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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.

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  • 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....

Reply
  • 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....

Children
  • in reply to Philip Oakley

    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....  A traditional mechanical balance trip RCD sort of did this as it 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 can change state at the same sort of speeds as those in the SMPS, faster probably, being 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 response to those sharp spikes to 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 roll-off needed to avoid tripping out when things are actually working as they should. It also reduces 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 not limited to imbalance currents.
    Mike.

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