RCDs: Neutral - Earth Faults

Why RCDs trip when faults between neutral and earth occur, and why switching off a circuit breaker / MCB is not isolating the circuit properly.

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[nowthenad]

Here I am again watching this video 2 years later. You are a national treasure JW.

[roberthorwat]

This used to drive me nuts in my old house. Such a simple explanation but all the same I'd have never figured it out in a zillion years. I am demystified at last. Thank you so much for this!!!

[benharris]

This guy is great. So informative and explains things clearly

[RandyDarkshade]

Thank you. I have always wondered why an RCD can still trip even if you have the power off on the circuit you are working on, now I know.

[tobybarker]

you have a knack of explaining stuff that even I can understand

[NOELTM]

Thank you for explaining this so clearly. The scenario you mentioned in the first 40 seconds was exactly what happened to me today!
I knew what I did - but didn't understand why this happened. Now I do - 9 years after you made the video!

[examplerkey]

Extremely helpful video as usual from John. I watched it so carefully that I noticed that you edited the video at 4:27 :) Thank you very much. You saved my day! Your video confirmed my suspicion of a N-E fault in a residential installation in which the responsible 30mA RCB was switching off every 5 minutes! despite all the MCBs switched off (and Live bus bar removed, just in case). Locating or narrowing down the fault is as easy as removing the Neutral wires one by one from the Neutral bar and see which one trips the RCD.

[glennbowers]

This guy is quality... Makes everything make perfect sense... Thanks alot John

[melliflousbufo]

Brilliant explanation - thanks John! As well as understanding why neutral/earth shorts trip the RCD I also finally understand why the RCD has no interest in the state of the earth!

[vhsable]

Thanks for the videos John, I find them very informative, easy to understand and a great back up for my level 3 2365 . Thumbs up on every video so far.
Thanks.

[ninjaman]

great video, explains what i was after. this happened to a street light while i was working on eastenders. the lamp wasnt working, it was disconnected from the supply but there was about 50v between earth and neutral. i have been trying to figure out the cause since. thats scratched an itch! cheers john, all the best.

[Paula-xobb]

This is one of the best explanatuons with demonstrations I've seen. Well articulated.

[derekincambs]

No one would fail their Electrical exams if you were the tutor. Great stuff. From a non electrical person!

[ionutpiglesan]

Great video ! I just saw it and I think one solution for this is to use 2poles or 1p+n MCB s instead of 1 pole. So when you disconnect the MCB both line and neutral are off . The regulations in my country recommends to use that kind of MCB s.
Of course RCBO s on all circuits is the best solution but it is more expensive.

[spiro]

I just love the occasional dry humour, makes for interesting listening.
many thanks for the detailed explanation.

[michaelherbert]

Hi John... Many thanks for your time too explain in depth the earth / neutral faults.

[burdenstephen]

You have answered a question from years ago when I replaced a socket in my house. I tripped the RCD even though the circuit I was working on was isolated and I suspect I did exactly this. Thank you

[jpegxguy]

What you're saying is that double pole mcbs are the way to go (especially since they exist in slim form factors), so that I can also isolate neutrals. The system of only disconnecting the line is a pain in fault-finding

[daviddavidson]

Cheers. Excellent explanation. This had me puzzled while doing some electrical work up in my gran's attic, I thought she was flipping the main breaker as she's terrified of electricity. I'm cautious; test with a NCV tester then a multimeter before work especially as whoever did the wiring in that attic had no idea what they were doing, flourescent strip light in *series* with a ceiling rose (not for lack of wire, they just left the live wire flapping in the breeze), steel cored speaker wire (or something similar) hooked up to live and neutral leading to seemingly nothing, if I had to hazard a guess it was for an old doorbell and just reconnected by every contractor who did some work there, though why it is on the attic lighting circuit is beyond me.
All of these things had me fairly worried about the house being plunged into darkness as I was concerned that there was a live (or intermittently live) conductor somewhere. Turns out it was neutral touching earth, which I'm sure of as I tested it twice. Measured with a multimeter and got .7 volts, guess the current was enough to trip the breaker though.

[mikeZLXD]

Excellent explanation, John.
I see this sort of things with inspecting caravans, where the owner has wired the damn thing themselves and left the link in between the
Neutral and Earth busbars at the little switchboard, that's normally in the smallest cupboard ever.
It's that time of the year at the moment, considering people can't fly out of New Zealand because of Covid-19 restrictions.