Additional connection to earth 411.4.2

There has, for a long time, been advice (via a note) that additional connections between PE and Earth can be made (e.g. from MET) in TN systems.

However, the thing that has changed in 2022 is the introduction of Chapter 82, which considers operation in island mode where (as Regulation 551.4.3.2.1 says) you can't rely on the distributor's means of earthing.

The IET Code of Practice for Electrical Energy Storage Systems contains details as to how the switch-over to island mode ought to be effected. It's briefly touched upon in this IET Webinar, which I believe you can register for free and view (even thought he event has passed).

Wasn't this the one that started off as a precaution against a broken PEN conductor in the supply ... but the sort of values to Earth for say a 100A installation turned out to be rather impractical (even foundation earthing might struggle in some conditions). But on the basis that something is better than nothing, and might be effective if you didn't have too much switched on at the time, it partly remained as a mere recommendation.

   -  Andy.

Not really ... always been permitted for TN systems (TN-S as well as PME), and even back in the 17th Ed BS 7671:2008, the Regulation had the Note regarding additional connections between PE and Earth.:

NOTE: The PE and PEN conductors may additionally be connected to Earth, such as at the point of entry into the building.

Even if we are talking about PME, though, additional earthing at MET has been noted for quite some time also, e.g. Appendix 3 (Fig 5) of the 15th Edition.

So, whilst foundation earthing has been explored in more recent drafts for public comment of BS 7671, there's nothing new in terms of the ability to provide additional Earth electrodes connected to PE or MET (noting the para and items (a) and (b) before NOTE 3 in 722.411.4.1) - but there are now cases, discussed by Chapter 82, that necessitate it.

Just like the "recommendation" to fit AFDDs to socket circuits, in this thread 

https://engx.theiet.org/f/wiring-and-regulations/28497/implications-of-the-term-recommended-in-7671

The "recommended" term does not equal "should", it is only recommended.

Why JPEL64 put that bit in the front of BS7671 about a recommendation equating to should, to clarify things, JPEL64 only knows. 

The change in 411.4.2 is from "may" - which indicates that you are permitted to connect an additional earth, without any suggestion of it being advantageous - to "recommended" - which indicates that it would be a good idea but not necessary. If it is to do with PEI then why not reference it as such within 411.4.2?

I think there is concern that if folk go bashing rods in willy nilly in built up areas then there will be a lot of surprisingly shallow gas mains and drains that are not on the map etc discovered the hard way.

However, when things are being done anyway for an extension or a new drive or 2hatever and there is a digger on site then the risk balance swings the other way, and adding an electrode does no harm.

Mike.

Thanks for the link. With AFDDs, accepting they provide additional protection, I can see a reason for recommending it to the client - "you can have this as it's recommended, these are the benefits, but it'll cost ££££".

With the additional earth connection (basically an earth rod), how am I backing up my recommendation to the client? "you can have this as it's recommended, for some unknown benefit, but it'll cost ££"

Yes, it could cause problems. Also, if every house on a terrace, for example, has an earth rod, the short distances between the spikes could actually introduce problems in the event of a fault, with voltage appearing in adjacent properties. If nothing has changed with regard to TN systems in general, I don't get why this move from a noted "may" to paragraphed "recommended" has happened in 411.4.2 - it should have been in Part 8 regarding PEI and island mode if that's the concern.

Round here (small market town in Hants) we have streets of 1930s houses with overhead TT, each with an electrode at tripping hazard height by the front door. The usual issue is the rod rotting off or the wire being cut, not induced voltages due to  faults from the neighbours. But of course on a fully TT street, the fault currents injected into the earth are quite low.

It is more complex when it is a mixture of tt and tnx on the same TX, and I suspect 'what is best' varies case by case,

Mike

If you have metallic water or gas supplies, it will hardly make any difference.