Sam Stubbington asks about whether a TNCS supply house RCBO can properly protect a TT outbuilding installation. His question has been answered by SELECT (Theme – Distribution Equipment including Cable Management):
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Question: I have fed an outbuilding shed from a TNCS supply on a 40A 30mA RCBO. The outbuilding has it own earth stake. Currently, there are just MCBs in the outbuilding mains board.
Is the RCBO in the house sufficient for protection?
Answer: The RCBO in the main building will provide fault protection for sub-main cable.
The earthing arrangements of the two systems will need to be separated - e.g. assuming the sub-main to be a SWA cable, the earthed armouring will need to be insulated from the earthing arrangements of the TT supply.
If there are any circuits in the outbuilding that do require additional protection by an RCD, then those circuits will require suitably rated separate RCD/RCBO protection.
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