Diagnosing instant circuit breaker tripping, with nothing connected

Question

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.

EDIT1:

@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.

Answer 1

Here's what you're supposed to have.

Live to neutral: some value of resistance depending on your load. Don't be surprised if the figure doesn't strictly follow Ohm's Law; many devices have their effective resistance change after they warm up. If there are no loads on the circuit, hot-neutral should be infinity ohms.

Live to ground/earth: this should always be infinity ohms if the breaker is switched off. If it is not, you have a ground fault on either hot or neutral. If hot-neutral is infinity ohms, then a lesser reading is definitely a ground fault.

Neutral to ground/earth: should be zero ohms if the proper neutral-ground bond is connected, and should be infinity ohms if the hot and neutral wire are lifted off the breaker or neutral bus.

Non-infinite resistance (conductivity) between hot and neutral is normal if any loads are attached. However, if you disconnect hot/neutral from the panel, there must always be infinity ohms/zero conductance between hot/neutral and ground. If there is conductance between hot/neutral and ground, then you have a problem that you must fix.

Answer 2

I'm assuming you made your measurements with nothing connected to the affected outlet. In that case there is a fault somewhere on that circuit and the Live-Neutral reading shows that.

The next step is to remove the outlet itself from the load-end of the circuit and see if anything changes. If so, you should replace the outlet itself.

If not, then the fault is in the wiring and you'll need to trace the circuit from end to end and find the problem.

Have you done any construction lately where a fastener might have been driven through the wire? Are there any junction boxes on that circuit where a fault might have developed? Pay particular attention to where the wire enters and exits boxes and bends around obstructions.

It's uncommon for a wire to just develop a fault all on its own unless it was sized improperly and was overloaded. Check that the wire size matches the load and the breaker rating.

In answer to your point in the comment, I may have misunderstood where you were taking your measurements. But it's also possible that the breaker itself is defective. You might try another one and see if that resolves the issue.

Answer 3

You need to measure the resistance of the wiring with the breaker disconnected, not just open. You normally use an insulation tester or megger for this. The values should be in the 10's of megaohms at least. If the insulation measures OK, replace the breaker.

Answer 4

My suggestion is to replace the RCBO: if with disconnected AC motor it still trip, it's higly probable that the faulty thing is the breaker itself. Also I suggest you to upgrade to "A" class [sinusoidal and pulsing currents] instead of your outdated "AC"[only sinusoidal currents] because it is more suitable for inductive loads.

Also check how it is wired: both neutral and live from the AC line should be connected DIRECTLY to the RCD, you absolutely don't have to connect anything coming directly from that circuit to the neutral bar if not the breaker itself.

If you feel safe ding so, please post a picture of the open consumer unit, so we could see if something has been badly wired. Otherweise just call an electrician.