Ground fault or not really

[Twoskinsoneman]

I don't do a lot of HVAC but when my brother's outside unit breaker tripped I was curious about it.
When I took the plug off his compressor and ohm'd it the main coil read 550k so I'd say it's shot. (The aux coil reads 2.0)
What has me curious is the common contact on the compressor reads 0.1 ohms to ground (checked at the EGC, unit housing, and copper tubing). Looking at the diagram this seems like a ground fault.
However when I closed the breaker it didn't fault like I expected. I had 1A on L1 and 0A on L2. Also the outdoor fan came on weakly, not full speed. With the contactor open it shouldn't have come on at all.
From the diagram I can see how if the compressor common is shorted to ground then that could provide a path for the fan motor to weakly run. What I don't really get is why that connection to ground in the compressor isn't a fault. It seems like it should have tripped the breaker as soon as I closed it but it didn't. It didn't even pull any current. Assuming the EGC path is good (I think it is) why isn't it a fault if the compressor motor coil is shorted to the compressor case?
Thanks. Like I said I was just curious about it since I happen to be visiting. He's got an HVAC service guy so I don't think I'll have time to do any other checks before it's fixed or replaced. Any insight?

 

[ptonsparky]

It's a a high resistance fault and the breaker sees it as a small load. I did not study your diagram.

 

[retirede]

Most of time, the problem ends up being a failed starting capacitor.

 

[jaggedben]

It's a a high resistance fault and the breaker sees it as a small load. I did not study your diagram.

0.1 ohms is not high resistance, that should draw ~1200A at 120V to ground. Something else is going on.

 

[Twoskinsoneman]

0.1 ohms is not high resistance, that should draw ~1200A at 120V to ground. Something else is going on.

That's what had me wondering. With the plug off I was checking the common terminal of the compressor to the housing, EGC, and copper refrigerant lines. It was a steady 0.1 ohms. Well in actuality my meter reads 0.1 ohms with leads touching each other. So pretty much no resistance.

 

[TwoBlocked]

I'm not an HVAC guy but let me give it a shot. Is this a 120V unit? If so, and L1 is connected to the neutral, then you would expect continuity between ground and "the common contact on the compressor" (one side of the overload?).

 

[TwoBlocked]

Oh! it does this with it unplugged?

 

[ptonsparky]

Do you have the CH and one or two pole contactor?

 

[Twoskinsoneman]

No CH or either of the other optional devices (start cap etc). One pole contactor. The L1 side has the factory installed jumper across it.
It's 240v, and though I didn't thoroughly check out the EGC path, it looked fine and I was getting 120v from L1 to the EGC, unit housing, and copper refrigerant lines.