Neighbour's power tools tripping my RCD?

[bernardgreen]

What about this,

The house where the RCD is tripping has a neutral to ground fault. With the incoming neutral at the same potential as ground this fault will not be passing any current so the RCD will not trip

Neighbour switches on a large motor with a high inrush current. The high in rush current along the network creates a transient potential spike on the network neutral. This raises the neutral above ground in houses in the area. The house with a neutral to ground fault will have a transient current through the fault and this coulr trip the RCD.

Adding a local ground rod to the incoming neutral would reduce the transient differences beten and maybe that was what the DNO did here

- each time the DNO came and had a look at the pole connections, denied it was anything to do with them .... but the tripping stopped happening anyway.

They added a ground rod at the bas of the pole.

 

[JohnW2]

What about this, ... The house where the RCD is tripping has a neutral to ground fault. With the incoming neutral at the same potential as ground this fault will not be passing any current so the RCD will not trip

I don't really get that at all. Even if the supply neutral and earth were at exactly the same potential (unlikely with the TT installation), an RCD would surely still be expected to operate if some of the return current from the house's installation was going to earth (vian an N-E fault), rather than through the N side of the RCD, wouldn't it? Am I missing something?

Kind Regards, John

 

[ceecjc1]

Firstly, as regards the trips you have been experiencing in recent weeks when the dishwasher was on, were these trips also always of the 100mA RCD, with no 30mA one ever tripping? Secondly, do I take it that your dishwasher was not on when you experienced the trips whilst your neighbour was sanding?

Hi John,

It's always the 100mA RCD which trips. The 30mA RCDs have never tripped.

The dishwasher was on every time the neighbour was sanding. The dishwasher is always on when the RCD trips but the RCD does not trip every time the dishwasher is on.

The element in the dishwasher has recently appeared a little corroded, or discoloured, I'm not sure how to describe it exactly. I was wondering if this was the cause of the RCD trips until the neigbour's sanding set off a flurry of trips.

Regarding the overhead power lines - it's impossible to tell how the power comes into the house. The lines come to a nearby pole and the power appears to come to the end of the three-house-terrace first, then to the middle, then to ours. I don't know if it's daisy chained or what.

The agents have just arranged an electrician to come and investigate

Thanks,
Chris

 

[bernardgreen]

Neutral to ground fault rather than neutral to CPC ( "earth" ) fault.

While the neutral potential is close to ground potential there will be some leakage but close to zero, some load current will go via the neutral-ground fault and by pass the RCD sensor. Normal load currents in the house with the tripping RCD will lift it's neutral above ground ( voltage drop due to current through the supply neutral's impedance creating a voltage drop along the network. )

This normal difference between neutral and ground will be small until a large inductive load is connected when the voltage drop along the neutral due to the inruch current will be much higher than normal.

When a past neighbour switched on a 2 Kw band saw the transient voltage drop in our house ( same phase ) was measured as between 20 to 30 volts suggesting a neutral to ground voltage of 10 to 15 volts. 15 volts on a 500 ohm neutral to ground fault would be enough to trip a 30 mA RCD

 

[JohnW2]

Hi John, It's always the 100mA RCD which trips. The 30mA RCDs have never tripped. ... The dishwasher was on every time the neighbour was sanding. The dishwasher is always on when the RCD trips but the RCD does not trip every time the dishwasher is on.

Right - that ought to give the electrician a good start in getting to the bottom of all this. It sounds to me as if there is probably something wrong with at least one of your RCDs. If the dishwasher is, indeed, the primary culprit, then the fact that it's taking out the 100mA (time-delayed) one, but not the associated 30mA one, would have to mean that the latter was faulty, and needs to be replaced. If the 100mA one is tripping spontaneously, 'for no good reason', then that is clearly faulty.

The element in the dishwasher has recently appeared a little corroded, or discoloured, I'm not sure how to describe it exactly. I was wondering if this was the cause of the RCD trips until the neigbour's sanding set off a flurry of trips.

It's obviously what is going on inside the element that matters, and you can't see that - but it's certainly far from an impossible cause. Your electrician may be able to help, but since the problem is clearly intermittent, it might not be all that simple - i.e. it may appear fine when he tests it, but not 10 minutes after he goes out of the door!

The agents have just arranged an electrician to come and investigate

Great. It would be good if you could let us have an update, after the electrician has investigated and deliberated!

Kind Regards, John

 

[JohnW2]

Neutral to ground fault rather than neutral to CPC ( "earth" ) fault.

I'm getting confused about where this hypothesised fault is meant to be. If within the installation, then (given required earthing and bonding) there shouldn't be any manifestations of 'ground' which aren't connected to CPC/MET - or are you talking about 'damp walls' again? Also, are you distinguishing between 'ground' and the OP's TT earth (which clearly will be connected to CPCs)?

