TT Agricultural RCD ISSUES

[PoultryMike]

Hi all,looking for some pointers to an RCD problem, I work entirely on large poultry farms, I've only been in post a year, at some point before I arrived when the regs changed probably on the 17th there was a huge rcd purge in our ccus where they were not needed, I've since in my spare time removing them and installing rcbo , as I don't see the need for 30ma personnel protection on feeders, ventilation and lighting which us all auto controlled and either insulated casings or lights in pvc conduit with no cpc anyway.
Had one of the sheds up front main rcd switch trip last night, it did reset, it is a 4pole 400v 30ma in front of 20 circuits, the test button was operational and it reset, I went out and isolated all motors and this morning with only lights on ( no cpc) it tripped, a colleague went out and replaced it with a 100ma, the test button didn't operate the rcd, he then replaced that one with another and again the test button didn't operate thr RCD, so now there is a 4 pole Isolator installed, when bringing back all 3 rcds to the workshop and hooking them up, none of the test buttons on all three of the RCD worked, the two replacements were new in a box and schneider , I'm going to ramp the original one tomorrow
Tonight another call from same farm, same problem, different shed, similar ccu, sporadic trip which then resets which points to no specific faulty circuit.
The farm supply is 250A 415v on a main rcd TT with a ze of 1.7
I've switched the farm onto the generator supply until 10am tommoriw as the load is low as the lights, fans snd feeders go off at 8pm, I've done this to see if it's a supply issue
Any help would be appreciated
Cheers

 

[pc1966]

it is a 4pole 400v 30ma in front of 20 circuits

Is it really a 30mA for 20 circuits!?

Usually for agriculture it was 300mA delay (type-S for selective) RCDs that were specified to reduce fire risk with rodents chewing cables, etc.

If the test button is not working it is suspicious, but be aware that some 3-phase RCD have the test between two of the L to bypass the sense transformer and not L - N of a typical single phase RCD so if you test on single phase (e.g. all L tied together and N) you might not get it to work.

Did the RCD test OK on an MFT or similar tester?

 

[pc1966]

By the way if it is a TT system you are almost certainly going to need RCD protection as it is very unlikely the earth impedance is low enough, so having an isolator switch in such a system in place of the RCD is VERY DANGEROUS unless every circuit is on RCBO, etc.

 

[loz2754]

By the way if it is a TT system you are almost certainly going to need RCD protection as it is very unlikely the earth impedance is low enough, so having an isolator switch in such a system in place of the RCD is VERY DANGEROUS unless every circuit is on RCBO, etc.

He states the farm supply is TT on an RCD

 

[PoultryMike]

Is it really a 30mA for 20 circuits!?

Usually for agriculture it was 300mA delay (type-S for selective) RCDs that were specified to reduce fire risk with rodents chewing cables, etc.

If the test button is not working it is suspicious, but be aware that some 3-phase RCD have the test between two of the L to bypass the sense transformer and not L - N of a typical single phase RCD so if you test on single phase (e.g. all L tied together and N) you might not get it to work.

Did the RCD test OK on an MFT or similar tester?

Cheers, yes can confirm that it really is a 30ma, 400v, I've raised this a year ago when I started and also the fact that a 415v operating rcd may not work linked 230v

 

[PoultryMike]

By the way if it is a TT system you are almost certainly going to need RCD protection as it is very unlikely the earth impedance is low enough, so having an isolator switch in such a system in place of the RCD is VERY DANGEROUS unless every circuit is on RCBO, etc.

The main incomer to the farm is rcd , then there is a 300ma on the swa that leads to the ccu in question, all circuits in the ccu in question that needed 30ma had rcbos put in before I arrived butcthe 30ma up front main switch was left in, Tont think to much risk involved with circuits left as they are all in insulated conduit and controlled auto via timers snd capacitive sensors

 

[pc1966]

The main incomer to the farm is rcd , then there is a 300ma on the swa that leads to the ccu in question, all circuits in the ccu in question that needed 30ma had rcbos put in before I arrived butcthe 30ma up front main switch was left in, Tont think to much risk involved with circuits left as they are all in insulated conduit and controlled auto via timers snd capacitive sensors

OK that is a much clearer explanation and makes sense.

I was worried the "up front" RCD really was at the front of the supply, not down-stream from other higher/delayed RCDs that would still disconnect if L-E fault.

If any final circuits that need additional protection have RCBOs then you don't really need a 30mA incomer for the board at all and it is just a liability. Having said that, many RCBO (in particular ones for 3P boards) only switch the L so a N-E fault can still take down the next RCD up-stream of that. In such a case a per-DB incomer RCD suitably chosen can at least restrict power outages to the single DB's circuits.