Draining Capacitors?

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Besoeker

Senior Member
Location
UK
FWIW.
We make power electronic equipment.
This the wording on one of the warning labels we attach to the panels.
This equipment contains power capacitors.
Before working on it:
Remove all supplies
WAIT AT LEAST TEN MINUTES
Check that all capacitors have discharged
There is always a similar warning on the front sheet of the Operation and Maintenance manual.
Typically we have discharge/sharing resistors that have a CR time constant of not more than 30 seconds. But you also need to consider the possibility of the failure of a discharge resistor failure or it's connection.
The only way to be sure is to measure the capacitor voltage.
It is a simple precautionary step.
I'm a little surprised that no one else has advocated such a simple safety measure.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The capacitor 'memory effect' is called 'dielectric adsorption'.

The electric field of the charged capacitor causes actual physical displacement of charges in the insulation. Some of this motion is quite slow, involving charged species (ions and the like) moving into different parts of the structure, or molecules with dipole moments getting pulled into different positions, etc.

These same _slow_ mechanisms of charge motion show up on the discharge side of things.

The easy way to visualize this is to imagine that a real capacitor is really a set of smaller capacitors each with its own internal resistance. The 'slow' capacitor that we are talking about is relatively tiny, with a big internal resistance.

If you discharge this set of capacitors for a few seconds (say by shorting the terminals), this tiny slow capacitor doesn't actually get discharged. Then when the discharge load is removed, the slow capacitor will start to recharge the rest of the capacitors via its internal resistance.

-Jon
 

ZZZ

Member
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

This is a great link, truly amazing. Screwdriver my tool of choice. Across the cap terminals a couple of times. Caps don't have a ground reference and will not discharge to a ground as such.
 
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Fulthrotl

~Autocorrect is My Worst Enema.~
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!

capacitors, even after discharged, can have a memory, and the next
day wallop you nicely.

the standard rule of thumb in the capacitor manufacturing arena is
one ohm per volt for the shorting jumper. i go a bit higher than that
for a number of reasons.

best move is a 5k ohm power resistor (10 watts, physical size
should be around half an inch in diameter, and 4 inches long)
and put a couple leads on it, with alligator clips. tape it up.

wearing your mittens, clip it across the cap's terminals, and leave it
there while you are handling the part. 3 or 4 minutes is enough.
after the part is removed from the fixture, before taking the
shorting resistor off, wrap a piece of #14 bare copper around
the caps leads, leaving them connected.

then remove the shorting jumper, leaving the cap with the #14 wire.
do not remove the #14 wire. end of problem.

hope that helps.


randy
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Caps don't have a ground reference and will not discharge to a ground as such.
They sure do if the supply circuit is grounded (and still connected, of course, which may have something to do with those new discharge-lighting disconnect requirements.)
 

ZZZ

Member
They sure do if the supply circuit is grounded (and still connected, of course, which may have something to do with those new discharge-lighting disconnect requirements.)


I agree, but many times the supply is not grounded such as on a 400 watt HPS if it was 208,230, and 480 volt where you don't have a neutral. The same goes with caps on 230 volt refrigeration compressors. Most of the caps I work on do not have a grounded conductor. Across the cap terminals always works.
 

Doug S.

Senior Member
Location
West Michigan
I too have never seen energized caps in a HID situation... but that's me.

I once had to de-energize caps for service work and made a 10kΩ POT with leads and alligator clips on it... start high and spin it down to low. This greatly reduces the drain time and lets you work safely. You can put additional resistor in series for comfort, or maybe someone makes a 100kΩ single turn pot?

My 2?
Doug S.
 
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