External garage electricity wrongly connected to neighbours supply

[JohnW2]

The order of testing: ... House CU: ... With RCD off, testing on RCD O/P IR L-N showed something too low to read on the IR Robin

OK - so when you said 'very low resistance', you meant 'too low to measure with an IR tester', rather than (necessarily) truly 'very low resistance' ?

.... Checked garage wires E-N still 1500Ω, E-N & L-N still O/C. .... At that point I've proved an unexpected low resistance E-N and L is clear, (>200MΩ) on the garage circuit.

OK. Fair enough - so what you had 'proved' at that point was that there was "an unexpectedly low resistance E-N on the 'wrong' garage circuit" ?

 

[SUNRAY]

OK - so when you said 'very low resistance', you meant 'too low to measure with an IR tester', rather than (necessarily) truly 'very low resistance' ?

OK. Fair enough - so what you had 'proved' at that point was that there was "an unexpectedly low resistance E-N on the 'wrong' garage circuit" ?

I was in the house CU where tripping proplems were occuring and I proved a fault on the only garage circuit.
It also depends on what one considers to be low resistance compared to ∞, the IR test thinks it is.

 

[JohnW2]

Assuming the N-E link is immediately before the RCD I'll place a figure of 1mΩ, assuming the faulty circuit is my garage example of say 25m of 2.5mm² so in the order of ½Ω it doesn't take too much calculation to establish that in order to get 30mA in the fault will rewuire 500x30mA or 15A so yes it will trip under a modest current

Yes, I should have acknowledged that a trip could occur 'with a modest load current' in a TN-C-S installation if the N-E fault was 'relatively close' to the RCD.

However, perhaps we can agree that an N-E fault of a given impedance at a point on the circuit which has a given R1+R2 is much less likely in a TN-C-S installation than in a non-TN-C-S one ?

However assuming there is no source side influence it can be seen it only requires a 60mA load (assuming the N-E fault is a direct short)

Yes, if the only load in the installation is on the circuit with the N-E fault, then essentially half of the neutral current will NOT go through the RCD - so, as you say,a load current of 60 mA should be enough to result in a trip

 

[JohnW2]

I was in the house CU where tripping proplems were occuring and I proved a fault on the only garage circuit.

Indeed - but the 'only garage circuit' which was not feeding the circuit you thought it was feeding

It also depends on what one considers to be low resistance compared to ∞, the IR test thinks it is.

True, but I personally would not use the phrase "very low resistance" to refer to a situation in which all I knew was that the resistance was lower than an IR test could quantify.