RCD neutral

[Gone West]

Is the direction of flow through the neutral of an RCD important?

[Mattg4321]

In relation to the line conductor, yes. 

[ProDave]

Explain the issue you are having?

[Gone West]

1 hour ago, ProDave said:

Explain the issue you are having?

I don't have an issue as such, it's more a theoretical question. The garage that blew down a year ago had a small consumer unit in it and it looked like the neutral to the RCD connected at the bottom but the live connected at the top with the busbar at the bottom as normal. So I wondered if the neutral could go either way round as I've not seen it like that before.

[Temp]

Got a picture?

[Temp]

They work by passing both live and neutrals through a transformer of sorts. They way its wound means the field created by each winding should cancel out the other when they are the same. When they are different they don't cancel and the field produced trips the breaker.

 

I think sending the current the wrong way on one would make it trip all the time because the field would add rather than subtract from each other.

[Nickfromwales]

The 'tripping coil' is kept in absolute equilibrium, regardless of direction, as power flows through line and neutral in equal 'quantities' in normal conditions. So in theory it will work with these reversed as there would be no pushing or pulling, just a constant current along both, ergo no potential present in the tripping coil.

 

 

The RCD / RCBO gets tripped by the difference of potential created when line or neutral becomes partly compromised by being 'shorted' to earth, with there then being an electromagnetic flux thus created from this imbalance and that is collected ( detected ) by the coil ( a-la a CT clamp ), hence the name Residual Current Device. It's why you can get an RCD / RCBO to trip by shorting neutral to earth as well as line to earth.

 

It's the reason birds can land on HV power lines and sit there, not becoming toast, as the left foot is at 132,000v, and the right foot is 132,000v so the difference between 132,000v and 132,000v is absolute zero. Poly happy. But, if they decided to sit in a row, and it was raining and the last one got his wet wing to ( or near to ) the earthed metal structure, then it's "en-flambe" for the lot of them.

The 20 live birds who were sitting there energised at the line voltage ( 132,000v to 400,000v but have no current passing though them ) happily minding their birdy-business suddenly just got grounded, and the difference of potential is then 132,000v.

Bye bye Polly.

[Nickfromwales]

4 hours ago, Gone West said:

Is the direction of flow through the neutral of an RCD important?

Absolutely. Yes, but for safety sake.

3 hours ago, Mattg4321 said:

In relation to the line conductor, yes. 

Indeed, as you wouldn't want to get zapped when working on something that had defective / incorrect cabling downstream of the CU, and you come to then work on it as a 3rd party. Switching 'off' the front of house RCD or main switch should give double-pole isolation of the entire electrical installation from both sides of the incoming supply.

[Gone West]

5 hours ago, Temp said:

Got a picture?

No I don't unfortunately. After the garage had blown down I just cut the SWA cable, took the CU apart and left all the bits. I don't even know if the power to the garage ever worked as the MCB in the house CU was off and having recently moved house had other things to sort out.

[Nickfromwales]

2 hours ago, Gone West said:

After the garage had blown down up.....

🙄

[Onoff]

No offence but this simplified picture may be better. An imbalance, caused by leakage to earth, gets picked up by the search coil:

 

Pushing the test button brings the resistor in and creates an imbalance.

 

It is critical that the in/out wiring is correct. (Same as with current transformers).

Edited April 2, 2023 by Onoff

[SteamyTea]

2 hours ago, Onoff said:

gets picked up by the search coil

Should be called Search and Destroy.