I appreciate that this subject has been touched on in the past.... and that the culprit is capacitive coupling from the electronics inside the washing machine..... However, today I was asked to investigated an electric shock incident with a washing machine and a stainless steel sink in a kitchen break room. The root cause of the shock was attributed to a loose earth wire in the fused spur connection to low level socket..... all wired in plastic trucking.

Having recreated the fault by removing the earth wire from the back of the socket to the dishwasher we measured 110V from the chasis of the machine to an adjacent socket earth reference/ sink. Proved with Martindale unit....

The question I have is 'Is there a maximum voltage that can be presented to earth by the electronics of these machines?' Surely 110V is a little too high given that a 'loose earth' can be a real possibility?

tis why we consider high protective conductor currents

What did you measure it with incidentally I'm guessing a good old fashioned moving iron meter wouldn't have shown that voltage

regards

OMS

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Failure is always an option

I would expect about 110v. Most modern washing machines have inteference suppression capacitors connected between phase, neutral and earth. The metal casing sitting around half mains potential is quite believable.

Regards,

Alan.

I disagree there, though it can happen, a loose earth is not a possibility considered in BS7671, If you think it's a possibility that someone may cock up the wiring of a plug, then you could provide a fixed earth to the appliance but this would seem to defeat the point of having a plug-in appliance.

Surely 110V is a little too high given that a 'loose earth' can be a real possibility?

That's why the machine has an earth conductor in it's mains lead, you can't blame the design of the machine just because it's connected to a faulty circuit!

Dave.

So what's 543.7 all about then ?

regards

OMS

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Failure is always an option

Never been quite sure on that - I have always made my earth connections with the best intention that they don't fail, having to provide 2 earth connections seems to me to be designed for the expectancy that one connection might fail, though if both connections have been made by the same person at the same time, using the same methods and materials then surely both are as likely to fail!

Statistics would show us that they won't - it's why you see say 3 relays in series on a safety interlock - each and every one of them has a failure rate - but the collective failure rate is significantly lower based on the probability of all 3 failing together to an unsafe condition - ie only one has to open to cause shutdown

same situation with the dual earth terminals in reverse

Regards

OMS

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Failure is always an option

Sorry to change direction here , but isn't it funny that installing 2 30mA RCD's in parallel is frowned upon!

Did you mean in series Weirdbeard? putting them in parallel could make a 60m/a trip if they both were to go at exactly the same time!

Dave.

No Dave, i mean't in parallel, for example in a domestic situation, if you have a 30mA protected main board, and a shed or garage is connected to an already RCD protected circuit, I have heard that it is frowned upon to put another RCD in the garage/shed board - though personally at home some of my sockets have upto 4 30mA in parallel including the upfront device, being clumsy as I am, if I need protection against a shock when in the garden I don't mind if the TV goes off in the house!

They would be in series then!

Thanks for the feedback.

The faulty wiring should have been picked up..... but these things do happen, poor workmanship.... etc.. upon inspection we found the sink was un bonded and 'floating' a little too....
I appreciate that having 'dual earths' would reduce the risk of a single earth failing and therefore reduce the risk of the electronics earth to 'float'.....

I didn't touch the 110V to see if you would get a 'belt' off the appliance..... someone else had already done this.... but surely the voltage presented should be below 50V to ensure that a 'faulty appliance' that is working does not harm those touching it for the sake of a loose earth wire? This is Class 1 equipment...... borrowing the earth....

The voltage was 'proved' with a Martindale unit..... LED's illuminated to 100V...... which I think 'proves' that we had a real voltage source.... ? I'm not sure what impedance the Martindale units present, but if it can drive the LED's on the Martindale that is good enough for me.....

Thinking about it now, maybe we should have taken a current reading from the appliance to earth to see what the 110V source could deliver? 3.5mA is the maximum, yes?

No Dave, as protective devices can't be in series, they wouldn't work as intended - what I mean is if I have an rcd adapter plugged into an extension lead that is plugged into a garden SRCD, that is supplied from the shed that has a 30mA trip, and the house has a 30mA main trip, then at the far end of the circuit if there is a problem that may be life threatening then anyone or all of the devices may trip, If I trip a device and survive to be able to walk up the garden to reset the device(s) then they are OK in my book!

The reason for such an arrangement is just what I had to hand at the time (the MK masterseal SRCD was worth £90 at the time, would have seemed a shame to let it go to waste! )

Perhaps you should also consider dual earths to the sink?

the sink was un bonded and 'floating' a little too

Metal sinks don't need to be bonded these days, in fact they're less of a shock hazzard unbonded than bonded!

Like alan said you expect the filter caps to sit it about half supply as these are often arranged as a delta type arrangement between L/N/E on single phase additionally any leakage thru them or thru the circuitry will likely want it to sit approx half way too.

Any high impedance tester will let it sit about its voltage wheras any loading it will pull it down as OMS elluded to/

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

I agree - they'd be in parallel in my mind if they had all their corresponding terminals commoned (Lin-Lin, Lout-Lout, Nin-Nin, Nout-Nout), connecting the output of one to the input of the next is in series to me (although perhaps not as obvious as simple lamps in series).


If you mean the input of one RCD connected to the output of another, then I'm not sure it is - indeed BS 7671 explicitly requires it in some circumstances - e.g. one feeding a caravan pitch and another inside the caravan. The consideration for discrimination/selectivity doesn't really apply if there aren't any branches between the two devices.

- Andy.

Spot on!

If you had a perfect volt meter - i.e. one that drew no current to measure the voltage, you'd get 230V get by holding the probe next to a line conductor - the conductor, insulation and probe would make a capacitor and with no current flowing the same potential would appear on both plates. The insulation of course keeps the current so small that no harm results - if anyone touches the insulation or tries to measure it with a real voltmeter, the current is drained away and the voltage collapses.

In this case the c.p.c. is connected to both L and N by capacitors - which effectively form a potential divider (like two equal value resistors in series across mains and measured to the centre point) hence if left floating it would at at about 115V. Any attempt to measure the voltage will cause a (small) current to flow, dragging the actual voltage down to earth (or whatever voltage you have the other probe at). The lower the internal resistance of the voltmeter, the more current is drawn and so lower the measured voltage - hence use an old moving coil meter and the problem seems much less.

Under the right (wrong) conditions, currents as low as 1mA can be felt (perceived electric shock) - but don't usually cause any direct harm a those sort of level. Always worth being wary of indirect harm though - e.g. instinctively pulling away and falling of a ladder as a result.

- Andy.

Weirdbeard, if you have one rcd feediing another rcd feeding another rcd you have three rcd's in SERIES, Sorry to keep on about this but that's the way it is! For them to be in parallel you would have to wire all the lives together and all the neutrals together. Think of batteries in series and batteries in parallel. Two 1.5 volt batteries in series, pos to neg, = 3 volts output. Two 1.5 volt batteries in parallel, pos to pos, neg to neg, = 1.5 volts but double the current,

Dave.