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Temporary Generator RCD Requirement

Gyaneshwar Kumar

Hello, on one of my projects, I have an existing LV feeder pillar (TN-S system) that is normally supplied from an upstream Switchboard via an XLPE/SWA cable with an integral CPC. We are relocating the upstream Switchboard and it is planned to connect small temporary generator (100A TPN) to the feeder pillar to maintain supply to associated loads. None of the loads inherently require any RCD protection.  

My understanding is that for temporary portable generators, Regulation 551.4.4.2 applies which requires every final circuit to be provided by additional protection by means of an RCD. 

My questions is- Would a single 30 mA four-pole RCD installed at the generator outgoing, protecting the entire feeder pillar, satisfy the requirements of Regulation 551.4.4.2, or is a separate 30 mA RCD required for each outgoing circuit?

I don't see why it wouldn't satisfy the intent of the particular clause. Of course, there will be minor risk of nuisance tripping (although unlikely as no high standing leakage current generating equipment connected) and risk of losing supply to entire FP if there is fault on any outgoing but that is acceptable given the constraints of design and temporary nature of this arrangement. 

Any thoughts would be much appreciated. 

Edit: I think I didn't read the clause properly, it says “every final circuit shall be provided with additional protection…”—it does not say that each final circuit must have its own individual RCD. Hence, I believe a single device upstream that all finals pass through is acceptable. 

  • 0

    I presume your feeder pillar (and perhaps downstream installation) are fixed? If so take a step back - 551.4.4 (the heading for 551.4.4.2) only applies where both the generator and the installation are not permanently fixed (e.g. where you have a temporary generator feeding a temporary installation) - so if the installation is "fixed" 551.4.4.x don't apply and you're back to general requirements.

    Where the installation is fixed, main bonding (if needed) is in place, and any final circuits that need them have 30mA RCD additional protection, it doesn't improve safety much to add in an upstream 30mA RCD ... indeed wanted tripping may potentially cause more hazards than it solves.

       - Andy.

  • +1 in reply to AJJewsbury

    Thanks a lot for your response and thinking about it you are right it says ‘Installation AND generator set’.

    However, because the Ze of generator is a bit high, it pushes the disconnection time slightly higher than 5 seconds for some of the distribution circuits. As this temporary arrangement is expected to be there for a few weeks, it may not be acceptable. So I would need RCD for ADS. Will it be more sensible in that case to use maybe 100mA or 300mA and/or time delayed RCD to reduce the risk of any nuisance tripping?

  • +1 in reply to Gyaneshwar Kumar

    Indeed - don't set the genest protection too tight or the whole lot will be off for every little thing. 

    In an amateur capacity I get involved with event power every 4 years or so for a very large Scout Jamboree. For that we tend to have hierarchical RCD protection - the gensets below 100kVA get set to 100mA /0.1 second, and the larger ones are 300mA 0.,3 seconds. The next tier of distribution down to 63A is protected by 100mA 0.1 sec and then anything at 32A 3 phase and less is 'final circuits' at 30mA instant.  In all but the oddest of corner cases, only the 30mA RCDs ever operate so the amount going off is limited.

    This is not your situation exactly, but it is an example of a tapered approach that has worked safely and very well for us in an adverse situation.  Do also watch where the NE bonding is - once running on genset you need there to  be no NE bond in the installation side, or the genset will see that as a fault and the earth trip (like  big programmable RCD) will refuse to stay on.

    Mike.

  • 0 in reply to mapj1

    That makes sense. Thanks Mike. Yes there won’t be any N-E bond downstream of the generator. It’s a private network and we always have single point N-E bond at the origin of installation, i.e, at transformer closed coupled feeder oillar or immediate LV switchboard where REF protection is required. 

  • 0

    I think your revised interpretation is reasonable. Regulation 551.4.4.2 requires that every final circuit is provided with additional protection by a 30 mA RCD, but it does not explicitly state that each final circuit must have its own dedicated RCD. If a single four-pole 30 mA RCD is installed at the generator output such that all final circuits are downstream of it, then each final circuit is indeed provided with the required additional protection.

    From a practical engineering perspective, the main drawbacks are reduced discrimination and the possibility of a single earth fault disconnecting the entire feeder pillar. For a temporary generator arrangement where continuity of supply is not critical and the associated risk has been assessed as acceptable, this may be a proportionate solution.

    The other aspect I’d verify is the generator earthing arrangement. Ensuring the temporary generator is correctly configured for the existing TN-S system, with appropriate neutral-earth bonding and fault loop impedance to achieve automatic disconnection of supply, is just as important as the RCD provision. A temporary connection should also be subject to a suitable risk assessment and testing before energisation.

    This acknowledges the regulation while also highlighting the practical considerations beyond simply installing the RCD.

  • 0 in reply to Terry osifo
    • Terry, thanks for your response. I am leaning more towards Andy’s response above. This clause probably doesn’t apply in my case as the installation is fixed. It’s only the generator that is temporary and portable in which case 30mA RCD is not a requirement. I would still consider RCD for ADS but won’t make it too tight and risk nuisnace tripping. 
  • 0

    Stupidly left a temporary changeover in a large joinery works to the last minute. Only a matter of plugging the 63A TPN lead from the an existing changeover switch into the genny. Planned for a Sunday night, a dry run was made earlier in the week with the nightshift supervisor given full instruction on the changeover strategy but without connecting load. 

    300mA S-type on generator did what it was paid to do and shut down due to a N/E fault on a piece of equipment. You can switch out the RCD on the genny but try explaining that from your bed to a joiner at 1.00am on a Monday morning! Had to get up, rain beating off the windows and go to site to fix. Won't make that mistake again!

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