
What I found was that using separate circuits, one can take advantage of the permissible volt drop being 5% of nominal supply voltage for non lighting circuits (lighting being 3% of course). With lighting rolled in on a 38A combined circuit, it meant a jump to the next size of cable (10mm2 in this case, rather than 6mm2 which would be fine for a 32A socket circuit). I have chosen 5A to be the design load on the leg of the lighting circuit going to the shed - should be suitably overstated.
The "wastage" of running a second bit of 1.5mm2 is mitigated by the the fact that there'll be a bit of 1.5mm2 running in that general direction from the house for lighting on another tool shed and any other lighting requirments in the garden. It's a very specific set of conditions I have, but teh twin circuit solution does buy guaranteed discrimination of shed lights vs shed angle grinder on shed socket, whereas that's a very close call for a submain.
It's not the classic solution, but I don't feel it's weird beyond it's merits.

I should have said - it's an all RCBO solution. That's one of the reasons I settled on Hager - generic single pole RCBOs are around 20-something + VAT, 30-something for type C which isn't bad for a quality brand.

Nice :)

Ow!
Cheers
Tim
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Tim S wrote:

Ah, sorry had missed that bit about RCBOs. Yes in which case separating outside circuits is less of an issue anyway. (about the only capability you lose is the ability to shut off all outside power with the flick of one switch)
--
Cheers,

John.

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Tim S wrote:

FWIW, that's the setup I have, and I can only remember one trip in the last year (which may have been associated with a blowing bulb). Of course, I've probably jinxed it now :-)
On the excellent advice from this group, though, I did add my shed supply as its own submain rather than tempt fate with the single spare CU way.
Pete
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Tim S wrote:

It would of course be essential if the earthing were TT. It may be required to get <5 s disconnection time with Ze ~ 0.8 ohm, which doesn't appear to apply in your case (and hopefully DNO's network alterations would only reduce Ze, not increase it).

You can be 100% certain, IMHO, that they will require a fuse (or cct breaker) and not just a switch-disconnector, regardless of discrimination (lack of) and however nonsensical it may appear from a purely technical point of view. Demarcation of design responsibility is the issue, see my previous comments here: http://groups.google.co.uk/group/uk.d-i-y/msg/113718da7daeb8e7?hl=en&dmode=source and http://groups.google.co.uk/group/uk.d-i-y/msg/bd1831bac67e6889?hl=en&dmode=source
BTW are you sure the main fuse is 100 A? TN-S suggests an older supply, likely to have a 60 or 80 A fuse fitted, perhaps.
--
Andy

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Hi Andy,
Andy Wade coughed up some electrons that declared:

Demarcation - yes that seems a good point.
http://groups.google.co.uk/group/uk.d-i-y/msg/113718da7daeb8e7?hl=en&dmode=source

http://groups.google.co.uk/group/uk.d-i-y/msg/bd1831bac67e6889?hl=en&dmode=source
Just read them - most informative, thank you.

No, it's really 100A by all indications. There's a sticker on the cutout that says BS1361 100A. Haven't looked at the fuse, obviously... An early conversation with EDF on the phone, they managed to dig up the records. The supply connection was replaced and a new head and meter fitted within the last 10 years and they confirmed the supply was good for 100A max.
They also told me the local network was suitable for TN-S and TN-C-S possibly explains the excellent Ze readings I'm getting?
Cheers
Tim
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Tim S wrote:

Fairynuff.
Your reading - around the 0.2 ohm mark - is pretty typical, IME, for urban domestic supplies.
--
Andy

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