Various electrial questions

I'm contemplating a serious rewire/CU replacement and had a few questions:

- I'll be using SWA to feed the CU some distance from the incoming supply. I've read the excellent terminating SWA article on the wiki, but it is unclear about exactly what size SWA can be cut using bolt croppers. I'll probably be using 10mm2 or 16mm2 SWA - is it reasonable to expect bolt croppers to be fine for cutting these sizes?

- Is my supply TN-S or TN-C-S?

While I have seperate PE and N entering my building, the PE and N are connect together on my property. I have two wires (11kV?) coming onto my property, which terminate on a larger transformer on a telegraph pole. I can see the PE is provided from underground at the pole, and is connected to the N supply from the transformer. So my thinking is that I'm TN-S if the transformer is considered the power source, but TN-C-S otherwise. Thoughts?

(Yes, I have asked my electricity supplier, but they have suggested that I contact the DNO. I haven't yet summoned up the energy to do this.)

- I'm considering using a metal CU. From doing a bit of research online it may be a requirement that the supply to this CU is protected by a 100mA time delayed RCD. However, I'm unclear as to whether this is only a requirement for TT systems or whether it's applicable to my scenario. I'll be digging out my 17th ed regs later, but can anyone let me know whether the RCD is a requirement here?

- I'm considering a three phase CU, and using this as a single phase CU. I'm unfamiliar with the internal construction of these CUs - is there generally any problems with using a TP CU as a single phase one? (The main obvious issue I've seen is that for the model I'm looking at a DP incoming switch is not available - so I'd have to use a TP one and leave one pole unused.

- Finally, any recommendations or brands to steer clear of? I've been looking at the Hager Invicta 3.

Thanks, Piers

Reply to
Piers Finlayson
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Yup they should hack it - you may need to "nibble" at it and take a couple of bites. If not then a hacksaw is your friend (at least to get you started).

Its not uncommon for TN-C-S to be provided on overhead supplies these days. The fact that there is a join inside your property would suggest TN-C-S. Are there no PME stickers on your head end gear?

The RCD protection of all circuits is nominally only for TT type systems (although the 17th edition nudges you closer to that anyway for all systems)

The fundamental question to answer is your earth loop impedance at the supply adequate to clear a line to earth fault in the CU. If it is then a metal CU is fine, and TT style protection is not required.

They tend to be of higher quality construction than the domestic ones. The main objections to using one would be cost and size. Do you anticipate having three phase supplied to it in the future then?

I would expect the incomer to have 4 poles - one for each phase and neutral. The CUs would normally supply a good number of single phase circuits.

In my limited experience of the three phase kit, the Memshield II stuff was nice with a very good range of MCBs (including some high breaking capacity ones as well (10kA)).

Reply to
John Rumm

Nope.

OK, thanks. I'll be asking the DNO for max Ze as well as supply type, but I'm guessing it's worth me measuring myself as well?

I see a number of benefits:

- Better quality construction.

- Better looks - more industrial and less cheap white plastic (the new CU will be fairly exposed, both due to the convenient location and also to ensure accessibility).

- More space inside for wiring (I hate fiddling around inside normal CUs).

- Many way - I've identified the need for at least 18 circuits and want to leave room for expansion. I've got my eyes on either an 8 way or 12 way TP CU.

You're right - most of them are 4 pole (but there are a few 3 pole ones).

Thanks - I'll take a look at these.

Reply to
Piers Finlayson

Assuming you are planning to use a type B distribution board (with two columns of MCBs mounted vertically) then you will need a piece of MCB busbar to short out the three phases on the output side of the (4-pole) incomer. You may be able to buy this as a single pole conversion kit.

Hager single phasing kit JK125BSP on page 12 here:

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there is a limit on the length of meter tails of 2m and the DNO will probably insist on 25mm2 meter tails.

If there isn't already one, you will undoubtedly need to install a a consumer unit within 2m tails-length of the meter and make your SWA a sub-main. You will probably need to make this sub-main 25mm2 as well to make the DNO happy (whether you can prove you need it to be 25mm2 or not).

