"Christian McArdle" wrote in message
. net...
Out of interest, maybe you could enlighten me on what a failed earth
loop impedence test would signify and what dangers does it put me at
risk from?

When you have an earth to live fault, in the worst case, the current has
to
flow from your incoming supply, through the consumer unit, along the
circuit
cables to the furthest point. It then has to flow back down the earth
conductor back to your incoming supply. The incoming supply will also have
some resistance along its live and earth connection.

The earth loop impedence is the total of all the resistances of the path
that the current has to flow down.

Now, a socket circuit must disconnect rapidly in the event of a fault.
Using
MCBs, to trip quickly enough, you must be able to guarantee that the
current
flowing will be 5 times the circuit rating. Therefore, a 32A circuit needs
160A to reliably flow. That requires a total earth loop impedence of
230/160
= 1.44 ohms, some of which will already come from the supply.

Now, the old cables you have found, I believe, had a smaller earth
conductor
than modern cables. The horrendously large ring, including your entire
house
has more of it, too. It is quite possible that the resistance of the cable
will be too much, either as is, or with your proposed extension.

The danger of not fixing this potential problem is that in the event of an
earth fault, it could take tens of seconds to disconnect the power, which
is
a fire and electrocution risk.

In reality, such circuits are normally RCD protected, so provided the RCD
is
present and working, then the danger is not real, although it is not
permitted to design circuits that rely on this, with the exception of TT
earthed (earth rod) systems, where it is simply not possible to comply.

Christian.

Thanks, Christian- that's probably the clearest explanation of a fairly
complicated (to a layman) subject I've ever seen.
If only my teachers at school could have been like you.........