Voltage dropped

davesparks · Aug 5, 2019

Unless you had the emergency light fully disconnected then the charging circuit will have been live. Disconnecting the neutral upstream of it will cause the black wire from the emergency light to cease to be neutral and rise to line voltage.

Lucien Nunes · Aug 5, 2019

I think HT's point was that all the lights on the affected section were unplugged from the Click fittings, so although the PL was present at the Click, there was no load, not even the EM charging load, to complete the circuit to the floating length of neutral to give him a shock w.r.t. the good section. I too would have been surprised to get a shock if all were as per his sketch.

Unfortunately we are dealing here with a section of circuit that is known to be faulty or incorrect, although we don't yet know how, where or why. So it is a leap of faith to assume that the circuit will behave in the way we expect. With luck, the cause of the shock path will be revealed at the same moment the original fault is found. Not much point guessing with so many variables.

marconi · Aug 5, 2019

Noting this is a big assembly hall with long cable runs perhaps this is the 'real' circuit (and you could add some insulation resistance too if you are feeling artistic(!)...) These capacitors are charged to +350V and the -350V every cycle.

[ElectriciansForums.net] Voltage dropped

TheReturnOfHightower · Aug 5, 2019

Thanks all, I appreciate all the ideas being bounced around. It's out of my hands now, I'm not allowed to touch it until a council spark comes and takes a look. At the moment I feel a bit foolish, and just hope tomorrow he has the same findings as me thus far. I've really enjoyed working on electrics this past 5 years or so but this has really humbled me, and I don't know how I'll feel should something be found that i really should have spotted at first sight.

DPG · Aug 5, 2019

I wouldn't worry about it. The wiring doesn't sound very straightforward.

TheReturnOfHightower · Aug 5, 2019

Hard not to mate, feeling a bit useless with it all...

marconi · Aug 5, 2019

It is a good story to tell others to illustrate the vital importance of safe isolation - it is your anecdote to tell which perhaps might keep others alive (a pun or a poor choice of word

).

westward10 · Aug 5, 2019

Most of us have been there and spent hours and days thinking if only I'd......... but we don't have the benefit of hindsight. As I said earlier lighting circuits can be a minefield even if you do use safe isolation procedures, few weeks time will all be forgotten about.

westward10 · Aug 5, 2019

Will be looking for a new thread tomorrow from a council spark

davesparks · Aug 5, 2019

Lucien Nunes said:
I think HT's point was that all the lights on the affected section were unplugged from the Click fittings, so although the PL was present at the Click, there was no load, not even the EM charging load, to complete the circuit to the floating length of neutral to give him a shock w.r.t. the good section. I too would have been surprised to get a shock if all were as per his sketch.

In which case my guess would be an, as yet, undiscovered load on the circuit. A light in a cupboard or something like that,which may turn out to be the location of the original problem.

PEG · Aug 6, 2019

It would be nice to get to the bottom of this fault,but,i would like to tip my hat,to the OP,for a refreshingly open and candid post.
The man that never made a mistake,never made anything,and it is job stories,such as this,which can keep even experienced hands,from forgetting the hazards which abound.

Marconis drawing,reminded of a large council workshop,where the spark had shut-down the power,and removed the main DB.....before discharging what may have been,an entire floor of broken florries,via his left thumb...

There was no site power,apart from a couple of small generators,and he was on for blaming the contractors for back-feeding,before it dawned on him...Believe nothing you hear...and only half of what you see

marconi · Aug 7, 2019

PEG's #36 and ROHT's anecdotes are great ways to warn and train people. When I was brought in to sort out the mess after a contractor was electrocuted by 11kV I introduced these 'tool box' talks for my technicians. I was always on the lookout for folk with a tale to tell which rammed home the point of 'no short cuts to safety'. Much, much better than a dry lecture on the subject. It's the setting and context which provides the engagement and learning. As humans we remember stories which is indeed how folk passed on information and learning in those days when they could not read or write or there was no paper or books.

One of my 'favourites' is the computer technician who shorted out the live 5Volt busbars of a Ferranti FM1600B computer with his gold wedding ring - yes he had done the training, there were warning signs for very high current and to remove hand and arm metal jewellery, he had checked busbar connections hundreds of times before but on this occasion he lost a finger, because ......

Wilko · Aug 7, 2019

Hi All, late to the party I know, but here’s 20p from me

.

Recently, I investigated an EICR C2 fault for high leakage. I did find the fault wiring with the usual application of time and patience to work through a couple of false starts (well no one’s perfect, not me anyway). I ended up with house global IR of about 90 meg Ohm and a leakage of about 6mA. Which made me think - why is it so?

