Adding an Earth Rod on a TN system and BS7671

[spinlondon]

I have checked out your note to 411.4.2., that indicates that a connection to earth may additionally be made at or near the installation.

What of it?

Systems for the distribution of electricity to the public, are outside of the scope of BS7671.

So is this note informing us that there may be a rod installed by the DNO at or near the installation, or informing us that we may install one?

 

[Bez]

Can't see why you would want to TT as well if you have pme earth takes all sorts though.

Can't see any mention of PME in the question, only TN. Remember, TNCS is not always PME.

Think this might have been covered before,

now where was that thread.............

over to you Steps

 

[ianmacd]

I have checked out your note to 411.4.2., that indicates that a connection to earth may additionally be made at or near the installation.What of it?

Systems for the distribution of electricity to the public, are outside of the scope of BS7671.

So is this note informing us that there may be a rod installed by the DNO at or near the installation, or informing us that we may install one?

My point exactly. Its outside the scope of BS7671 so how can you possibly justify saying yes now you have said that?

 

[spinlondon]

My point exactly. Its outside the scope of BS7671 so how can you possibly justify saying yes now you have said that?

Think about it.

How can there there be a Regulation for something that is outside of the scope of the Regulations?

If there is a Regulation for something, then that thing must be within the scope of the Regulations.

 

[davetheglitz]

I'm voting for ambiguous - but whilst I can't see the harm in it - I really can't see any point.

Consider a worst case of a TNCS system with an earth rod in a house. How would this be any different to a TNCS with a bonded water pipe? Could be done with no harm.

Consider a TNCS supplying an outbuilding with no added services - which is allowed - an earth rod would be just a 500 ohm impedance to the local earth - so again no harm.

Now consider a TNCS system supplying an outbuilding with services - if the earth is beefed up and the services are locally bonded - then the earth rod acts in the same way as the bonding for the most part - so again no harm.

Now in the light of the above why is TNCS prohibited in camp sites? Without an earth rod the loss of a neutral by any means would leave all metalwork at line potential. Now assuming a 16A load - and earth rod at say 500 ohms to the local earth, there is now a potential divider to the metalwork of approx 14 ohms to 500 - so the voltage at the metalwork is now 223v. Still pretty damn high!!

Overall the conclusion is that you could add an earth rod - but what is the point! Second conclusion is that you would be far safer if you TT'd the job anyway!

 

[Adrian]

TT may have its benefits but the reality is that the rcd we rely on for disconnection of a fault and not short circuit are not reliable

I have come across loads of faulty rcd,s on TT but never a lost neutral on tncs

 

[kme]

Okay. I`ve read, and perused this thread, and the "relevant" section of 7671 (although taking ONE section out of context can also provide misleading info.....), and I`m voting "Yes, you can".

1. As has been said, the rod is *functionally* no different to a lead or soft copper water pipe; so how could it be regulated against? Although I understand the thrust of the argument ("one" of the methods), there is no reg. specifically precluding this.

2. As I posted on the "other" thread recently; IF it was against reg, surely we`d be expected to REMOVE bonding from any services which entered the ground with a conductive material. Not going to happen, is it?

I`m not "backing my answer with a reg", as there isn`t one. However, conversely, there isn`t one to preclude it, either.

KME

 

[PBC_1966]

Only just noticed this thread following on from the other one.

I've voted yes, it's allowed. To summarize my take on it:

* I do not read anything into 542.1 which says it can't be done.

* You don't need a regulation to say explicitly that you can do it; like the law, if there's nothing to say it's prohibited, then it's permissible by default.

* Connecting a purpose-made electrode to the system is electrically no different from bonding a buried metallic water pipe, so for the regs. to allow one but prohibit the other would make no sense.

* Electrically speaking, arguments over whether the rod is connected directly to the supplier's neutral at the cut-out or to the consumer's MET all of 6 inches away are quite pointless since the two are solidly connected together.

* Long before the days of the Wiring Regs. becoming BS7671, it was actually recommended that an additional earth rod be used as an extra safeguard.

* It is standard practice (and required by codes) to provide such an electrode in other countries.

 

[ian932]

Regulations on the DNO equiv or on the installer?

Installer, the DNO does not give the installer power until they show the installation is ready for it by way of signing off the 1st stage of the install which is all the dead tests on the new installations, ET101:2008