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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
    This rather depends on the internal design of the 24V supply - we are assuming isolation between the primary and secondary side, so in old money is PELV or SELV.

    If however it is FELV, there may be a connection between one pole of the incoming mains and one pole of the DC output, for example.

    The other case is when the supply characteristics are unknown, in terms  of Zs, so that there may be quite a high earth impedance (TT supply or TT island anyone ?) Then an RCBO  (well a double pole switching one anyway) neatly removes the need to install a separate RCD . May be found in kit to be installed in a cabinet outdoors for example. (There are cases when exporting a PME like earth is not desirable - perhaps some electronics on a garage forecourt for example)
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  • 0
    This rather depends on the internal design of the 24V supply - we are assuming isolation between the primary and secondary side, so in old money is PELV or SELV.

    If however it is FELV, there may be a connection between one pole of the incoming mains and one pole of the DC output, for example.

    The other case is when the supply characteristics are unknown, in terms  of Zs, so that there may be quite a high earth impedance (TT supply or TT island anyone ?) Then an RCBO  (well a double pole switching one anyway) neatly removes the need to install a separate RCD . May be found in kit to be installed in a cabinet outdoors for example. (There are cases when exporting a PME like earth is not desirable - perhaps some electronics on a garage forecourt for example)
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