What size RCD for farm submains?

[electrospark]

Hello,
I have a friend who works on a farm and has had some work done on adding/changing the metering to various buildings. The supplier has requested RCD's for the 3 supplies.
There is a 3 phase 100a head feeding 3 SWA's, which have the meter and isolator in line before the swa.
I have been asked to put a RCD in the line to protect the swa's etc. My question is: There is 30mA RCD on the outbuildings, so to protect the SWA's and discriminate between the RCD on the local DB and the RCD on the swa it should be higher... Yes?
Thanks

 

[RF Lighting]

For what reason is this new RCD required?

 

[ericmark]

There are 5 sizes of RCD in common use.
500 ma only used on construction sites see 704.411.3.2.1
300 ma for fire protection.
100 ma = 1/3 of 300 ma used for discrimination after main 300 ma.
30 ma for personnel protection.
10 ma = 1/3 of 30 ma used for discrimination after main 30 ma.
There is also S type which is delayed and also for discrimination.

So first question has to be what is the RCD there to protect? As well as fire and personnel protection there is also where a TT supply is used. In this case it would depend on reading taken on earth rod but cost wise common sizes are likely cheaper than specials. You can get variable types with up to 1A tripping current and 1 minute delay often fitted as standard on generators but I would be testing to see if a 300 ma would fit the bill then both fire and protection where a TT supply is used are both covered with a standard size (freely available RCD.

So why have they asked for a RCD and if because it is a TT supply what is the earth loop impedance?

This is not really DIY stuff.

 

[electrospark]

I don't know why they have asked for it to be honest as i'm not dealing with them personally. I will have to find out.
Its not a TT supply its a TNCS from what I have been told. Its only a farm out in Wembly so there is still an under ground supply there (not over head)
Like I said, all there is, 1 x 3 phase supply which feeds 3 seperate single phase meters and then 3 x fused isolators in which 3 x swa's are connected which feed 3 seperate buildings, such as farm house, barn etc.
I know that the DB which controls each building should be RCD protected but do these supply cables (that run under ground) need an RCD or is the supplier talking rubbish?
Thanks again

 

[flameport]

Underground SWA from a TNCS supply would not require an RCD.

An underground supply could easily be TT, you need to confirm exactly what the arrangements are.
Even if a TNCS supply is provided, the installation could still be TT.

 

[electrospark]

Yes of course it could be, sorry. I'm going to check exactly what they want today. But in the mean time, if its TNS or TNCS it does not need an RCD to cover the sub mains, correct?
I know that TT systems always have RCD's but I just can't understand why they are requesting them to cover the submain. (maybe it is a TT after all).