Is it legal to for me to change the garage consumer unit

[David Hansen]

On Mon, 23 Jan 2006 15:58:56 +0000 someone who may be Nigel Molesworth wrote this:-

Assuming one wants to comply with the Wiring Regulations, the two options are:

1) providing an earth rod, to which the garage earthing system is connected, but not that of the house or 2) extending the house earthing to the garage

Multiple earthing is not an option in this sort of situation, for the usual reasons in particular broken neutral conductors.

[John Rumm]

This is another reason why outbuildings are often made TT if at a distance from the house. Otherwise you risk having a potential difference between local "earth" and the imported one. If you can create an equipotential zone by bonding the lot together then you may make it safe - however it is not always possible (imagine a barn with a bare earth floor for example).

[Andy Wade]

It doesn't work like that when you have a very low impedance supply (the mains) and much higher resistance to ground. Earthing traditionally works by being able to pass large fault currents and blow fuses. During a fault the earth conductors ("protective conductors" in the jargon) may be raised well above the local ground potential. It's relative voltage (touch voltage) that matters and this is reduced by equipotential bonding.

Another scenario is the o/c neutral: suppose you are on a PME supply, your cooker and electric shower are switched on and then the supplier's combined neutral/earth conductor to your house goes o/c (and yes this does occasionally happen). How good a local earth system would you now need to stay safe in the absence of equipotential bonding?

Safety in the TN-C-S/PME system is _fundamentally_ dependent on the equipotential zone concept. In outdoor & 'semi-outdoor' situations (especially where Classs 1 equipment is to be used outside the equipotential zone) it makes a lot of sense to rely on your own earth (TT system, with RCDs). See uk-d.i.y archives /passim/ on supplies to sheds & outbuildings...

[Nigel Molesworth]

Thinking about it, perhaps I'm unusual. I have an earth spike outside the kitchen, the utility room, the bathrooms, the shed, the garage and the swimming pool. All in a 20m radius.