Discharge Lighting Capicitor Question.

[andyb]

Looking for advice on capacitors.
Been to a site where a couple have melted (for want of a better word)
These are 400v floodlights and all the ballast and capacitors are in a big wooden cabinet, and some of the ignitors.
My knowledge of pf capacitors is limited to my course theory from a long time ago and of course seeing them in discharge lighting and on motors. Never had to give them too much thought.
So some questions,

Do they run warm or hot? I had the other lights on whilst I was there and the capacitors all were cold to touch.
Is this type of leakage common?
What would cause it?

Also why would each fitting have 2 capacitors in parallel? do the rating of the capacitors need to be that precise?

Finally, was it common to put this type of arrangement in wooden cabinets, this was done by SSE, the last ones about 12/15 years ago.

Thanks.

 

[plugsandsparks]

Capacitors age, so don't last forever. As they age the characteristics change and they can start to generate heat which leads to a vicious cycle. Ultimately can lead to short circuit hence all the fire and brimstone. Also capacitors generally don't like harmonics, which again generate heat etc etc. So they need to be accurately specced against the VAR and harmonic content and maintained to the MIs.

Thats all I know... lol

 

[Marvo]

That's a neat trick getting your sidecutters to hang in mid air like that whilst you took the first photo, very impressive .

Usually when caps are paralleled like that it's just because it's cheaper to use 2 smaller value caps instead of a single big one.

The caps should run cool, if they're hot to the touch there's normally a problem such as a lamp that's not igniting properly or maybe harmonics as mentioned although it's a long shot.

 

[andyb]

Thanks Plugs and Marvo.

That's more than I knew a couple of hours ago.
It's a worry that they run hot when they start to break down, especially as they're mounted in a wooden enclosure and as you can see there is quite a few of them. 48 in fact.
If I bought a capacitor tester would it help in determining whether they are breaking down, or would it just tell me it's not
working?

Thanks again.

 

[Marvo]

The tolerances on caps are so damn slack at the best of times that a capacitance tester is unlikely to give you conclusive results. A Flir camera / thermal imager would probably be the weapon of choice to see which are on their way out, it's what we use on PFC banks to detect problems.

 

[andyb]

The tolerances on caps are so damn slack at the best of times that a capacitance tester is unlikely to give you conclusive results. A Flir camera / thermal imager would probably be the weapon of choice to see which are on their way out, it's what we use on PFC banks to detect problems.

PFC, would that be power factor capacitor banks? Seems like I should take a look at one.
I'd like to know that the capacitors are doing their job as well. These are 2Kw lamps so if they stop working that would be quite expensive, and as I said earlier they are quite old.

 

[Lucien Nunes]

Small caps for discharge lighting do not give out appreciable heat in normal use. Capacitors fail in different ways but the usual cause of them blowing up is plain ordinary dielectric leakage. Moisture gets in, or the film breaks down due to age, and it starts behaving like a resistor and dissipating heat. You can usually test for impending failure with a 250 or 500V IR test between the leads, unless the cap contains an integral bleeder resistor which swamps the test result. A capacitance tester will not always warn you about this problem, because the cap often still functions as a cap right up to the moment it fails. Some types go quickly when they go, one minute they test fine and then there's a short, a bang, the case cracks open and the fuse goes. Others fizzle, though they don't often give out enough heat to melt things, ballasts do but rarely caps because they self-destruct first.

Testing with a capacitance meter can be interesting but not always useful. A low reading can indicate that a self-healing metallised type has been subjected to excessive stress, broken down in many places and healed around the faults, losing active surface area as it does. A high reading is usually caused by the capacitance meter being fooled by leakage (handheld meters often can't tell the difference). Any cap that reads high beyond its tolerance will usually show leakage on an IR test.

There are other ways of identifying a suspect cap but most are more suited to an electronics lab than electrical installations. FWIW a lot of electronic equipment (TVs, PCs, LED drivers...) fail due to dead caps. I spend many (happy?) hours changing them.

 

[davesparks]

No, it shouldn't really be mounted on a plywood board in a wooden box these days.

 

[DPG]

Small caps for discharge lighting do not give out appreciable heat in normal use. Capacitors fail in different ways but the usual cause of them blowing up is plain ordinary dielectric leakage. Moisture gets in, or the film breaks down due to age, and it starts behaving like a resistor and dissipating heat. You can usually test for impending failure with a 250 or 500V IR test between the leads, unless the cap contains an integral bleeder resistor which swamps the test result. A capacitance tester will not always warn you about this problem, because the cap often still functions as a cap right up to the moment it fails. Some types go quickly when they go, one minute they test fine and then there's a short, a bang, the case cracks open and the fuse goes. Others fizzle, though they don't often give out enough heat to melt things, ballasts do but rarely caps because they self-destruct first.

Testing with a capacitance meter can be interesting but not always useful. A low reading can indicate that a self-healing metallised type has been subjected to excessive stress, broken down in many places and healed around the faults, losing active surface area as it does. A high reading is usually caused by the capacitance meter being fooled by leakage (handheld meters often can't tell the difference). Any cap that reads high beyond its tolerance will usually show leakage on an IR test.

There are other ways of identifying a suspect cap but most are more suited to an electronics lab than electrical installations. FWIW a lot of electronic equipment (TVs, PCs, LED drivers...) fail due to dead caps. I spend many (happy?) hours changing them.

Excellent answer from Lucien there. And yes, a capacitance meter can give a seemingly good result even on a faulty capacitor. A good way of checking electrolytic capacitors is an ESR meter - these are very useful when repairing electronic equipment. It measures the 'goodness' of the device for want of a better phrase. Daz