TT and exported earth

[Knobhead]

This has been bugging me for a while, why with a TT system shouldn’t the earth be exported? Surely the more earthed points the better, whether they be in the building or a remote building.
I’m not interested in the regs, but a sound reasoning.
To my mind if a SWA is earthed only at one end and the remote installation is soundly but independently earthed there’s a possibility of a voltage gradient between the armouring and the installation it serves. If the were connected it can not occure.

 

[malcolmsanford]

In many situations in order to lower the value of Ra you can insert rods either one on top of another or inserting then in a cluster.

If you had a rod in building 1 and then 20 mtetres away had building 2 with another rod IMO opinion there is a case for making them 2 individual systems, or linking the rods to lower perhaps a high Ra.

With todays technology and the reliance of RCD as additional protection the Ra of a rod is perhaps not taken as critical, unlike when I first started, but I still like to achieve as low as possible Ra, and if that is achieved by linking the rods then I would be happy to do that.

In reality a PME system is just really a systems using several interlocked spikes in the neutral conductor................... not to dis-similar to a TT

 

[NoSparks]

I totally agree. The problem is where the TT installation is between a PME user with a fault and the source of supply. Then their fault will create a gradient across the TT installation premises. Having one earthing point for the TT system at each location means that location has one equipotential reference point.

At least that is my understanding of why we need bonding. (Ref diagam in Electrician of about August last year.)