Lamps blowing cont...

[HappyHippyDad]

Evening all,

A while ago I started a thread about 11 halogen lamps blowing fairly often.

Over the course of the last few months the problem seemed to improve after I found 2 junction boxes that were pretty leaky and sealed them. The problem now seems to be as bad as ever with one blowing perhaps every week or two.

They are standard bulkhead light fittings with halogen lamps located on a fence along a long driveway. The lightfitting itself seems well sealed and there was no sign of water ingress or condensation. Underneath each lamp is a junction box which I think may be the (main) problem. After opening a few, there was quite a bit of condensation in each. They all have a 4mm drain hole.

Apart from this problem they are all getting switched on/off a fair bit as they are on 2 sensors and a timer so this will shorten the lifespan, but not to one bulb blowing every week.

They are good quality lamps.
All connections are sound, including the busbar.
The lampholder connection is nice and shiny.
It is not an excessively windy location.

Apart from the amount of times they are getting switched on/off which I guess would be 10% of the problem I believe it will be due to the condensation in the JB's.

My question is would putting a potting compound (magic gel) in each of the JB's be a good idea? If it is the condensation causing the problem, would this alleviate it?

 

[davesparks]

How does condensation in a joint cause a lamp to blow?

What type of lamp is it?

 

[ian.settle1]

How does condensation in a joint cause a lamp to blow?

What type of lamp is it?

My thoughts as well

 

[Kamikaze]

I haven’t read the other thread but halogen lamp quality is poor in general. Their biggest killer is over-voltage, closely followed by fingerprints on the glass. They are also sensitive to vibration. Just a bad lamp for outside lighting in general. Time to bite the bullet and install something better!

 

[Leesparkykent]

I would probably go with vibration.

 

[davesparks]

I hadn't noticed that it's halogen lamps. Fixed to a fence they are going to be moving around a bit.
Have you considered changing the lamps for something less sensitive to movement? What lamp base are they? If ES/BC them put rough service lamps in an see how it goes.

 

[Lucien Nunes]

Exactly what lamps are fitted now? Are they the halogen replacements for GLS lamps with the quartz capsule inside a glass bulb? If so, I can't imagine condensation has any effect as it can't get at the capsule (and the glass is not sensitive to thermal stresses) but I can totally see it being vibration. They are much more fragile than the GLS types they replaced, especially the low wattages on account of their very thin filaments.

I cannot see what objection the owner would have to upgrading to LED, as it can't be cost-effective to be replacing these things constantly. Just watch out for the inrush on the timer / PIRs!

 

[HappyHippyDad]

Cheers guys? I have asked them to change 2 of them to LEDS to see what effect that has. If they don't blow over the next few months that may be the solution. There was quite a bit of condensation. I was thinking that at times this may cause a short circuit and blow the lamp.

 

[davesparks]

How do you think a short circuit will blow a lamp?
If the circuit is short then current through the lamp will be reduced, not increased

 

[HappyHippyDad]

PS... Yes, they are 'halogen replacements for GLS lamps with the quartz capsule inside a glass bulb' Lucien. Also they are ES fittings.

 

[davesparks]

Swap them for rough service lamps if they're ES, at least as a temporary measure to see how it goes.

 

[Lucien Nunes]

There was quite a bit of condensation. I was thinking that at times this may cause a short circuit and blow the lamp.

This is a bizarre notion! If you're watering the garden with a hose and someone rips a big hole in it with the mower, would you expect the pressure at the nozzle to increase? The words 'short circuit' indicate that the current 'stops short' of some intended destination, i.e. by definition a short circuit across a lamp prevents current reaching the lamp. By convention a short is taken to mean a low or catastrophically low resistance relative to the rest of the circuit, otherwise we would just call it leakage.

Think about the resistances involved too. Can you really create a short circuit, of low enough resistance to affect the circuit in any way, with condensation alone? You might be surprised to find that if you completely filled the fitting with water, in the absence of an RCD it would at first carry on working. There would be leakage, the water would become loaded with metal ions and its resistance would fall. Eventually it would either trip the MCB or boil itself dry and leave some nasty metallc tracking across the insulation. But it wouldn't for one moment affect the lamp.

 

[HappyHippyDad]

I think it really is time for the 2365 that I keep threatening to take!

 

[Welchyboy1]

If they are external im assuming they are enclosed fittings?, are the lamps the correct wattage?, they usually lower the max wattage for enclosed fittings, may be overheating if they are too high wattage?.?..?

 

[Lucien Nunes]

I don't think it's overheating. Fitting an over-rated lamp tends to overheat and damage the fitting and wiring but not the lamp itself. The fitting might turn into a pool of melted gloop with a 75°C excess temp rise but the effect of this on the lamp filament is negligible because it runs at around 2750°C anyway and almost all its heat is dissipated by radiation, whereas the fitting loses it by convection. Even a glass envelope will withstand a few hundred.

The only filament lamps that are really vulnerable to overheating are high power ones in small envelopes. A traditional non-halogen 1000W projector lamp the size of a cigar needs fan cooling or the glass will melt, although the filament will be OK until it does.

 

[HappyHippyDad]

I just remembered this oldish thread!

Just to update and finish it.. I advised the customer change 2 of the lamps for rough service and 2 for LED's to get a picture of what worked well for their environment. They immediately went out and bought 11 LED's! I went back last week for another job and they said that none had blown since changing.

Thanks to all of you for the advice.