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Friend of mine has phoned. He needs my help because his lights in the hallway keep flashing dimly whilst in the off position.

He has measured nominal voltage when the lights are turned on and 50v when the lights are turned off.

I dont have much information to work with yet but thought I would ask for a heads up on how to approach this. Iv not had a fault like this before.

The offending lights are via a 3 gang 2 way switch. Could this be induced voltage? If so, how would I confirm this and how can I remedy it?

He has said he changed the lamps for LED and they flash even more obvious now.

I will obviously check connections and test Zs, probably I/R switch live to neutral/earth etc...

I thought I'd just ask for a heads up on how to approach it.

My friend is an electronics wiz so he has some knowledge on the basics. I would imagine he bas tried the obvious
 
Tell him to put an incandescent lamp back in for now, this should stop the flashing until it can be sorted properly.

He will more than likely need a snubber fitting.
 
Tell him to put an incandescent lamp back in for now, this should stop the flashing until it can be sorted properly.

He will more than likely need a snubber fitting.

How is a snubbed fitted? Is it effectively a resistor?
 
How is a snubbed fitted? Is it effectively a resistor?
It is usually a R-C combination, say 0.1uF in series with 100R, so it has a path to dampen contact-opening surges, but is not too low impedance for closing switches to get burned.

The issue with LED fittings is they light on very low currents (even neon indicators can light them effectively in a dark room), so it seems something is leaking enough to be a problem so should be checked.
[automerge]1589661137[/automerge]
This is the sort of thing, but I remember seeing ones with insulated leads:
[automerge]1589661250[/automerge]
This looks like it:
[automerge]1589661329[/automerge]
But in this case you wire across the LEDs, not across the switch contacts!
 
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Common problem caused by many mains-voltage LED (and CFL) lamps being made down to a price and not having any means to discharge accumulated leakage without lighting up briefly. There could be an insulation problem causing excessive leakage, but it is unlikely, since this nuisance behaviour often occurs with just the normal capacitive leakage in the cables.

All cables have capacitance between cores, so even if the insulation resistance is infinite, there will still be a small current passing through the capacitance whenever an AC voltage exists across the cable. The switch drop has one core PL and one core SL with mains across, and will pass a tiny current into the lamp(s) with the switch off. Because the lamps have an electronic power supply built in, they rectify and smooth the incoming supply. The leakage through the switch drop capacitance slowly charges up the smoothing capacitor until the voltage is high enough for the lamp's voltage converter circuit to start up, the lamp lights, the smoothing capacitor is immediately drained and the lamp goes out.

You mention a measurement of 50V from SL-N with the switch off. In theory you can't measure the voltage on an isolated piece of wire with a multimeter, because there's no complete circuit to allow the measuring input of the meter to function. If you do get a reading, it's not very meaningful, as it depends on the input resistance of the meter as much as it does on the wiring under investigation. I.e. one meter might say it's 50V, another 10V, (i.e. what is popularly called 'ghost voltage'.)

But, because leakage from the line consists of a consists of 230V passing a small current through a high impedance, it's the current that is nearly constant, not the voltage (hence why we talk of leakage current, not leakage voltage) and because most multimeters have a standard input resistance of 10 or 11MΩ we can estimate the current flowing through the meter.
50V/10MΩ = 0.005mA.
This will not be the true value of the leakage current because just as there is leakage from PL to SL, there is leakage from SL to N and to E and all over the shop. It just illustrates roughly the order of magnitude of currents we are dealing with.

We can also estimate, very roughly, how much of the time a lamp will light on that current. Suppose a 7W LED lamp with a running current of 30mA at 230V gradually collects the rectified leakage in its smoothing capacitor. As it builds up voltage across its input the leakage current will reduce, so if it's capable of lighting at 115V and tends to hover around this voltage, we might estimate the average lamp input current at 0.003mA. If the lamp flashes on to full brightness at 115V it will use 60mA from the capacitor so there is enough leakage current to light the lamp for 0.003/60 of the time, or 0.18 seconds per hour. If each flash is 10 ms (determined by the capacitance of its smoother) it will flash 18 times per hour or every 3 minutes. Rough figures indeed, a Fermi approximation, but it shows how the small amount of information given can sometimes still provide a useful insight into what is going on.

How to fix it? Yes, test the insulation resistance as a precaution, just in case there is a wet switch or a rodent-damaged cable, but it's unlikely. Then, provide a path to neutral for that 0.005mA of capacitive leakage. That could in theory be a resistor, but as mentioned, a snubber is a popular solution. It's called a snubber because its normal usage is something completely different, but it's really just a capacitor with a small series resistor. The advantage of using a capacitor to dump the leakage current is that when the switch is on, the current passing through the capacitor is still capacitive, i.e. wattless, so it doesn't dissipate heat or increase the energy consumption. The series resistor just limits the peak current during any voltage transients.
 
