Earth loop impedence

16 Jun 2008
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United Kingdom
I have added a summerhouse to the existing supply, which is a TT system.

The earth loop impedance is too high at over 11.5 Ohms. The Earth rod impedance tests at 11 ohms.

I think I can get Scottish power to prove a TNS connection but at a minimum of £200.

Would it be worth trying to get a better TT earth (new rod etc) or just pay up.

I haven't confirmed the possibility of a TNS connection yet, and there is a complication. We are on a single overhead supply that terminates at my house but then feeds 5 or 6 other neighbours, will they all have to convert to TNS.
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What's wrong with a TT with a Ze of 11 ohms?
I take it all circuits are RCD protected?
There is a 30ma RCD in the main CU and another 30ma RCD in the summerhouse CU.

The summerhouse is fed from a 25A MCB, what is the maximum permissable Zs
1666 ohms when protected by a 30mA RCD - however if one RCD supplies another of the same size & type they won't discriminate.
In reality if the reading was over 200 ohms then it would be rejected, also if more different than expected to the Ze.
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25A MCBs don't exist.

What size is yours?

The maximum permissible Ze for a TT system is 200Ω to ensure reliability of connection (although anything upto 1666Ω would actually work)

What are you using to measure Ze?
25A MCBs do exist, just not very common ;)
Put 2 in at the theatre for the lighting circuits sub mains with 6mm cable.
my mistake, its a 20A MCB.

I used a ROBIN digital multi function tester KTS 1620.
Did you test the rod in isolation (i.e. with all the main protective bonding etc disconnected)?
Yep, as has been said the max Zs is technically 1666 ohms as ADS (fault protection) is being provided by a 30mA RCD, however a value not exceeding 200 ohms should only be considered for reliability.
As pointed out I did test with the supplementary earthing connected.

On re-test the Ze came out at 36 ohm.
As to 25A MCB Try here and just see how many.

But as to ELI and TT supply outside the consumers premises it is only required to be 21 ohms so even with a full earth nest with a reading of less than 1 ohm to earth using an earth rod testing kit the earth measured with an earth loop impedance meter may still be 21 ohms.

Normally we will look at around 40 ohms but up to 200 is allowed as already stated over 200 is considered as unstable.

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