TN-C-S supply - worth adding an earth electrode?

[EFLImpudence]

I'm not sure I understanf this "lot of cases".

Well, any installation with which you are not familiar - customers.

There is surely only a need for bonding if any of the pipework (or things connected to it) 'exits' the property.

I'm sure that in a property such as yours you cannot be certain that there is nothing that would be making the pipe extraneous.

If there is a gas supply, then there will usually be continuity between that and the water pipework (e.g. at boiler), but provided the gas pipe is properly bonded, that would not, in itself, create a need to bond the water pipes, would it?

No, not with regard to main bonding but that's not the point.
You must be certain that the water pipe is NOT extraneous itself.

 

[WellKnownSid]

Loss of it where?

At the transformer, either via a rogue cow / tractor / or 'person known to travel'.

The supply disappears underground (straight concentric) and re-appears at the house with no additional earthing point in between or at the house end.

My understanding of PME was that there was an M in it... although I also appreciate that TN-C-S doesn't necessarily imply PME.

 

[westie101]

Though there are one or two interesting assumptions here that need answering.

The OP states there is a single earth rod at the transformer

Is this an HV earth? Which will be required

or an LV earth? Which normally needs to be 10m from the HV earth

Or a combined earth? Which requires a very low impedance (below 1 ohm) and is very rare on OH systems

Was this supply originally PNB which would have seen a N/E earth rod at the cut-out (was this provided by the metal water pipe in the past? It should not have done but you never know)

Before going down the line of a detailed analysis of earthing and bonding I think the actual situation should have been discovered!

So WellKnownSid I suggest you get onto the DNO and find those answers

 

[ban-all-sheds]

If you mean the bond is connected to the plastic itself then obviously this is pointless.

Yes, wrapped around a plastic pipe and clearly pointless...

What prat did that?

You should take that as a clear indication that at some point someone staggeringly ignorant has been fiddling with your electrical installation.

 

[WellKnownSid]

Yep, I think the Hydro boys will be the only ones who can answer this one... We'll give them a ring and keep you posted...

 

[JohnW2]

There is surely only a need for bonding if any of the pipework (or things connected to it) 'exits' the property.

I'm sure that in a property such as yours you cannot be certain that there is nothing that would be making the pipe extraneous.

Well, mine obviously is extraneous. However, if the supply entered the house as plastic, I can't think of anything which could possibly make any of the internal pipework 'extraneous'. Metal waste/soil pipes are long since gone, and there is nowhere 'outside' that any metal pipes could go (and I would have noticed them, if they did!). Am I missing some other possibilities?

Kind Regards, John

 

[WellKnownSid]

What prat did that?

You should take that as a clear indication that at some point someone staggeringly ignorant has been fiddling with your electrical installation.

That much has been sussed already.

On the face of it, the electrics looked really solid. It was originally two houses - before they mysteriously burnt to the ground (presumably in some earth-related electrical fault ) about 30 years ago.

So - early 1980s build - CPC in all the lighting circuits, every faceplate and switch an MK, Wylex standard CUs. Pretty cool, huh?

On closer inspection:

• Plastic services bonded to CPC
  Undersized shower feed
  Mixed and undersized tails
  Undersized CPCs
  Henley block after Henley block feeding two main CUs and three subs
  Spurs off of spurs

...and much more. Hence a re-wire of the kitchen and lighting circuits has become a whole house rewire. Almost a mile of t&e later and the end of the first fix is in sight.

 

[JohnW2]

Loss of it where?

At the transformer, either via a rogue cow / tractor / or 'person known to travel'. The supply disappears underground (straight concentric) and re-appears at the house with no additional earthing point in between or at the house end. My understanding of PME was that there was an M in it... although I also appreciate that TN-C-S doesn't necessarily imply PME.

We need westie here. My understanding is that (when mutiple services are being served) it is not allowed to have TN-C-S without PME - although I have been somewhat disillusioned (by westie!) in discovered that 'M' is often (maybe usually) only 2 - one at the tranny and the other at the end of the main. However, I've no idea what the situation is when, as in your case, a TN-C-S supply only serves one property - i.e. whether a second earth is required (thereby creating PME).

