Rcd protection on 3 phase system

[wozzit99]

Hi

I'm looking at a job where the proposed pv system will be terminated back into a 3 phase dis board. The dis board will be exclusively for the pv and will consist of several individual systems.

My question is how would you protect the systems with an rcd. Individual systems protected individually or 1 rccb at the incomer switch?

TIA

Wozzit

 

[wozzit99]

Hi

Been doing a bit more digging on requirements for rcd protection on inverters. Taken from this board is a document "the criteria for selecting an rcd for sma inverters"


Operational differential currents

If operating a transformerless inverter, operational DC differential currents are created (caused by theinsulation resistance and through capacities of the PV generator). In order to prevent unwanted triggeringduring operation, the rated residual current of the RCD has to be min. 100 mA.For every connected inverter, a rated residual current of 100 mA has to be planned. The RCD's rated residualcurrent must at least correspond to the sum of the rated residual currents of the connected inverters. That meansthat, if, for example, 3 transformerless inverters are connected, the rated residual current of the RCD has to bea minimum of 300 mA.If this requirement is met, the following residual current devices are approved:• RCD type A ABB type F202A-xx/0,x oder F204A-xx/0,x• RCD type A Siemens type 5SM1.... or 5SM3….Additional RCDs from other manufacturers are being tested at the moment.In this case it must be taken into account that the tripping currents resulting from the differential DC currentsfrom the operation can be slightly over the rated residual current of the used RCD (0 - 30 %). Therefore, thecondition of 2 b changes (cf. Page 6):If , then the RCD can meet the conditions of protection through automatic disconnection.When taking into account the criteria mentioned before, PV systems can be installed both in compliance withthe appropriate standard and in a cost-optimized way. Especially the suitability of the mentioned​

transformerless inverters for RCD type A enable an affordable installatio

The inverters I am proposing to use in the system are tl's sma, the earthing arrangement is tt hence the requirement for an RCD, the pv system will be a ground mount. So am I right in thinking I can protect the whole system with 1, 300ma rcd, I thought tt systems had to be protected with 100ma rcd's?

 

[BruceB]

If the only reason you are putting a rcd in is because it is TT, then the regs do not define a value, they just say there must be an rcd. It was customary in 16th edition days to use a 100mA rcd for TT for domestics, but that value was not prescribed in the regs.

I guess you have x3 Tripowers, in which case at 100mA each (from the installation instructions) then the minimum value of a single rcd must be 300mA.

Is it agricultural? If so there is a 300mA rcd requirement for that which ties in nicely.

Regards
Bruce

 

[wozzit99]

Hi Bruce

Gave NICEIC A call earlier to get some clarity on the situation. The guy was adament that because the three inverters were under 32 amp (3 phase ) final circuit that 30ma protection is required as a mimimum. I think nuisance tripping might be an issue here..


Thre's no way out of getting away from the tt system as the distribution is a fair distance away from the service which is tncs.


Wozzit

 

[BruceB]

Well I am afraid the NIC man is talking ********. Ask him to cite the regulation requiring a 30mA rcd. The inverter is not 'mobile equipment' if he is thinking of 411.3.3(ii). Provided you can meet disconnection times etc then not required for that reason. Also not allowed because against manufacturers instructions.

Also you are in danger of getting confused answers here because initially you implied the installation was TT, but now you are saying it is TN-C-S.
Why do you need any rcd at all? There is no regulation that requires it as far as I can tell. Just run your cable out to the inverters, probably SWA if outdoors, and follow the DTI guide which will lead you to earth the array with a local spike. If you are uncomfortable earthing the inverter all the way back to the main DB by the service head, then just do it to the local earth rod also.

Regards
Bruce

 

[yellowvanman]

In 16th Edition it was generally recognised that you put in a 100mA Time delayed RCD on the circuits that didn't need 30mA protection, so as to give discrimination.

Agree with Bruce that the NIC man is talking a load of b****s. Inverter circuit is a distribution circuit so if you have TT disconnection time is 1 second, and any RCD (correctly working) would cope with that.

RCD would only be required it other factors were overriding - concealed cable <50mm, farm building etc.

 

[wozzit99]

Thats cleared things up, cheers.

Bruce, the service is 100m+ away from the proposesd 3 phase distribution. The origin is tncs. Where the 3 phase board is going it will be earthed via its own spike.


Wozzit