551.4.3.2.1 Flats how to provide earth rod in island mode

Interesting question. As it is probably impossible to isolate the Earth from incoming services and thus the Mains Earth, why should I fit an Earth rod? If the island supply is from a battery, thus effectively IT, do I need an Earth connection anyway? In such an installation is there a significant shock risk, or is the risk of a battery fire greater? Please discuss.

Lightning protection might also have to be considered.

Thanks for the input David. Yes I need an earth to provide ADS. I think the shock risk is sufficient given the multiple circuits that the island CU supplies.

The MET earth stays connected (but cannot be solely relied on) as indicated in 826.1.1.2.1 which also states the need for an earth electrode - which gave rise to my original question:

How to provide an earth electrode in installations where this is tricky. Eg flats.

Depending how the flats are wired, earthing needs to be considered as a whole building issue, as the services to many flats are interconnected - the installation origin, and the site of the main MET and any electrode is not in the flat. Co-ordination between leaseholder and free-holder is required, which is often a bit tricky. However, solar panels and so on need this co-operation anyway.
Mike.

I have a job on the go for a bathroom company in a top floor flat, three storeys up, that has had PV installed on the roof with the inverter in the loft above the flat along with battery storage.

Although the installation and its consumer unit are three stories up and the inverter with the batteries are really four stories up, the supply is taken from the DNO underground supply cable and meters in an external cupboard with the supply taken into the communal cupboard under the stairs on the ground floor, so part of the installation the consumers fused switch and distribution circuit to the flat are on the ground floor, but that still doesn't make it practical or possible to install an earth rod.

But if it is ever configured to support an island mode of generation, and class 1 appliances can be used,  then it will need some sort of electrode, but maybe not a rod - are there foundation steels for example. There may be a need for some trades other than electrical to dig up the car park or whatever.

'Impossible'  may be solved other ways in other places. Going back over 20 years in Germany it was common to see a TNC incomer to a basement of an 'altbau' pre war block and then electrodes drilled out more or less horizontally through the sides of the walls  Holes in masonry then re-proofed with some sort of tarry goop. (Only in the West. The East Germans did not really do earthing, or indeed neutral/earth differentiation. All rewired and much better now.)

Mike.

David Stone said:

As it is probably impossible to isolate the Earth from incoming services and thus the Mains Earth, why should I fit an Earth rod

There is no requirement to isolate from a DNO provided earthing arrangement, and as David said that might be a fool's errand, but:

• we know we can't always rely on extraneous-conductive-parts as the fault current path for fault protection
• the normal fault current path, in the case of a distributor's earthing terminal, could well be interrupted by repairs to network cables.

This is what Regulation 551.4.3.2.1 is all about. 551.4.3.2.1 is not a new Regulation, it's been in BS 7671 in some form or another for a very, very long time.

It also depends on whether the system is for the individual flat, or whether a shared PEI approach is used.

I think Mike's previous response covers most of the considerations - and the electrode doesn't have to be a 'rod'.

I would add there's probably not a single solution that is right for all cases we might ever come across.

As an aside, what about over-current/short-circuit protection? 

Will the alternative supply (inverter?) be able to source enough current to make the 'normal' protection operate?

If not, what other mechanisms are in-place to allow detection of a short circuit & how would it cleared?

 AdrianWint 

Yes, this is a design consideration also - alongside the issue of load-shedding to prevent damage to the battery and unwanted tripping (well, unwanted overcurrent limit operation more correctly) of the inverter.

These topics for island mode operation are among those covered in the IET Code of Practice for Electrical Energy Storage Systems