Distance between other electrical systems

I've been asked this time and time again and it could be a case that I've looked at this so much that it's made me go insane, but I'd like a 2nd opinion.


When installing a TT feeder pillar on the street, does the 2.5m rule come into effect when it's located near to a PME system or does this need to be 2.5m away from all other systems which are not connected to the same earthing system (not connected to the earth mat/rod under the feeder pillar?  

Yes, this is the case. See Reg 411.3.1.1 "Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively."


You can't simply bond separate installations together either. Regulation 542.1.3.3 puts a stop to that if you consider you've also got to account for HV faults and impulses. In addition, DNOs may not like it, if there is a risk of you transferring fault voltages or currents from their earthing systems (again, the behavior of HV faults also to be considered).


In the case of certain Part 7 "special locations", (e.g. Sections 708 caravan pitch supplies, Section 717 supplies for mobile and transportable units, section 722 supplies for EV charging installation), where the original supply is PME, you're often providing a TT earthing system for safety, to cover the case of broken PEN conductors - and you wouldn't want to bond in any case.


Further, it's not the above-ground separation that's important, but also the separation below ground. And this might have to be greater.

For example, voltage from earth potential rise on the PME system due to a broken PEN conductor can be transferred via buried metalwork connected to the PME earthing system (e.g. structural elements, metal pipes, etc.) through the ground, to the TT earth electrode.

What separation distances are we talking about? Figure 16 of BS 7430 provides an illustration - this is presented in GN7 as Figure 7.3.

• For caravan sites, GN 7 (section 7.5.6) recommends 10 m separation of the TT earth electrode from buried metalwork, electrodes etc., connected to the PME system earth.

• Some DNOs require earth electrode separation of at least 3.5 m for EV charging equipment installations - others require "at least 2 m where practicable" or similar - of course, the designer of the EV charging equipment installation will have to decide whether this separation returns a hand-to-foot touch voltage, if it is likely someone will be standing above buried metalwork connected to the PME earthing system, whilst simultaneously touching the vehicle, when the earth potential on the PME system rises due to a broken PEN conductor.

I've been asked this time and time again and it could be a case that I've looked at this so much that it's made me go insane, but I'd like a 2nd opinion.


When installing a TT feeder pillar on the street, does the 2.5m rule come into effect when it's located near to a PME system or does this need to be 2.5m away from all other systems which are not connected to the same earthing system (not connected to the earth mat/rod under the feeder pillar?  

Yes, this is the case. See Reg 411.3.1.1 "Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively."


You can't simply bond separate installations together either. Regulation 542.1.3.3 puts a stop to that if you consider you've also got to account for HV faults and impulses. In addition, DNOs may not like it, if there is a risk of you transferring fault voltages or currents from their earthing systems (again, the behavior of HV faults also to be considered).


In the case of certain Part 7 "special locations", (e.g. Sections 708 caravan pitch supplies, Section 717 supplies for mobile and transportable units, section 722 supplies for EV charging installation), where the original supply is PME, you're often providing a TT earthing system for safety, to cover the case of broken PEN conductors - and you wouldn't want to bond in any case.


Further, it's not the above-ground separation that's important, but also the separation below ground. And this might have to be greater.

For example, voltage from earth potential rise on the PME system due to a broken PEN conductor can be transferred via buried metalwork connected to the PME earthing system (e.g. structural elements, metal pipes, etc.) through the ground, to the TT earth electrode.

What separation distances are we talking about? Figure 16 of BS 7430 provides an illustration - this is presented in GN7 as Figure 7.3.

• For caravan sites, GN 7 (section 7.5.6) recommends 10 m separation of the TT earth electrode from buried metalwork, electrodes etc., connected to the PME system earth.

• Some DNOs require earth electrode separation of at least 3.5 m for EV charging equipment installations - others require "at least 2 m where practicable" or similar - of course, the designer of the EV charging equipment installation will have to decide whether this separation returns a hand-to-foot touch voltage, if it is likely someone will be standing above buried metalwork connected to the PME earthing system, whilst simultaneously touching the vehicle, when the earth potential on the PME system rises due to a broken PEN conductor.