4 Pole RCD tripping

[Pat Tester]

Could some one offer some advice please. I have been asked to look at a farm supply that has just started tripping out. 100 A per, 3 phase supply, + nutral, TNCS, + an earth spike. Client said that when EDF moved the supply from overhead to underground in SWA they suggested putting in the rod as backup. Ze 0.34
without spike

The supply feeds through meter into 4 pole RCD 80A 30mA. The whole farm is then split from this into varoius buildings. The problem is that one unit constructed from portacabins appears to produce an intermittant earth leakage of around 16 - 20mA. I have tested all circuits and they test out fine.
The causes of the tripping is the switching of any circuit within this unit.
I have had a similar problem in a domestic situation when next doors electric strimmer tripped the house
RCD. This was sorted by fitting a time delay RCD in its place.
Question: 1) Can you get 4 pole time delay RCDs ?
2) Can the RCD become more and more sensitive for some reason ? it does test out ok when all
other circuits are removed.
3) Is 5.4mA earth leakage acceptable for a flourescent fitting, all 4 of them produce around 14mA
4) I can't get a Zs from the DB supplying the unit as the RCD drops out, should I dissconnect all
Power to the farm and do an R1+R2 on the cable ?
Many thanks for all help, Pat

 

[Des 56]

http://www.protekuk.co.uk/portal/default.asp?id=1041〈=ita&sez=prodotti

Bottom of the page of this link are time delay 4 pole rcds

I cant see how or why any rcd should deteriorate,but deteriorate they do
The 14m/amp leakage does not leave much room for any additional leakage
It seems quite a lot for just a few fittings
Diffiicult to give any opinion 2nd hand,but you seem to have narrowed the problem down
tI would temporarily bridge out the Main rcd to get the Zs reading

When you say the switching of any circuit in this unit would make me think that an earth fault is present somewhere and that the loading of any other circuit is feeding through that fault
Not much help but you will probably locate the problem with your methodical approach
Good luck with it

 

[Pat Tester]

Thanks Des for input.
Decided best option was to swap out 30mA RCD for new 300mA rcd as main trip for whole farm.
This cured the tripping problem from the unit. This was still going through another 30mA 4 pole RCD
as it was being supplied from another DB, without tripping.
I then had to replace two other DB incommers with 4 pole 30mA RCDs to give each of them protection as I had swaped out the 30 for a 300mA. The faulty RCD was burnt on the rear, due I suppose to repeated tripping.

The problem I think was a combination of things; the installation of a 30mA rcd to cover the whole farm,
The high earth leakage from the unit, along with the rest of the farm ran the rcd right to the edge, the constant tripping further degraded the unit to such an extent that switching of any circuits at any point in the farm tripped it.

I tested the rest of the feeds to the other units at 500V without any problems. None of the other DBs produced more than 4 - 8mA leakage

The problem unit had, the cover broken on the check meter, this fed the lower portacabin into a Wylex
2 fuse DB via a single piece of 2.5 t&e, then out from the top of the switch this fed another Wylex board
in the other Portacabin with 2 off 1.5kW heaters two 13A double sockets, and a 6A lighting circuit !
this was all fed from a 60A breaker in another DB. This unit was supplied through four 30mA RCDs in series.

Problem now sorted, but expensive for the client, 4 pole RCDs are not cheap.

Pat