Sanity Check - RCD Trips with Safe Isolation

[dizz]

Ring feeds 5A FCU

FCU -> Junction Box -> Switch / Feed to Transformer

Transformer feeds 4 downlights

Need to replace Transformer to have four lights off 240V

Put in new 1mm T&E from JB to feed four lights

Just got the cable to the last light and cut it and the RCD trips

Before my cabling, I Isolated the JB by removing the 5A fuse from the FCU -> checked voltage before and after at the JB. 240V before switch and 216V after switch (dimmer). With 5A fuse removed, no pd.

Am I going mad - no-one else touched anything, and the instantaneous trip at precisley the time I cut the cable implies the cable was live?

 

[EFLImpudence]

Just got the cable to the last light and cut it and the RCD trips

Not going mad - that's what happens when you connect Neutral & Earth even when MCB/Fuse open.

 

[sparkticus]

No, you shorted Neutral and Earth. That will trip an RCD due to small currents flowing between N - E

On a TN-C-S system when you are close to the intake then it may not happen since N&E are quite close in potential at that point.

 

[dizz]

Ok.

1. So it was perfectly safe doing what I did?

2. So, I guess I should be cutting the wires before I make the connection at the JB.

3. I thought the RCD measured the difference in current flowing through Line and back through Neutral - and tripped when there was a difference. Could you explain how the neutral in an open circuit affects the current flowing back to the RCD - That would really help me.

 

[sparkiemike]

No, you shorted Neutral and Earth. That will trip an RCD due to small currents flowing between N - E

On a TN-C-S system when you are close to the intake then it may not happen since N&E are quite close in potential at that point.

No quiet, the reason the RCD trips even though you have isolated (using single pole isolation) is that you have created a parallell path for the neutral.

If you still have a load(s) connected on other circuits, when you cut the cable you have effectively joined the neutral and earth. All the current returning to the transformer on the neutral now has a parallell path via the cpc/earth, this creates an imbalance in the RCD and trips it. This does not happen on double pole isolation.

 

[sparkticus]

Ok.

1. So it was perfectly safe doing what I did?

2. So, I guess I should be cutting the wires before I make the connection at the JB.

3. I thought the RCD measured the difference in current flowing through Line and back through Neutral - and tripped when there was a difference. Could you explain how the neutral in an open circuit affects the current flowing back to the RCD - That would really help me.

I would not class what you did as safe isolation.

Yes, build the circuit first then make the final connection.

No, an RCD detects and responds to the difference in current L - N
If L & N see the same current there is no difference and no trip.
You connected E to N via your wire cutters. In doing so you caused a small current to flow in the loop. Since the current did not flow in L then the RCD saw it as a current difference and tripped.

 

[dizz]

No, you shorted Neutral and Earth. That will trip an RCD due to small currents flowing between N - E

On a TN-C-S system when you are close to the intake then it may not happen since N&E are quite close in potential at that point.

No quiet, the reason the RCD trips even though you have isolated (using single pole isolation) is that you have created a parallell path for the neutral.

If you still have a load(s) connected on other circuits, when you cut the cable you have effectively joined the neutral and earth. All the current returning to the transformer on the neutral now has a parallell path via the cpc/earth, this creates an imbalance in the RCD and trips it. This does not happen on double pole isolation.

Brilliant - Thanks - So, the single pole FCU meant that all the other neutrals on the ring where the FCU resides was effectively connected, and ultimately, had a path to earth when I made the cut, as the neutral wasn't isolated at the FCU - with a double pole FCU, this would have been safe isolation. For single pole I would have had to open the MCB and the RCD would not have tripped - Is that right?

 

[sparkticus]

No, you shorted Neutral and Earth. That will trip an RCD due to small currents flowing between N - E

On a TN-C-S system when you are close to the intake then it may not happen since N&E are quite close in potential at that point.

No quiet, the reason the RCD trips even though you have isolated (using single pole isolation) is that you have created a parallell path for the neutral.

If you still have a load(s) connected on other circuits, when you cut the cable you have effectively joined the neutral and earth. All the current returning to the transformer on the neutral now has a parallell path via the cpc/earth, this creates an imbalance in the RCD and trips it. This does not happen on double pole isolation.

Brilliant - Thanks - So, the single pole FCU meant that all the other neutrals on the ring where the FCU resides was effectively connected, and ultimately, had a path to earth when I made the cut, as the neutral wasn't isolated at the FCU - with a double pole FCU, this would have been safe isolation. For single pole I would have had to open the MCB and the RCD would not have tripped - Is that right?

It is really more simple than that. Lets say you only have one single circuit from a CU (no other circuits)

Lets say it is a 1.5mm T&E for a single lamp.

You have an RCD in the CU and the single circuit is protected by that RCD.

