TT systems and Amendment 3

This Q & A is one of thousands posted in our Technical Expertise area, and answered by our Voltimum Experts.

Question: 

Regarding fitting an upstream 100mA RCD to a TT system using a metal consumer unit.

I understand that the RCD would also have to be in a metal enclosure,therefore would it be safer not to earth the RCD enclosure as a meter tail earth fault would energise all of the metalwork on the premises whereas this would contain the earth fault solely in the enclosure?

I feel that the fire risk is far outweighed by the greater risk of fatal electric shock.

Answer:

Gary Parker

Hi there

In many cases there is no need to provide an upstream 100 mA RCD to TT systems, it may be worth reviewing the design to see if there is a better way of doing this (if it is an actual installation you are talking about rather than a hypothetical situation).

But yes, you are right that in a dwelling the enclosure would have to be metal.  In this case, however, if you provide adequate mechanical protection of the cables the risk of damaging them on the enclosure is minimal i.e. you would not be likely to cause a fault.

Therefore earthing the enclosure is fine.

Hope this helps