While the neutral potential is close to ground potential there will be some leakage but close to zero, some load current will go via the neutral-ground fault and by pass the RCD sensor. Normal load currents in the house with the tripping RCD will lift it's neutral above ground ( voltage drop due to current through the supply neutral's impedance creating a voltage drop along the network. ) ... This normal difference between neutral and ground will be small until a large inductive load is connected when the voltage drop along the neutral due to the inruch current will be much higher than normal.

That's largely true, but why are you concentrating on 'large inductive loads'? Even a normal, largely resistive, load will result in a voltage drop in the supply cable. As we all (I imagine!) know only two well, an N to CPC (and it ought to be the same with N-ground) short will usually cause an RCD to trip immediately, with only 'ordinary' loads present, even in a TT or TN-S installation.

Kind Regards, John

 

[bernardgreen]

but why are you concentrating on 'large inductive loads'?

Because they have high inrush currents many time the normal running current. In the case of a transformer the inrush current lasts at most one cycle of the mains. In the case of a motor the current remains high ( greater than nornal running current ) until the motor has reached normal operating speed.

 

[JohnW2]

but why are you concentrating on 'large inductive loads'?

Because they have high inrush currents many time the normal running current. In the case of a transformer the inrush current lasts at most one cycle of the mains. In the case of a motor the current remains high ( greater than nornal running current ) until the motor has reached normal operating speed.

Yes, I understand all that but, as I said, a low-impedance N-E (or N-G) fault will usually result in immediate operation of an RCD even if there are only 'ordinary' loads, drawing modest currents. If it happens with 'ordinary, modest' current loads, it's surely hardly surprising that it also happens with 'abnormally large' current loads!

Kind Regards, John

 

[bernardgreen]

Apply Kirchoff's laws the to closed circuit formed by the neutral, ground and fault.

Untitled

• bernardgreen
• 21 Oct 2013
• 1

The load current will create a voltage drop along the neutral as the load current passes through the network impedance Z and this voltage will drive current through the sensor on the neutral conductor and then to the ground via the fault.

With a 500 ohm fault resistance a volt drop of 15 volts across Z will create enough current through the fault to cause the RCD to trip.

Assume Z to be say 0.1 ohm then 150 amps of load current in the house would be needed to create the 15 volts. Or 50 amps in the house and 100 amps from next door's motor and other loads.

An in-rush current of 150 amps in the motor next door would create that 15 volts without any load in the house.

The resistance of a stationary 1500 watt motor in a chop saw measures 4.1 ohms, resulting in a single half cycle worse case starting current of 58 amps

With other values of Z and fault resistance and the load current in the house adding to the voltage drop across Z the trip of the RCD can be triggered by the transient from next door.

 

[JohnW2]

With a 500 ohm fault resistance ....

That's the crucial bit you failed to mention when you proposed this scenario/mechanism

Yes, if there is a N-E fault in the installation which, in itself, is of too high an impedance to result in an RCD trip under normal load conditions, then (with TT or TN-S, but not TN-C-S) any sufficiently large load on the local part of the supply network (whether due to the inrush current of the neighbour's motor or anything else) could push the L-N imbalance 'over the threshold' and result in the RCD tripping.

Kind Regards, John

 

[ceecjc1]

Great. It would be good if you could let us have an update, after the electrician has investigated and deliberated!

Will do.

Thanks for everyone's help!

Cheers,
Chris

 

[westie101]

but not TN-C-S

I've seen it happen on TN-C-S with a too sensitive RCD. That there was a high resistance N-E fault was not checked but as it was a Spa room it was quite possible

 

[EFLImpudence]

Have I missed something?

Why would this fault of 500Ω not cause tripping when anything was switched on within the installation?

 

[JohnW2]

but not TN-C-S

I've seen it happen on TN-C-S with a too sensitive RCD. That there was a high resistance N-E fault was not checked but as it was a Spa room it was quite possible

Interesting. However, I don't really see how the mechanism Bernard was discussing could be applicable with TN-C-S unless there was fault in the installation from N to 'true earth' ('ground', as Bernard was calling it), rather than the installation's (TN-C-S) earth ... so maybe the mechanism was somewhat different in the case you're describing?

Kind Regards, John

 

[JohnW2]

Have I missed something? Why would this fault of 500Ω not cause tripping when anything was switched on within the installation?

It might do (which is why I talked about an N-E fault of too high a resistance to, in itself, cause an RCD trip with whatever load was present - without actually mentioning any numbers ).

AsI see it, for a 500Ω N-E fault to result in a 30 mA imbalance in the RCD would require that, with whatever loads were drawing current, the N of the installation was 15V above the potential of the (TT or TN-S) earth. For 15 mA imbalance (the minimum that might result in a trip), the pd would have to be 7.5V. If the N-E potential difference at the time was not as high as those figures, then the 500Ω N-E fault would not produce enough imbalance for the RCD to trip.

... so, as I see it (and, as we know, I've often been wrong!) it really all depends upon what the N-E pd happens to be at the time.

Kind Regards, John