One more thought (I know it's a bit late for the DPC, but just in case it contributes to the wider debate...)

Method M3 - open "When the protective conductor current is 21 mA or more". I presume the idea here is to protect against current flowing from the (possibly hazardous MET/PEN) to true Earth via someone touching the car while in contact with the general mass of the earth. (Mostly to protect from open-PEN situations that the L-N voltage method doesn't detect). I'm wondering if measuring the current in the c.p.c. alone is actually a good proxy for what could be flowing through a victim - after all current can arrive on a c.p.c. through a number of routes - e.g. from the EV itself, or the wall mounted controlled, by leakage current or even uncleared faults - and furthermore current returning through the c.p.c. won't be passing through the victim (possibly increasing the chances of nuisance trips?). Has 21mA been chosen rather than say the more conventional 30mA to allow for leakage current? Is 9mA a reasonable assumption for that?

My thought is that what we're really looking for is current that's flowing along the c.p.c. that isn't part of the normal circuit 'return' current (including leakage, L-PE fault currents etc) - so would it make sense to do that by feeding all three wires (L/N/PE) through an RCD? In this case a normal 3P or 4P off-the-shelf 30mA one of an appropriate type would suffice. Of course the RCD wouldn't trip if the L-N voltage was low, but in that case the L-N voltage mechanism (M2) would have opened everything including the .c.p.c. anyway, so that's OK I think.

Maybe M3a - measure the c.p.c. for ≥21mA
M3b - L/N/PE imbalance of ≥30mA.

    - Andy,

AJJewsbury said:

My thought is that what we're really looking for is current that's flowing along the c.p.c. that isn't part of the normal circuit 'return' current (including leakage, L-PE fault currents etc) - so would it make sense to do that by feeding all three wires (L/N/PE) through an RCD? In this case a normal 3P or 4P off-the-shelf 30mA one of an appropriate type would suffice.

The standard isn't there to tell the manufacturer precisely how to design and manufacture the product, just the relevant parameters it ought to meet.

I'm sure a similar approach could be taken to actually building a product, but perhaps this would be "mis-use" of a COTS 3P (three-phase) RCD for single-phase, and whilst a 4P (TP&N) RCD might appear to do the trick for three-phase:

• it doesn't contain 5 contacts to open if the arrangement is TP&N; and
• whilst there is a contact to open N, I'm not sure they all pass the N through the sensing coil (and I don't think it's necessary either) - so if that were the case and we decided to use the 4-pole device for TP+PE (as it were), it wouldn't pass the PE through the sensing coil to detect current flowing from the grid to the person (i.e. from the raised PEN voltage at the installation, and not via L1, L2 or L3 supplied to the EV).

I'm not sure they all pass the N through the sensing coil (and I don't think it's necessary either)

A 3P+N RCD does sense on the neutral - if it did not you could not connect a single phase load to one phase and N and not expect it to trip.

(And any out of balance 3 phase can be considered equivalent to a balanced load  plus in parallel with some  single phase part that represents the neutral current.)

Somewhere, years ago there was a South African publication aimed at detecting neutral copper theft, that did something more or less equivalent to PEN loss detection,  by using an RCD breaking Phase(s) and what we would call PEN prior to the NE split if the neutral voltage rose enough to drive more than a certain amount of current into an electrode on the load side.

Mike

mapj1 said:

A 3P+N RCD does sense on the neutral - if it did not you could not connect a single phase load to one phase and N and not expect it to trip.

Yes on reflection it's definitely necessary ... but there's still no way of feeding the PE through (to measure the additional current coming from the "grid".

mapj1 said:

Somewhere, years ago there was a South African publication aimed at detecting neutral copper theft, that did something more or less equivalent to PEN loss detection,  by using an RCD breaking Phase(s) and what we would call PEN prior to the NE split if the neutral voltage rose enough to drive more than a certain amount of current into an electrode on the load side.

Agreed - the PE current tied up with the PEN would permit that detection ... but in the consumer's installation, where we are not allowed PEN conductors, we'd need the PE to be fed through.