Whatever, I still think what I said before will probably still apply. If you totally lost the outer of the straight con (pretty difficult without also losing the inner core), you would lose your neutral as well as your earth - and no 'ordinary' domestic TT electrode would have a low enough resistance to make much difference to the resulting situation. The installation's N (connected to your MET, hence CPCs, via the TN-C-S head) would rise in potential (through any connected loads), possibly close to line potential, and if an earth rod were also connected to that MET, the potential of that electrode (and surrounding bits of your garden) would also rise - again, possibly to nearly line potential. If all required bonding was in place, the inside of your house would remain equipotential, hence 'safe', but I'm not sure that the TT rod would achieve anything particularly useful.

Kind Regards, John

 

[westie101]

We need westie here.

As I commented earlier the earth rod mentioned could be one of 3 things. There are certainly aspects of what has been described that need further investigation by the DNO

 

[mfarrow]

Just happens to be in the MF DIYnot archive, this is PME, the tranny is 30ft away, and feeds 3 very rural supplies wthin a 50ft radius:

View media item 20581
In relation to the OPs query, I don't see much point in installing an earth rod. You're in an equipotential zone, it's not your responsibility to maintain the earth. The above hasn't been nicked in 30 years, it's just not that big a prize.

Bonded plastic pipe - are you sure it's not PVC coated copper? Probably a plumber come in and done some work. No excuse for his cluelessness though.

And will you please use the correct context for CPC. It's a Circuit Protective Conductor.

 

[JohnW2]

The OP states there is a single earth rod at the transformer ... Is this an HV earth? Which will be required ... or an LV earth? Which normally needs to be 10m from the HV earth ... Or a combined earth? Which requires a very low impedance (below 1 ohm) and is very rare on OH systems

I'm a little confused here, particularly in terms of the relevence of the above (interesting) questions to the question posed by the OP.

We have been told that the OP has been provided with a TN-C-S supply from the pole/transformer via underground straight con cable. That surely means that the (LV) CNE conductor must be earthed 'at' the transformer (and presumably nowhere else, given what we've been told) and the OP has merely asked about the possible 'benefit' of connecting an additional earth rod to the TN-C-S earth at his end.

Kind Regards, John

 

[WellKnownSid]

Bonded plastic pipe - are you sure it's not PVC coated copper? Probably a plumber come in and done some work. No excuse for his cluelessness though.

Definitely a plastic pipe...

And will you please use the correct context for CPC. It's a Circuit Protective Conductor.

Okay, okay, I'm new to this electrickery lark... The main protective bonding conductor cross sectional areas did not meet the minimum requirements of table 54.8...

 

[EFLImpudence]

The main protective bonding conductor cross sectional areas did not meet the minimum requirements of table 54.8...

Table 54.8 only applies to PME. Are we still of the opinion that yours is not PME?

If so, then half of the required, N.B. not actual, c.s.a. of the earthing conductor.

 

[EFLImpudence]

However, if the supply entered the house as plastic, I can't think of anything which could possibly make any of the internal pipework 'extraneous'. Metal waste/soil pipes are long since gone, and there is nowhere 'outside' that any metal pipes could go (and I would have noticed them, if they did!). Am I missing some other possibilities?

You, probably, are not missing anything as you have inspected the premises because you live there and are interested - but the pipes could be in contact with the ground or through damp walls near the ground somewhere.

If I were to visit a house like yours for Mrs.Typical-Customer would she know or would I be able to tell without extensive work?

 

[JohnW2]

Table 54.8 only applies to PME. Are we still of the opinion that yours is not PME?

That's an interesting one. I know it's dangerous to try to 'read the mind' of the regs, but I somewhat suspect that when they wrote "PME", what they actually meant was TN-C-S (on the assumption that all TN-C-S is also PME) - lets face it, it''s not obvious why bonding conductor size should alter because there is an earth at both ends of the supply main, rather than just at one end, is it?

Kind Regards, John