It needs to be low enough for a live to earth fault to trip the breaker
in the maximum time allowed. If I had the figures to hand it would be
easy to work out. I would be aiming for low milliohms rather than ohms.
Sorry to interject, but that is completely incorrect.
a) You will not get an earth rod Ze to low milliohms, even if you bury
it in marconite;
b) The aim is to keep the touch voltage down to safe limits up until the
RCD (which there must be) trip current is reached.
The exact allowable earth loop impedance is a matter of debate - there
is an upper bound which is supply-voltage/RCD-tri-current but a lot of
leeway needs to be added as the ground in which the rod is buried is an
unstable and slightly unpredictable medium.
Here is a bit of reasoning:
100 Ohms is just about the limit for a stable resistance, so you really
want to get below that.
In real life, you can probably get less then 20 ohms with 2 rods either
joined together going 2 metres deep, or in separate areas.
Of course, it all depends on what your ground is like around the rods.
And if Dennis can get his down to milliohms, then he really should be
lecturing on how to do it.
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