24VDC Power Supplies & Leakage Current

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.

Very interesting, the part that says "any leakage between the 24VDC and earth will not trip" therefore having the RCBO is purely for 1 metre of cable (in my case) and for the power supply itself.

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)

It would be a switch mode power supply and SELV,  I usually earth the 0V of the output of the power supply. I had been taught to do this many years ago as it means that any single fault condition short to 0V trips the circuit. Off topic slightly but that still means its SELV correct?

Out of curiosity - is this 24V DC supply for a special location - ex environment or similar? 


or a Motor control centre maybe?

It is a Control Panel for Industrial Application so falls into Safety Of Machinery rather than BS7671 however I would imagine what we are talking about is relevant to both.

No, you have changed it to PELV - now a fault on the supply side that bounces the earth voltage  is impressed on the output. SELV is immune to that, but has higher specs on insulation of transformers etc.

Consider what happens in the case of a fault like the one discusssed on this thread (shocks due to neighbours earth/ RCD fault)  in the two cases.