Socket behind dishwasher pt.2 - are we being had?

[ban-all-sheds]

I was thinking more of a much lower resistance L-E fault (approaching a 'dead short') which did have the potential to cause a dangerous touch voltage - but was pointing out that, in most situations, the circuit's OPD would immediately clear such a fault, even in the absence of an RCD. Is my reasoning not correct? (and that' a question as much for BAS as for you)

Well I was thinking of anything but a "dead short". EFLI talked about "partly worn elements", and one doesn't need to postulate a remarkably unlikely dodgy fault loop (Ze a bit too high because of a corroded cable and a TN-S supply, a long-ish flex, slightly worn/grubby socket contacts) to end up with a touch voltage of >50V and a fault current that doesn't blow, or quickly blow, a 13A BS1362, let alone trip a B32. You get half-way to the impedance needed with the maximum allowable Ze and 5m of 1mm² flex....

 

[JohnW2]

Well I was thinking of anything but a "dead short". EFLI talked about "partly worn elements", and one doesn't need to postulate a remarkably unlikely dodgy fault loop (Ze a bit too high because of a corroded cable and a TN-S supply, a long-ish flex, slightly worn/grubby socket contacts) to end up with a touch voltage of >50V and a fault current that doesn't blow, or quickly blow, a 13A BS1362, let alone trip a B32. You get half-way to the impedance needed with the maximum allowable Ze and 5m of 1mm² flex....

I don't think 1mm² flex is realistic for feeding a cooker . Let's assume 1.5mm². 5 metres is a rather excessive for a cooker lead - let's say (still generous!) 3m, which equates to a CPC impedance of about 0.048Ω. Assuming a B32 is protecting the circuit, the maximum Zs (if in the UK) would be about 1.37Ω, and (being lazy) let's guess that the 'earth' part of that is about 0.9Ω. The total path to earth is therefore about 0.948Ω, which would require a current of about 53A to get a touch voltage of 50V, even if the 'leak' were right at the L end of the element (higher current if the leak were further downstream) - which would trip a B32 within about 1.5 minutes ... and, incidentally, not far off being a "dead short" (fault impedance about 2.8Ω). With such a fault (in the UK) a 13A BS1362 fuse would blow in about 0.1 seconds.

Admittedly, that is far from 'instant tripping' of the MCB, but it's very improbable that someone would be touching the appliance (and a path to true earth) in the ~1.5 mins after a fault right at the L end of the element first appeared. In any event, in the UK, if there were a 13A BS1362 fuse in the equation, then that should blow within about 0.1 seconds of the onset of a fault which resulted in a 50V touch voltage - which I think would probably be acceptable.

... unless, of course, I've got my sums wrong!

Kind Regards, John