linking earth rods

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Two outbuildings side by side with a party wall.

Each is supplied as a TT by its own sub main from the main building via 4 core SWA. Each has its own CU each with 30 mA RCD and earth rod. The CUs are fitted almost back to back on each side of the party wall. The rods are internal through the floor at the opposite ends of the party wall.

Each earth rod has an impedance of about 190 ohms which calculates as allowing a maximum of 6 volts on the CPC before the RCD will trip on 30 mA.

Question. Is there any good reason to NOT link the two CPCs by a link between consumer unit earth bars (and hence the earth rods are linked) to obtain effectively an even lower maximum fault voltage on the CPC's and also to provide a back up if one rod should for any reason fail.
 
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Hi Bernard,

A lot will depend on how close together the rods are. Under certain circumstances the impedance can actually increase when two rods are connected together. (There's lots of white papers on this with lots of formulae via Google)

Practically I would firstly connect the two rods together and test to see if the earth impedance reduces. You'll never get 190/2 but it may be less. If it does reduce you're in good shape!
 
The rods are 3 metres apart at the moment and probably are in each others "resistance area" so the effect of linking may not as you say become 190/2 ohms. I will link them and measure out of interest.

To be honest I was bit surprised to get that low impedance from rods through the concrete floor although they are less than a foot from the outside of the building so moisture will get to them.

When the garage renovation into workshop and garage is fully finished there will be a third rod that will be out of the "resistance area" of the two at present.
 
For multiple earth rods we usually us the rule of thumb that the separation distance should be at least the length of the rods.
We put a lot of earthing systems using 4 rods coupled together and connected with bare 70mm copper
 
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I fitted 100's of earth rods in the Point of Aye gas terminus with each electrode having between four and eight rods and each having to measure 8 ohms or less including earth mats with 70mm bar copper between them at that time I was just an electrician but I am sure with a job so large there were so really clever guys would had decided these were required it must have cost a lot to have two guys for 6 months doing nothing else but sink rods.

I see no problem in multi earth rods and in fact I would think there is an issue with buildings so close not having the same earth as pre-RCD one could have a fault which could raise/lower the metalwork of the two buildings to a point where for example touching the soil pipes of both buildings could produce a lethal shock.

This is after all why the DNO not us has the option of deciding the earth system as two close houses one with TN and other with TT could present a danger. Although the house with TT would trip their RCD the earth is not disconnected so house with TN could provide a lethal current to house with TT.
 
This is after all why the DNO not us has the option of deciding the earth system as two close houses one with TN and other with TT could present a danger.
As I've described before, that's the very situation I appear to have here, and the presumed explanation for why I have a Ze of around 0.3Ω with a TT installation, provided I have the main protective bonding connected (and not a lot higher if I disconnect the MPB, presumably because of all the parallel paths between my installation and the incoming water supply pipe). However, there's nothing I can do about this this, short of inserting an insulating section into my water supply pipe to isolate my house from the neighbour's TN 'earth' (via their main bonding).

Kind Regards, John.
 

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