Definition of high protective conductor currents

Thanks guys - this has been very useful!

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 there (I'm sure I recall a 400Hz system for aircraft that was meant to have a reduced shock risk). But all the same we still have no solid evidence that higher frequencies can be safely ignored.

For me that calls into question the effectiveness of specifying 30mA RCDs for additional protection in this case. I guess in the case of direct contact with live conductors on the a.c. supply side the residual current will still be 50Hz and so the RCD should trip as normal. But for other cases - e.g. a broken c.p.c. (single fault) the current flowing from the equipment case via a victim could well have high frequency high current components the RCD would ignore, but could still prove fatal. So my first question - whether we should have high integrity c.p.c.s seems justified,

Or just design out the need for a RCD of any kind

Indeed - that was my starting point. I've still got an open question with the manufacturers whether their statement that ALL the heat pumps need an B-HP type RCD should be read as requiring RCD protection where BS 7671 makes no such demands - or whether they just meant "where an RCD is present it shall be a B-HP type".

Given the supply wiring is fixed, unlikely to be damaged and everything else is contained within an earthed metal box, the need for additional protection seems very small.

In the mean time I'll be looking to ensure my setup complies with 543.7...

   - Andy.

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.

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.