Separating PME & TT earthing for outbuildings

[marcuswareham]

It's (theoretically) possible that by applying the calculations for minimum supplementary bonding csa that if it comes back in small enough (min 4mm) that your SWA armoured might be up to the job, in which case just extend the PME. Have a read of GN8.

The minimum size for a bonding conductors with a PME supply is 10mm

see Reg 544.1.1 & table 54.8

I believe this is todo with high current under PEN fault conditions and the potential for diverted neutral currents

 

[Msitekkie]

It is up to the designer to assess the risks associated with a broken PEN conductor, and then put mitigation in place to reduce the risk, if the risks are unacceptable.

One way of mitigating against a broken PEN conductor risk is by making the outbuilding TT, although this is not as simple as a fix-all solution sometimes using TT can increase the risk and careful consideration should be taken into account when setting up TT systems very close to TN systems

Yes this is the scenario I am working with.

 

[marcuswareham]

There is no supplementary bonding requirement. This is mainly about mitigating the risks of a broken PEN conductor.

Although you say currently you have an earth rod connected to the PME earth at the workshop, this earth rod could be counted as an extraneous conductive part, if you were to leave it this way with that earth there would need to be a minimum of a 10mm conductor connecting that earth rod to the PME MET

This is also the case for any other extraneous conductive part in either of the buildings, like water pipes etc

 

[Msitekkie]

The minimum size for a bonding conductors with a PME supply is 10mm

see Reg 544.1.1 & table 54.8

I believe this is todo with high current under PEN fault conditions and the potential for diverted neutral currents

There is no bonding requirement. No water pipes or anything like that in the outbuildings.

 

[Rockingit]

The minimum size for a bonding conductors with a PME supply is 10mm

see Reg 544.1.1 & table 54.8

I believe this is todo with high current under PEN fault conditions and the potential for diverted neutral currents

But are we discussing main bonding or supplementary??

 

[Rockingit]

There is no bonding requirement. No water pipes or anything like that in the outbuildings.

But there ARE extraneous conductive parts???

 

[Msitekkie]

Although you say currently you have an earth rod connected to the PME earth at the workshop, this earth rod could be counted as an extraneous conductive part, if you were to leave it this way with that earth there would need to be a minimum of a 10mm conductor connecting that earth rod to the PME MET

This is also the case for any other extraneous conductive part in either of the buildings, like water pipes etc

It's (theoretically) possible that by applying the calculations for minimum supplementary bonding csa that if it comes back in small enough (min 4mm) that your SWA armoured might be up to the job, in which case just extend the PME. Have a read of GN8.

Sorry I don't understand how that would protect against a broken PEN conductor?

 

[marcuswareham]

There is no bonding requirement. No water pipes or anything like that in the outbuildings.

That make TT easier then, H

But are we discussing main bonding or supplementary??

Wouldn't it be main bonding

 

[Msitekkie]

But there ARE extraneous conductive parts???

Exposed conductive parts yes, extraneous conductive parts no, unless you include the earth rod itself in that category.

 

[marcuswareham]

Exposed conductive parts yes, extraneous conductive parts no, unless you include the earth rod itself in that category.

In its current configuration, the earth rod is classed as an extraneous conductive part as currently the PME earth is in use at the workshop.

However you want to isolate the PME earth and change to TT, in that case the earth rod will become the earth electrode for the workshop TT system.