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Definition of high protective conductor currents

AJJewsbury

I'm currently installing a heat pump and noticed that some of the manufacturer's information says of the RCDs to be used with it:

...now requires the use of a Type B RCD/RCBO with the following specification:

  • Minimum detection capability up to 20 kHz
  • Minimum trip threshold of 150 mA above 1 kHz

and looking at the devices the manufacturer suggests, these seem to be nominally 30mA types.

So it seems to me these suggested devices may trip at 15-30mA at 50Hz, but may tolerate more than 150mA above 1kHz.

My first thought was where does this leave me with respect to section 543.7 (equipment having high protective conductor currents)? Can I assume that the 10mA limit only applies to 50Hz currents? or given the way the words are written should it be read as applying to all frequencies?

The other (possibly more important question) is how do currents at higher frequencies affect the human body - if I have a device that maybe doesn't trip until over 150mA (at 1kHz) do I still have additional protection? I think I recall that 50 or 60Hz is about the worst possible choice of frequency for shock considerations, but can currents at higher frequencies be safely ignored entirely?

I guess similar considerations might potentially arise anywhere we have power inverters .. so my heat pump might be just the tip of the iceberg,

   - Andy.

  • 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....

  • 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

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