Do wou want a box to test equipment with or a thing that sits in the dis board and raises an alerm if the current is too high ?

Or do you just want to check that the RCD  is still responding to AC faults and not blinded.

For things that can be unplugged a trick 'extension lead' with a plug and a socket and a meter that measures DC mA in series with the green and yellow core is probably the easiest.

To monitor DC it is possible to build non-invasive things to clip on the wiring based on hall effect current sensors that look like current transformers but go down to DC , but quite a bit of electronics knowledge is needed to do that .

To verify the RCD is not blinded you just need to introduce an AC fault, and see if it trips to order.

Mike

The issue for RCDs is not simply "DC leakage current" but DC components in currents flowing during a residual current fault (such as an earth fault).

For this reason, measuring quiescent DC leakage current alone is not sufficient to determine the RCD Type that ought to be selected.

See the section 'Possible fault currents in systems with semiconductors' and Table A53.1 on pages 194 and 195 of BS 7671:2018+A2:2022

I ought to have added, that this is important, because whilst in some installations RCDs are only provided for additional protection, in other installations (for example, TT earthing arrangements, or where overcurrent protective devices can't operate quickly enough), RCDs are used to achieve automatic disconnection for protection against electric shock.

Thanks for the viewpoints gents. What I'm trying to do is satisfy the requirement below from the wiring matters article.

Which RCD Type? - Electrical (theiet.org) 

What if I discover a Type AC RCD whilst carrying out an electrical inspection during an electrical installation condition report? If the inspector is concerned that residual DC current may affect the operation of Type AC RCDs, the client must be informed. The client should be informed of the potential dangers which may arise and an assessment of the amount of residual DC fault current should be made to determine if the RCD is suitable for continued use. Depending on the amount of residual DC fault current, an RCD which is blinded by residual DC fault current is likely not to operate which could be as dangerous as not having an RCD installed in the first place.

What I am wondering is HOW I can quantify the amount of DC leakage current that is potentially affecting the installed RCDs.

Maybe that is too complicated and Mike you have given me an option, that would be to run everything at full power, and then perform an RCD test to see if it operates. However in this scenario we have multiple socket outlets so we do not know exacylt how much equipment is going to be plugged in and used at any one time.

If I could check each piece of kit, quantify the DC leakage, and label, we could then control how much is affecting the RCD.

The RCDs we have on the consumer units are Type AC, and some Type A, which will only be good for 6mA. 

Maybe I will need to recommend upgrading to Type F, or even Type B.

The problem with your question is that there can never be a definitive answer! There is also a problem that the various makes of RCD are not identical in performance, and that the specifications of the types are OK as they stand as worst case examples, but the possible worst case is unlikely in any installation. The pages of BS7671 referenced by Graham K above contain huge assumptions, but these are not considered in the text.

I will ask a couple of questions and you can consider the answers:

Are your RCDs for "ADDITIONAL PROTECTION", this would be the normal case in a lab?

Do they get tripped often, or essentially never?

Do staff work on live exposed mains connected equipment, or are the items enclosed in protection against direct contact?

If the equipment has commercial power supplies fitted, how could direct contact actually happen?

Are your staff properly trained to work under possible direct contact conditions?

Is the equipment controlled under a calibration / inspection cycle, making faults very unlikely, and has any equipment ever been found mains faulty?

I suggest that the training question is the most important, it is probably much more so than the questions about the other things which have easy answers. It may be answered by a procedure document for working on live equipment. Just a note, I have worked on high powered transmitting equipment. Mains is much the least danger to such work, and an RCD would make zero difference! 100kV DC or 500 kW of RF is quite dangerous enough.

I think the wiring matters article is perhaps a bit alarmist.  It is just about credible that there may be the pulsed AC sort of fault where a device which rectifies the mains directly (switch mode supply based thing, thyristor based speed controller/ lamp dimmer maybe) develops a fault that in effect puts a diode and a resistor between live and earth. If the resistance is high enough that the dide id not blown out, then the likely rise in cpc voltage should be small.
Personal experience suggests that commonly when the over stressed diode fails soon after,  it blows to a short condition, so creating an AC fault that then trips the RCD anyway
A smooth DC fault is quite unlikely except in very odd cases - car chargers that run DC pilot signals over the CPC are an exception.
Most RCDs are not blinded by 6mA anyway, it is just a very safe limit to assume in the absence of better maker's data.

In this case is such a fault likely to persist undetected for any length of time ? If yes something should be done, but before rushing off, it is important to be sure it is the case.

M.