I had a previous question regarding an AC, but now that I have disconnected the AC and the problem remains, I want to diagnose this problem a bit more.
First step was seeing if this specific breaker trips by itself. With all other breakers opened on the main panel, this one lone breaker trips.
I then turned off the main power, the breaker closes fine. The moment I flip on the main breaker (with all others opened), this one trips.
With main breaker opened, along with all other breakers opened (off), here are the measurements:
Specific breaker opened:
Live-Neutral: 20,000 Ω Live-Ground: 4,000 Ω Neutral-Ground: 20,000 Ω
Specific breaker closed:
Live-Neutral: 400 Ω Live-Ground: 4,000 Ω Neutral-Ground: 4,000 Ω
For the sake of learning about electrical problems, what would be the potential problems that can cause this? Or is my diagnosis not enough? This circuit dedicated to an A/C has been working for about 3 months. It tripped one time before and I just reset it, not sure what the problem was, but now it has tripped a second time and does not reset.
@JACK: "Remove the two wires going to the circuit breaker. Now try to reset the breaker....."
I unplugged the live from the breaker to the circuit, doesn't trip.
@Ilmari Karonen: "where along the circuit you took those measurements?"
I took the measurements at where the AC connects to the junction box, while the circuit is still connected to the breaker.
Regarding the circuit, it should have no other sources of problem, because it goes from the AC junction box straight to the main panel. It is buried inside a concrete wall straight down into the ground, then runs along the ground to the main panel, so very minimal chance for renovation damage. It has been working for about 3 months.
My next test is to connect this circuit to an adjacent breaker and see if it'll trip. It has 3 dedicated breaker to 3 separate AC units, so I have a confirmed functioning breaker right beside it. I'll update the question when that's done. Need to wait for the weekend when I have light and can shut off the power to the apartment.
One possible source of problem is that the wiring to the main panel was not long enough for some of the wires, so the electrician had to extend it a bit. It's a lot of work so I'll want to confirm it is 100% the wiring there before I take them apart and potentially cause other problems.
EDIT 2019-11-04: Breakers aren't tripping when completely disconnected from the circuit. I've confirmed that the wiring is a bit messed up. Somewhere in the extended connection to the panel there is induced current (non-infinite resistance; 200k to 500k Ω). I'm also getting a hot neutral on another circuit now... so that's kind of messed up. I highly suspect the extension was done incorrectly, since it is wires twisted together with electrical tap (no wire nut, no pig tails, etc).
I'll have to take my time and properly deal with this... so that'll be a completely separate question.