Physical installation of earth electrode

Your worry about touch voltages for people is probably displaced. Remember that we expect a TT installation to disconnect in less than 200 ms (depending on the fault current) and even at 230 Volts this is considered safe(ish). There should not be a situation where an Earth electrode is "live" for a significant period, the only time being as part of a TNC-S system with an open circuit PEN conductor. Earth faults on a TT system should result in quick ADS, and so be cleared, but this does not apply to a few particular faults on TNC-S systems. Earthing systems should not be considered a particular risk, and this is probably why you find little advice.

Ziggie said:

it is clear that the information is not well known and I would have thought something would be in both the On-line guide and the Regs.

You may find this changes very very soon. More installations will require earth electrodes ... not simply TT ... for example, if you want island mode ('backup power') capability if you have a battery storage system fitted, then TN-S and TN-C-S earthing arrangements would require an additional consumer earth electrode connected to MET, as BS 7671 says you can't rely on the distributor's earthing terminal for switched alternative supplies (in case the DNO is working on your cable specifically).

I am slightly concerned that a child could be standing in water puddle with the fault being present and gets electrocuted by the difference in voltage that might exist.

The risk is the same for any metallic part connected to the consumer's earthing system - especially outside - everything from outside taps to class I light fittings to electric vehicles on charge. In practice the risk is very low - in an installation that complies with BS 7671 the earthing system shouldn't persist at above 50V for any significant lengths of time (e.g. due to leakage currents) - and earth faults that could raise the system above that should clear within 1s (if on a large final circuit or distribution circuit) or 0.2s on more usual final circuits.

   - Andy.

gkenyon said:

(b) Terminations should ideally be 250 mm below ground, in a suitable inspection pit (the pit itself being approx 500 mm deep).

I have a stop cock which is about a forearm's length deep. I am pretty sure that if it were the top of an earth rod, I would be unable to inspect it.

I cannot quite see the problem of step voltages when an electrode is placed as close as possible to the building (allowing for the fact that foundations tend to be wider than walls, though there may be none at all). It would be pretty bad luck if somebody were doing some gardening right in that spot just as a fault occurred. Or perhaps some of you plant electrodes in the middle of the lawn?

The biggest risks from rods with exposed tops is probably from falling over them, or damage to the wiring.  If the TT installation has working RCDs as has been noted the dangerous time before the ADS kicks in is quite short, and as we all know, I hope, shorter than half a heartbeat is what the turning point on the graphs of permissible shock voltages versus duration. There will be odd cases of stuck RCDs or badly maintained systems where the fault is persistent and not realised.  In such a case making the electrodes and anything connected to them inaccessible or hostile to bare feet is worthwhile.

In the same way that a short length of plastic pipe in the feed to any outside taps or other plumbing limits the shock current, nothing beats having some extra R between you and it.

A lot of townie sparks are not very  familiar with TT, and tend to worry unduly, done well it is an excellent system with limited fault currents and less risk of tingles than PME. It is of course sometimes not done well -but that is true of all systems.

There are some funnies worth looking for e.g. metal gates and bars in milking parlours that are not licked and rubbed smooth by the cows as they queue up may be an indicator of poor earthing and leakage into ground causing voltage gradients.

If the transformer is small and serves only a few properties, it is possible that the customers earth is lower Z than the one(s) at the transformer neutral and then a fault to earth may not pull up the voltage of the customers earth, but rather depress the live, and in doing so push the neutral to earth voltage well off centre. Such faults on overhead lines can cause great consternation, and is the reason that double pole isolation is required for front end ADS and safe working.

Mike

I recently visited a thatched house in The Norfolk Broads area with a large detached garage. The garage supply was from the house with P.M.E. earthing. In the garage there is a split load consumer unit. The sockets are 30mA R.C.D. protected, but the light circuits are just protected by a B6 M.C.B. The garage earthing is TT.

So, the metal fluorescent lights and outside are just protected by a B6 M.C.B. with no R.C.D. protection.

The earth rod is outside with its top buried. I had no chance to test the resistance of the rod. But the water table was about 3 to 4 feet below ground level, so the rod soil will not dry out presumably.

My concern was that a fault with a resistance may liven up the rod top and surrounding soil without tripping off a M.C.B. for an extended period of time.

Z.

Thanks Mike.

If a rod is sticking above the ground, could this be an insurance claim - assuming that there is a yellow warning notice isn’t nearby.

yet another warning notice .

Exposed earth rod tops can be covered with temporary things like large upturned flower pots, a traffic cone, old bike or deceased pet. Some people build a small wall around the earth rod top with a few decorative stones or house bricks with no mortar. 

Z.