100mA RCD location TT??

So I believe a 100mA time delay RCD is required for a TT system if a split load board but not an RCBO board, why the difference?? Also any regs reference please?

I have been told that I can just swap out the isolator switch in the board for an RCD however I read that the RCD should be upstream in a plastic enclosure?

Any input appriciated Slight smile

Metal enclosure BTW...

Parents
  • It depends. It may be that an upfront time-delay rcd is needed anyway. If you look at the standard, the rcd element of an rcbo must trip at 30mA (assuming a 30mA device), so its worst case leakage is 30mA. In theory, the worst case total leakage of 10 rcbos is 300mA. Is your earth rod good enough for this? Something to think about, especially with the larger rcbo boards. 

    Regards,

    Alan. 

Reply
  • It depends. It may be that an upfront time-delay rcd is needed anyway. If you look at the standard, the rcd element of an rcbo must trip at 30mA (assuming a 30mA device), so its worst case leakage is 30mA. In theory, the worst case total leakage of 10 rcbos is 300mA. Is your earth rod good enough for this? Something to think about, especially with the larger rcbo boards. 

    Regards,

    Alan. 

Children
  • Thanks Alan,

    So its a metal enclosure split load board with 2x 30mA RCDs, I have a Ze of 23.8 ohms and pefc of 10A. The board has a 2 pole isolator incomer with no RCBO upstream.

  • so, look at the 2 RCD board and run exactly the same questions about insulated terminals and pre RCD wiring, and ask could a CPC or a non insulated  live credibly come loose in a dangerous way .

    What you want to avoid is a single fault to danger condition where one loose screw or wire off-cut could be fatal.

    It may be just as OK as soem designs of all-RCBO board, or not, depending on the  design.

    Despite what you seem to think, there is no reg that says that a dual RCD board can never be OK on TT, nor is there one that says RCBO boards never need an up-front RCD - some might and others might not depending on the internal details.

    Mike.