Why would a pendant light keep burning out bulbs?

[Ask_Itee]

For the last 11 years, one 3-light pendent in our loungeroom (we have three identical units but on different switches) has gone through bulbs. 

In the time this pendent has been in place we've had 3 electricians investigate the issue with the aim of stopping this lone pendant from killing bulbs.  Bulbs last between 4 and 12 weeks regardless of type.  We really should have bought shares in Osram or Mirabella to subsidise the cost of bulbs.

Before Xmas I swapped the "offending" pendant with another in the same room - the aim to see if anything changed - and it did.  The good pendant started killing bulbs, the bad guy has been well-behaved.  So from this, I am assuming its a circuit/wiring issue.

The electricians have not been terribly effective and none suggested I/they swap pendants to see it brings about any changes.

My question to the masses.  Aside from telling the electrician about the above change, what do I ask the electrician to check and what questions should I ask at the end of their visit to ensure they have covered all that needs checking?

[Walzz]

@Ask_Itee  Vibration can cause early failure of incandescent bulbs due to movement of the hot filament. LED bulbs are more or less immune from this problem. If you have an air-conditioning unit nearby, vibration may be conducted through framing timbers in wall/ceiling and reaching the affected light fitting.

Can you provide more info?

[Ask_Itee]

Thanks Walzz.

We have tried LED's, incandescent, halogen, compact fluorescent... and many many other types over the years.  There is a 2.5 HP split about 4m from the pendent and we've had it replaced about 18 months ago.  After your suggestion, I had a feel of the wall and ceiling and can't detect any vibration at the moment.  You've given me some food for thought though - trying to think what I could mount on the wall to amplify any vibration to see if any is present.

The room has a raked ceiling which is approx 8.5m at its highest point, on which sits the tiled roof, all pendants are suspended from a sheet material that lays under the sarking for the tiles.

[MitchellMc]

Hi @Ask_Itee,

It's great to see that @Walzz has already contributed to the discussion.

I'd be keen to hear what these three electricians have come up with and what options they provided you.  

You seem to have determined that the fixture itself is not the cause, and if there are no external issues affecting the globes as per @Walzz's suggestion, then I see no other reason for globes blowing that quickly unless there is a fault in the line. Have you had any issues with rats in the ceiling space chewing on cables? I would have thought that the electricians would have picked up on chewed cables. I presume at least one of the three advised that they could rewire the circuit, which could resolve the issue.

Any leaks in the roof above the light fixture that could be allowing water into the light fitting?

Keen to hear more about your issue and answer any questions you have.

Mitchell

[Ask_Itee]

Hi MitchellMc

To add

• Nil stock on the Rats and water leaks.  I had popped tiles above that pendant years ago to see if that was the cause,

• I replaced the bayonet fitting in one single light pendant - I was looking for any traces of arcing inside, but I had a spare and thought i might as well try something.

The Electricians had no answer, I was disappointed in each but that being said I know issues like these can be hard to track.

I am trying to make sure I check on what has been done on the day - its a bit like the time my trike spent a month in the shop under warranty and the dealership  kept saying they found nothing wrong - their words "the computer says all is fine and perfect". but when I asked had they checked wheel bearings, tie rods, ball joints for play I got a blank stare.  I really didn't think I'd have to ask these sorts of questions of the "experts".    Two days later they called to say they had replaced the Ball joint assembly on one side .... they hadn't even given it a thought to check. I don't want the 4th visit by a leco to be the same outcome.

[MitchellMc]

Hi @Ask_Itee,

I can't see any reason why the electricians would not have offered to rewire the circuit for you. It's the most likely method of resolving the issue without spending time trying to track down issues. The process of elimination would suggest that if it's not the light fitting itself killing the globes, then the next thing to check and replace would be the cabling. If that doesn't resolve the issue, then you'd be considering something going on at the switchboard.

Mitchell

[Walzz]

@Ask_Itee  I spent a number of years working as an electrician (now retired), and I can't see how a rewire would make any difference.  From your description all three lights are on a common circuit from the switchboard, but separately switched.  The only three things that I can think of that would kill bulbs are voltage spikes, vibration, or bulb orientation. (Some bulbs die prematurely when mounted horizontally rather than vertical and vice versa).

If indeed there is some sort of wiring/ mis-wiring problem. it will be confined to the branch feeding the problem pendant and nothing to do with power from the switchboard to the wall switch (which is common to all three lights. Since you have tried LED bulbs as well, which in theory are not affected by vibration or orientation issues, so I'm surprised LEDs are dying as well.  Without careful inspection and testing it is difficult to pinpoint what is really going on! One thing I would check is that all three pendants are definitely connected to the same neutral.

Regarding vibration, the indoor a/c unit may not be vibrating all the time. An old mechanic's trick (poor man's stethoscope) was to place the handle end of a large screwdriver against the ear, and the blade end at various points around the motor to listen for noises. Maybe that will work for you?
One final thought is that the vibration may be coming down the cord from the ceiling to the pendant, and some sort of rubber isolation mount may mitigate that happening.
[edit] one final final thought is that the cord and light fitting will be mechanically resonant at a certain frequency, and if that coincides with vibration frequencies from the A/C, that will greatly magnify the problem, if indeed vibration is your issue.
Just to demonstrate what I'm talking about, here is a practical demo from a physics class.
https://www.youtube.com/watch?v=UQ7tuUxiLkg 

Sorry I can't offer any more ideas. Will be very interested to hear the final solution to this mystery..

Good luck!
Walzz

[Jason]

Fascinating to read the advice from @Walzz. Many thanks for sharing your knowledge and experience. 

Let me also add a couple of other electricians to the discussion to see if they might like to add their thoughts: @MikeTNZ and @CSParnell.

Jason

[CSParnell]

Hey @Ask_Itee, 

Few things it could be.

1. Over voltage.

2. Loose connection as in the center pin is pressed in to far and it's slightly arcing

3. High resistance.

4. Bad neutral connection.

5. Incorrect Dimmer if applicable 

Give that a go see how u end up.

[MikeTNZ]

Hi @Ask_Itee,

This sounds quite difficult now that you're at this end of the job.

Has the Electrician tested the Earth Loop Impedance from that light fitting, through all of the lighting points.

This is quite difficult to do, but it will tell you/them how good/poor the connection between that fitting and the connection back to the switchboard is.

Any Electrician should be able to do this test, if they can't, I would seriously doubt their qualifications.

One other thing, if the bulbs (lamps) point upward, there is the risk that if a filament in one lamp fails, it will by attrition cause the others to fail by the short circuit current, this is because when the filament fails, it falls down across the feed wires inside the bulb itself.

Do the bulbs fail when you first switch the pendant fitting on?

Reason I ask this (and your Electrician should have checked this) is because if your voltage is too high (ie 250V AC) and you are only using 230V lamps, that extra 20 volts can cause bulbs to fail left right and centre.

One other thing I would suggest, is this, get an Electrician to put a data-logger in your switchboard for a week and get them to download any voltage/current disturbances.

Also, I'm not sure if this has been done yet, but surely they would have checked all of the terminal screws in the switchboard, especially the Earth and Neutral busbar screws, having a few of these loose is a house fire waiting to happen.

I am reminded of a case in Perth a number of years ago, where they have to use 250V AC lamps because the supply authority there didn't want to turn down the single phase voltage to 235V AC or it would affect the high voltage side of things.

Please, by all means, let me know how you get on with this, I've only suggested a couple of things to do, if you don't get any satisfaction, let us know.

Cheers,

Mike T.