Earth Resistance to garage

[neerday]

Following my previous question about the mcb rating I've nor measured the wiring resistance to the garage. What I did was to join all 3 conductors in the swa into a brass earth block in the garage and measure between them at the house end.

I get approximately 0.8 ohms between each pair (live to neutral, neutral to earth, earth to live). So the resistance of the earth wire seems to be around 0.4 ohms. Is this low enough to be safe? Thanks.

 

[roberta1000]

The short answer is yes, 0.4 ohms earth resistance would be OK. Actually it is likely to be a good deal higher than this in practice due to garage end wiring and the earth source resistance from your supply but it is still fine because you have an RCD. From a purely practical point of view any extension cable you use around the house or garden has this 'problem' in that the longer it is the higher the earth resistance and in the limiting case the highest shock voltage is around half of the supply.

But the RCD alters all that. By the way have you noted that if you only have a single RCD a fault in your garage supply will take the house out too. If you don't fancy this (for instance if you don't fancy your freezer dropping out while you're away on holiday) you could consider a separate RCD for the garage circuit.

 

[securespark]

Actually it is likely to be a good deal higher than this in practice due to garage end wiring and the earth source resistance from your supply but it is still fine because you have an RCD.

Even if the circuit has RCD/RCBO protection, the Zs (or Zdb) values should still be within spec.

The only time this rule is bent is when the supply is TT.