Draining Capacitors?

[Mattula]

I have to remove some capacitors from a 400w HPS fixture. In my experiences as an electrician normally I install new fixtures. Now I have to remove the old ones and install new ones. Id like to ground them so I do not have to fear being shocked by them as I remove them. What is the safest way to achieve this? Thanks!

 

[electricalperson]

for smaller capacitors you can short it with your screwdriver

 

[Mattula]

Hahaha....thats what my Dad said too.

By this you mean......take the shaft of the screw driver and short it across the hot and common post on the capacitor?

...or

......have the slotted end of the screw driver touv=ch the grounded metal of the fixture and them touch the shaft of the screw driver on the hot post of the capacitor????

 

[jdsmith]

Hahaha....thats what my Dad said too.

By this you mean......take the shaft of the screw driver and short it across the hot and common post on the capacitor?

Yep, like this

 

[topgone]

shorting caps?

shorting caps?

for smaller capacitors you can short it with your screwdriver

I would advise against that practice of shorting capacitors! I mean how small is small? Capacitors will discharge on the circuit given the time. Since we are in hurry, we "drain" the charge, not by shorting the terminals first, but draining to ground/ chassis. Then, I would suggest the use of a resistor a few ohms resistance to dissipate the charge of the capacitor (shorting it). No bare hands please!

 

[iaov]

I would suggest a resistor with quite a few ohms(10K) and give it some time .A minute or two.Shorting caps with a screw driver may wreck your screwdriver and has the potential to be an arc flash hazard.

 

[nolabama]

i was wondering about the arc flash potential from shorting the capacitor
and i found this thread
lord have mercy on their souls
you guys have got to read this electrical advice ....
http://www.rollitup.org/indoor-growing/5713-hps-fixture.html

 

[jrannis]

Ive seen some with resistors soldered between the terminals. I guess you dont have that.

Big screwdriver is my first choice for the small ones. Time for the big ones.

 

[iaov]

Big drives with large cap banks always have resitors between the leads for dis -charging when de-energized. I have heard of, but never actualy seen caps that are big enough to charge when just sitting hooked to nothing. In big RADAR transmitters. Thecs have to discharge them and short the terminals to work on the gear.

 

[76nemo]

There is no universal answer for this question. If you'd like a deeper side to this, look here:


http://www.repairfaq.org/sam/captest.htm#ctdtk

 

[Besoeker]

I have to remove some capacitors from a 400w HPS fixture. In my experiences as an electrician normally I install new fixtures. Now I have to remove the old ones and install new ones. Id like to ground them so I do not have to fear being shocked by them as I remove them. What is the safest way to achieve this? Thanks!

If the fixture is isolated from the supply, grounding the capacitor isn't really the answer. If one side of the capacitor is still connected to neutral, shorting it to ground won't do a lot and shorting the other side to ground is not much different to shorting it out. If the fixture is totally isolated from the supply grounding either leg won't provide a discharge path.
Safest way?
Use a meter to check the voltage across the capacitor (DC) and to ground (AC and DC). Wouldn't you do that as a matter of course anyway?
If the capacitor is charged, then discharge it with a suitable resistor. I'd normally use 100 ohms or so depending on the voltage. Once you have done that, measure the voltage again.
Repeat as necessary.

As my father was fond of saying:
"Measure it twice. Cut it once."

 

[ptonsparky]

Has anyone ever found a charged cap in a HPS fixture? I, quite frankly, have never checked for one. Now I must look at ballast diagram to see if it is a potential problem.

 

[steve66]

Most likely the capacitors in a light fixture will discharge through other circuit components.

But, it pays to be safe. And discharging capacitors slow is always better than discharging them fast. For everything except large or HV cap's, I would put a wiggy across it and call that good. The solenoid in the wiggy should give it some resistance.

If you don't have a wiggy, then I would use a voltmeter, followed by a screwdriver.

High voltage caps tend to recharge by themselves (they have a memory effect). For these its very important to discharge it slowly, give it some time, and disharge it again. I've seen this with cathode ray tubes in TV's - even after you discharged it a couple times, if you got your finger too close, a spark would jump out and get you.

Steve

 

[LarryFine]

High voltage caps tend to recharge by themselves (they have a memory effect). For these its very important to discharge it slowly, give it some time, and disharge it again. I've seen this with cathode ray tubes in TV's - even after you discharged it a couple times, if you got your finger too close, a spark would jump out and get you.

I haven't researched it, but I'll bet that only applies to DC applications.

 

[Karl H]

I use a 100ohm resitor.Depending on the size of the CAP I wouldnt use
the screwdriver method or that screwdriver will end up in your Tool Graveyard. lol

 

[Karl H]

Most likely the capacitors in a light fixture will discharge through other circuit components.

But, it pays to be safe. And discharging capacitors slow is always better than discharging them fast. For everything except large or HV cap's, I would put a wiggy across it and call that good. The solenoid in the wiggy should give it some resistance.

If you don't have a wiggy, then I would use a voltmeter, followed by a screwdriver.

High voltage caps tend to recharge by themselves (they have a memory effect). For these its very important to discharge it slowly, give it some time, and disharge it again. I've seen this with cathode ray tubes in TV's - even after you discharged it a couple times, if you got your finger too close, a spark would jump out and get you.

Steve

I've never seen a CAP "Re-charge" itself without current passing through
it.That's very interresting .Can you explain?

 

[steve66]

I haven't researched it, but I'll bet that only applies to DC applications.

That could very well be. I wasn't at all suggesting this would be a factor with the caps the Poster was talking about.

I've never seen a CAP "Re-charge" itself without current passing through
it.That's very interresting .Can you explain?

I didn't find any references to this on the internet (too many electronic "capacitor memory" devices out there to just search for capacitor memory).

And this may be more specific to CRT's than capacitors in general. The glass makes a very good insulator, with very little leakage current through it. In contrast, oil filled and electrolytic capactiors probably tend to self-discharge faster than they self recharge - making this a non-issue.

And I'm not sure anyone really knew exactly how CRT capacitors would recharge. (I would like to find out if more is know about this now days. I first heard about this in the 80's.)

Anyhow, my main point was to be careful with any HV caps, even if you have discharged them.

 

[glene77is]

IAOV,

I would suggest a resistor with quite a few ohms(10K) and give it some time .A minute or two.Shorting caps with a screw driver may wreck your screwdriver and has the potential to be an arc flash hazard.

I suggest a 100Kohm resistor.
(I don't like sparks.)

 

[Cow]

Hmm, I've changed quite a few ballasts(HPS and MH) and I've never been hit. There's always been a minute or two from the time I shut the circuit off till I take the ballast wiring apart though. Never needed to short one with my screwdriver yet.

A while back I watched my coworker unplug a 400w MH low bay and immediately touch the prongs of the cord end. He got a pretty good shock out of that one. The potential is there but if you wait a minute for the charge to dissipate you should be alright.

 

[ATSman]

Shorting Caps

Shorting Caps

To be safe I usually take a 120V, 100W incandescent light bulb with leads
to short it. It glows bright then dies down fast. Use HV gloves and safety glasses