Snags with submain, extending meter tails

[John Rumm]

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

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 ;-)

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!

[Tim S]

John Rumm coughed up some electrons that declared:

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

[John Rumm]

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)

[BigWallop]

Hi

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[Andy Wade]

It would of course be essential if the earthing were TT. It may be required to get [...]

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:

?hl=en&dmode=sourceBTW 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.

[Tim S]

Hi Andy,

Andy Wade coughed up some electrons that declared:

Demarcation - yes that seems a good point.

and

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

[Pete Verdon]

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

[Andy Wade]

Fairynuff.

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