How could I measure DC leakage current of a particular piece of equipment?

Question

I need to ensure we have the correct type of RCD, protecting the weird and wonderful equipment in our labs. 
 Is their a way to measure exactly how much DC leakage a piece of equipment will leak whilst in operation. 
 Some of the laboratories have single phase consumer units with an RCD main switch supplying multiple radials out to socket outlets, into which various pieces of kit are plugged in. 
 My concern is, that without knowing how much DC the equipment is leaking, how can I determine if the correct RCD has been installed. 
 Ideally, I would have the Main Switch changed to a regular type and the MCBs changed for RCBOs, but before that happens, I just need to satisfy myself that the RCD installed is actually performing its function. 
 Thanks you for any help in advance.

mapj1 · Accepted Answer

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

gkenyon · Answer

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

David Stone · Answer

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.

How could I measure DC leakage current of a particular piece of equipment?

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