Physical installation of earth electrode

During fault the to of the rod and the ground around it become live relative to the rest of the ground further away and there is a touch and step shock hazard if the area is likely to be accessible

To that end insulating the top of the rod is a good idea, so that the full mains voltage only starts below ground, and by the time it reaches the surface the available shock current is limited by the resistance of some inches of earth and maybe some  free draining gravel or paving as well.

But joints below ground corrode and need to be accessible for inspection. A wire and clamp up in the air can be seen if it is failing but equally it can be hit by a lawnmower and forgotten in long grass...

Personally I think the access pit method is best, but it is dearer.

The nice way to verify fully buried electrodes is to make two connections in not quite the same place so that you can ring round with a meter on the low ohms range and verify that clamps have not rotted off, but as it uses twice as much material and is not mandated in the UK, it tends not to be done.

Mike.

Thanks Mike. I consider that the pit would have an inspection cover to gain access to the connection. I believe that animals are at risk from a voltage difference if standing on the ground across area up to 2.5 meters.

The pit method is not in the regs ( though it may be in the British Standards)

surprised that the regs and on-site guide don’t even cover protection needed.

All depends on location and use of the rod. Most domestic rods will just have the small plastic enclosure around the top just sticking out enough to avoid damage etc but can easily be located for testing complete with the required label. Ideally a proper recessed pit is the best option depending on budget. GN8 has more info but basically you need to protect the earth connection against damage and corrosion whilst being able to safely test.

Thanks Colin,

I would have expected some rules like the enclosure should only be opened by a tool.

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.

trials have shown that voltage at the surface moves from 240 volts to 25 volts at around 2.5 meters from the rod .

clearly an animal with front legs within 2.5 meters of the rod and back legs over this, then the diff is 25v which could be fatal.

cats and dogs and young children might be at risk. 

the safe distance relates to the step voltage during fault, and is particularly a problem for longer baseline animals like cows and horses,where the front feet may be near the rod at more than 200V and the back feet may be  further away on a bit of ground only a few volts above the voltage of the plate at the end of the universe. For barefoot humans the step distance is taken to be 1m, and then calculate how far away you have to go to get to two voltage rings 50V apart at that separation. Chickens have small dry feet close together and do not straddle significant voltage drops.

(imagine the voltage on the surface during faults to be more or less the shape of a fried egg, with a 'yolk' of the ground near the electrode more or less at electrode voltage, and then current spreading and getting thinner so less steep voltage drops with distance, falling to non at all far away, but with odd ripples in ir depending on the varying composition of the ground beneath.)

As a quick rule of thumb more than half the voltage has gone from around a rod electrode by a rod length away. Only true for sensible sized rods in sensibly uniform soil free of buried metal, conductive salt deposits etc.

If need be, fencing off an area around the electrode, or something like gravel hostile to bare feet and a poor electrical conductor as a surface finish  can help reduce the hazards.

M,

surprised that the regs and on-site guide don’t even cover protection needed.

Fair comment. I guess the problem is that earth electrodes can take many and varied forms - a huge amount depends on the ground and the surrounding environment. In the UK the old school approach was simply to bang in a rod leaving a few inches exposed above ground and have an exposed clamp onto that. In some situations that may well still be perfectly acceptable - say it's in a quiet corner unlikely to be damaged or have 'long-wheelbase' 4-footed livestock about. In other situation clamping onto existing structural steelwork offers a better solution, or where soil conditions are poor longer buried tapes or grids (or foundation electrodes) might be needed. Not to mention the need to avoid other buried services. Soil conditions can easily change numbers considerably - e.g. in some areas 2.5m separation might be adequate, in other situations the specification is often closer to 8m. In some parts of the world standard practice is to insulate the upper portion of the rod to reduce step potentials at the surface (but they tend to use 2.4m rods minimum and have soil conditions that allow that). There are so many variables and challenges the designer already has to deal with that it's impossible for the regs to be prescriptive and sill be applicable to a wide range of situations.  Once the designer has sorted out the bigger problems, small details like protecting the connection tend to come out in the wash.

I think there is some guidance in GN 8 and of course BS 7430 (to which BS 7671 refers) goes into a lot of detail.

   - Andy.

The answer is not in BS 7671, but in the standard that BS 7671 refers to for this ... BS 7430.

If you are looking at driven rods, there are a couple of things to consider, which would be usual good practice:

(a) The parts of the earth electrode that are not buried 600 mm below ground are ineffective in freezing conditions
(b) Terminations should ideally be 250 mm below ground, in a suitable inspection pit (the pit itself being approx 500 mm deep).
(c) All terminations should be suitably protected against corrosion - suitable compounds are readily available for this.

Reasons not to leave rods protruding above ground (even if there is a suitable length at a depth of 600 mm and below) are:
(a) Rods protruding from the ground where people move around are seen as a health & safety hazard, with risk of impaling, and this must also be considered under CDM.
(b) Terminations above ground outdoors are more susceptible to contamination and damage.


There are, however, other alternative earth electrode types, which are potentially easier to install, and help avoid risks of striking anything buried in the ground (including underground services), which may well offer an overall cheaper approach when all of the time, trouble and H&S risks are considered. For example, a suitable earth mat, say 600 mm x 600 mm, buried at a depth of 600 mm or more, with terminations in a suitable inspection pit.

Thanks gkenton,

Really great info.

I am aware of the other “earth rod alternatives” .l and the list exists in the regs.

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.

i know I wouldn’t want my granddaughters to be rear a rod not buried to sufficient depth!

Thanks Andy very useful - I still believe there is a need for something in the regs and on- site guide.

Thanks for this .