Another advantage of the snubber (R-C) type of device is they measure open on an insulation test if they are wired in to a light socket, etc, so you don't have to find the location of someone 330k resistor ruining the results!

Lucien Nunes has covered the background as to why it happens in some details, but one thing to add is in the UK it is often less of an issue due to our use of T&E with an earthed 'shield' between wires reducing the capacitive coupling (as much of the capacitive current flow terminated on the mid wire). If the OP's friend is seeing a lot of stray current it could well be that some of their lighting wiring has a floating CPC.

So while a snubber across one of the lights L & N terminals ought to kill the problem, I would also be checking all of the cables / accessories are actually earthed!
 
Another source of increased capacitive leakage is 2-way switching using 3C+E, where depending on the position of the switches and the allocation of cores to switch terminals, sometimes the cores connected to SL and PL are adjacent rather than either side of the CPC. The worst case is when of the two adjacent insulated cores, the one furthest from the CPC is SL and the two either side of the CPC are PL. Capacitive leakage can also occur between two sheathed cables run together, but is usually minimal as the conductors are usually much further apart than in a single cable.
 
I popped over this morning. I didn't have long because I got called home because one of the family has been taken into hospital.

My findings are...

There are 2 downstairs fittings, 1 upstairs landing fitting.

All are 2D 16w tubes. All have had fluorescent tubes replaced for led retrofit. The gear doesnt need adjusting as the lamps state they run from tbe gear.

Measured 79v because live and earth when lights were in the off position. Nominal voltage when the lights were on.

I fitted a neon indicator across in a hope of eating the ghost voyage and it indeed did light up as youd expect, very dim. Once the lights were turned on the tube and neon lit up. This didn't solve the ghost voltage and even with tbe neon across and dimly lit, still getting stray voltage, although now 57v.

I next decided to look at tbe switching. It's a little bit of a mess. There is a 2 way 3 gang switch at the bottom of the stairs, a 2 gang 2 way at the top of the stairs and an intermediate downstairs near the kitchen.

There are 2 circuits between these switches. Upstairs lighting and downstairs.

What I did find was that once you disconnect one of the strappers from the bottom of the stairs (3 gang 2 way) the flashing (ghost voltage) disappeared. The same happens when you disconnect one if the strappers upstairs.

So, because I had to go I everything back as it was and will return.

My suspicion is that the wiring between switches are mixed up. In all honesty I dont know how mixing strappers etc would cause this problem but it certainly seems those cables are the smoking gun....

I have taken quite a few photos but not had chance to look properly yet
 
Hope your family member is OK.

It certainly sounds like simple cable capacitance is the reason, but if there is on obvious screw-up in the use of 3C+E cable (or missing CPC ground) then the use of a snubber ought to fix it. Typically a neon has something of the order of a 100k-470k series resistor for current limiting, but a 100nF + 100R snubber has a 50Hz impedance of around 32k so ought to be far more effective at bleeding off the current.
 
Friend of mine has phoned. He needs my help because his lights in the hallway keep flashing dimly whilst in the off position.

He has measured nominal voltage when the lights are turned on and 50v when the lights are turned off.

I dont have much information to work with yet but thought I would ask for a heads up on how to approach this. Iv not had a fault like this before.

The offending lights are via a 3 gang 2 way switch. Could this be induced voltage? If so, how would I confirm this and how can I remedy it?

He has said he changed the lamps for LED and they flash even more obvious now.

I will obviously check connections and test Zs, probably I/R switch live to neutral/earth etc...

I thought I'd just ask for a heads up on how to approach it.

My friend is an electronics wiz so he has some knowledge on the basics. I would imagine he bas tried the obvious
be aware that some multi meters show 50V when ther is no voltage but the setting you are on is wrong
 
be aware that some multi meters show 50V when ther is no voltage but the setting you are on is wrong

I'm not familiar with this behaviour and wonder why they should do so. Can you give me some examples of meters?
 
I meant, to display 50V when on the wrong setting. But thinking about it again, I recall some MFTs giving a warning that the circuit is live with something like '>50V' when resistance is selected. Perhaps some multimeters do that too (none of mine do, not that I am in the habit of trying to test the resistance of a voltage source) but presumably only when a voltage is actually present?
 
I meant, to display 50V when on the wrong setting. But thinking about it again, I recall some MFTs giving a warning that the circuit is live with something like '>50V' when resistance is selected. Perhaps some multimeters do that too (none of mine do) but presumably only when a voltage is present?
When I assess the AM2 candidates doing their I&T part it is very common that they get a 50V reading due to wrong setting.
 
And presumably most MFTs display a similar '>50V' message for excess touch voltage on a loop impedance test.
 

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