Of course it’s the capacitance to earth of the L conductor and a direct consequence of running the CPC together with the Lives. But how much leakage is reasonable? I got a new 100m roll of 2.5/1.5 and just connected it up - 1mA earth leakage. Add up 10 circuits and even with no loads we see another reason why we don’t remove the Earth on a live installation...

I’ve added the hurt graph to illustrate - blue is ok, green is not good, yellow makes me bellow and red I may not be posting again.

TheReturnOfHightower · Aug 9, 2019

I haven't forgotten about this fellas, just still waiting for somebody to come and investigate a bit further.

TheReturnOfHightower · Aug 13, 2019

Right then chaps, sorry I didn't post yesterday but council sparks came out Monday and got to the bottom of it. Bottom line is, it was a loose neutral causing the fault in 3 lights.

So, to talk through the actually wiring of it, all lights on the bank are fed from a single switched line, with the exception of 2 & 5 that have an additional perm line for emergency. Lights 1-3 are fed by 1 neutral, lights 4-6 are fed by another totally separate neutral (independent all the way back to the board). Other banks are seemingly wired the exact same, 1 line 2 neutrals, so this looks original wiring instead of bodged repair. The loose neutral was at a junction box upstream from lights 4-6, hence why only these 3 lights were faulty.

So, I got a belt, and I think the reason is, I separated the neutral after light 3, what I thought was the link from 3 to 4. Because I'd left the emergency connected and had only isolated at the switched line (wrongly thinking light 2 was upstream and therefore wouldn't be affected by me breaking the neutrals). Instead of breaking 3-4 neutral link, I broke 3's neutral back to the DB and hence the belt. Yep, it's my own fault - safe isolation would have prevented it, and I've learned a massive lesson.

However, anyone know why it may be wired in such a way?

James · Aug 13, 2019

Could be that originally it was two separate circuits.
At some point, one of the switches has been removed.

TheReturnOfHightower · Aug 13, 2019

James the Spark1976 said:
Could be that originally it was two separate circuits.
At some point, one of the switches has been removed.

For 5 banks though? It all looks original, I know it's hard to tell but there's no real sign of alterations.

James · Aug 13, 2019

Don’t know the layout,
Could be it was designed so it could be split into 2 separate zones with a partition?

TheReturnOfHightower · Aug 13, 2019

James the Spark1976 said:
Don’t know the layout,
Could be it was designed so it could be split into 2 separate zones with a partition?

Nah, big assembly hall, always been like that since day dot. I'd surely expect to see spare line conductors in the trunking though if they'd altered the circuit over the years. I'm not writing you off by the way, I'd love someone to tell me the answer but me and 4 council sparks couldn't come up with it.

James · Aug 13, 2019

Well that makes me the 6th spark in the dark then!

Lucien Nunes · Aug 13, 2019

Maybe they were just too tight on voltage drop, decided to spread the load between two cables but only bothered on the long straight run of neutral rather than doing the same with the switched line?

TheReturnOfHightower · Aug 13, 2019

Lucien Nunes said:
Maybe they were just too tight on voltage drop, decided to spread the load between two cables but only bothered on the long straight run of neutral rather than doing the same with the switched line?

I like your thinking - it's got to be something like this surely, but the circuit is run in 2.5mm which for a lighting circuit that has a furthest point about 30 metres from the DB, doesn't sound like it would struggle.

davesparks · Aug 13, 2019

I've wired circuits in a similar fashion once or twice with duplicated cables for no apparent reason.
What actually happened is that with two of us working on the job we both thought the other was pulling a different circuit/cable but actually we were doing the same as each other. Usually when we're working long into the night to get a job back on schedule.

TheReturnOfHightower · Aug 13, 2019

davesparks said:
I've wired circuits in a similar fashion once or twice with duplicated cables for no apparent reason.
What actually happened is that with two of us working on the job we both thought the other was pulling a different circuit/cable but actually we were doing the same as each other. Usually when we're working long into the night to get a job back on schedule.

Have you ever done it with 5 circuits, in mirror of each other, by total accident?

davesparks · Aug 13, 2019

TheReturnOfHightower said:
Have you ever done it with 5 circuits, in mirror of each other, by total accident?

I haven't, but I recently found 12 duplicates lurking in the trunking when replacing dimmers in a theatre. Some poor sod must have run these same 12 4mm circuits twice, and it was only around a 60m run in a stuffed 9x9 trunking.

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