TN-C-S Danger - Broken PEN Conductor (Combined Earth & Neutral)

Description of the dangerous situation which can occur when the combined earth & neutral supply wire is broken on a TN-C-S electricity supply, leaving the line conductor connected.

Any RCD fitted in the installation will NOT disconnect in the event of this type of fault, and dangerous voltages will appear on exposed metalwork within the installation.

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[eduardtomasek]

Finally someone did it brief, clean, easy understandable. Excellent work.

[sgordon]

Hi John, great explanation . Back in the late 70s as an apprentice electrician in Glasgow, i was nearly killed due to the above situation on what was Called then PME system .It was a factory unit and the SSEB were connecting mains supply, 3phase & N (earrth) + meter . I was at the other end of the factory unit connecting MK grid swtiches . The part i was working on was isolated .The SSEB Electrician had finished and left site, and our Electricians began powering on finished/tested part of the installation .
I then received a severe shock through metal MK grid plate as this had earth attached.I fell to my knees and broke the circuit to earth .Further investigation from our electricians found the SSEB had screwed onto the Neutral insulation which had the same effect you described above . Im now retired after 44 years, but a lesson never forgot.
Enjoy your videos, keep up the good work.

[AleX-unbw]

This is basicly simple, but no one knows to explain it simple, thanks.

[eriklingarde]

Good video! In addition, if the supply is three-phase (standard in for example Scandinavia) you get the additional problem that a lost neutral will render any voltage in between 0-400 V on the standard one-phase 230 V circuits that might destroy plugged-in equipment and cause a fire. This is due to one-phase loads with different impedances will back feed the neutral. In essence, two loads are connected in series between two phases (400 V) and if one of them is an immersion heater, the other a computer, essentially all 400 V will appear over the computer due to its higher impedance and the formula of voltage division. This combined with what's explained in the video makes a lost PEN a very nasty fault.

[lurkzie]

Had a hard time understanding this up until now, thanks for making it so simple

[jokerash]

Basically you should split PEN to PE+N on your first distribution box and connect PE from PEN to your house PE which is mandatory in EU. Then, if N is disconnected from supplier, the L will flow to PE via the house PE connection. In my country the national company already splits the PEN before connecting the house so i have 3 phases + N + PE in the junction box and supplier PE si connected to house PE.

[gibsonmb]

A number of people on here have mentioned about also fitting a local earth or connecting to pipework as well. The local network provider here (Western Power) strongly advised against this because in the rural hamlet I’m in on overhead cables, the break is likely to be some distance away and thus I would be suddenly providing the earth for all 100 homes. Dependent on what goes wrong elsewhere, I’d better be sure my earthing was capable of a good few hundred amps!

[eGroup]

6 years later... JW was ahead of his time 👍

[dummwiebrot-zw]

thank you very much, this is hands down the best video on earthing systems that I've found so far.

[numbers]

A girl in Australia got a bad shock from a similar scenario (TT/earth stake) a few years back from an outdoor tap. The report said the ground stake was correctly fitted however being the heat of summer it appears the girl unfortunately made a better connection to earth than the stake did. Poor girl survived but is badly disabled.

[Iyengaryogawithshaun]

Thank you John, love the 2 different marker pen towers 🙃

[cpcnw]

John, you have a very listenable voice. You should do voice overs and audio books. You'd make a bomb!

[steveday]

Hi JW - I have watched this Video a couple of times, I really enjoy the way you explain things even though I thought I understood them you always seem to add another way of looking at things that I would not have thought of!

[Azam]

33:55 I was going to ask if this is a short circuit, but I realized the path is not complete. line->heating metal part and then no connection back to line. thanks Mr john for the clear explanation.

[pauldavies]

Hi J.W, you are the best teacher on the whole Internet. You explain everything so well, you make it easy to understand. You are funny sometimes too but I don't think you even know it. Thank you so much. 🙂

[michaelcostello]

Thanks for another amazing video. These videos make one aware of how little knowledge we have.

[gfvt]

Another excellent video (I know I am late to it). This has quite serious implications for some radio hobbyists whereby equipment inside the house with it's normal safety ground is connected to structures outside the house via coaxial cable.

[sharr]

Thanks John. I'm in NZ where I understand we use a MENS earthing system. We had a situation where a mouse bit through the supply wiring in the wall between the meter and switch panel causing a similar problem to what you describe here (earth was severed and some of the supply cable damaged). I think the house side of the earth connected to either the live or neutral cable at this point. All of the bonded items began carrying a charge (130V at the metal front doorknob discovered by my son as he exited for work). No fatalities (apart from the mouse) and an emergency call out to the electricians put things right. There may be an issue with our earth rod (on the MENS system) which theoretically should have carried the charge back but need to speak to speak to an electrician after the holidays.
I'd value any input from others to put my mind at ease in the meantime. I think we've been very lucky but until the electrician has a thorough assessment I remain twitchy. Thanks.

[alantorrance]

Picture situation at 3:55. Neutral Return cable is broken on Supply Side of the boundary, no earth connection shown. In New Zealand it is mandatory for the Neutral to be connected to Earth at or close to the point of entry to the property -- normally by a special suitably heavy cable running from the Earth Bus in the Fuse Box direct to the Earth Stake. The Neutral Bus is also connected in the Fuse Box to the Earth Bus by suitable heavy jumper. In domestic situations, the Earth Cable is an insulated (green and yellow, but may be green only) minimum 6 sq mm, stranded cable.

[winner]

You could install a light bulb right before the RCD. When you notice the light is off, then there must be something wrong.

Or you could install something like under voltage detector. When the PEN is breaking, the under voltage detector will tripped.

Or how could you make this safe?