Definition of high protective conductor currents

I cannot second guess the intentions of the authors, but physically, higher freqency AC has less effect on the body -  actually the frequency range between DC and about 100Hz-1000Hz  is as bad as it gets in terms of muscle lock-up to which the nasty heart-disrupting effects of electric shock relate to. Also above a few KHz the sensation / perception falls, so the flinch response at HF also falls

By radio frequencies an RF burn is possible where the flesh cooks near the point of contact with no sensation at all, until the actual burning. And then, speaking from personal experience, it really hurts, for days, just like any other deep burn.

Texts on the subject produce graphs like this but there is not much original work out there.

Or more formally

AJJewsbury said:

• Minimum detection capability up to 20 kHz

• Minimum trip threshold of 150 mA above 1 kHz

I'm trying to get my head around what these mean technically (as opposed to physiologically) and how the various limits are interrelated (and perhaps related to EMC/RFI filters in the internal switching units)

Does it say that the trip threshold should be higher than 150mA [minimum being no lower than..] for the sum of all components above 1kHz, up to and including the 20kHz figure?

There can be a lot of switch RF leakage currents to earth as part of the designs that seek to make the supply (live) and return (neutral) conductors 'appear' to be anti-phase at the RF (just as the Yellow transformers convert 110VAC to +/-55VAC, though for sightly different purposes/methods).

Edit: it could be that previous, more simpler designs, were subject to nuisance tripping from those deliberate RF/EMC suppression leakage currents, and the newer types now have their own filters....

Certainly the leakage current from a switch mode power supply is far from simply sinusoidal and contains pulses of both polarities that may be only 10s of micoseconds apart and of very short duration (sub microsecond). If you average these over the period of a half sinewave at 50Hz, you measure a very low residual, as the ups are cancelled by the downs and furthere more, for say 90% of the time there is nothing there at all....  A traditional mechanical balance trip RCD sort of did this for free as the short spiky pulses simply could not get the armature moving, giving a natural low-pass (HF reject) effect.
A first generation electronic RCD however uses a comparator that on-chip has transistors that certainly can change state at the same sort of speeds as those in the SMPS, actually faster probably, being as large power devices are slower due to increased transit times than the smaller devices on-chip.

Therefore there has to be a deliberate low pass filtering added between the sense coil and the comparator/ amplifier to roll-off the unwanted response to those sharp spikes, to make the RCD behave more calmly and in a similar way to a good old mechanical trip.
This expectation of a roll-off by 5 (about 16dB) between 50Hz and 1kHz is an attempt to  recognize and accommodate the sort of filtering needed to avoid tripping out when things are actually working as they should. It also helps to reduce the mis-firing on other one shot transients such as arcing light switches.

There is a certain irony that in an AFDD that high frequency sensitivity has to be build back in, but then tamed by a multiple 'if this and that but only when this happens too' type of algorithm and of course sensing the line as well as the  imbalance currents.
Mike.

AJJewsbury said:

The other (possibly more important question) is how do currents at higher frequencies affect the human body

Afraid that all this talk of various frequencies makes my eyes glaze over, but v. high frequencies are used in diathermy. Bipolar is easy to understand - the current passes between the tips of a pair of forceps and cooks the tissue, which stops any bleeding. Rather more old-fashioned monopolar diathermy has a large "earth" plate attached e.g. to a buttock where the current density is low, and a pointy instrument where the current density is high, and accordingly, able to coagulate (stop bleeding) or cut.

Then you have things like cardioablation, where the electrode is poked into yer heart tissue and the current fries it, so clearly HF currents can have very beneficial effects. :-)

After posting, I did some extra web searching, as you do ;-) 

I found a quite good tech note from Schaffner on the RS web site that provides a good bit of background (and slightly slides past other nuance aspects) on Low Leakage current EMC filters with full RCD compatibility   I Also found Hager's "Why Use Type B HP RCDs for Heat Pump Applications" which has comments about the VDE B+ standard.

The main clue is it's the "Type B" problem of anything with switch mode electronic loads which [could] effectively create (on an average, 20ms cycle) a DC load through the residual/leakage sensing coils. The 'could' part is that most of the current is very high speed switching transients, which if not internally filtered in the circuit breaker device look like live leakage (up to a point where the equipment needs to filter as well, no doubt also for RFI reasons). 

The nuance bit that I hadn't implicitly realised is that the electronic device's diode rectifier, and capacitor, has a store and delay effect on the way supply current is converted to leakage current, such that from the supply side the leakage is in phase (goes through the expected rectifier phase diodes), but on the load side it is, on average, DC.

The Schaffner fig 1 shows the leakage paths. Diag 4 shows the RCD tripping, diag 6 shows a situation where the equipment switching currents are too high (without filter, labels in German;-) and then diag 7how their filter saves the day on the equipment side and stops nuisance tripping relative to a type B RCD..

There is another approach which completely removes the issue, localising earth leakage to the heat pump itself. That is to use a transformer with simple separation.

Example for single-phase in Figure 12.4 (p104) of GN5.

If manufacturers built that into the heat pump, or used an inverter with separation (possibly cheaper transformer like in a DC SMPSU), no special precautions are necessary in the installation.

That is to use a transformer with simple separation.

Very true, but to a manufacturer, quite undesirable for a number of reasons. The obvious penalties are size and weight, although that only increases manufacturing, materials, storage, and shipment costs. Then there is also a problem in terms of selling the efficiency ratings to the user (not just explaining an increased floor or height claim) for reasons of transformer losses once in use - its an extra few % of the kVA going as heat that has to go somewhere - trivial if its a few % of less than a kilowatt, but actually really awkward once you get to  using VSDs that are ten times that or higher. 

It is for the same reasons that it is now almost impossible to buy any domestic appliance that does not contain a switch mode supply of some kind - and the UKCA/ CE efficiency requirements have all but removed traditional linear power supplies even from the small 'wall-wart' applications, so the modern house is buzzing with RFI generating switchers.


And of course its not the maker's problem to say 'use an RCD as required by local regulations', however unhelpful it may be to us, it's very easy for them, the extra ink in the handbook is almost free.

So while the transformer idea is technically appealing, and personally I'd love it , I'm not holding my breath for many makers to adopt it.
Mike.

mapj1 said:

And of course its not the maker's problem to say 'use an RCD as required by local regulations',

I'm led to understand some inverters (e.g. heat pumps) need an RCD with a performance that doesn't yet have a classification in standards (but does exist in the market place).

On top of that, the cost of even a standard Type B is an issue of cost to someone, as well as the fact that it may also use power (but that may well be less than the losses of a transformer).

mapj1 said:

Very true, but to a manufacturer, quite undesirable for a number of reasons. The obvious penalties are size and weight, although that only increases manufacturing, materials, storage, and shipment costs. Then there is also a problem in terms of selling the efficiency ratings to the user (not just explaining an increased floor or height claim) for reasons of transformer losses once in use - its an extra few % of the kVA going as heat that has to go somewhere - trivial if its a few % of less than a kilowatt, but actually really awkward once you get to  using VSDs that are ten times that or higher. 

Perhaps another approach could be to use a DC-DC converter stage with HF separating transformer, between rectified AC input, and inverter output, to provide the separation, and drastically reducing the impact of providing separation on losses, cost and weight!

gkenyon said:

Perhaps another approach could be to use a DC-DC converter stage with HF separating transformer, between rectified AC input, and inverter output, to provide the separation, and drastically reducing the impact of providing separation on losses, cost and weight!

Or just design out the need for a RCD of any kind. Appropriate design would negate the need for RCD protection on TN systems or additional protection on TT systems. 

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.