Metal CU in garage T.T system

[rattlehead85]

Would like a bit of advice on how others have tackled this.
Got a situation where we have a TN-C-S supply to a house with a 6mm 2core swa feeding a detached garage using a T.T. Earthing arrangement with a service water pipe within the garage which has a 10mm bond to it. My question is the client has provided a new Metal clad Rcd 30mA front line board to install in the garage, what would be the best and most effective way of separating the earthed armour of the swa feeding the garage from the earthing system employed to the garage itself T.T. [emoji106]

 

[happysteve]

Would like a bit of advice on how others have tackled this.
Got a situation where we have a TN-C-S supply to a house with a 6mm 2core swa feeding a detached garage using a T.T. Earthing arrangement with a service water pipe within the garage which has a 10mm bond to it. My question is the client has provided a new Metal clad Rcd 30mA front line board to install in the garage, what would be the best and most effective way of separating the earthed armour of the swa feeding the garage from the earthing system employed to the garage itself T.T. [emoji106]

Sorry if I'm being dim here (only a trainee), but wouldn't a PVC adaptable box solve this problem? Gland SWA to adaptable box to separate the two earthing arrangements, put the adaptable box just below your metal DB. Either butt the two up to each other (and use 20mm PVC grommets to prevent cable damage) for the tails, or a conduit coupler and a couple of bushes for a slightly neater job.

Or have I missed summat?

 

[spark 68]

Sorry if I'm being dim here (only a trainee), but wouldn't a PVC adaptable box solve this problem? Gland SWA to adaptable box to separate the two earthing arrangements, put the adaptable box just below your metal DB. Either butt the two up to each other (and use 20mm PVC grommets to prevent cable damage) for the tails, or a conduit coupler and a couple of bushes for a slightly neater job.

Or have I missed summat?

You could do that, or an insulated gland plate might be possible, but you have to be sure you cannot simultaneously touch the exposed conductive parts of two different earthing systems, as a potential difference could occur say from a PME earthed metal gland and the TT earthed metal CU/DB.

Another problem is that the TT earth fault loop impedance may be too high to reliably disconnect the upstream OCPD prior to the metal clad board, leaving a possibility during a fault of the metal CU becoming live as well as the TT CPCs, there are ways around this such as insulated (effectively class 2 cableways for the main tails), an upfront 100mA (or whatever is suitable for the particular RA) RCD to protect the tails prior to the CU, careful insulated glanding etc.

 

[davesparks]

Plastic stuffing gland and heatshrink over where the armour is stripped.

 

[happysteve]

You could do that, or an insulated gland plate might be possible, but you have to be sure you cannot simultaneously touch the exposed conductive parts of two different earthing systems, as a potential difference could occur say from a PME earthed metal gland and the TT earthed metal CU/DB.

Another problem is that the TT earth fault loop impedance may be too high to reliably disconnect the upstream OCPD prior to the metal clad board, leaving a possibility during a fault of the metal CU becoming live as well as the TT CPCs, there are ways around this such as insulated (effectively class 2 cableways for the main tails), an upfront 100mA (or whatever is suitable for the particular RA) RCD to protect the tails prior to the CU, careful insulated glanding etc.

Thank you Knew I'd missed summat! Cheers.