Andrew Gabriel wrote:

The right hand earth wire coming out of the Voltage Operated ELCB
should go to an earth rod, and nothing else.

It currently runs to an earth terminal block, from there is a connection
to the gas main, plus one other (as yet undefined) connection that runs
up to somewhere else in the building.

My plan was to measure the fault loop impeadance of what is there first
to see what the starting point is, and then add at least one proper
earth rod and carry on unitl I get a decent low enough value. Finally
add the cross bonding to the water main.


There's no point with the ELCB in the circuit -- its coil will be
of the order 100-300 ohms most likely.

Good point - I am certainly planning to ditch the ELCB anyway (space is
a bit limited in the cupboard) so that is not to much of a problem to
replace it with an RCD first.

If you add more earth rods to such a setup, they should be spaced
well away from the existing one, and all connected to the main
earthing terminal, not the existing earth rod. Same goes for cross
bonding. The right hand earth terminal must only go to an earth rod,
which should be outside the resistance area of any other earth rods

Keeping away from the existing one should be easy ;-) (since it's not
there). Keeping away from the other services is not quite as simple
since they both run down the side of the property. I also want to avoid
banging an earth rod through any drains if I can help it.

or equivalent resistance area around bonded services. (The resistance
area is the area around an earth rod where the ground changes potential
due to current flow in the earth rod.) The ELCB works by monitoring

I was thinking of placing the earth rod toward the front of the property
just beyond the last drain gully. That way I can be reasonably sure of
being clear of the drains (and other services). It also means I can run
a 10mm^2 earth wire inside the building to level with the rod and exit
through the wall right by it (about a 6m cable run it total).

The distance from the rod to the gas main (where it enters the property)
would then be approx 4m, and about 6m to the water main. The underground
pipes would however still pass within a couple of meters of the earth
rod in this situation. The other alternative would be to try and find a
route out the back of the house to the back garden - this is a little
more awkward however since there is a substantial patio area to cross.

Where do they normally site earth rods on new builds these days?

You should add an RCD nowadays, but you can leave the ELCB there
harmlessly if you lower the overall earth impedance as above, and it
provides you with a means to isolate the CU.

Looking through the TLC catalogue shows MK RCDs at twice to three times
the price of the Contactum, Hager, Volex ones etc. Any one got any
preference for brand? Are the MK ones worth the extra?

--
Cheers,

John.

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