What Happened Here?

220/221

T-shoot resi A/C condenser circuit.

Get there and see this.




I open the panel to find a typical burned A/C breaker and bus. The pics aren't very clear but the bus is burned, melted, pitted and the breaker is discolored as well as the breakers next to it.



1st question is WHY do the AC breakers AND bus burn up here ALL the freaking time?

This panel is only 7 or 8 years old. It's out of warranty but stilll....stuff should last longer than that.

I get a dozen calls like this in the summer. This was not the worst case I've seen. Often the bus is beyond repair.


2nd question is, can the arc burn at the panel/meyers hub be associated with the burning of the breaker/bus?

I think not. No EGC pulled into the conduit. I suspect a ground fault at the unit at some point in the past 7 years. Homeowner claims that the burn JUST happened.

I was able to clean up the bus and move the AC breaker to a pristine spot. AC condenser worked fine drawing only 25 amps.

Homeowner claims that no one else has worked on the panel but I know otherwise. Someone has installed a homeline breaker and double/triple tapped a couple of other things since I had been there last.

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Just because it's within code doesn't make it safe.

Just because it's against code doesn't make it unsafe.

Dustball

Is the panel in direct sunlight a lot of the time? Just wondering if the panel is being baked by heat build-up? That in combination with a heavy load running a lot of the time may do it.

InPhase277

Perhaps the breakers are the minimum size allowed by the unit? I mean this might cause the breaker to run a little on the hot side, coupled with a high load kicking on and off. Maybe going to the maximum size OCPD allowed would alleviate the problem. And you could wire brush the bus and put some noalox on the stabs.

In addition, it wouldn't hurt to get the paint off the area around where the hub seats. It just can't be making a good ground connection through that grey paint.

InPhase277

Speedy Petey

All I will say is this is a PRIME example of why ALWAYS pulling a ground WIRE, regardless of conduit type, is a good idea.

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mdshunk

Your picture, to me, shows 2 common problems. Yours are coincidently on the same circuit.

First, that burned fitting coming out of the LB is just typical of a hard ground fault, and it will spit some sparks at any loose fitting. This is why pulling a ground is always recommended by me. I've seen all sorts of stuff blackened like that. Couplings, steel beam attachments, box adaptors, etc.

Second, your picture of the burned up breaker and buss stab. Residential plug-on panels just don't tolerate continuous load at higher currents very well. You're in the southwest, and you find this condition on AC breakers. I'm in the northeast, and do you care to guess what we find this condition on? Electric heat and heat pump breakers, which can be quite a few breakers in the case of baseboard heat. The HOM and other breakers patterned after the old 1" Westinghouse plug-on breakers seem to be the worst offenders to me, followed by QO. For what it's worth, I seldom (if ever) find this condition with Cutler Hammer "CH" breakers. This condition never happens with properly installed bolt-on breakers. I have used NQOD panels on higher end resi work just for this reason.

220/221

Outside but not in the direct sunlight after early morning.

50 amp breaker, #6 copper wire, condenser draws about 25 amps.

Absolutely. Heh heh...I thought it was code to pull a groound until I started reading the electrical forums.


That is what I am thinking. Two separate issues. I think the hard fault happened on a prior occaision....shorted out compressor or something. The beast was about 30-40 years old and had been serviced MANY times over the years.

Another good reason to install full 40 space panels. You need to relocate the AC breaker every 5 years . At the time (7-8 years ago) this was the standard 200 amp panel we always used. Now we always go with the full 40's.


Question #3. Have you ever seen a full copper bus burn like this? It doesn't seem like it would cost much more for a little bit of copper.

__________________
Just because it's within code doesn't make it safe.

Just because it's against code doesn't make it unsafe.

Thomp

220/221 I think if you megger or read phrases to ground you are going to find a fault in that line. I would think of replacing the conductors.

220/221

Yeah....you are right. I need to go back and look deeper. I suppose that there is a slight chance that the thing faulted, cleared and is still operational. There is more of a chance that the AC service guy caused the fault or a compressor shorted out.

I was going to buy a megger anyway. I might as well do it now that I have something to use it on.

__________________
Just because it's within code doesn't make it safe.

Just because it's against code doesn't make it unsafe.

idoelectric

There's no mention of any fusible service disconnect at the condensor unit for overload protection. If this is so, then it's a possibility that the breaker is not properly sized for the circuit. Breaker may be the weakest point in which loose connections at the buss will cause arcing.

Copper doesn't melt in cases of ground fault or arcing, it dissipates and turns into a gas. Ground faults reaches temps of 10,000 degrees F and arc flashes reaches temps of about 35,000 degrees F. Cleaning the buss and continue to use the same buss may not be a good idea for repairing.

220/221

Went back yesterday. HO said it tripped again. It was reset and working when I got there.

Fuses were oversized @ 60 amps. Circuit is 50 amp. THHN to SE (jb in attic), back to thw (jb under eave) to disco/unit.

AC guy took covers off the unit and inspected. Unit draws 25 amps and sounds good.

Next step...get a megger or belly crawl about 12' into the attic to access jb (about 24" height) or replace the feed (60' thru trusses appx 3' clearance max)

I'm thinking just pull new circuit. The old cloth SE from 1960 something, along with the 2 jb's is coming out.


PS. I moved the AC breaker to a clean spot.

__________________
Just because it's within code doesn't make it safe.

Just because it's against code doesn't make it unsafe.

nap

Didn't see where you listed the max OCPD (missed it if you did).

(Not that this would cause the arcing but) Is the breaker HACR rated? (should be)

The arcing at the LB is obviously not from an overload but the bus burn problem could be or it could also be from whatever caused the arc burns at the LB. Since you haven't figured it out and you aren't there when the thing trips, I presume you have not determined if there is actually an overload at all and not just some short circuit causing the trips.

I would size the breaker properly, if not already correct.

Then, I would look for a possibility of a short circuit. Not sure what was already looked at but I would look at the unit itself very closely. The contactor or any junction box and any wiring within the unit would recieve close scrutiny.

If there is a bare wire or anything that may make electrical contact that is not always making contact to ground, when the unit starts, the jerk and vibration may be causing the problem to move and come into contact with ground and tripping things. If it does not weld, it may simply be a continually intermittant problem that occasionally touches ground.

You might ask if the owners have noticed if the thing trips on start up or after running for a minute or two. I would make a wild guess and say it was on start up, if they actually noticed at all.

You may try running a temp feed to bypass all the existing if you seriously believe the wiring in place now could be the problem.

loganm

In most cases when the compressor kicks on it will pull 100amps or more on the start. Now this is when the capacitor discharges and brings the compressor up and running. However, if the capacitor is weak it may not be doing its job as fast or as well as it should. Also depending on the condition of the compressor a good capacitor may not be enough, in which case a hard start kit would be in order.

I would say to look at the capacitor and compressor in situations like this.

Logan

bjgelect

The panel shown is a Murray/Siemens AL buss, the #1 rated burnt buss panel in my book. A few heat cycles from high current draw and the connection weakens, and then it goes downhill exponentially. Whenever I install an aluminum buss panel (usually it's CU) I ALWAYS put a dab of Penetrox or Contax on the stab. The panel shown has anothe strike against it, it's a 3R with AL buss, double whammy. These thing corrode inside by themselves just sitting there.

ROOFERSCOTT

Good luck with that buddy