ebee:
Interesting Lyle,
It does enhance a bit my perception of two cascading RCDs to a TT to add resiliance.
In this instance even a Delayed 100mA RCD prior to a 30mA non delayed might have saved the day.
Different makes in different locations might help a little too.
OK a 100 mA and a delayed RCD does not count as personal protection but in instances such as this a bad fault and a non functioning RCD might have caused the TD 100 to trip seconds before touching occoured thereby giving a warning that something was wrong
Indeed Ebee! Where enhanced resilience is required following the assessment of the risk, it might be difficult to contend that a single RCD is an adequate control measure. Just because 7671 doesn’t require it doesn’t mean it isn’t required!
lyledunn:Indeed Ebee! Where enhanced resilience is required following the assessment of the risk, it might be difficult to contend that a single RCD is an adequate control measure. Just because 7671 doesn’t require it doesn’t mean it isn’t required!
It is not clear whether the failed RCD was a 300mA, 100mA or 30mA. 705.411.1 mentions all three types.
Assuming it was a 30mA, you would have thought, it being a barn, that 705.422.7 would come into play, and an additional 300mA RCD , possibly time-delayed, would need to have been fitted.
John Peckham:
I am thinking that the ground is sitting at around 0V and the outside tap is at 187V with respect to the general mass of earth as I cannot see the line to earth fault lifting the voltage of the general mass of earth to 187V some distance away from the site of the fault.
If the copper water pipe in the house is bonded it is directly connected to the transformer neutral so how can it sit at 187V to Earth?
If the water pipe is not bonded and/or the transformer earth is disconnected then that is a different story.
I would like to think that the DNO would have checked the transformer earthing.
It was I who asked whether there was a common metal water supply and I don't think that Lyle has answered that one, but if that is the case, the absence of adequate main bonding in the house would explain the situation.
lyledunn:
Above is my interpretation. There are no common services.
OK, thank you, that's interesting. The tap can only be at 0 V if it is bonded; but if the water supply is metal, why has it not reduced the ground potential by the tap? Or is it a plastic supply, and if so does this illustrate that under a rather unusual set of circumstances, unnecessary bonding may be hazardous?
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