Voltage dropped

[TheReturnOfHightower]

So after what I thought was a productive day yesterday replacing old lights with LED panels, 3 of them weren't working. Turns out the voltage between line and neutral was about 210 at each of the 3 points, 245 between line and cpc. Voltage was 245 between line and neutral at all other points on the circuit.

I'm testing it coming Monday, but just racking my brain about it. The only thing I can think of is an insulation breakdown somewhere, either cables or in the click connectors. First test is naturally going to be IR, but thought I'd ask to see if anyone had any other ideas too so I can try it all Monday. Don't mind a bit of fault finding but man do I not have time for it at the moment.

 

[telectrix]

can't be an insulation break. seems more like a poor connection on the neutral at the first non-working light or the one before it. check connections in the click connectors.

 

[TheReturnOfHightower]

Why can't it be insulation degraded dropping some voltage across like a resistor? I've checked all connections (3 click connectors where the lights aren't working and also the next light upstream) - all seem solid. On Monday I was going to remove connections from all the clicks and reterminate temporarily in wagos to test again.

 

[James]

Poor joint in cable or at a fitting would be by best guess, put a temporary cable from 1 st not working to last working light.
210v would be enough to start most led lamps, if there is a break in the cable then that voltage could just be tracking across a cable break or bad joint.
Voltage could be dropping to near zero for short durations when the driver tries to start.

 

[GBDamo]

If these are the end of a line then the issue lies at the point of failure may be worth disconecting the drivers for each of the low voltage pannels form mains and testing the voltages.

Think its going to be fairly simple find with a fresh head on monday.

 

[telectrix]

Why can't it be insulation degraded dropping some voltage across like a resistor? I've checked all connections (3 click connectors where the lights aren't working and also the next light upstream) - all seem solid. On Monday I was going to remove connections from all the clicks and reterminate temporarily in wagos to test again.

you mean like leakage N-E? i would have thought that that would trip RCD (assuming there is one on the circuit).

 

[TheReturnOfHightower]

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?

All lights that weren't working were disconnected for testing the voltage. Think my plan is quick IR test, then after that I'm just going to rewire it as haven't got time to be messing about

 

[telectrix]

try dissing the first non-working light, but leaving the other 2 connected. could be a faulty fitting. I had 2 out of 24 duff out of the box on a recent job. 1 went pop on connecting, and the other flickerd on when hit with a large screwdriver handle ( an old trick from when TVs had valves inside. )

 

[TheReturnOfHightower]

I will, but with all 4 (3 non-working and last working one in the line) disconnected voltage was still around 210 at all 3 non-working points

 

[telectrix]

it's an intriguing one. let us know the outcome once you get it sorted.

 

[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