RCD discrimination

[malcolmsanford]

Take a seperate garage/shed/workshop installation. and say it is 15 metres from the house. If you run a sub main from the CU with say your RCBO and it's a 32amp 30mA kind. Then at the outbuilding you put a little 2 way board with say a 30mA double pole RCD and then a couple of MCBs for your lights and sockets.

If you have a fault in the outbuilding the chances are it will trip the RCBO in the main CU and your RCD in the outhouse. This is because the RCBO and the RCD both give you 30mA on the earth fault protection, the RCD will though not give you overload protection as the RCBO, which is why when we use RCD protection we fit MCBs

Now say you decide right to give me discrimination I'll fit a 100mA RCBO in the main CU and then a 30mA RCD in the local outbuilding board as that will give me discrimination. No that is not right if you go to appendix 3 pg 243 in the BRB you will see a normal 10mA, 30mA, 100mA, 300mA and the daddy 500mA will all trip in 40mS at 5x I delta N what is the difference is the mA that is needed to trip it in that time so 30mA is 150mA and and 100mA is 500ma. So under fault conditions the amps in a circuit the fault current can rise to many 100's of amps, so within milliseconds it will rise well over the 500mA it takes to trip the 100mA and so will take both RCDs out.

An S type works differently as you can see in appendix 3. A 100mA RCD though still will take 500mA to trip it at I delta N, but the times have altered to 40mS min max 150mS, so a manufacterer will increase the time to perhaps 90mS, so in this case the 30mA will trip before the S type will. That is your discrimination

 

[pushrod]

Ok thanks pushrod that clears things up a bit for me. But leaves me wondering what is the correct device to put in if your adding a separate C/U.

Can you just put a 30mA rcd in the second board if I you put the cable supplying the sub main on an RCBO?

Sorry but I have seen different things done and heard different things I just would like to understand what is the correct way.

No - well certainly not the best thing to do as you have effectively 2 RCDs in series.

Best to supply the submain from a non rcd protected way in the main CU using SWA (it does not need rcd protection) then just have an rcd at your submain.

or you could(if the submain is say at the bottom of the garden) separate the earths of the 2 consumer units and use an RCD at the shed but earth it via an earth electrode (TT system). Then the supply cable is being protected by an RCD at the original CU and the submain is being protected by its own RCD. As the 2 systems are now on separate earthing systems only one RCD can trip


edit : another option, you could supply the submain from an rcbo at the main CU and have no rcd at the submain. The drawback is that in the event of a fault, or when testing, you have to keep going back to the main CU to reset it.

 

[SparkusMaximus]

Very helpfull there thanks I appreciate that.