What about using a CT (Current Transformer) on the Earth/CPC at the MET as a checksum.  mA would be OK but anything over lets say 1 or 5 amp would tringger the device.  This would be for domestic single phase.

Sergio Fernandez said:

What about using a CT (Current Transformer) on the Earth/CPC at the MET as a checksum.  mA would be OK but anything over lets say 1 or 5 amp would tringger the device. 

Depending on the area in which you live, if you have extraneous-conductive-parts that are shared with other installations, such as metallic gas and water pipes, it's not unknown for a portion of your installation load current to return through those pipes.

So, I can see where you're coming from with 1 A and 5 A ... but there could be frequent unwanted operation of the device in some installations.

Hmm If we have access to the concentric main to the service head we could perhaps have a clip on clamp around the whole supply cable, and then in the manner of the 'Vigi' earth leakage relays, where the current camp and the breaker are separate, break conductors on the consumer side.

however, such an arrangements would  trip on any diverted neutral currents that occur during a normal PME with parallel paths via water mains and binding in neighbours properties etc.

I fear the safer thing to trip on is Neutral to terra firma voltage, rather than out of balance current, despite the ghostly echos of a technique found to be inadequate in the 1970s, and deprecated in the regs from the 1980s onward.

Mike

The standard isn't there to tell the manufacturer precisely how to design and manufacture the product, just the relevant parameters it ought to meet.

Indeed - but if the standard says to measure one parameter (_ICPC) with no alternative options and the desired design measures something different (SUM(I_L+I_N_I_CPC)) with the effect that under some circumstances passing >21mA through the c.p.c wouldn't register as a problem (because it was partially or fully balanced by L or N currents) even though the overall effect was just good, it's going to be hard to show that the new design complies with the standard.

whilst a 4P (TP&N) RCD might appear to do the trick for three-phase:

No, I was suggesting a 3P or 4P RCD might be good for a single phase application (all three wires (L/N/PE)) - for example by feeding the c.p.c. through one of the otherwise unused L2 or L3 contacts. Some care would be needed - e.g. not using a pole that was involved in the test circuit, or powering the RCD itself, but doable with most designs on the market I suspect (even if relying on specific manufacturer data rather than jut generic BS EN data). For 3 phase it's a lot less of a problem since you could use the artificial N approach which doesn't have the myopia of the L-N voltage measuring approach, so M3 is far less critical - or if you did need it then the earth leakage relay approach where you can feed all the conductors you like through the torroid might work.

   - Andy.

The standard isn't there to tell the manufacturer precisely how to design and manufacture the product, just the relevant parameters it ought to meet.

Indeed - but if the standard says to measure one parameter (_ICPC) with no alternative options and the desired design measures something different (SUM(I_L+I_N_I_CPC)) with the effect that under some circumstances passing >21mA through the c.p.c wouldn't register as a problem (because it was partially or fully balanced by L or N currents) even though the overall effect was just good, it's going to be hard to show that the new design complies with the standard.

whilst a 4P (TP&N) RCD might appear to do the trick for three-phase:

No, I was suggesting a 3P or 4P RCD might be good for a single phase application (all three wires (L/N/PE)) - for example by feeding the c.p.c. through one of the otherwise unused L2 or L3 contacts. Some care would be needed - e.g. not using a pole that was involved in the test circuit, or powering the RCD itself, but doable with most designs on the market I suspect (even if relying on specific manufacturer data rather than jut generic BS EN data). For 3 phase it's a lot less of a problem since you could use the artificial N approach which doesn't have the myopia of the L-N voltage measuring approach, so M3 is far less critical - or if you did need it then the earth leakage relay approach where you can feed all the conductors you like through the torroid might work.

   - Andy.

... also if you did got down the all-wires (L/N/PE) residual monitoring approach, there presumably would be less need to tolerate normal protective conductor currents - so maybe the limit could be set significantly lower than 30mA (or 21mA) - 10mA say or maybe even lower (don't the Yanks like 6mA devices?) - giving better protection where the path to Earth has a high resistance (due to footwear etc) but still allowing enough current to flow to give a nasty perceived shock.

   - Andy.