lighting fault...

[dawsirn]

hello,
i'm hoping someone here can help. been looking at a lighting fault today thats been doing my head in.

This is a new build town house with metal stud internal walls, this could be of some importance but not sure. 3 separate lighting circuits, one for each floor.

customer has a low energy pendant on top floor landing that has never worked. started off with simple voltage check: 230v live and neutral
0v Earth neutral
230v switch live and neutral (switch on)
5v switch live and neutral ( switch off)

checked all other lights in house and every light was giving me proper voltage readings except two more low energy pendants on middle floor landing (these two are linked so they switch on together). these lights were working to begin with but then the furthest light from the switch stopped working for some reason. But the readings were the same as above except i was getting 16v between switch live and neutral with the switch off.

thought i'd start breaking down the circuits checking for damaged cables and making sure all the connections were good. found plenty of loose connections which i sorted. but then i did find after alot of narrowing down that the cpc for the top floor lights feed from consumer unit was shorting out (0.02Mohm) with the live conductor feeding the 1st of those two lights on the middle floor landing.
not sure how this could of happened, maybe somehow they were both scagged on the metal stud as these cables do run near to each other.

annoyingly after cutting a massive hole in the ceiling hoping to find the evidence i found nothing.

I'll point out that the cpc for top and middle floor circuits are connected through 2way switch boxes.

but for some reason after i give up for the day, connected back everything and hey presto the 3 lights are all now working.....but 2 middle floor lights still have 16v sl and N and top floor light is completely sorted with no strange voltages.

i did plenty of other tests but be typing all night.

so to conclude i think my brain is damamged and i'm sure someone here can tell me that i've missed something pretty simple.
cheers

 

[pushrod]

but for some reason after i give up for the day, connected back everything and hey presto the 3 lights are all now working.....but 2 middle floor lights still have 16v sl and N and top floor light is completely sorted with no strange voltages.

cheers

Hate that when you get something to work and you don't know what did it, takes all the satisfaction away... maybe just loose terminations?

... and i've got some similar strange voltages in my own place that i haven't got round to sorting yet, so will be interested in what people suggest.

 

[dixon9]

customer has a low energy pendant on top floor landing that has never worked. started off with simple voltage check: 230v live and neutral
0v Earth neutral
230v switch live and neutral (switch on)
5v switch live and neutral ( switch off)

Where did you take these readings from? i.e Where did you put your probes? Sounds like it could be a faulty rose/pendant. If the fault has mysteriously disappeared then it could be a loose/intermittant internal connection within the rose & pendant (might be well worth changing this anyway).

 

[telectrix]

sounds suspiciously like a lost neutral to me. check from the working light to the next one down the loop. broken core at terminal is favourite , or terminal clamped to insulation.

 

[dawsirn]

Where did you take these readings from? i.e Where did you put your probes? Sounds like it could be a faulty rose/pendant. If the fault has mysteriously disappeared then it could be a loose/intermittant internal connection within the rose & pendant (might be well worth changing this anyway).

probes between switch live and neutral at the ceiling rose with the flex from the pendants disconnected at the base.
the 5V reading i was getting i presume buggered the top floor fitting. when i tried putting a new replacement Low energy pendant in you could actually hear a buzzing from the pendant so i disconnected it quickly before that one broke too. this was with the switch off!!

i couldn't hear any buzzing from the 2 lights on the middle floor giving the 16V readings.

 

[dawsirn]

sounds suspiciously like a lost neutral to me. check from the working light to the next one down the loop. broken core at terminal is favourite , or terminal clamped to insulation.

i'm going there again soon i think, so i will check every light on the circuits for this kind of thing. just find it odd that its happening on two different circuits and only on the low energy pendants and even with the pendants disconnected from their bases!!

 

[dawsirn]

right then, had another look at this today, and after chatting to my boss have concluded its something to do with the inductance of the 3 cores running up the walls. This could explain the voltage at the sl to N/E.
did a little google search and come across a few other stories of this happening. still not sure why this only effects these low energy pendants and even when the pendants are disconnected from their bases.
the damaged cable seems to be a different problem altogether.

Does anyone here have any similar experiences with inductance.

 

[WDMDL]

It's the stray capacitance between line and switched line on your switch drops. Obviously with 2-way or multiple pneumatic switches the effect will be worse.

Now a normal filament lamp has quite a low resistance when cold - if you measured the resistance of a cold 60W lamp and then applied Ohm's law you would get an answer in the region of a few hundred Watts.
Basically the high-impedance voltage source (Megohms) is shorted out by the lamp filament (few Ohms).

A low energy lamp has a very high resistance until there is enough voltage for the circuit to start up.
The little bit of induced voltage goes through the rectifier and capacitor in the lamp - charging up the capacitor.
When the voltage across the capacitor reaches a certain level, the circuit will try to do something.
There's not enough current for the circuit to work properly, but the noise you may hear is from small pulses the circuit is managing to drive the lamp transformer with.
Proper electronic ballasts have undervoltage lock-out to prevent unwanted circuit operation - but the induced voltage would still be there.

Some brands of lamp have a special driver chip rather than a discrete circuit - the chip has the undervoltage lockout feature.
Maybe worth looking at the higher spec Osram and Philips lamps if the noise is a problem.

Simon.