Voltage dropped

[Lucien Nunes]

No RCD tel, but why can't current leak line to neutral through a breakdown in insulation. Not a dead short, but enough to drop voltage across?

Because it would have tripped the MCB. Suppose the circuit has R1+Rn of one ohm (pure guess but close enough for the explanation) that R1=Rn, Zdb is low and that the faulty insulation is concentrated in one area near the end. The difference between voltages L-N and L-E was 245-210 = 35 volts, which would have to be the voltage drop across Rn between the DB and the fault. The current through the fault would be 35/0.5 = 70 amps. If it hadn't tripped the MCB for whatever reason (e.g. the circuit was wrongly connected to a 63A MCB) the heat dissipated would be 70 x 210 = 14.7kW which would rapidly start a fire. You can't dissipate the heat of twelve 2-slice toasters at a fault without it getting, er, toasted. Since a further 35V would also be dropped across R1, the supply would have to be 245+35 = 280V which would be causing other problems elsewhere. FWIW the 'insulation' resistance would be 210/70 = 3 ohms, pretty close to what one would legitimately call a dead short.

All lights that weren't working were disconnected for testing the voltage

This is key. With the lights connected, the voltage would probably have collapsed to a much lower level. The neutral is almost certainly open-circuit at a terminal and what you are reading is more or less a ghost voltage.

 

[TheReturnOfHightower]

cheers fellas, I'm looking forward to getting in on Monday and letting you know what I find.

 

[TheReturnOfHightower]

Here's a question then... If you were to rewire this segment would you just rewire the neutral or would you rewire the lot (keeping in mind this is on old colours so a blue alongside red may be deemed confusing)?

 

[Lucien Nunes]

From this I take it that it is a conduit installation? If so it is most unlikely that the faulty connection is hidden... just open the boxes until you find it, and remake it. I doubt you will have to draw in new cable unless there is extensive physical damage.

 

[TheReturnOfHightower]

It's in lighting trunking, but I've inspected and reterminated all 4 click connectors. I'll get the wander lead out on Monday and test for continuity. I wouldn't mind rewiring it anyway because from what I could tell quickly on Friday, the line goes from light 3 to 4 to 5 to 6, but the neutral, as far as I can tell goes 3 to 6 to 5 to 4

 

[Risteard]

It has to be a loose neutral.

 

[Lucien Nunes]

Agree it's not ideal to have L & N visiting points in random order. This anomaly hints at a known existing disconnect between points 3 & 4.

Fun fact... Historically, long incandescent lighting circuits run in singles, particularly for display and signage work, often had the line and neutral run in opposite directions to spread the voltage drop more evenly amongst the points. The overall power loss is the same but the lamps receive more uniform voltage and therefore one end of the sign is not visibly dimmer than the other.

 

[TheReturnOfHightower]

This is a big old assembly Hall with 30 lights, it would have been wired around 40 years ago so that is really useful information - I wonder if that was their thinking at the time, however I haven't investigated 1 and 2 to see what order they are in.

 

[TheReturnOfHightower]

Actually, now I think about it, when I first fitted the lights and powered on the 6th light (which is 4th in line confusingly on the neutral order) was flickering ever so dimly.... It stopped flickering after I did a bit of pulling about so i wonder if my actions have made the final break. The trouble is, this Hall had so many lights out I'm not sure which were working and which weren't before I started work

 

[Lucien Nunes]

I wonder if that was their thinking at the time,

No, more likely it's just muddled up and spaghettified.