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.

... 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.

Good morning all, quick one from me - Ive heard a lot of people within industry discussing this. A quick question from myself in terms of the overall installation process and the table released in the publication on line - wiring matters 96 July 23. In the table it references 230 volts, which is a fair starting point. However I very rarely see this constant voltage and almost all the issues I have seen with O-pen and problem tripping occur with an overvolatge. 

The question I have - is the figure of 3 % a generic guidance/illustration figure as I am now seeing this being used as the figure for Max VD in cable calculations? And people using terms such as we can only use 39 metres of cable  6mm? 

How is this viewed in terms of an install where the voltage sits at 246 volts for example - where the voltage above 230 is more than the 3% tolerance discussed? I appreciate volatges will always fluctuate - but restricting location and length of run based on this ????

I would love to know the thoughts of others on this - are we saying this is a guidance piece in terms of the 3% and open to change re the voltage at site? 

Thanks 

The supply voltage is rarely constant - Ohm's Law is at play in the DNO's cables as well as within installations. So 246V in the middle of the afternoon might well drop to something significantly lower at 1am when next door's storage heaters kick in on their E7 tariff, or when the factory down the road starts up in the morning, or (possibly more relevant these days) when a street full of EVs switch on when a cheap tariff starts. So 'acceptable' voltage drop has always been a bit arbitrary - it's been anything between 2.5% per final circuit to 5% for entire installation within my memory.

Operational voltage at the appliance isn't the only consideration though - 3% v.d. also means 3% wasted power - and with large loads - e.g. 7.2kW EV means over 200W lost in the cabling - that's quite a few pence every charging session, maybe tens of pounds a year, hundreds of pounds over the lifetime of the installation - so the cost of going up a cable size or two rapidly becomes easy to justify both economically and environmentally.

   - Andy.

Thanks as always Andy for the detailed answer.

I agree with every point you have made above. In regards to the new publication, how do you view that? Are you of the opinion that the 3% is a guidance or would you look to use that figure when designing a circuit?