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 always find it amusing when the use of the concrete floor rebar as bonding/ earthing is mentioned. 
The chances of getting on site during the installation of the concrete floor in my experience is remote at best. Often the electricians won’t know that the job exists at this stage. Builders can’t even stop boarding over cables let alone involve us at the initial stages. 
Then there is the questions of the type of rebar used, welded or tied with bits of ‘bendy’ metal, what is the resistance of the rebar is it a reliable earthed structure capable of accurate results?
Using the structural steelwork is a better option, often with very low resistance and often exposed when the electricians arrive on site. 

This is something that is mentioned in EV charging, with the same questions, quality and availability of access to the rebar, I think may apply. 

It may ease your worry to realise that the rebar does not need to be welded to get a decently lo electrode effect unless you connect to a very short length. - the connections between the bars via the bulk of the concrete is enough, and the fairly scratchy connections where the bars cross, - so the twist and tie rebar is still worth connecting to.  It is  worth a play with a meter if you get the chance.

Note however that a concrete encased electrode is not the best idea for lightning protection - a really high current, far higher than a few hundred volts of mains could ever push through,  tends to crack the concrete.

It is a good idea,  and required in some countries regs, but not ours, to make two connections to the buried metal, so that  you can do a ring-round continuity test with an ohm meter between the two wires and have confidence that it has not rotted off or snapped below ground.

Mike

I always find it amusing when the use of the concrete floor rebar as bonding/ earthing is mentioned. 
The chances of getting on site during the installation of the concrete floor in my experience is remote at best. Often the electricians won’t know that the job exists at this stage. Builders can’t even stop boarding over cables let alone involve us at the initial stages. 

I worked on a particular job where they forgot to construct an earth mat for an HV substation during construction. It's unusual, but illustrates the point with a little humour. There was still a requirement for HV earthing, though, which had to be addressed ...

What I would say is that other countries do have a requirement for a foundation earth electrode without any problem. Once it becomes a requirement that's known and understood, there would be no real issue.

Yes the idea of multiple connections would seem to be a valuable one. But as I said putting this into practise on any site other than a prestigious one, or perhaps one governed by an architect seems unlikely in my experience. 
builders tend to ‘crack on’ and let electricians worry about their job. 

Point taken, upgrading a HV mat after concrete is laid is a nightmare, did you get part of it taken up to expose the edges of the rebar to get onto it? 
We had a similar story with a concrete access ramp and a damaged under floor heating cable. 
On your other point with this type of install common abroad, let’s hope that we can go forward with this. 

It will only become common in UK general building if the building control approved docs get updated to say 'leave two ends of Rebar poking up in diagonally opposite corners for the electricians.  Dress the damp course round this to the satisfaction of the LA inspector'

About 6 months after that everyone will do it if it  is needed or not.

M.

SL1 said:

Point taken, upgrading a HV mat after concrete is laid is a nightmare, did you get part of it taken up to expose the edges of the rebar to get onto it? 
We had a similar story with a concrete access ramp and a damaged under floor heating cable. 
On your other point with this type of install common abroad, let’s hope that we can go forward with this. 

A trench was dug around the substation building, with two radial trenches, and bear copper bonding ring conductor with radials was installed as the earth electrode system. If memory serves, this was one option provided for foundation earth electrodes in the DPC guidance for BS 7671.

Sounds like that might work. 
Or perhaps an earth pit in the diagonal corners over those sticking up rebar ends, as you say, like we install on Lightning Protection installs? We would be able to test it then and service/ inspect it  

Sounds like that might work. 
Or perhaps an earth pit in the diagonal corners over those sticking up rebar ends, as you say, like we install on Lightning Protection installs? We would be able to test it then and service/ inspect it