SELV light prob and mistery voltage

[alchemist]

Have just been out to a job to look at a fancy SELV halogen light where it stays on for a few mins, then goes off and wont work for 30 mins. Assuming it's the transformer so cut it out and will try to get a replacement tomorrow.

I checked the supply voltage and across switched live and neutral I was getting 238V with the switch on, and 53V with it off.

I haven't looked yet, but my guess is that there is a loose wire in the switch, and the live is touching the switched live, maybe loss of seperation in the switch mechasnism itself or even damaged insulation on the switch drop?

Does this sound right, or can you suggest an alternative cause i'm missing.

The cables in the ceiling looked well terminated and isolated from each other.

 

[sparkytim]

Self re-setting Thermal cut-out in Transformer.

Common problem due to Over heating.

Strange voltage may be several explanations

Search other threads on subject.

 

[NozSpark]

I wouldn't worry about the voltage, but it does sound like the thermal fuse is tripping... probably due to your customer using higher wattage replacement lamps than they should.

 

[sparkytim]

Or not enough air circulation around Transformer

 

[Apache]

53V could well be induced voltage. What did you use to measure it?

 

[alchemist]

Good points, thanks all.

Used a Metrel Eurotest Easi multi tester. It's usually spot on, even when my fluke T100 is giving odd readings.

How does the induced voltage work? I thought whereever you have T+E the neutral and live fields cancel out.

 

[m4tty]

Think digital meters get fooled by induced voltage whereas analogue meters don't. If the cables are running next to other circuit cables it could induce voltage. I stand to be corrected tho. Cheers

 

[Apache]

How does the induced voltage work? I thought whereever you have T+E the neutral and live fields cancel out.

Basically having a cable with a magnetic field in proximity to another cable. More noticeable when cables bunched together and greater at hight amperages.

The voltage may creep up but usually tiny current, and it's the amps that kill you!

 

[alchemist]

Another couple of points on this subject. When testing the transformer by connecting it to a normal 13a plug, I found that there was no output voltage, except when I connected a lamp to the transformer.

The voltage I then measured across the load was only 3V, despite it being a 12V transformer.

I'm guessing that the transformer doesn't output except when the load is within a prescribed range, perhaps to protect against short circuit or not having a load connected, although I've no idea why I only got 3V when a load was connected - the lamp worked fine (halogen down lighter).

I got the same result with two transformers, so I know it's not a fault.

 

[Andy™]

12v transformers are AC - you do have the scale set to AC and not DC, dont you?

 

[alchemist]

Yeah, I'm using the same Metrel tester which measures AC and it showed between 2 and 3 volts.

With my multimeter on AC I didn't get anything at all, but perhaps it's not sensitive enough to pick up low AC voltages.