Troubleshooting - A/C circuit tripping

[PetrosA]

Hi all

I'm getting called in to assist an HVAC guy with troubleshooting a roof unit that's tripping a BR230 240V circuit. I've already checked the roof disconnect, the BR230 was changed by the A/C guy, and I had a looksee at the guts of the unit about two weeks ago. What I found then (and should have been spotted by the other guy) was a burned out spade connector on one of the capacitors which I replaced (the connector, not the capacitor). Aside from that one connection, I couldn't find any strictly electrical issues in the circuit. The unit seemed to work fine for about a week after I was there, then he got called back in for a tripped breaker again. He's telling me that his amp meter is showing 16-17 A per leg, the breaker is warm, then it trips. The warmth is probably normal (I'll take a temp reading when I get back there), but a 30A breaker tripping on a 16A load is strange, and I'm at a loss. Any ideas out there?

Thanks,

Peter

 

[ultramegabob]

either the breaker is weak, or you have an intermittant short that is tripping the breaker.

 

[DanZ]

If you can, check the capacitor. If there was a burned out connector, there's a chance that it "shorted", fried, or in some other manner discombobulated it.

 

[km5qd]

Sounds like a compressor to me. I have ran across this several times. Unplug compressor and check all 3 terminals to ground and all 3 terminal across each other.

 

[Ebow]

Yes, I would change the capacitor. But I would check everything else also. It seems from your original past that the A/C guy just figured a bad breaker and replaced it without checking out anything else. Maybe he had a swing needle amp meter and misread the line for the setting. Was it a dual capacitor? Which wire burnt off the terminal? I am leaning towards a bad compressor, but without being there its hard to say. Maybe a bad feed to the disconnect itself.

Gene
_________________________________
Remember - Speed Kills and it may not always be you

 

[brian john]

Perform an FOP if OK then replace CB, if FOP shows an issue determine if it is a loose connection, bad connection or internal CB problem.

 

[ramsy]

If you can, check the capacitor. If there was a burned out connector, there's a chance that it "shorted", fried, or in some other manner discombobulated it.

Who's got a capacitor-check procedure, without blasting meters or people with a discharge?

 

[ultramegabob]

Who's got a capacitor-check procedure, without blasting meters or people with a discharge?

http://www.applianceaid.com/micro4.html

 

[Fishin-trician]

check for loose connections at the load center circ breaker, and the connections in the r.t.u. disco. with the disco off, check thru the wiring compartment inside the r.t.u., and contactors for loose or burned wires. also chek wiring going into condeser fan and blower fan for burn signs. then restart the unit (amprobe attched) and watch it running for a bit. a good ac guy would have done this prior to calling in an electrician. if nothing is found to be a definite cause, then just replace the breaker, as our responsibility usually ends at the disco. hope this helps- goodluck

 

[ramsy]

http://www.applianceaid.com/micro4.html

Superb.. Thanks Bob.

We are on on a role. What's normal resistance for the fan, or if on separate circuits, what else shorts out HVAC equipment, besides the condenser and compressor?

 

[PetrosA]

Ok, I'll try and respond to you all

UMBob, I'm leaning towards an intermittent short or overload fault.

Dan, I didn't check the cap at the time since it started fine when we hooked power back up, but I probably should have - my bad.

KM, I'll do this when I get back there. I'm assuming I should be checking for continuity, correct?

Gene, I visually checked what I could, but I'm no specialist in AC units. You kind of hit the nail on the head when you wrote that all the other guy seemed to do was change the breaker. Supposedly he's been out there like 6-7 times already. They did change one of the fan motors, but I'll bet the whole unit is ready for replacement at this point (it looked about 25-30 yrs. old.). You're also right that when one guy gets troubleshooting results from another that he doesn't know, procedures and competence can only be assumed...

Brian, You lost me at FOP I need a list of commonly used abbreviations for this forum...

Fishin, That's basically what I did the last time I was there and how I found that burned connector. Thanks for the wishes.

 

[brian john]

Fishin

check for loose connections at the load center circ breaker, and the connections in the r.t.u. disco. with the disco off, check thru the wiring compartment inside the r.t.u., and contactors for loose or burned wires

If you perform a FOP you'll cover all of this, though a good visual is ALWAYS a part of any inspection for electrical anomalies

 

[ptonsparky]

Who's got a capacitor-check procedure, without blasting meters or people with a discharge?

A 40va 24v transformer will usually work for this.

1 Make sure the capacitor is not shorted.
2 Series an ammeter with the cap and xfmr
3 Connect Voltmeter across cap
4 Briefly apply power to capacitor. Take note of voltage and current.
5 (2650*I)/E = mfd

 

[R Bob]

Who's got a capacitor-check procedure, without blasting meters or people with a discharge?

I have a Fluke 87 that measures capacitance and also has a bar graph.
You can't beat it if you do a lot of this kind of work. Very versatile.

 

[wptski]

One thing that hasn't been mentioned is "short cycling". If the compressor tries to cycle ON before the head pressure drops, it could cause a OL. Some modern thermostats have a built in timer that's supposed to make sure that the OFF time is at least five minutes of so to prevent this.

I accidently did this with a basement dehumidifier a while back. I went to adjust the setting while it was running, forced it OFF, started to adjust back to a lesser setting but it tried to start and POOOF!

 

[R Bob]

One thing that hasn't been mentioned is "short cycling". If the compressor tries to cycle ON before the head pressure drops, it could cause a OL. Some modern thermostats have a built in timer that's supposed to make sure that the OFF time is at least five minutes of so to prevent this.

I accidently did this with a basement dehumidifier a while back. I went to adjust the setting while it was running, forced it OFF, started to adjust back to a lesser setting but it tried to start and POOOF!

A power outage of short duration would yield the same result.

There are time delays, about $10, that can be placed in the control circuit in the RTU to give the compressor time to equalize before allowing it to start again. Good idea even if your not having problems. Highly recommended.

After 7 or more service calls for well over $1000 coupled with eroding customer confidence and disatisfaction, I think that I would seriously consider a recording meter and try to narrow down the sequence of events.

If nothing else, short cycle the unit with an amprobe in place and see if the breaker trips.

 

[DanZ]

Uhm...three more things, since no one else has thrown them out...

-Check the refrigerant, if it's low or bad, it could cause an overload that would trip the breaker, more so if it's GFCI

-Megger. I can't believe Brian didn't mention it, but megger the lines and make sure it's the unit that's causing the problem.

 

[ramsy]

I have a Fluke 87 that measures capacitance and also has a bar graph. You can't beat it if you do a lot of this kind of work. Very versatile.

Is the value your checking stamped somewhere on the capacitor?

My Extech scope meter manual says, "After Capacitor is de-energized & discharged, Touch the test lead to the capacitor to be tested. The auto-ranging display will indicate the proper decimal point, value and symbol."

What nF values do you get from those HVAC capacitors?

 

[john m. caloggero]

Air Cond tripping breaker

Air Cond tripping breaker

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.

 

[R Bob]

Is the value your checking stamped somewhere on the capacitor?

My Extech scope meter manual says, "After Capacitor is de-energized & discharged, Touch the test lead to the capacitor to be tested. The auto-ranging display will indicate the proper decimal point, value and symbol."

What nF values do you get from those HVAC capacitors?

The uf value is usually stamped on the capacitor.

I've seen a full range of values depending on the characteristics of the motor/compressor. 15...30, 35...etc.