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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. 

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    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. 
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  • 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. 
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