Earthing in 2 flats

[user56565]

hi. This may look quite long but im sure theres an easy solution to this.

There are 2 flats, one above the other in a detached property. The main supply came into a cutout in the downstairs flat, and from here, it was split into 2 sets of cables, one set for each flat. Earthing was provided by cable sheath (TNS), and this was also run upstairs. This earth cable is very minimal, approx 4mm. Both flats have main equipotential bonding in place.

The decision was made to install a brand new supply (PME) for the upstairs flat, and to disconnect the upstairs flat from the submains.

Siemens metering services arrived yesterday and initially refused to energise and install the new meter for the upstairs flat. They said that it could be unsafe for the upstairs flat to be on PME, and the downstairs on cable sheath. Any electrical fault upstairs could raise the metalwork in the downstairs flat (via water/gas pipes). They wanted the downstairs cutout changed to PME (remaining on its own supply cables), and then they would be happy. (I do not see how this would avoid voltages rising under fault conditions. Also as downstairs has main bonding, any user downstairs would remain in their equipotential zone)

Siemens came back the same day and did install the meter under pressure from London Energy, supplier for the upstairs flat (it would have otherwise been off-supply). He acknowledged that any risk would be very minimal, but if someone could reach metalwork in both flats, there could be a possibility of slight shock.

So: downstairs is on cable sheath earth, upstairs is (for now) also on cable sheath using the earthing cable that was already present. It has not been connected (yet) to the PME earthing terminal. This has been unofficially OK'd by Siemens, and officially OK'd by london energy.

My question: where do we go from here?? I would like to connect the upstairs onto PME terminal as it has lower external earth impedance, and i feel the cable sheath earth at 4mm is not sufficient for both flats. To keep the whole property in one equipotential zone, I would like the upstairs MET connected to the downstairs MET with 16mm cable. (note that due to main equipotential bonding in both flats, they would be connected anyway, but this would reduce any volt differences). The downstairs flat would remain on cable sheath too.

Does anyone feel they have any valid arguments, or quote specific regs against doing this?? Many thanks for your help on this matter.

 

[Adam_151]

I think they do have a valid concern, there would be two paths for the neutral from the upstairs, one through the incomming neutral connection, and one through the neutral block, to the MET, through the bonding, onto the pipework, then downstairs, off the bonding, to the met and down the cable sheath.

now thats a bad enough situation in itself, but now picture someone replaceing the main water stock cock in the downstairs flat...

 

[user56565]

@adam151

Thanks for your reply. Parallel paths are inherent when PME is in use even in a single property + installation, example main bonding to earth via gas/water services. These also provide paths for neutral currents to flow and are relied upon if the combined PEN were to be broken. In these typical PME installations there is assumed to be no risk to eg a plumber working on a stopcock. Wouldn't the same apply here??

Also, if a 16mm earth cable were installed between the METs, this would provide a low resistance path for the neutral current to flow, as opposed to higher resistance from the service pipes which interlink the flats.

I do however seem to have overlooked that potentially a lot of current could flow through the downstairs cable sheath earth (4mm) through the parallel path it provides. If the downstairs installation were to be converted by the REC to PME, as suggested by Siemens, then i assume that there are no risks??

 

[mapj1]

If both flats are PME, the earth fault currents in either flat will be carried by neutral conductor, so the risk of a half and half fault involving live plumbing and earthed plumbing is reduced.
But one has to ask why a higher fault current is a good thing - once it is above a few hundred amps any self respecting MCB will clear in near-instant time in any case. Any more current (lower Ze) just adds to the burn damage and sense of dissapointment. Surely both flats on TNS would be so much safer ?

Please don't think ruptured PENs in a PME set-up do not occur - yes it is very rare, and fatal accidents rarer, but certainly not unknown. It may be that if you only have 2 wires it is better to have PME than TT at least if buried feed, but if you have the earth wire, then TNS is almost always prefferable.
If you do bond the PME MET to the TNS one, you should assume it may one day carry the full neutral current (100A perhaps?) of the flat above, and size accordingly. 16 or 25mm will likely be needed.
If service pipes are shared between the 2 flats then there is equally a risk they will end up carrying a large current, and the main bonding should really be of PME size, even in the lower flat.

 

[ban-all-sheds]

Surely both flats on TNS would be so much safer ?
.
.
.
TNS is almost always prefferable.

It's not something the customer has a choice over.

 

[HDRW]

There are 2 flats, one above the other in a detached property. The main supply came into a cutout in the downstairs flat, and from here, it was split into 2 sets of cables, one set for each flat. Earthing was provided by cable sheath (TNS), and this was also run upstairs. This earth cable is very minimal, approx 4mm. Both flats have main equipotential bonding in place.

The decision was made to install a brand new supply (PME) for the upstairs flat, and to disconnect the upstairs flat from the submains.

I seem to be missing something here (not unusual! Why wasn't the new supply TNS, since the downstairs flat is already that? Surely the facility for a TNS supply must be there already, so why didn't they go that way?

Further, isn't it possible to ask for TNS (and maybe pay extra) when it's ordered?

Cheers,

Howard

 

[mapj1]

In the days of the electrcity council, when PME was new, any PME customer had the right to insist on its removal, although the RECs could charge significantly to do so, and did, because they did not want to do such work.
Usually this was only ever done for specials like radio mast installations, where a TT installation with a clean local earth is vastly preferable to injecting RF interferece onto the mains.
Normally in such cases PME and TT are the options, not PME or TNS, as it implies a 2 wire feed.
The only exception I have been told, is for very old TN-S installations where the armour is either inadequate from the beginning or now rotting away underground, and the 0.8 ohm earth impedance requirement cannot be met, these are then converted to PME, as a way of extending the life of the old installation, and saving some digging.

