Conversion from TT to PME

[jpadie]

Hello all

 

UKPN have been to increase the house supply to 100A and have installed a PME earth block.  The house is currently TT.  

 

Am I right in thinking that reg 411.4.2 recommends (impliedly) bonding the existing earth rod to the PME earth block?

 

I assume, if so, the earth cable size for the leg to the earth rod still needs to be 16mm2 if not in conduit?

 

Thanks

Justin 

[Nickfromwales]

I'd be very careful as the combination of the two is a not-well-recognised widow maker. Why aren't you simply getting rid of the earth rod?

[jpadie]

Because of the recommendation in 411.4.2.

 

Why is it an issue?  I think pme max PD is 18v to true earth.  Creating an additional true earth reference feels like a sensible thing and also protects against the unlikely event of a lost supply neutral doesn't it?

[Nickfromwales]

  On 06/04/2025 at 12:03, jpadie said:

Because of the recommendation in 411.4.2.

 

Why is it an issue?  I think pme max PD is 18v to true earth.  Creating an additional true earth reference feels like a sensible thing and also protects against the unlikely event of a lost supply neutral doesn't it?

Expand  

You usually have one or the other, but on a project with PME and an annex which was metal clad with PV, it was too far away to export the earth.

For that one we terminated the SWA in a PVC adaptable box and lifted the earths there and then added a rod to the annex CU. That is recognised practice, connecting two together, not, afaik.

 

There are two very different potentials at work here, so conduit a qualified electrician to advise you properly, as you say you think it’s not an issue when it is . It is not sensible at all imo, it’s dangerous.
 

 

[Mattg4321]

  On 06/04/2025 at 09:49, jpadie said:

Hello all

 

UKPN have been to increase the house supply to 100A and have installed a PME earth block.  The house is currently TT.  

 

Am I right in thinking that reg 411.4.2 recommends (impliedly) bonding the existing earth rod to the PME earth block?

 

I assume, if so, the earth cable size for the leg to the earth rod still needs to be 16mm2 if not in conduit?

 

Thanks

Justin 

Expand  

Yes, it’s recommended in 411.4.2 that a supplementary electrode is installed. It’s been strongly considered a number of times that this be made mandatory. If you have an inverter which allows island mode, then it is definitely necessary to install one. 
 

Table 54.1 shows min earth conductor sizes. It’s 16mm2 for no mechanical protection, but cable has sheath for protection against corrosion. 
 

I’d be leaving it connected to the MET assuming it’s installed to a good standard, with a stable Ra. 

[jpadie]

  On 06/04/2025 at 15:19, Nickfromwales said:

You usually have one or the other, but on a project with PME and an annex which was metal clad with PV, it was too far away to export the earth.

For that one we terminated the SWA in a PVC adaptable box and lifted the earths there and then added a rod to the annex CU. That is recognised practice, connecting two together, not, afaik.

 

There are two very different potentials at work here, so conduit a qualified electrician to advise you properly, as you say you think it’s not an issue when it is . It is not sensible at all imo, it’s dangerous.
 

 

Expand  

 

I ask again, why is it a recommendation (likely to become mandatory, if I hear correctly) in reg 411.4.2 if it is, as you say,  contraindicated?  

 

In non-fault situations this adds a local earth to the PME.  In a PEN fault situation (where it might be possible to float at phase to phase PD) there is then a path for fault current.  I'd like to understand from a physics perspective where your concern is, please.  

[jpadie]

  On 06/04/2025 at 15:34, Mattg4321 said:

Yes, it’s recommended in 411.4.2 that a supplementary electrode is installed. It’s been strongly considered a number of times that this be made mandatory. If you have an inverter which allows island mode, then it is definitely necessary to install one. 
 

Table 54.1 shows min earth conductor sizes. It’s 16mm2 for no mechanical protection, but cable has sheath for protection against corrosion. 
 

I’d be leaving it connected to the MET assuming it’s installed to a good standard, with a stable Ra. 

Expand  

 

Thanks.  Currently the earth is TT.  The new TNCS earth exists but is not connected to the CU.  Having PME solves a headache with exporting SWA but I was uncomfortable not having an earth rod or similar in the event of a PEN fault.  When I read 411.4.2 it resonated with what I'd thought was a good idea!

 

I'm happy to be schooled as to why it might not be of course 

[Mattg4321]

  On 06/04/2025 at 15:41, jpadie said:

 

Thanks.  Currently the earth is TT.  The new TNCS earth exists but is not connected to the CU.  Having PME solves a headache with exporting SWA but I was uncomfortable not having an earth rod or similar in the event of a PEN fault.  When I read 411.4.2 it resonated with what I'd thought was a good idea!

 

I'm happy to be schooled as to why it might not be of course 

Expand  

Personally I’d have the supplementary electrode connected, and I do in my own house, due to island mode. 
 

FYI for an SWA sub main/distribution circuit on TT, then the best way is a switchfuse with BS88-3 fuse and a 100mA S type RCD. Then use switched neutral RCBO’s downstream to maintain selectivity. It’s really not much worse than TN system and having a switchfuse with a main switch isolator rather than the RCD as above. It’s much better than a sub main from an MCB which I see all the time!