BS7671:2018 DPC - Local Earth Electrode with TN systems

[SimonH2]

Most neutral faults are caused by Consac cable which has a plastic covered aluminium sheath ... water gets in and turns the aluminium to dust

BT also had (still have) that problem. At a previous job, we had a remote site with a total of 9 lines into the building - mix of analogue phone, ISDN, and leased lines - which would randomly fail. Report fault, fault gets fixed, next week, different random failure. This went on for some time before the fessed up to a run of alli cable with that problem. Eventually they pulled that cable out and replaced it.

There was a period (in the 60s IIRC) when copper got so expensive that they used alli cable instead as a cost cutting measure. I bet it's cost them a lot more over the years dealing with all the faults caused by that.

Having seem how alli cable goes with water, I can well understand the problems having it in a DNO network.

 

[ban-all-sheds]

As for whether it's actually safe to have a rod connected in each property, I'm not sure how I feel about that, having seen singed bonding cables, gas pipes, and internal wiring following neutral faults where it has tried to find any conductive path back to the transformer, it would only be acceptable if every property had one I reckon.

But it will take several decades for rods to become universal.

 

[JohnW2]

As for whether it's actually safe to have a rod connected in each property, I'm not sure how I feel about that, having seen singed bonding cables, gas pipes, and internal wiring following neutral faults where it has tried to find any conductive path back to the transformer, it would only be acceptable if every property had one I reckon.

If there are already bonded extraneous-c-ps (at risk of "singing" under the circumstances you mention), then I would have thought that any additional connections to true earth (even if only in a small proportion of properties) would improve, rather than deteriorate, the situation?

I don't know the facts but, despite the progressive appearance of plastic water supply pipes, I would have thought that a substantial majority of properties still have extraneous-c-ps - so, in areas where that is the case, as above, gradual introduction of local earth rods cannot really do any harm, but would do some good (in the event of 'rare' faults).

Kind Regards, John

 

[EFLImpudence]

What people probably don't realise is the hourly rate of a contract jointer is huge, especially by the time the DNO has put their uplift on.

No one is expecting the jointer to do menial tasks. DNO uplift?

It makes no sense to tie up a team of two to bang rods in and hang meter boxes when you're going to have an electrician there anyway to install your electrical installation,

It may if that were the job at hand.
The reason for this new electrode requirement is the lack of satisfactory maintenance of infrastructure by the supply companies.
Perhaps they could have a team to fit rods in everyone's property for a reasonable amount. Either it is needed - now - or it isn't.

and a labourer is capable of hanging meter boxes.

Are you saying the DNO/Supplier have no labourers? Is that the problem?

Then they can use the jointer for more productive tasks like another new connection.

Obviously.

As for what they charge, I don't really get involved with that, but broadly speaking it's not too different to other utilities.

I am afraid that is not a reasoned justification.



We all know that these companies are now so big that they no longer have 'customers'; merely payers of whatever is demanded who must do as instructed or not get a 'service'.

 

[JohnW2]

DNO uplift?

"Rake off", I presume!

Kind Regards, John

 

[TheKeymeister]

BT also had (still have) that problem. At a previous job, we had a remote site with a total of 9 lines into the building - mix of analogue phone, ISDN, and leased lines - which would randomly fail. Report fault, fault gets fixed, next week, different random failure. This went on for some time before the fessed up to a run of alli cable with that problem. Eventually they pulled that cable out and replaced it.

There was a period (in the 60s IIRC) when copper got so expensive that they used alli cable instead as a cost cutting measure. I bet it's cost them a lot more over the years dealing with all the faults caused by that.

Having seem how alli cable goes with water, I can well understand the problems having it in a DNO network.

I used to work for BT before I went into working for a DNO, so I've heard all about it!

The advantage BT have is that much of their system is ducted, not the case in the DNO world, and where DNO cables are ducted, the joints are just buried, unlike at BT where they usually build an inspection chamber for each joint. So theoretically BT could replace an ali cable with little/no excavation, whereas a DNO would probably have to open cut a trench for the whole run, and make the street look like a golf course as they pick up each service joint.

 

[TheKeymeister]

But it will take several decades for rods to become universal.

The only thing I can think of is to only allow it on completely new build sites (no idea how you would enforce/apply that), and everyone else would have to use TT earthing, if it's so much of a requirement to have a rod at each property.

 

[TheKeymeister]

If there are already bonded extraneous-c-ps (at risk of "singing" under the circumstances you mention), then I would have thought that any additional connections to true earth (even if only in a small proportion of properties) would improve, rather than deteriorate, the situation?

I don't know the facts but, despite the progressive appearance of plastic water supply pipes, I would have thought that a substantial majority of properties still have extraneous-c-ps - so, in areas where that is the case, as above, gradual introduction of local earth rods cannot really do any harm, but would do some good (in the event of 'rare' faults).

Kind Regards, John

That is true. The problem would be in estates built since the 70s when plastic water and gas services started coming in (so nothing extraneous to connect to earth) and before earthing at each service joint started happening. As soon as one property gets a rod in, it immediately becomes the path of least resistance and that house potentially burns down if there's a neutral fault just because their electrics are up to current regs.

 

[TheKeymeister]

No one is expecting the jointer to do menial tasks. DNO uplift?


