Good Morning everyone.
Quick question regarding a sub main from a TNCS supply feeding an outbuilding on TT.
This is a domestic installation not agricultural.
TNCS supply to house feeds High Int. CU from which 4mmsq 2 core SWA feeds an non attached rear building.
THE SWA of the sub main is glanded and connected to the TNCS earth at the front end and R1 + R2 measured between Line and SWA at the extremity is 0.31Ohms. (measured before final termination).
At the outbuilding end of the sub main the armour has been stripped flush to outer sheath, taped and then glanded through a nylon stuffing gland via a WISKA box which is itself nylon glanded hard up to the metal 2 way garage type CU which is TT earthed. In effect the two earthing systems are completely separated by 100mm or so of nylon enclosure.
Within the 2 way CU the live conductors of the sub main are terminated in the 40A RCCB main switch.
My question regards front end protection of the sub main. My plan was, in fact I have, fed it via a 20A mcb from a non rcd protected way of the upstream CU. It being earthed SWA it doesn't require RCD protection and 30mA protection of the finals is via the RCCB at the 2 way unit.
Max disconnection time for TN distribution ccts. is 5 seconds but the EFLI of this cct. is eaily low enough to cause instant activation of the 20A mcb.
So all good except.... the last 8 inches or so of the sub main after the armour is stripped where the two earthing systems are separated and inside the 2 way CU.
An earth fault here would have a never ending duration. I have measured Ra at 48.5 Ohms so such a fault would be less than 5 amps which the 20A mcb would supply forever.
Now I don't envisage that such a fault would ever occur... But, my question is - Does this present arrangement comply with regs in terms of disconnection of the Dist. Cct.?
In effect the issue is exactly the same as when supply/meter tails enter a post 17th ed. 3rd. ammend. CU on almost any TT install.
Thanks in advance for your considerations.
Quick question regarding a sub main from a TNCS supply feeding an outbuilding on TT.
This is a domestic installation not agricultural.
TNCS supply to house feeds High Int. CU from which 4mmsq 2 core SWA feeds an non attached rear building.
THE SWA of the sub main is glanded and connected to the TNCS earth at the front end and R1 + R2 measured between Line and SWA at the extremity is 0.31Ohms. (measured before final termination).
At the outbuilding end of the sub main the armour has been stripped flush to outer sheath, taped and then glanded through a nylon stuffing gland via a WISKA box which is itself nylon glanded hard up to the metal 2 way garage type CU which is TT earthed. In effect the two earthing systems are completely separated by 100mm or so of nylon enclosure.
Within the 2 way CU the live conductors of the sub main are terminated in the 40A RCCB main switch.
My question regards front end protection of the sub main. My plan was, in fact I have, fed it via a 20A mcb from a non rcd protected way of the upstream CU. It being earthed SWA it doesn't require RCD protection and 30mA protection of the finals is via the RCCB at the 2 way unit.
Max disconnection time for TN distribution ccts. is 5 seconds but the EFLI of this cct. is eaily low enough to cause instant activation of the 20A mcb.
So all good except.... the last 8 inches or so of the sub main after the armour is stripped where the two earthing systems are separated and inside the 2 way CU.
An earth fault here would have a never ending duration. I have measured Ra at 48.5 Ohms so such a fault would be less than 5 amps which the 20A mcb would supply forever.
Now I don't envisage that such a fault would ever occur... But, my question is - Does this present arrangement comply with regs in terms of disconnection of the Dist. Cct.?
In effect the issue is exactly the same as when supply/meter tails enter a post 17th ed. 3rd. ammend. CU on almost any TT install.
Thanks in advance for your considerations.