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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.

  • in reply to AJJewsbury

    Are you planning to use a separate consumer unit fed independently via Henley blocks ?

  • in reply to AJJewsbury

    A 30mA 50Hz shock (the electricians classic shock) is not the same, physiologically, as [say] a 60kHz 30mA 'shock' leakage. The RFI leakage could be (depending on switching frequency design) anywhere between say 8kHz and 200kHz (or higher).

    Then there is the "phasing effect" that it is capacitively coupled, and from the leak perspective, it's always in-phase with itself, but 90 degrees out of phase with the switching clock, and has no phase at DC, making discussions hard as each group uses it's own 'reference' concepts, while ignoring those [unappreciated] aspects from the other group.

    It's real tricky stuff. (see https://tv.theiet.org/?videoid=1327, and my comment about 2/3rds the way through)

  • in reply to AMK

    Are you planning to use a separate consumer unit fed independently via Henley blocks ?

    Nope, not initially at least (longer term I might go for a 2nd CU in the house so that the PV could be removed from the main house CU where the RCBO isn't strictly speaking of the correct directionality and overload protection doesn't meet the letter of the current regs, and give me a spare way or two for anything else that might come along in the future). At present I have a HBC fuse DB at the intake in the cellar (feeing the house CU on the ground floor and another in the detached garaged) - that has a spare way, so I was going to pop a 16A fuse in that and take the HP supply from there.

       - Andy.

  • This has been an interesting discussion, but what exactly is the risk?

    Andy's proposal deals with the risk to the cable and I agree that the circuit does not need to be RCD-protected.

    So consider the heat pump itself. Let's assume that there is 150 mA of earth leakage at > 1 kHz - any more and the RCD will trip.

    Whilst the cpc is intact, 150 mA flows down it. Should somebody touch an exposed conductive part of the HP, some of the current will travel through the person in proportion to the resistance of the two paths. Even with bare feet on a rainy day, only a small proportion will flow through the person. however, the proportions are irrelevant because there is still a 150 mA offset between L and N, so the RCD will not trip.

    Not let's assume that the cpc has become detached. Now there is no L-N offset, so the RCD will not trip. Then somebody comes along (with bare feet, etc., if you wish) and touches an ecp. Up to 150 mA will flow through the person, but this will still not trip the RCD.

    I conclude that the specified RCD cannot protect a person from up to 150 mA at > 1 kHz, so that cannot be its purpose. However, the householder may want additional protection against 15 - 30 mA at 50 Hz. If so, a type B+ RCD must be used else it will be susceptible to blinding or nuisance tripping, or both.

  • AJJewsbury said:

    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

    To me, that reads as the requirement being a 300 mA Type B RCD.

    The current is presumably a mix of alternating, pulsating DC and pure DC? If it is, don’t forget to multiply by the square root of two as required when considering the leakage currents and testing.

  • in reply to Chris Pearson

    I conclude that the specified RCD cannot protect a person from up to 150 mA at > 1 kHz, so that cannot be its purpose.

    Exactly. I do wonder if the prescribed tests for 30mA RCDs for additional protection only consider 50Hz currents though (as nobody considered much else in the old days of filament lamps and resistive heating) - so these new B-HP devices appear to satisfy all the requirements for devices specified for additional protection ... even though enlightened common sense might now beg to differ.

       - Andy.

  • in reply to Sparkingchip

    To me, that reads as the requirement being a 300 mA Type B RCD.

    You might very well think so ... but all the examples of the recommended "B HP" devices I can find are 30mA ones - e.g.https://hager.com/uk/search?q=heat+pump

       - Andy.

  • Worcester Bosch do not specify the type of RCD to be installed, just 30 mA fast tripping with a requirement to trip in less than 100 mS.

    www.worcester-bosch.co.uk/.../Installation Operating Instructions CS2000 4-30_6721859133_01.pdf

  • in reply to Sparkingchip
    Sparkingchip said:
    To me, that reads as the requirement being a 300 mA Type B RCD.

    I do not think that can be correct at all - in that case you would have no additional protection.

    In BS EN 61008-12012+A12:2017, the "preferred value of the rated frequency is 50 Hz" (5.3.7) and I think that must mean that IΔn is the value at 50 Hz (or 60 Hz) unless stated otherwise.

    The distinguishing feature, and reason for the eye-watering price of HP RCDs, seems to be that IΔn is frequency dependent.

  • in reply to Chris Pearson

    “let's assume that the cpc has become detached “

    Hi Chris. I experienced a shock from a mobile mixer in an autoclave room, controlled by a built-in inverter. The issue arose from a disconnected CPC in the mixers standard 13A plug. The mixer, frequently moved between multiple bins before operation, the leakage current with no path back was on the motor housing, resulting in a shock when I touched it while my other hand was on a stainless steel table whilst trying to move past. Despite the circuit being protected by a 30mA RCBO, it did not trip, I installed an ECM Series Earth Continuity Monitor and replaced the 13A plug with a commando-type.

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