Aircon earthing: what it does and why it gets skipped

Earthing gives fault current a deliberate route back to source. A live conductor that touches the metal casing of an aircon has to go somewhere. Where an earth conductor is bonded to that casing, the current takes the easy path through the wire. Where there is none, the casing holds mains voltage and waits.

The wire is not there to make the aircon run. It carries nothing while the system is healthy, and the unit would work identically if it were cut out on the day of the install. Its only function is readiness for a fault that may never arrive. That is what makes it easy to leave out and hard to notice missing.

Low resistance is the property that matters. Current returning along a proper earth conductor meets almost no opposition, so the fault current is large. That size is what a protective device is built to notice. A route through a person is thousands of times more resistant, so the current is smaller and far harder for any device to act on.

Two terms get used loosely here and they name different jobs. Earthing links the metal parts of an installation back to the general mass of earth through the supply. Bonding ties separate metal parts to one another so they cannot sit at different voltages. An aircon needs both, and on a domestic job the same green and yellow conductor usually delivers them together.

Neither half of the protection functions properly alone. The earth path makes a fault detectable. The earth-leakage device performs the disconnection. Take away either one and the arrangement stops behaving the way its design assumed.

An earth-leakage device compares what leaves on the live conductor against what comes back on the neutral. A fault reaching a properly earthed casing sends current away through the earth wire instead, the two flows stop matching, and the device opens. The earth conductor is what creates the very imbalance the device exists to watch for.

Strip the earth out and that imbalance has to arrive some other way. On an unearthed casing holding voltage, nothing flows until a person or a wet surface completes the circuit. The device does still operate at that moment, which is worth knowing. It has become protection triggered by contact rather than protection that acted before contact, and those are not the same standard.

The opposite case is quieter and more common than people expect. Earthing present, no earth-leakage device on that circuit, and fault current runs down the earth conductor as designed. Whether anything disconnects then rests on the fault being big enough to operate the overcurrent breaker. A poor-quality fault can sit below that level, energised and undisturbed, for as long as it likes.

The condensing unit is where an earthing failure carries the most weight. It is a metal box, wired to mains, bolted to an exposed ledge, and rained on in every storm. Very little else in a flat brings those four conditions together.

Water is what makes the combination unusual. Indoor circuits stay dry, so insulation that degrades there tends to degrade slowly and stay contained. The outdoor unit takes weather directly at its cable entry, its terminal cover and its enclosure seams. A film of moisture across a dusty terminal conducts, and it never has to pool to do so.

An unearthed metal enclosure carrying a casing fault is live metal in a place people reach. Ledges, service yards and aircon compartments are where laundry poles go, where drains get cleared, and where cleaners and technicians put their hands without thinking about it. The earth conductor is what keeps that casing from holding voltage, and it is the only thing in the installation doing that job.

Vibration then works on the connection across the life of the system. The compressor runs, the enclosure shakes, and a terminal that was nipped up rather than tightened backs itself off. A conductor landed properly stays landed. One pushed into a terminal and left there is the one that comes out, and it comes out without a sound.

Corrosion supplies the second slow failure. Coastal air reaches most ledges in Singapore, and salt settles on bare copper and screw threads. A joint that was already loose corrodes at its contact face, resistance climbs, and the earth path degrades without ever cleanly breaking. A path too poor to clear a fault can still look entirely intact.

Good practice here is unremarkable, which is half the problem. A continuous earth conductor runs from the distribution board to the indoor unit, to the outdoor unit, and to the isolator sitting between them. Every metal enclosure in that chain is connected. Nothing gets left out for being too small to bother with.

Termination is where quality shows itself. The conductor lands on the dedicated earth terminal inside each enclosure, under a screw tightened to hold it. It is not looped round a fixing bolt, not taped off, not folded back into the box, and not cut so short that it barely reaches. A conductor present but unconnected is worse than an obviously missing one, because it looks finished.

The isolator is the enclosure most often treated as a pass-through. Its job is to cut the supply so an outdoor unit can be worked on safely, and where its body or gland plate is metal it needs earthing like anything else. Where the earth runs through it, that continuity has to survive the journey rather than stop inside the box.

Replacement work brings its own way of losing the earth. A new unit dropped onto an existing supply inherits whatever arrangement the previous install left behind, and every fresh termination is made by whoever happens to be on site that day. An earth that was sound before the swap is not automatically sound after it.

• •The conductor is continuous from the board to both units, with no break introduced at the isolator
• •Each enclosure lands its earth on the dedicated terminal, not on a convenient bolt or mounting bracket
• •The conductor is the size the cable design calls for, not a thinner offcut that happened to be in the van
• •No joint sits outside an enclosure, and none sits anywhere rain can find it
• •The isolator is earthed in its own right rather than assumed to be covered by the units either side of it

An aircon with no earth connection cools exactly as well as one with. Same airflow, same temperature, same electricity drawn, same quiet outdoor unit. There is no performance penalty, no noise, no error code and no warning light. The system has no mechanism for reporting the absence and no reason to acquire one.

That silence is the whole reason the shortcut survives. Faults affecting cooling get found because the customer notices something and picks up the phone. A missing earth surfaces when a fault occurs, when a licensed worker tests the installation for some unrelated reason, or never. Never is the usual answer.

Covers complete the concealment. Once the terminal cover is back on the outdoor unit and the isolator lid is screwed down, everything that would have revealed the omission sits behind a panel. Handover happens, the unit runs, and the installation gets judged on the two things a customer can observe, which are whether it cools and whether it drips.

Anyone selling a shortcut is trading on that gap between what can be observed and what matters. The uncomfortable version is short. On earthing, a customer has no route to catching a bad install alone, so asking before the covers go back on is the whole of the protection available.

Verification is a Licensed Electrical Worker's job and nobody else's. Earth continuity and loop impedance are read with instruments. The circuit has to be made dead first. The person doing it holds a licence under the Electricity Act. None of that is homeowner work. Looking at an enclosure replaces none of it either. Which aircon contractor credentials the law actually requires is a separate question, and one worth settling before a job is booked.

Say the limit plainly. Do not open the isolator, the outdoor terminal cover or the distribution board to hunt for the conductor yourself. The same enclosures that hold earth terminations hold live parts, and an installation with a doubtful earth is precisely the one where opening it is least safe. The question goes to a licensed worker, and so does the looking.

Timing decides whether asking achieves anything. Put before the covers go back on, an earthing question is an ordinary part of handover and costs the contractor a moment. Put afterwards, it means reopening enclosures on a finished installation, and the same question now carries a real cost for whoever has to answer it.

Evidence is what separates an answer from an assurance. Each row below is something a contractor can produce if the work was done, and something that turns vague if it was not.