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Molten contacts and a lesson in observation

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So, it’s hot days and the house AC runs a lot during daytime at our ranch.  But even though the inside fan unit runs, nothing is stirring in the outside compressor unit.  I check the breaker, and it’s tripped.   I reset it and the fan on the compressor unit works but can’t determine from sound if the compressor is going.   It’s not; no cool air from the inside ducts.

I wondered why the breaker tripped and soon found out.   I turned the breaker off and opened the electrical compartment on the compressor unit.   Wow … soot, molten metal, melted insulation and more!  See the first pic.  Looked like catastrophic failure of the contactor.  But wait … the fan and compressor both get power from the contactor, and the fan works.  Closer inspection reveals it was the connection between one of the compressor wires and a lug on the contactor that failed.

I decide to replace the contactor.  A call to the local electrical supplier revealed they could get a replacement shipped from Utah.   I could get one immediately from a different supplier in the big city (Sacramento), but that would be a 2-3 hour round trip in heavy traffic.  I noticed the contactor was 3-pole even though we have single phase power.  Guess the AC manufacturer has a 3-phase compressor option for units going to apartment and commercial buildings.  The contactor still worked, so I could do a “temporary” fix by trimming the burnt wire ends, applying new terminals where needed, and moving wires from the damaged pole to the unused pole.

Before getting started, I made a careful drawing of connections and consulted a picture I made 4 years ago when the capacitor had to be replaced.  The pic is below.  You should be able to see what I missed 4 years ago.   I had an HVAC tech out to the ranch 3 years ago to repair different equipment.  For a decent price, he offered to do a thorough inspection and evaluation of the AC system of the house.  Yeah, he missed it too.

A picture of the “temporary” fix is below.   I ordered a replacement contactor.  Chances are it’ll sit in reserve.

As a final note, all the inside wire of the compressor unit is AWM (appliance wiring material).   This wire is not legal for building wiring in the US, but is common in, yup, appliances.  This stuff can be strange, and the AWM in the compressor unit was no exception.  The wire to the fan was 18 AWG, but the outside diameter was the same as 10 AWG building wire … massively thick insulation.  The wire to the compressor was 10 AWG and same OD as the wire to the fan.   The conductor was stranded and composed of numerous fine wires … much finer than in building wire.

Mike in California

Put a #6 stranded wire in a lug similar to that, tighten it as much as you want, then move the wire around a bit.  Sometimes you can just pull the wire out of the lug.  Vibration can do the same thing.
I had that happen on a 480V 200kW generator breaker (on the generator side), Melted the top half of breaker, the wires, and the front panel. 

I wonder if a ferrule would prevent that failure


--- Quote from: coppercone2 on July 21, 2021, 06:25:23 am ---I wonder if a ferrule would prevent that failure

--- End quote ---

It probably would have if it was the right kind of ferrule properly applied.   I think the stranded wire with many fine strands was a culprit, and a ferrule would have rectified its mushy, fragile nature.  Building wire of that size is used frequently in set-screw lugs with little problem.  But building wire has fewer, thicker strands.


it seems that only a certain conductor class range has good performance with screw terminals


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