Definition of high protective conductor currents

DC-DC converter stage with HF separating transformer, between rectified AC input, and inverter output, 

Yes, and that sort of thing may well happen outside the R & D labs eventually (currently its still in the long haired 'too hard to be worth it' pile I think )
It will still bring some primary side earth current problems similar to those in the average EV, that more  or less does the first step  of this already. However MF  transformers that allow the core and inter-winding foils that connect to the primary DC help significantly by breaking the capacitive path between primary and earth referenced secondary..

Once cost - effective semiconductors and magnetic materials allow that sort of power level, then at about the same time we may also see a move to get rid of some of the mechanical tap switching and 50Hz transformers at smaller substations where power levels to be switched are comparable.

Mike.

ensure my setup complies with 543.7...

A sensible precautionary approach.

I notice Mike's graph is based on pain rather than effects that cause permanent damage or Ventricular fibrillation, so maybe there's still a bit of room for debate

Oddly its not an area that most folk are keen to research and there is not a lot out there  !!

This from ICNIRP may be of interest to you. 

GUIDELINES FOR LIMITING EXPOSURE TO TIME-VARYING ELECTRIC AND MAGNETIC FIELDS (1 Hz TO 100 kHz)

While predominantly about radiation, it does cover direct contact as well.

Note however that the numbers here are a factor of 30 or so lower than those used by BS7671 and the standards from which it derives its specifications... And, given results of rather worryingly simple/ dangerous tests like this, perhaps correctly so.

https://pmc.ncbi.nlm.nih.gov/articles/PMC2763825/ scroll down to the rather childish picture of someone with feet in two buckets of water.

Fresh (not salt) water with conductivity of 320 µmho/cm filled each bucket to a level near the hip. It was found that electrically induced muscle contractions were greatly modified by leg position in the water.

Initial testing has shown that with 3.05 V (60-Hz AC rms) applied between the plates, a current of 8.65 mA flowed, resulting in involuntary flexion of the knee to 90°. This flexion could not be overcome with voluntary effort. ..

Now that's a voltage most regulation writers feel is safe, even near a swimming pool  ;-)

Then there are folk measuring people and arguing the existing body resistance model is wrong (well, more charitably, incomplete perhaps)

Assessment of Human Body Impedance for Safety Requirements Against Contact Currents for Frequencies up to 110 MHz

It is fair to say, the science is not settled. I'd prefer screened or armoured  cables for now.

Mike.

One of the big problems here is that as 'electricians' we are often unaware of the complexity of other competing regulations, especially the EMC regulations, which also require the measurement of (residual) electrical signals relative to the well connected mass of earth to ensure that the energy we supply doesn't re-emerge as RF radiation, relative to our common ground/earth.

Everybody thinks it's somebody else's problem, anybody could add extra costly components, nobody wanted to do it. 

And the RFI/EMC specs are just as susceptible to fudging as the electrical specs (e.g. see clock dither on microprocessors..) .

The requirements are nothing to do with the 'shock', rather it's nuisance tripping and 'blinding' (failure to trip) of the circuit, that is being coordinated.

That's a bit sweeping - why do we worry about it tripping (or not) if the currents under discussion are not potentially a threat - put another way, if electric shock was not a thing, we would not need CPCs, let alone RCDs that tripped on earth currents,  at all. 
Mike.

Let's get back to the start. The question was about the technical spec of an RCD.

That spec (the special aspects) is not about the shock potential for the human, rather it's about all the side effects of the load and how it interacts with the RCD's detection apparatus, and the coordination between the two.

Things like needing to be a type B, and needing certain rejection characteristics. None of those characteristics, as best as I can see, are anything to do with effects on the human, and far more to do with a coordination between EMC/RFI requirements, filtering capabilities at the load and filtering within the RCD. The EMI/RFI filtering is the one that creates earth leakage that looks like residual current, and we need discrimination between real fault leakage and EMC/RFI leakage.

Hi Andy,

I’m curious about your heat pump install setup. Would using SWA cable eliminate the need for RCDs in your case? Also, does your heat pumps have a dedicated earth connection point, similar to solar PV inverters, where you could attach a 10mm copper conductor to handle high protective conductor currents? 

Have you seen the Heat Pump Association updated guidance from March 2025? Lot of useful information on RCDs, although doesn’t mention isolation transformers as an alternative to manage leakage.

To save others from spending time  looking for it that guidance is on-line here HPA-RCD-Guidance-March-2025.

Mike.

Philip Oakley said:

and we need discrimination between real fault leakage and EMC/RFI leakage.

I think there could be some cross-over there. Normally we'd expect a 30mA RCD to provide a decent level of protection against shock in a wide variety of circumstances - including the single fault condition of a broken c.p.c. where "protective conductor current" becomes the same as shock current. If we've now got (non-standard) "30mA" RCDs that actually allow 150mA to flow without disconnecting (if not 50Hz components) it feels to me we might have lost something here.

   - Andy. 

I’m curious about your heat pump install setup. Would using SWA cable eliminate the need for RCDs in your case? Also, does your heat pumps have a dedicated earth connection point, similar to solar PV inverters, where you could attach a 10mm copper conductor to handle high protective conductor currents? 

In my setup, it's TN so Zs will be adequate, the supply HP cables won't be concealed in a wall or under a floor, or run underground, won't run through a bathroom, aren't supplying sockets or domestic lighting, and the HP although outside weighing in at over 100kg and being bolted down onto a concrete foundation (via large rubber feet) I would suggest wont count as "mobile". To my reckoning therefore I don't need an RCD (30mA or otherwise) from a BS 7671 perspective, regardless of the type of wiring system. If the cables were concealed or run underground I reckon SWA or similar might be needed.

Manufacturer's instructions are confused - the printed manual that came with the HP only asks for an RCD where the sit requires one, and diagrams show overcurrent protective devices, but no RCDs. Other (later?) on-line documentation, says "all" their HPs need a B-HP RCD, I've got an open enquiry with the manufacturer to try and clarify that point.

In my case I'm opting for 4mm² BS 8436 cable (partly because I rather like shielded cables, mostly because I have some left on a roll from previously) - run in flexible conduit outside to protect the sheath from UV, I've not spotted a dedicated stud (just a PE terminal on the main PCB) - but suspect I will be able to improvise something easily enough. I'm hoping to go down the 543.7.1.203 (ii) route - i.e. single 4mm with the BS 8436 cable providing equivalent protection to flexible conduit,

  - Andy,