Troubleshooting - A/C circuit tripping

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Lots of good info here for sure!

Fishin, I'm still not sure what FOP means here...


Short cycling could be the issue, but I think that should be up to the AC guy to check, as should a recording meter.

Meggering - I spoke to my boss about getting hold of one (he doesn't have one) so I can check the feeds up to the disconnect and into the unit. We'll see if he can handle that request ;)

John, unfortunately, it's got a printed (not enameled or stamped) nameplate. I'm guessing the unit is at least 25-30 years old and nothing is legible on the nameplate anymore. It's being fed with #8 CU and the run can't be more than 50' or so, so I don't think Vdrop is the issue.

My wife's getting more and more ready to go into labor, but if it doesn't happen over the weekend, I'll probably be back there Monday or Tuesday and I'll be able to give some more info then.

Thanks for all your ideas!
 
ramsy said:
Thanks Bob,

Would a shorted capacitor have a very low or Zero value?
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I usually check for a short with the ohm meter setting prior to checking capacitance. I'm pretty sure it would read 0uf on the capacitance setting if it were shorted.
 
FOP means Fall of Potential. You are checking for the voltage drop across the contacts of the circuit breaker. 250mv drop from line to load of all phases may be okay. 250, 750, and 125 means your load is imbalanced or the connections thru the CB are bad. Internally or at either end.
 
PetrosA said:
Lots of good info here for sure!

Fishin, I'm still not sure what FOP means here...


Short cycling could be the issue, but I think that should be up to the AC guy to check, as should a recording meter.

Meggering - I spoke to my boss about getting hold of one (he doesn't have one) so I can check the feeds up to the disconnect and into the unit. We'll see if he can handle that request ;)

John, unfortunately, it's got a printed (not enameled or stamped) nameplate. I'm guessing the unit is at least 25-30 years old and nothing is legible on the nameplate anymore. It's being fed with #8 CU and the run can't be more than 50' or so, so I don't think Vdrop is the issue.

My wife's getting more and more ready to go into labor, but if it doesn't happen over the weekend, I'll probably be back there Monday or Tuesday and I'll be able to give some more info then.

Thanks for all your ideas!
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FOP=Fall of Potential...a measurement of the voltage drop between the line and the load terminals of a ckt brkr, disco switch, fuse, contactor, etc.

Any indication of voltage drop warrants further investigation.

I usually perform this test on ckt brkrs when the proper function/integrity of the CB is in question.

Check with Brian if you need further info, he appears to be far more versed in this than I.
 
john m. caloggero said:
Hello Peter, Unfortunately you have not provided the nameplate data of the air cond, such as full load amps and voltage. But assume that the FLA is 16amps, at 230V.

Refrigerant motor compressors are covered in Art 440. The mfgs information provides the minimum conductor size and the maximum ground-fault short-circuit protection. Assuming that the nameplate rated load current is 16 amperes, the circuit breaker could be rated 175 % of that or 1.75 x 16A = 28A, therefore a 30A CB is allowed. However, that is if the proper voltage is available at the compressor location. If the branch-circuit is a rather long, a voltage drop will cause the compressor to draw more current and trip the breaker.
If this is the cause, larger gauge conductors will have to be installed to minimize voltage drop. In addition, Sect 440-22 of the 2005 NEC allows the Breaker to be increased to 225 % of the rated load. However, it cannot exceed the branch-circuit selection current.
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That's a good idea, but it's more of a new install issue. It sounds like this has been in place and working for a few years now, so I doubt it's just starting to become a problem.
 
DanZ said:
It sounds like this has been in place and working for a few years now..
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Its common for oil saturated conductors to deteriorate over time, inside a hermetically sealed A/C motor to compressor unit. When insulation resistance measures < 20k Ohms to compressor housing, breakers start tripping. I agree with DanZ, if your boss won't get you a Megger its time to bring your own beer (BYOB).
 
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make sure you dont megger line to line. just megger line to ground on each leg (hot to ground neutral to ground) or a good capacitor will suddenly become a bad capacitor. you can get a megger on amazon.com and have it in a couple days if you pay for the shipping. also make sure you set the voltage accordingly. check the name plate for the voltage and set the megger to about double. if its 120 lets say set the megger for 250. 240 volt set the megger to 500 and so on
 
electricalperson said:
make sure you dont megger line to line. ..or a good capacitor will suddenly become a bad capacitor. ..if its 120 lets say set the megger for 250. 240 volt set the megger to 500 and so on
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Learning sponge is soaking. That kind of help is rare, even for this forum.

Minuteman said:
Typically, a bad Cap will be open, not shorted.
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Another unexpected tidbit; along with R Bob's CAP resistance procedure. Whats wrong with this forum, you people are killing each other with kindness.
 
I think the problem is throwing all this money into a piece of junk 20-30 year

old unit. It's time to pony up and get a new unit installed, how many years do

you expect to get out of it anyway ??
 
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