BS7671:2018 DPC - Local Earth Electrode with TN systems

[JohnW2]

Apologies if this has already been discussed during my absence ...

We have often had discussions about the ‘acceptability’ of connecting a local earth electrode to a a TN installation, with at least some people suggesting that it is ‘not allowed’ (particularly with TN-C-S).

In response, many of us (including myself) have pointed out that a number of major countries require a local earth electrode (again, particularly with TN-C-S), that such an electrode offers an additional degree of protection and that, in any event, to ‘not allow’ an earth electrode to be connected would be ridiculous, given that there is a requirement to bond extraneous-c-ps to the MET of a TN installation.

Anyway, if I’m reading it correctly, it seems that we are probably about to join those ‘other countries’, since this appears in the BS7671:2018 DPC ...

542.1.201 The main earthing terminal shall be connected with Earth by one of the methods described in Regulations 542.1.2.1 to 3, as appropriate to the type of system of which the installation is to form a part and in compliance with Regulations 542.1.3.1 and 542.1.3.2. Additionally, there shall be an earth electrode, supplementing any earthing facility provided by the distributor, in accordance with one of the requirements of Regulation 542.2.3, to prevent the appearance of a dangerous touch voltage in the event of the loss of the main connection to Earth.

Have I interpreted this correctly? If so, it might be Christmas for those who manufacture earth electrodes! Whist it’s straightforward for standard houses and many commercial premises, I wonder how this will work with multi-floor blocks of flats and office blocks?

Kind Regards, John

 

[ban-all-sheds]

A long, slow, Christmas, if you try and estimate a realistic rate of installation.

 

[JohnW2]

A long, slow, Christmas, if you try and estimate a realistic rate of installation.

True, but a steady source of additional sales for many years!

Kind Regards, John

 

[securespark]

This from another thread about the 18th DPC:

Re 542.1.201, this is one of the comments in response:

If in an existing installation an earth electrode is installed in accordance with the requirements of 542.1.201, a potential danger could be introduced by

• Creating a low resistance path to the property with an electrode installed that can be shared by several properties in the event of a lost Neutral dependant on the supply configuration.
• The installation of the low resistance path would potentially mask a lost supply fault, which could be left in service undetected.
• Any person working on or testing and inspecting an installation with an electrode can be subject to additional risk, particularly when disconnecting the earthing conductor.

Also achieving the values required for touch voltage are likely not to be met, and it is likely that an electrode cannot practicably be installed, e.g. space restrictions, property types etc.

Read more: https://www.diynot.com/diy/threads/...lable-for-comment.483595/page-3#ixzz4kukion85

 

[flameport]

Have I interpreted this correctly?

Yes, but consideration of 542.2.3 is an important element of this - electrodes can consist of many things.

For TN-S, the electrode is most likely the outer covering of the underground cable.

 

[JohnW2]

This from another thread about the 18th DPC: Re 542.1.201, this is one of the comments in response: ... If in an existing installation an earth electrode is installed in accordance with the requirements of 542.1.201, a potential danger could be introduced by .....

Thanks.

That's all very well, but, as I said in my initial post, all of those "potential dangers" already exist if (as is usually the case) there are any extraneous-c-ps which are (necessarily) bonded to the MET.

Kind Regards, John

 

[JohnW2]

Yes, but consideration of 542.2.3 is an important element of this - electrodes can consist of many things. For TN-S, the electrode is most likely the outer covering of the underground cable.

With TN-S, is not the outer covering of the underground cable the "earthing facility provided by the distributor"?? If so, it surely can't also be "...supplementing any earthing facility provided by the distributor", can it?

Kind Regards, John

 

[flameport]

Many of the comments on the draft are from people who apparently have a fundamental deficit in understanding.

 

[flameport]

The earthing is provided by virtue of the outer covering of the cable being connected at the transformer. If that or some other underground joint fails, the outer covering remains in contact with the ground and can function as an earth electrode.

If the connection inside the property fails or is disconnected then it will not, but that is no different to any other earth or bonding connection failing - they are usually all connected together at a single point.

 

[bernardgreen]

Creating a low resistance path to the property with an electrode installed that can be shared by several properties in the event of a lost Neutral dependant on the supply configuration.

Through which the Neutral currnt flows without any fuse or other current limiting device. True that the Live current to each property will be limited by the supply fuse in the cut out but it is the sum of the Neutral currents from several properties in a un-balanced distribution ( several properties all on the same phase ) that has to be considered.

If the fault in the network cable connects the Neutral to properties to a Phase then the current from the CPC to Ground via the low resstance electrode will be limited only by the fuse at the substation.

 

[JohnW2]

The earthing is provided by virtue of the outer covering of the cable being connected at the transformer. If that or some other underground joint fails, the outer covering remains in contact with the ground and can function as an earth electrode.

That's all true but I still don't think it's really within the spirit of the proposed regulation to use the same incoming conductor as both the "earthing facility provided by the distributor" and as "an earth electrode supplementing the earthing facility provided by the distributor".

After all, a break in the cable sheath close to the consumer's property would result in substantial loss of the 'supplementary earthing' as well as the primary ('distributor provided') earthing.

Kind Regards, John

 

[JohnW2]

.... If the fault in the network cable connects the Neutral to properties to a Phase then the current from the CPC to Ground via the low resstance electrode will be limited only by the fuse at the substation.

Unless, I'm misunderstanding, this "low resistance electrode" is, at least in domestic properties, likely to have an impedance of at least a few dozen ohms, isn't it - in which case that would very much be the limiting factor, limiting the current to just a few amps even if full phase voltage were applied to it??

In contrast, extraneous-c-ps, already required by regs to be bonded to the MET, might, in some cases, have very low impedances to earth.

Kind Regards, John

 

[bernardgreen]

extraneous-c-ps, already required by regs to be bonded to the MET, might, in some cases, have very low impedances to earth.

They do, and when the Network Neutral is compromised, the current through the bond to the extraneous-c-ps can be enough to melt the bond cable.

 

[EFLImpudence]

I like the comment in another forum -

The need for this is the fault of the DNOs and it should be up to them to correct it.

 

[JohnW2]

They do, and when the Network Neutral is compromised, the current through the bond to the extraneous-c-ps can be enough to melt the bond cable.

Indeed - but, returning to context, it terms of a standard domestic earth electrode, you're usually not even going to get enough current to melt a 0.5mm² conductor, let alone a 10mm² or larger one.

Kind Regards, John