I never quite understood why when wiring an out building it is not advised to use the main property earthing . In most cases the only option is an earth spike that is a dubious alternative?

I don't think there's a blanket "not advised".

PME Earthing does pose some challenges, which should be taken into consideration. Usual reasons for not using a PME earth might include:

• Not being able to create a reliable equipotential zone - e.g. building with an solid floor without a damp proof membrane - anything from victorian quarry tiles to concrete in an old garage - or a steel clad construction that can't be reliably bonded.
• The presence of extraneous-conductive-parts that would require main bonding and the cost of running a full sized bonding conductor out to the outbuilding is prohibitive
• Where there is a "special location or installation" involved that has a particular aversion to PME - e.g. swimming pools or EV charge points.

otherwise you're normally quite free to use extend the main building's earthing out to an outbuilding.

   - Andy.

If you had it you'd also use the outbuilding rebar, or steel piles or any other metal that was in the ground or in concrete in the ground  that was part of the building. A lone spike as such is the least you can have.

Now you can and would extend the house earth to any old outbuilding if the supply was TT or TNS, the only problem is that a great many areas are wired with a  supply earthing that is TNC-s (PME)  and I presume you live in one of them.

So with PME the system earth is really the DNO neutral - and is very good as a CPC for blowing fuses if there is a live to earth fault, but it is not guaranteed to be at the same potential as the ground outside - the "terra-firma" earth voltage if you will. For garden tools, being pretty much always double insulated we do not care, but a supply to an earthed object that can be touched is a bit of a stickier wicket. The risk varies and so does the advice about how best to organise the earthing. Not all cases are equal.

The newest panic is some one in bare feet on wet grass touching a metal car door handle while it is plugged into an EV charger.  Things like painted lamp posts are less of a hazard, and tend to be in part their own electrode. For years folk have wondered about mains supplies into caravans and boats.

As above a building with an earth floor, like a barn, or a damp concrete floor, like some garages, poses a similar hazard, but things that are free draining and hostile to bare feet (gravel perhaps)  are less of an issue.

So what is in this outbuiding, how is it constructed and how is it to be used, and it may or may not be OK on a PME system, or best moved to TT.

Mike.

If you had it you'd also use the outbuilding rebar, or steel piles or any other metal that was in the ground or in concrete in the ground  that was part of the building. A lone spike as such is the least you can have.

Now you can and would extend the house earth to any old outbuilding if the supply was TT or TNS, the only problem is that a great many areas are wired with a  supply earthing that is TNC-s (PME)  and I presume you live in one of them.

So with PME the system earth is really the DNO neutral - and is very good as a CPC for blowing fuses if there is a live to earth fault, but it is not guaranteed to be at the same potential as the ground outside - the "terra-firma" earth voltage if you will. For garden tools, being pretty much always double insulated we do not care, but a supply to an earthed object that can be touched is a bit of a stickier wicket. The risk varies and so does the advice about how best to organise the earthing. Not all cases are equal.

The newest panic is some one in bare feet on wet grass touching a metal car door handle while it is plugged into an EV charger.  Things like painted lamp posts are less of a hazard, and tend to be in part their own electrode. For years folk have wondered about mains supplies into caravans and boats.

As above a building with an earth floor, like a barn, or a damp concrete floor, like some garages, poses a similar hazard, but things that are free draining and hostile to bare feet (gravel perhaps)  are less of an issue.

So what is in this outbuiding, how is it constructed and how is it to be used, and it may or may not be OK on a PME system, or best moved to TT.

Mike.