Reply to
Dave Osborne

Actually, one other complexity here is that I'll be installing a incoming feed from my generator behind a break before make switch, before the SWA to the CU. In the event of a power cut I can't rely on the earth from the supply being present, so I will be providing my own via earth spike. In this case I'll be TT and Ze will be much larger, so even if I don't need to protect the mains supply to the metal CU via an RCD I will need to protect the feed from the genny supply. But then, I may also need to protect the feed from the genny to the house anyway (I haven't figured out how I'm going to run this yet).

Reply to
Piers Finlayson

not uncommon; unless its be upgraded in-situ relatively recently.

Yup. If you do have PME, then it will in effect be the same as your supply impedance.

Yup, you will certainly get those...

Also probably true - although there are some domestic ones that offer reasonable space. Many of the more recent ranges have been made a bit bigger to allow easy use of single module RCBOs.

Nothing stopping you having more than one CU either if it makes it easier neater etc.

Being TT here, I have one small split load CU for all exterior / outbuilding circuits, and another larger one for stuff in the house. Time delay RCDs on the main incomers of both.

Indeed - but only really suitable for boards feeding proper three phase loads only I would have though?

Reply to
John Rumm

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thanks - I hadn't spotted this kit.

Yes, I was aware of this.

Hmm, I wasn't aware of this :-(.

Reply to
Piers Finlayson

Once the DNO gives you the supply type then the max Ze is defined by type of supply.

OKAY. That is most unusual but there is nothing wrong with it if you use a conversion kit. How many circuits do you want or need?

You can buy very good single phase CUs.

Adam

Reply to
ARWadsworth

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Hi, Merlin Gerlin do some big 23 way industrial single phase units

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Reply to
James Salisbury

Well, I'm being pessimistic. It's not something I've personally experienced, but it's come up before on this n.g.

I guess It largely depends on whether you are going to get the DNO in to disconnect/reconnect your tails. If you are, then they can refuse to re-connect if they don't like the look of your arrangement. If you aren't, then they'll in all likelihood never know.

I'm pretty sure they won't like it if you don't have a CU of some sort at the point of supply. As a minimum, you should have a 1-way metalclad CU like this:

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Page 1.23 (page 29 in pdf reader)

IU4-16 or IU44-11 or IU44-18 as appropriate.

As to whether you need 25mm2 SWA for your sub-main, I couldn't say, but be aware that it *might* be a problem.

Reply to
Dave Osborne

There is one gotcha which catches out electricians with this... If it's rated at, say, 3 x 100A, then as a single phase CU, it's going to be 1 x 100A, and not 1 x 300A. This is because the neutral bus bar will only be rated for 100A.

Reply to
Andrew Gabriel

Ah, that changes things somewhat! If you do have PME, then you can add your own local earth provision to it anyway (yours just becomes one of the "multiple"). Your genny feed would need to include suitable RCD protection to cope with the local Ze ignoring whatever you are getting via the supply (which as you say, can't be relied on in the event of an interruption).

Alternatively you could make the the whole install TT...

Reply to
John Rumm

A switchfuse would be ok rather than a CU as such:

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't say I like the look of the price of the mem ones though! ;-)

Reply to
John Rumm

For single-phase the Wylex 110M (or 108 / 108M if 60 A is enough) is OK and won't break the bank:

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Reply to
Andy Wade

I've identified the need for 18 circuits, but want room for expansion - so at least an additional 5 circuits.

Reply to
Piers Finlayson

Thanks for pointing that out - but that's fine because I won't be needing more than 100A.

Reply to
Piers Finlayson

Why metal? (I don't have any requirement for it to look good in this location, and this may require an RCD depending on my earth type/Ze.)

I actually already have a Proteus (nasty stuff) 1-way CU with a 63A fuse which will be surplus to requirements and I can reuse.

Reply to
Piers Finlayson

Remember that your generator switch has to have a suitable load-make rating unless there is some other method of protecting the operator (like a suitable contactor circuit) if it is closed onto full load. It will also have to switch Neutral so that you can work on the genny with the mains live (unless you have a separate means of isolation). Ideally (and

*always* on 3-phase systems) the Neutral pole should make first, break last.
Reply to
mick

There is a 28 way A board on the Hager webside.

Adam

Reply to
ARWadsworth

Only because you can make off the swa directly to it. Saves time, which may not be an issue for you ;-)

Reply to
Dave Osborne

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