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.

 

[SUNRAY]

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 ?

Yes

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

Which is way above that considered by some to be lethal.

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

It was certainly feeding a different location than expected but what are you trying to get at?

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.

I consider myself chastised but believe later comments did correct the error (wherever it is - I haven't gone back over to see).

 

[SUNRAY]

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 ?

Yes

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

Which is way above that considered by some to be lethal.

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

It was certainly feeding a different location than expected but what are you trying to get at?

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.

I consider myself chastised but hopefully recent comments are correct, I haven't gone back over to see where I made the confusion.

 

[JohnW2]

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 ?

Yes

[A 'double post' from you again ! ]
Glad you agree!

Which is way above that considered by some to be lethal.

I think you may be getting yourself confused here? 60 mA through a person may well be lethal, but we are talking about the current through (the N side of) a load protected by the same RCD that would cause an N-E fault on that circuit to result in a trip of the RCD. For that to happen would presumably require someone to be in contact with both L and N (with a body resistance of about 3.8 kΩ) and for there simultaneously to be a N-E fault - which sounds like an almost unbelievable combination of circumstances, doesn't it?

It was certainly feeding a different location than expected but what are you trying to get at?

The only thing I was trying to 'get at' was that, had it not been for subsequent events/revelations, you might have wasted a fair bit of time searching for the cause of a N-E fault in the wring garage!

I consider myself chastised but hopefully recent comments are correct, I haven't gone back over to see where I made the confusion.

Yes, you have subsequently clarified - but at the time when I had only seen your original reference to "a very low resistance", that choice of words resulted my in interpreting them (I would say 'reasonably'!) as meaning something which was not necessarily the case!