Farm Installation help

[Welshsparky]

Hi all

Went to look at a job today on a farm , 3 phase board with 75mm supply on a 100 amp 100 ma rcd mainswitch, with a submains out to the house , which is also protected by another 100 ma rcd, problem is every time theirs a trip in the house it trips at the main distribution board also which is situated in a barn, What would be the best way to rectify this? would it be possible to swop the domestic consumer unit for 30ma dual rcd, and change the mainswitch at the incomer for a 100 ma time delay?

 

[Rockingit]

One upstream RCD to protect an entire estate is never a good idea. You need to think about some proper cascading discrimination which will no doubt make use of time delay RCD's at some point.

 

[johnboy6083]

Agree with above. The submains don't have to be fed by 100mA S types, thru could be fed by 30mA s types, as it's the time delay that provides the discrimination.

 

[ringer]

Are you saying that you can not guarantee discrimination with a 100mA non-time delay RCD at main supply feeding a sub board with a 30mA non-time delay RCD? In other words, its is only time delay that can provide adequate discrimination, and not 100mA vs 30mA RCDs?

 

[Dan the spark]

Are you saying that you can not guarantee discrimination with a 100mA non-time delay RCD at main supply feeding a sub board with a 30mA non-time delay RCD? In other words, its is only time delay that can provide adequate discrimination, and not 100mA vs 30mA RCDs?

Thats right, it's pot luck which one trips first if you don't have a time delay.

Welshsparky, I'm guessing that it's a TT supply which is why you have a main switch 100a/100mA?

 

[Welshsparky]

Yes its a TT, i think ill have to go for the time delay RCD mainswitch, problem being customer has had a celler conversion, If the pumps are knocked off for more than 20 minutes it could potentially flood the cellar argh!

 

[Dan the spark]

Is that a bs en 60947-2 with rcd incorporated to be able to take the 75mm supply? If so those are pretty expensive

 

[Engineer54]

Well to my knowledge there isn't any such 75mm cable, so i'm taking this as a typo error!! My guess for a 3 x 100A supply is 3 x 25mm conductors!!! lolAs

been previously suggested, this installation needs a 300mA S type at the origin, that will discriminate with any 100mA RCDs downstream. I would also suggest that being a TT system, any CU should be all 30mA RCBO protected rather than split or single RCD protected CU's....