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24VDC Power Supplies & Leakage Current

Les Spiteri
Hi, looking for some thoughts.


I recently had a customer asking for an RCBO upstream of a 24VDC power supply (i.e. on the 240Vac incoming) and I thought "never seen that before".
  • What possible reason is there for doing this?

  • Would this be considered "safer" than using an MCB?

  • What would happen without RCBO one if there was leakage inside the 24Vdc power supply?

  • Only time I have previously used RCBOs was where you provide a socket in enclosures as per all guidelines now.


Thanks Folks

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  • 0
    Probably not required, and MIGHT be a blanket policy of the customer, called for without any real thought as to actual need. An RCD is arguably safer than an MCB, but only very slightly in most circumstances. If there is a significant length of cable between the RCD and and the PSU, then an RCD is prudent and may be a requirement. In such circumstances it could be argued that the RCD is primarily to protect the cable rather than the PSU.


    Any leakage between the mains side of the PSU and earth will trip the RCD if it exceeds the trip threshold.

    Any leakage between the ELV output and earth wont trip the RCD.

    Any leakage between the mains and ELV sides of the PSU might trip the RCD, depends on whether the ELV side is earthed either by design or fortuitously.


    A very large 24 volt power supply might have a large enough earth leakage current to risk nuisance tripping of an RCD, most unlikely for a small one though.
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  • 0
    Probably not required, and MIGHT be a blanket policy of the customer, called for without any real thought as to actual need. An RCD is arguably safer than an MCB, but only very slightly in most circumstances. If there is a significant length of cable between the RCD and and the PSU, then an RCD is prudent and may be a requirement. In such circumstances it could be argued that the RCD is primarily to protect the cable rather than the PSU.


    Any leakage between the mains side of the PSU and earth will trip the RCD if it exceeds the trip threshold.

    Any leakage between the ELV output and earth wont trip the RCD.

    Any leakage between the mains and ELV sides of the PSU might trip the RCD, depends on whether the ELV side is earthed either by design or fortuitously.


    A very large 24 volt power supply might have a large enough earth leakage current to risk nuisance tripping of an RCD, most unlikely for a small one though.
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