I have a RCBO circuit dedicated to an AC unit. Obviously the AC is now off due to the fault. It has tripped once before, but I flipped it back on and it seemed ok. The AC stays off until I power it back on.

I noticed it has tripped again today, but when flipping the RCBO up, it immediately trips.

Since it is a dedicated line to just the AC, there is nothing else that could be pulling power and causing the trip. What should I be testing and what can the potential problems be?

Edit 1: There is no intermediate switch. I know enough (although unlicensed) to disconnect the AC unit from the wiring, insulate the ends, and see if the RCBO resets.

Edit 2: I've disconnected the AC from the wiring and the breaker trips immediately. That's going to be a new question...

  • 1
    By the AC unit there should be a disconnect switch. I'd operate that switch and them try to reset the breaker. That would tell you whether the fault was in the wiring or the unit.
    – JACK
    Oct 12, 2019 at 13:59
  • Yeah, I'd turn off the AC at its local service disconnect (if present) and try to reset the breaker. If it still doesn't reset, the wiring run is damaged somewhere...if it does, its time to get an HVAC tech out. Oct 12, 2019 at 14:08
  • Hm....no disconnect. So yeah, I'd flip the breaker off, unhook the AC at its junction box outside, insulate the wire ends in whatever way is approved in your area, and try flipping the breaker back on Oct 12, 2019 at 15:14

2 Answers 2


Some possible bad news here. Air conditioning systems have a sealed compressor, the oil and Freon (or refrigerant) is in a sealed loop, one common failure mode is some moisture gets in the system and mixes with the oil, then the mix becomes acidic and etches the varnish off the motor windings, causing small ground faults at first and later full blown shorts inside the compressor.

I would verify it is not a wiring issue as jack and 3 phase suggest. If it is OK with the unit turned off or disconnected, this may be the issue. A HVAC tech should have a megger (a high voltage ohm meter) that can verify if leakage from the motor windings to ground are the cause of the fault.

A “window” type is usually not worth repairing, a small mini split may be worth repairing. I don’t know your laws but on this side of the pond repairs like this require a licensed HVAC person to recover the Freon and replace the compressor and filter dryer. Then the system is pumped down and flushed, the Freon, if not damaged, can be filtered and reused but, if it has turned black, it is replaced with fresh refrigerant and the oil charge verified. So you can see why we usually replace window units.


Surely, the RCBO detected the thing it's there to detect. One must now confront the sobering reality of either a wiring-fault bug hunt, or that a beloved appliance in fact needs a trip to the shop (or crusher).

The first thing you should do is relocate the A/C (electrically) to a different RCBO device. Either the problem moves with the A/C (in which case it is the A/C) or the problem stays on that circuit (and the A/C is innocent). When I say "move electrically" I mean move how it is connected; e.g. If it is cord-and-plug connected, use an appropriate-duty extension cord. (Yankees, remember the rest of the world is 240V so half the amperage, e.g. 4-8 amps for a window A/C, so a beastly extension cord is not required.)

If it is hardwired, and dedicated, then you'd [have someone?] move its supply wires to a different RCBO.

If it is hardwired but not dedicated, use the usal troubleshooting techniques to narrow down which appliance is failing.

Ground is vital for the test to work, but IIRC ground is in every UK cord.


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