I regard this trick with some suspiscion, rather like those countries installing multi-phase pumps that can pump oil/water/sand mix onto old oil wells that are nearly depleted and now give out oil with more than 50% water cut, its one of those things that doesn't really instill me with great confidence in what is coming in the near future.
Still it works for today so it must be OK, must it not!

 

[user56565]

thanks to all for your replies. i have spent a couple of minutes making this neat little diagram to show whats happening: http://i7.photobucket.com/albums/y278/user56565/earthing.jpg

there is an earth link between the METs, also run in 4mm however i will be replacing this with 16mm once the REC has removed some old live cables and a cutout

@ mapj1

regarding your comments on fault currents. i feel things would be a lot safer with a lower Ze (0.2ohm in this case), as any faulty circuits would be isolated much quicker due to lower disconnection times. Yes, fault currents would be higher, however due to the shorter times that they would be in operation, damage (if any) would be minimal (in my opinion).


@HDRW + BanAllSheds

TNS was not an option given by EDF. However if you look at the above picture, you'll see I've left them both connected to the cable sheath. i do however have the choice of whether i wish to connect to the PME earth terminal.


The problem is, any fault currents in the above layout from either or both flats would have to be transmitted to earth via the tiny 4mm cable. surely this can't be a healthy situation, or can it?? i can't and don't want to upgrade the 4mm cable going to the downstairs MET as it is directly connected to the sheath, and the REC will most likely be very unhappy if this were tampered with.

i can't use the PME terminal only and remove connection to the cable sheath, as the downstairs flat will be relying on someone else's PME terminal for earth!

i can't use both in it's present state, as potentially the full neutral current could flow through the parallel path to the sheath, and the cable is undersized.

if the REC changed the downstairs cutout to PME, then i could link all the earths together and that'd be safe...

 

[plugwash]

am i the only one who thinks it dodgy to connect a TN-S earth which is supposed to be availible for places that can't have PME up to a PME earth?

much as i dislike PME i'd say best bet in this situation is to have the whole building on PME and then run a 16mm or bigger link between the main earth terminals to make sure they are at the same potential and can't end up dumping huge currents down thin earth wires that end up connecting between them.

 

[user56565]

@plugwash

the TNS has been there for 40years at least, before PME even started.

i can't use both in it's present state, as potentially the full neutral current could flow through the parallel path to the sheath, and the cable is undersized. i definitely would not connect the 2 earths together.

updating the earthing cable between the METs is first on priority list, however as i said earlier, there are live cables and a cutout which are completely in the way.

do you know whether it is possible to replace the cable around the sheath itself?? ie uprate from 4mm to 16mm?? or whether this is something that only the REC will do??

 

[HDRW]

updating the earthing cable between the METs is first on priority list, however as i said earlier, there are live cables and a cutout which are completely in the way.

I don't understand what you mean here - how are they "in the way"? Do you mean there is no way physically to run the cable?

do you know whether it is possible to replace the cable around the sheath itself?? ie uprate from 4mm to 16mm?? or whether this is something that only the REC will do??

Not sure what you mean by "cable around the sheath" - there should be an earthing clamp around the sheath, and the cable should be connected to that. The REC's responsibility for the earthing ends there - it's up to you (within the regs, obviously) what you do with it - they certainly would NOT change the cable!

When I updated my understairs cupboard (some of it dating back to 1937) I found that the 4-fuse CU, and thence the rest of the house, was earthed by a single black-sheathed wire that was 7/.029" (about 2.5mm in new money) ! A decent fault may well have melted that before it blew the service fuse... (although I don't know this for sure - the load to melt a cable isn't often quoted in the specs!

This wire ran from the clamp around the incoming cable's lead sheath to an earth-block and then to the CU, so I replaced it with 16mm which is the current standard, and not only do I think I did the right thing, as an improvement to the earthing/bonding arrangements, I believe it's outside Part P! (Hang on, I hear the thunder of hundreds of professional sparkys heading for my door, half of them carrying tar, the other half feathers!)

There may well be a good reason not to, but from here it looks like the first job should be to replace those 4mm cables with 16mm... but remember that this advice may be worth no more than you've paid for it

Cheers,

Howard

 

[ban-all-sheds]

The cable is more likely to be sweated onto the sheath, and is not user-serviceable.

 

[user56565]

@hdrw

yes, sorry for not being clear. the layout is such that the shortest route between METs is obstructed by the old upstairs supply.

unfortunately ban-all-sheds is correct, and the cable has been sweated onto the sheath. i can't replace it.

for now im going to leave things as they are and leave both earthed to the sheath via the 4mm cable. this although not ideal, can't be a dangerous situation, otherwise it wouldn't have been firstly installed in many homes, and secondly allowed to remain as is. also the 60A service downstairs has been completely RCD'd by their electrician (max earth load 30mA), whereas upstairs only has 2 MCBs (32A + 6A) that aren't RCD protected. i would imagine a short circuit from these would have little effect on the earth cable.

once the REC removes the old live cables, i can update the earth link between METs to 16mm. i can also have siemens round to assess the downstairs earth and raise a job with REC if they see necessary.

many thanks for your replies. you have all been very helpful!!