You isolate the MCB so there is no LIVE. But there is a Neutral and Earth connection.

Lets say you have a TN-S earthing arrangement where the supplier provides an earth. That earth is connected to the Neutral at the sub-station transformer.

If you (at the house) then connect N to E with your wire cutters or any low impedance conductor such as a piece of wire then you have closed that loop. Small currents will then flow in that loop as a result of the small potential difference in the long lengths of cables.

That current is only seen by the N side of the RCD. The L side of the RCD does not see that small current. The RCD trips.

You do not need other loads on the circuit. You have closed the circuit with your wire cutters.


If you had a TN-C-S system where the N&E are joined at the service position then there is a possibility that when you join N&E that the RCD will not trip because the opportunity for a a PD to exist in the wiring is much reduced. However you can not always rely on that because in reality other circuits may capacitively couple to your circuit and cause small PDs.

 

[dizz]

OK - when isolating the MCB, I will only get small currents in the way you describe - when I think I am isolating a SP switch, it could be a lot worse, because, apart from the diffrence in potential of earth and neutral, which is still there for all circuits, I also have parallel paths of much more dangerous currents.

 

[EFLImpudence]

So, the single pole FCU meant that all the other neutrals on the ring where the FCU resides was effectively connected

Yes, all the neutrals are connected in the CU.

, and ultimately, had a path to earth when I made the cut

Yes.

as the neutral wasn't isolated at the FCU

Yes.

with a double pole FCU, this would have been safe isolation

Yes.

For single pole I would have had to open the MCB and the RCD would not have tripped - Is that right?

No, that would be the same. MCBs are single pole too.

When wiring start at the load and work your way back to the supply and connect last - after testing, of course.

 

[sparkticus]

OK - when isolating the MCB, I will only get small currents in the way you describe - when I think I am isolating a SP switch, it could be a lot worse, because, apart from the diffrence in potential of earth and neutral, which is still there for all circuits, I also have parallel paths of much more dangerous currents.

If I have understood you correctly then yes. Other energized circuits under load may raise the neutral potential with respect to earth. But you do not need that situation to see the effect that you observed. It can happen with just one single circuit - nothing else connected.

 

[PrenticeBoyofDerry]

When wiring start at the load and work your way back to the supply and connect last - after testing, of course.

It is also a good idea to test the cable on the reel prior to installing, as it can be a right pain if you have a faulty reel and you have already fitted without doing preliminary tests.

 

[sparkiemike]

Brilliant - Thanks - So, the single pole FCU meant that all the other neutrals on the ring where the FCU resides was effectively connected, and ultimately, had a path to earth when I made the cut, as the neutral wasn't isolated at the FCU - with a double pole FCU, this would have been safe isolation. For single pole I would have had to open the MCB and the RCD would not have tripped - Is that right?

No, double pole isolation means Line and Neutral are both open.
Even if you had opened the MCB, if the RCD is shared with other circuits it could have still tripped.

 

[sparkiemike]

It is really more simple than that. Lets say you only have one single circuit from a CU (no other circuits)

Lets say it is a 1.5mm T&E for a single lamp.

You have an RCD in the CU and the single circuit is protected by that RCD.

You isolate the MCB so there is no LIVE. But there is a Neutral and Earth connection.

Lets say you have a TN-S earthing arrangement where the supplier provides an earth. That earth is connected to the Neutral at the sub-station transformer.

If you (at the house) then connect N to E with your wire cutters or any low impedance conductor such as a piece of wire then you have closed that loop. Small currents will then flow in that loop as a result of the small potential difference in the long lengths of cables.

That current is only seen by the N side of the RCD. The L side of the RCD does not see that small current. The RCD trips.

You do not need other loads on the circuit. You have closed the circuit with your wire cutters.

If you had a TN-C-S system where the N&E are joined at the service position then there is a possibility that when you join N&E that the RCD will not trip because the opportunity for a a PD to exist in the wiring is much reduced. However you can not always rely on that because in reality other circuits may capacitively couple to your circuit and cause small PDs.

This probelm exists regardless of the earthing arrangement. TN-S, TN-C-S and TT all have their neutral point connected to earth at the transformer. If the neutral has a parrallel path back to the transformer then it will trip the RCD

 

[sparkticus]

No, double pole isolation means Line and Neutral are both open.
Even if you had opened the MCB, if the RCD is shared with other circuits it could have still tripped.

You do not need other circuits on the load side of that RCD to witness this effect. The small PD in the actual circuit you are working on including the loop all the way to the sub-station transformer and back is enough to cause a sufficient PD to allow a sufficient current to flow to trip a 30mA RCD when you short N-E on the load side of the RCD.

That may be slightly different with a TN-C-S system where the N&E loop is much smaller and within the property.