External garage electricity wrongly connected to neighbours supply

[ericmark]

Working with this

as the volts increase, so does the mA leakage, at 4 volts within limits, at 12 volts near the edge, and by 15 volts tripped.

 

[JohnW2]

Working with this ...... as the volts increase, so does the mA leakage, at 4 volts within limits, at 12 volts near the edge, and by 15 volts tripped.

Yes, but only because you have introduced a 500Ω N-E fault.

As I've written, if there were any loads on the installation, introducing such a fault path would cause the RCD to trip even if the N-E voltage were zero.

 

[SUNRAY]

is it 1A? Do I get a lollie?
Until the rcd trips

Give me your address, I'll put a cornetto in the post.

 

[SUNRAY]

So 1 amp for around 20 milliseconds, but would not be able to set the RCD on.

Well you can when the ammeter is taken away.

 

[SUNRAY]

Many electricians in my experience, do not understand that a N to E short can trip an RCD.

Far too many in my opinion.

 

[SUNRAY]

Yes, but only because you have introduced a 500Ω N-E fault.

As I've written, if there were any loads on the installation, introducing such a fault path would cause the RCD to trip even if the N-E voltage were zero.

Well in the above mentioned garage there were NO LOADS running, are you trying to say the RCD shouldn't trip?

 

[SUNRAY]

lol

That would have been more amusing to find out they that been in the wrong garage for some time, esp. if they had been modifying it with shelving, workshop things etc.

That is what happened further along the row

...Apparently further along the row there had been a mix-up with which garage was being used when a house sold and buyer found his garage keys didn't fit ...

 

[SUNRAY]

Other than in TN-C-S installations, there is not uncommonly a significant N-E potential difference (due to VD in the supply neutral) - and, indeed, even with TN-C-S there is not uncommonly a significant PD between the CNE/PEN (hence N & E within the installation) and 'true earth' (for the same reason). However, in no case does that result in any 'potential for tripping' of an RCD.

Some 50 years ago we usually had enough voltage N-E to be able to rectify it and float a car battery on it to run my 19set on it's dynamotor. I ran it like that for a couple of years without any other form of 'charging'.

 

[JohnW2]

Well in the above mentioned garage there were NO LOADS running

Maybe, but (a) you hadn't told us that, having merely written ...

.... N-E has over 1V so some potential for tripping, .....

... and (b) I have consistently made it clear that I was talking about the situation in which there were some active loads in the installation., and ....

in the above mentioned garage there were NO LOADS running, are you trying to say the RCD shouldn't trip?

Not at all. On the contrary, I'm not only saying that the RCD should trip in the situation you described, but that it should also trip (in almost all real-world situations) even if the N-E potential difference were zero. I say that because, even if the N-E pd were zero, the RCD would be expected to trip if there were any significant loads anywhere in the installation (not necessarily in the same final circuit, nor even a final circuit protected by the same RCD) - and (other than for an unoccupied property) it would be an extremely unusual real-world situation for there to be no significant loads on any of the circuits for any appreciable period of time.

 

[ebee]

Yup, without a load an NE short would not cause an RCD to trip. Once a load is running then once the N to L balance gets to the threshold of an RCD trip current then yes it will trip.
If that RCD short is at or near zero ohms the using a 30mA it would be in the region above 15mA , if that "short" has some more resistance (impedance) then a higher current would be the practical trip point of an "NE short". It all depends of the actual value of the short and the actual value of current running to reach the actual value of the trip current.

 

[JohnW2]

Yup, without a load an NE short would not cause an RCD to trip. Once a load is running then once the N to L balance gets to the threshold of an RCD trip current then yes it will trip.

Indeed, and that remains the case if the load is on a different final circuit from that with the N-E short (even, I think, a final circuit protected by a different RCD).

However, this bit of the discussion all arose because Sunray wrote/implied that a trip due to an N-E fault would be more likely if there were a significant pre-existing N-E potential difference. Whilst that would be true if there were absolutely no loads on the installation (on any final circuit), that would seem to be a very rare situation (other than in an unoccupied building) - and if there are any loads on the installation (as there nearly always will be), then an N-E fault will usually cause an RCD trip even if the prior N-E potential difference were zero.

 

[EFLImpudence]

(even, I think, a final circuit protected by a different RCD).

Surely that cannot be right?

If it were, would it not mean that all RCDs (e.g. a row of RCBOs) would trip with a single fault?

 

[ericmark]

Yup, without a load an NE short would not cause an RCD to trip.

With a TN-C-S supply, any other supply there is no way to be sure neutral and ground have no potential difference, I know with some supplies we put diodes in the earth connection to allow a small difference without current flowing, all part of the cathodic protection, but only seen that with canal boats.

Since the early days, we have seen how a fault with a toaster can trip a RCD when the kettle is used. But permitted volt drop of 5% means 11.5 volt at 30 mA = 380 Ω approx. To get that low, nearly a short circuit. Even if we consider a background leakage of 9 mA, and it trips at 15 mA (half) needs around 2 kΩ to trip. And since only 11.5 volt, only a standard multi-meter required.

 

[JohnW2]

Surely that cannot be right?

It is right Part of the N current from a load protected by a different RCD goes 'backwards' through the N side of the RCD protecting the circuit with a N-E fault, thereby creating an L/N imbalance (hence a trip, if enough imbalance) of that RCD ...

If it were, would it not mean that all RCDs (e.g. a row of RCBOs) would trip with a single fault?

No. The full N current from load (as well as the full L current from the load) goes through the RCD protecting the circuit(s) with a load ("RCD1" in diagram above), hence no trip of that/those RCD(s)

 

[JohnW2]

With a TN-C-S supply, any other supply there is no way to be sure neutral and ground have no potential difference,

With anything other than a TN-C-S it is surely inevitable that there will be some N-E potential difference, isn't it? Absence of some N-E pd could only occur if the length of neutral conductor from transformer to installation is zero (impossible), or if no customer served by the transformer is drawing any current (incredibly unlikley)?