Electrical problem revisited

A couple of weeks ago I asked for advice about the wiring in my daughter's new flat under the thread "Improving an old electrical installation". There was some helpful advice, but it all got rather buried in a discussion about the relative merits of voltage-operated trips and RCDs.

Thus when I followed up with more information and questions on 23 June, I guess some of those who could have offered advice were no longer following the thread. Would anyone care to revisit it, please, or would it be better netiquette to repeat my last questions under this new thread?

Reply to
Autolycus
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IMHO(&E) on usenet you're better to break down your problem into a logical order of questions & post them one by one, that way your thread isn't so easily waylaid or hijacked & you tend to get each aspect sorted out properly before moving onto the next.

HTH

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Reply to
jim_in_sussex

OK, I have copied the new info and posted below... Might as well carry on here!

end fuse block...

(Most TN-C-S (PME) setups I have seen would have an earth wire vanish into the neutral connector block right next to where you have the two neutrals exit currently)

the whole layout?

That earth connector does not seem to be attached to anything other than the two wires? Is it just resting against the side of the box, or actually connected to it in some way?

Next to is not the same as connected to though...

Not convinced personally (depends a bit on that terminal - but even if it is attached to the box it could still be a TN-S setup with the description you give of the flat tab on the cable sheath).

16mm^2 is indeed adequate for a 60A main fuse

Not used either, so can't comment. Used a few contactum ones from TLC recently, which while a bit "plasticy" do have plenty of space to work with.

Could do...

It matters in the sense that if it does, and that is your only earth (i.e. you have a TT supply) then you will need RCD protection for all circuits - not just the usual suspects.

Probably better to remove it when not required. Are the tails into it long enough to reach to the new CU position? If so that saves you a join.

If you decide you do have a TT setup then that may be a good place to stick a 100mA RCD - but the time delay ones are much more pricey than the ordinary ones. It may be more cost effective to have a Henley there and feed two CUs, one with an "ordinary" 100mA RCD and the other with a

30mA one.

flying well above the head of the person who installed that!

Since you are changing the CU you could even go for a nice DIN rail mounting transformer - you would just need a cable run to the bell then with no extra external transformer.

Reply to
John Rumm

SNIP

The service units I am familiar with have the earth block as shown which is actually an extension of an internal copper bar link straight off the neutral connector inside the plastic of the unit

see above

That is slightly confusing as it could be an extension of the cable sheath and may point to a TN-S system although the mounted earth block on your phot suggests TN-C-S

I'd very strongly suggest you investigate further, hiring an ELI tester if you have not access to one. It might be possible to use a low ohms meter to find out if the unit earth block is actually a sheath earth (block to sheath no voltage present, practically zero ohms) or is as suspected a neutral coupling (small voltage possibly present block to sheath and a slightly higher resistance if your gear can measure with a voltage present, practically zero ohms block to neutral). (Switch off all offtakes then remove the wires to avoid false path readings. Restore correctly on completion!) If you are not fully competent get someone in who is. Use fused test probes

Reply to
John

The low-voltage cable run inside the CU must be insulated or sleeved to

240V - most bell wire is only rated up to 60V.

If it's flats, the opportunity might be taken to upgrade to entry intercom and emergency lighting on the stairs.

Owain

Reply to
Owain

No, sorry: wider views get very messy because so many of the bits relate to the upstairs flat.

It's connected to the box, though without delving rather deeper in than

3-phase boxes encourage, I can't tell whether it's electrically as well as mechanically connected - I assume so.

True.

No, I'm not convinced. I wonder if the supply company would have a record?

I think it a fair assumption that the connection on the side of the service head is genuinely earthed, either via the cable sheath or the Neutral: the upstairs flat has obviously been rewired relatively recently, and has no other earth connection

The CU itself is 6 or 7 yards away, inside the flat: the meters etc are in a shared access hallway. The tails that would need to stretch are the ones from the meter to the VO trip at present, which would need to go straight into the 60A switchfuse - but I don't think they're long enough.

Can one reconfigure split-load CUs so that they have two RCDs, instead of one plus an isolator? Without wishing to bring heavens or posters with firm views crashing down on me, I've been running an entirely RCD protected house (2 separate CUs) for well over 15 years with no nuisance tripping, and lighting circuits split between the two to ensure nowhere blacks out completely with just one fault. Otherwise it would need a second Henley near two new small CUs.

As I said, the CU is some way away so we'd be into floorboard lifting territory, which may not be ideal in the present circumstances. If I put a whole-flat 100mA time delayed RCD in a 3 or 4-module box next to the meter and isolator, I could cheaply enough add either a DIN rail transformer or a single MCB feeding an adjacent fused connection unit and thence the existing bell transformer.

We're getting there...

Reply to
Autolycus

Thanks for the suggestions: I can easily enough check some voltages with my DMM, but anything more subtle would require liaison with the many inhabitants of the upstairs flat before turning off their supply, and may not be so easy. I'm looking into buying most or all of the required Part P test instruments - if only so that I don't get caught looking daft by a professional test. I'd always sooner spend money on tools than someone else's services.

Reply to
Autolycus

I'm working on her about an entry intercom or camera, having pointed out that there's not much point in having a door chain on her inner "front door" if she's had to go through it into a small entrance hall to open the outer door to a caller who's rung the bell.

Fortunately the stairs are entirely part of the upstairs flat, so that's one worry I don't have.

Reply to
Autolycus

On Tue, 28 Jun 2005 15:33:00 +0100,it is alleged that "Autolycus" spake thusly in uk.d-i-y:

Yep, just did that the other day, 100mA time delay RCD on the main, protecting the lighting and the single 16A radial feeding the fridge/freezer, and a normal 30mA feeding the second half of the CU protecting everything else.

The setup is unusual in one other way, the 100mA protected side also has a 30A/30mA RCBO in it (it was there already for a shower which is now on the 30mA protected side) which feeds the outside sockets, to prevent the likely nuisance tripping on the outside circuit from killing everything. Expect however to pay much more/suffer less availability for a time delay RCD, a 100mA RCD of the more usual sort probably wouldn't prevent it tripping as well as the 30mA one in the event of an earth fault in the range above 100mA.

Reply to
Chip

Ah, OK. I had not seen one like in the picture before (I guess the different electricity companies have different "pet" brands they use and hence you will get different things in different parts of the country)

Yup, that is what occured to me as well.

He could also possibly simply measure the voltage between earth and neutral. On PME it ought to be zero (close to the bond anyway), on TN-S is may be a small voltage.

Reply to
John Rumm

Given John's comments above it does sound more like PME then (the 16mm^2 earth wire is also a clue). Often these days the supply company sticks a sticker on the setup to say so, but I guess you cannot rely on that.

They ought to have... might be worth asking.

Yup, I expect you are right.

You can, and this is common practice on TT setups. However it looks pretty unlikely that you are TT so there is no need for a 100mA RCD at all.

What many people call nuisance trips are in fact trips indicating a fault of some description anyway. Although use of a "whole house" RCD is deprecated, the main objection is the plunging into darkness aspect. It is a problem you can solve in several ways - you have found one having separately protected CUs.

Reply to
John Rumm

There's the additional problem that the transformer will still need to be supplied via an MCB and so some judicious cutting of the busbar is likely to be required. Also it'll be 2 units wide which reduces the space available for circuit MCBs.

Hwyl!

M.

Reply to
Martin Angove

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