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#48772 02/19/05 04:00 PM
Joined: Dec 2001
Posts: 5
Junior Member
I am about to change out 7 sections of an old Westinghouse MCC and directly above the main lugs in a 6" space there are two capacitors, I imagin they are power factor corection capacitors. Am I corect in assuming that, and how would I go about safely discharging them?

#48773 02/20/05 05:57 PM
Joined: Jan 2001
Posts: 1,044
Tom Offline
I don't know if your assumption is correct or not. IBut if those are capacitors, they may have a discharge resistor connected across the terminals, so start by looking for that.

No resistor? With the power to the MCC turned off, measure the voltage at the terminals, if those caps are real old, they might not even be in working condition and there will be no voltage present. If there is a voltage present, determine what the caps are connected to. If it is a specific contactor, with the power still off, you can manually close that contactor & let the load discharge the cap.


Few things are harder to put up with than the annoyance of a good example.
#48774 02/20/05 06:03 PM
Joined: Sep 2001
Posts: 806
Just a quick note that the voltage check at the cap terminals should be made with the meter set for DC voltage, not AC.

NEVER trust that the bleeder resistors will do their job. They are connected across the powerline all the time, and fail open quite regularly with no warning sign.

[This message has been edited by NJwirenut (edited 02-20-2005).]

#48775 02/21/05 03:10 AM
Joined: Oct 2004
Posts: 48
I agree with NJwirenut, never trust the bleeder resistors. I have successfully discharged such caps (assuming they are PFCs and at a reasonable voltage i.e. less than 1kV) using incandescent lamps of equal or greater than the rated DC voltage of the caps (the peak voltage on the caps could be 1.414 x AC voltage) - you can always wire lamps in series if required.

Leave the lamps on the caps for no less than 1 hour after the DC voltage across the caps reaches zero (preferably leave the lamps in circuit while all work is carried out - does two jobs; first is the caps remain discharged, and second you know if some twit re-energises them without your knowledge!). Reason for leaving lamps in circuit for some time is such caps are known to "recharge" and can develop hideous voltages across the terminals without any power applied.

Don't know about there, but here (UK) it is now regulation that PFCs have bleeder resistors. While re-working the caps, you may want to test for and install bleeders if required.


#48776 02/22/05 08:00 PM
Joined: Jul 2002
Posts: 8,443
Likes: 3
According to my '99 NEC book:
Article 460 gives guidance on capacitors used on electric circuits.
460-12 states that the nameplate on a capacitor shall indicate if a capacitor has a discharge device inside the case.

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