RCD Discrimination Help

[Loui Palmer]

I am not yet fully qualified and this is not a job I am going to take on so have passed it onto a qualified electrician , but out of interest I would like to understand it.

Domestic TT. CCU in house feeds (SWA) to a garage CCU (with an independent earth rod). Now someone has taken the feed from a ring main!! so looks like the earth in the SWA is not connected in the garage only in the house hence earth rod.

Both CCU's have 63A Rcd's, firstly If I remember rightly discrimination is not relevant on RCDs?

There was a fault (must be earth leakage) on a pump in the garage and the RCD in the house tripped but bypassed the one in the garage??

Obviously because this is connected to the ring it took out half the board in the house! I have heard of time delay s type RCD's, could this be used in the house?

What would you do?. I do intend to go down whilst the electrician is there and have a chat with him but also am scratching for some voluntary work with him so would be nice if I seemed a bit more clued up!!

Cheers chaps :stooge_curly:

 

[telectrix]

firstly, the 63A rating of the RCDs is irrelevant. that's just their max. operating load. the RCD on the house RFC needs to be 30mA so cannot be uprated. the SWA should have been taken from the CU, either from a non-RCD way or by splitting the tails and fitting a switch fuse. also the TT system is pointless. house earthing should be extended to cover the garage. ( bearing in mind bonding requirements, if needed ). basically, the job's a F.UP.

 

[driverman]

firstly, the 63A rating of the RCDs is irrelevant. that's just their max. operating load. the RCD on the house RFC needs to be 30mA so cannot be uprated. the SWA should have been taken from the CU, either from a non-RCD way or by splitting the tails and fitting a switch fuse. also the TT system is pointless. house earthing should be extended to cover the garage. ( bearing in mind bonding requirements, if needed ). basically, the job's a F.UP.

The way I'm reading this is.........that the whole system is TT. But yes garage should of been wired separately.
Generally though, if 2 x RCD's are wired in parallel the one nearest the mains will operate first, like the situation above. Must be a reason for that re earth parth /speed etc.

 

[Brightspark2]

You will get no discrimination between the 2 RCD's. The first one to react the quickest will go first. So it hasnt bypassed the one in the garage, the one the house was the quickest to react.

You dont put an RCD feeding an RCD unless the first one is a time delayed (Where permitted). Otherwise its just pointless as you already have your RCD protection with the first one.

Tel has described above the way it should of been designed...

 

[Loui Palmer]

Hi
Thank you all for the help and advice, I realise its an F up, apparently it was an IT guy who installed the SWA and connected it to a ring!!! the CCU is full so I guess this is why he did this!

Be interesting to see what the Sparky does to sort it out.

Thanks again chaps

 

[telectrix]

IT guys should be welded to their keyboards. surprised he didn't wire it in cat6.