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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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  • 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.

  • 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 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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  • 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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