It may if that were the job at hand.
The reason for this new electrode requirement is the lack of satisfactory maintenance of infrastructure by the supply companies.
Perhaps they could have a team to fit rods in everyone's property for a reasonable amount. Either it is needed - now - or it isn't.


Are you saying the DNO/Supplier have no labourers? Is that the problem?


Obviously.


I am afraid that is not a reasoned justification.



We all know that these companies are now so big that they no longer have 'customers'; merely payers of whatever is demanded who must do as instructed or not get a 'service'.

So they should employ labourers in addition to a jointer and a jointer's mate when on the whole, customers prefer to do these things themselves to save money? Righto. And no, the jointer's mate can't do it because the jointer can't work live unless the mate is with him.

Can you quantify your statement on lack of maintenance? What maintenance haven't they done? Nobody can predict where these neutral faults will occur, and in 'hot spot' areas the whole main is often overlaid. Usually at great annoyance to the residents who don't understand why we want to dig the road up and make a mess then turn their electric off when it's working fine now and they're sat watching the telly. There's hundreds of miles of Consac/Alpex in the ground, it can't be easily replaced like with gas/water because it's usually laid direct rather than ducted, and the worst 'hot spot' sections are being replaced bit by bit as they're found. The cost of pushing a button and doing it all now would be insane, and it'd get paid for by going on everyone's electric bill anyway. Banging rods down in every property wouldn't cure the problem either, the faulty cable would still need digging up and replacing, rods would just lessen the effects and cost more money. I don't particularly agree with it being added to the regs either, because neutral faults causing huge amounts of damage aren't exactly a daily occurrence.

Average breakdown for a single phase new connection would be something like,
Jointer and jointers mate - 400/day - so 200 for half a day
Excavation team to dig joint hole in footway and reinstate - 750
Cable and joints - 250+ depending on length
Cost of planner designing scheme and project manager organising everything - 200
20% uplift (profit margin)
...and of course you have the option to go elsewhere to an ICP. Water connections usually cost less because the excavation team carry out the connection as well. Not sure what the issue with having uplift costs are, I'm sure you don't do everything at cost price? If your quote comes in at massively more than that, there's either reinforcement required or no main nearby. People often bemoan paying for reinforcement, but it would otherwise come from everyone else's use of system charges, so why should everyone else pay for your connection? It sounds like you've got an issue with DNOs for whatever reason, I take it you had an expensive quote for a connection at some point.

 

[JohnW2]

That is true. The problem would be in estates built since the 70s when plastic water and gas services started coming in (so nothing extraneous to connect to earth) and before earthing at each service joint started happening. As soon as one property gets a rod in, it immediately becomes the path of least resistance and that house potentially burns down if there's a neutral fault just because their electrics are up to current regs.

I think that risk gets over-stated. The rod might be "the path of least resistance", but a domestic earth rod will not often have a resistance/impedance much less than about 50Ω, in which case the most current it (and the conductor to it) would have to carry would usually not be much more than 5A - not enough to even significantly warm up a decent sized earthing conductor, let alone enough to make a house "burn down"! Even a 'very rare' 10Ω domestic rod could only result in a maximum current of 23A - again not enough to start a fire (given appropriate conductors).

... and before earthing at each service joint started happening

You're probably not talking about just TN-C-S but, in my innocence I used to assume that the 'M' of PME actually meant 'quite a lot', until much-missed westie told us that it was quite common for M=2 - i.e. neutral was connected to earth just at the transformer and at the end of the main!

Kind Regards, John

 

[EFLImpudence]

So they should employ labourers in addition to a jointer and a jointer's mate when on the whole, customers prefer to do these things themselves to save money? Righto. And no, the jointer's mate can't do it because the jointer can't work live unless the mate is with him.

Well, of course they should. Don't they?
No one expects the most highly skilled/expensive personnel to do all the menial work. Isn't that what happens everywhere else?

Can you quantify your statement on lack of maintenance? What maintenance haven't they done?

This has all come about because the DNOs did not maintain/repair the TN-S supplies but merely connected the householders earthing to the supply neutral with scant regard for the consequences which, now, everyone has to try and remedy.

Nobody can predict where these neutral faults will occur, and in 'hot spot' areas the whole main is often overlaid. Usually at great annoyance to the residents who don't understand why we want to dig the road up and make a mess then turn their electric off when it's working fine now and they're sat watching the telly. There's hundreds of miles of Consac/Alpex in the ground, it can't be easily replaced like with gas/water because it's usually laid direct rather than ducted, and the worst 'hot spot' sections are being replaced bit by bit as they're found.

Whose fault is that?

Average breakdown for a single phase new connection would be something like,
Jointer and jointers mate - 400/day - so 200 for half a day
Excavation team to dig joint hole in footway and reinstate - 750
Cable and joints - 250+ depending on length
Cost of planner designing scheme and project manager organising everything - 200
20% uplift (profit margin)

I cannot argue with your figures but they are a lot less than I have experienced with things like new gas supplies and BT repairs.
Does the excavation "team" not consist of labourers?

As you say, it probably all works out the same in the end but Tesco, for example, does not charge the community in advance for building a new shop. They invest in the hope of attracting customers.
Plus, the fact that employing someone else to do the allowed work instead would be cheaper implies that it is more expensive than it needs to be.

Anyway, you cannot deny that these huge companies have little regard for the 'customer' and the prices charged by the likes of British Gas would make a private contractor's eyes water.