Do I really need a 20,000 ohm, 5 watt resister to de-energize a capacitor?

[simplyrollin]

no

[mgenius33]

First, a meter should not be used to discharge a capacitor, unless you like buying new meters often.

Second, capacitors will hold a charge depending on how it is placed in a circuit.

The reason a run cap doesn't hold a charge is because the motor windings discharge it very quickly. It does have low resistance, but it also has greater mass to dissipate the charge via heat.

Unless it is placed in the circuit as a trickle charge capacitor. That is why there is a resistor on those aswell. To discharge the cap before a tech touches it. If you don't believe me, wire in a trickle circuit without a resistor across the terminals. Allow it to run, then pull the disconnect and check the voltage across the cap terminals.

A start cap uses the bleed resistor for two reasons. One, to protect the potential relay from welding it's contacts, and two to save a tech from getting shocked.

Yes, you need a 20kΩ 5watt minimum resistor.
Theory... higher wattage resistors can dissipate heat better. That is pretty much what you're asking it to do, in order to discharge the cap.

Typical caps are 370v when charging.

Ohms law tells us E/R = I, and IE = P.

So, when charging at 370V/20000Ω we get < 0.0185A. (charging is by reactance which is lower or < resistance of the resistor)
When discharging to line voltage at 230V
we get 230V * 0.0185A = 4.255W

Using a screwdriver to discharge a cap is a good idea. Not so much for the equipment, but for you. I would much rather eat up a screwdriver or cap than get shocked. In most cases, the cap should be discharged before you have time to touch it. If it's not wired properly or an issue with the circuit, it could hold a charge. I don't know any tech that carries a 20kΩ resistor in his/her pocket. So a screwdriver is a good sacrificial alternative.

[mgenius33]

The charging part of that last post is wrong. Was just going off the top of my head.

[-80guru]

Notice the arc flash when you short the cap. For your start relay it will experience that arc flash on every start up if your start cap does not have the resistor. As for discharging when wanting drain the energy then short it or bleed it (speaking of capacitors in residential or small electric motor applications),NOT VFDs.

[Rushmore]

I don't know anyone who carries around a 20k ohm resistor either. Definitely use a screwdriver. Your teachers just tell you that because they have to give you the most professional methods available.

[markvilleman]

How big the cap is,if small charge inside I just use screwdriver.

[MicahWes]

Well over 90% of the time, the run caps you will encounter on a day-to-day basis discharge themselves very quickly through the motor windings when power is removed. Most start caps have a bleeder resistor. It isn't good for an electrolytic capacitor to discharge it by direct shorting (it harms the plates and the electrolyte between them), but since most all caps are already discharged when you put your screwdriver across them anyway it doesn't really matter.

[ironpit]

Should not direct short the cap terminals, or hook your meter to a charged cap. If you don't want to buy a resister, take one off a used or old start cap. 15k ohms is plenty. Discard the altered cap in a responsible way, please!

[walklong]

As an electrician for 20+ years, my 2 cents: big enough start and run capacitors, if charged, CAN vaporize a nice divot in the screwdriver and render the cap's terminal a useless stub. If you suspect the cap might have a charge, use a screwdriver you can afford to lose, and land the blade first, then complete the short with the shaft: the arc will spring from the terminal to the shaft and you'll still have a usable blade end on the screwdriver. As others have said, most caps will be fully discharged through the compressor within a few minutes of shut-down. I short them all because I'd rather replace a yard sale screwdriver for a buck than my fieldpiece meter.

Caps encountered in residential HP equipment I'm used to aren't big enough to do much damage UNLESS you forget to pull the disconnect! An energized system can yield: 1) welded/blown equipment; 2) a screwdriver converted to a lethal missile; 3) a scorched spot on the earth where your boots where when you smoked yourself.

I've been to more than one funeral for guys who KNEW the circuit wasn't energized.

[Lucid]

what kinda cap are you trying to discharge is the right question...if its a run cap under 80mfds

it will discharge withing 30seconds MAX

now a car audio cap..thats a different story....or a cap bank for a diesel power back up...totally different story

instructors at trade schools tell new techs all sorts of crazy stuff

i had a new hire tell me i was ruining package units becuse i used a drill to take of panel screws...lmao

[broc123]

I use any tool with rubber handle to verify discharge of run or start caps . Get into habit just because the disconnect looks off doesnt mean all legs are dead bad disconnects will kill .

[walklong]

I use any tool with rubber handle to verify discharge of run or start caps . Get into habit just because the disconnect looks off doesnt mean all legs are dead bad disconnects will kill .

I will ALWAYS check the high voltage side of the contactor, either with the NCV on my meter or if I'm really paranoid, with the leads and that's BOTH legs of the supply wiring.

You've got the service panel off: why NOT check it: the life you save may be your own!