Aircon only trips when it rains: water in the outdoor switch box

Mitsubishi Electric Wall-mounted·14 years old·Landed·Changi, Singapore

The breaker had been tripping for about two months, almost always on rainy evenings. On dry days the system ran for hours without fault. Previous advice was that the compressor was likely failing, which pointed to a large repair bill.

The weather link was the strongest clue. A failing compressor trips in any weather, so we looked at the outdoor power path first. Basic compressor checks came back normal. We then opened the outdoor switch box, mounted on an exterior wall in full sun and rain. Inside were rust deposits on the wire connections and a seal that had hardened and cracked along the cable entry.

Fourteen years of sun and rain had worn out the switch box seal. The rubber had hardened, shrunk, and cracked at the cable entry. During heavy rain, water ran along the cable and into the box through the failed seal, then pooled on the wire connections. The wet connections let current leak across from the live wire to the earthed metal mounting plate. The earth leakage breaker sensed that small leak to earth and tripped to keep the circuit safe. On dry days the moisture dried off, the leak path closed, and the system ran normally. The compressor was never the problem. The breaker was doing its job, reacting to a real earth fault caused by water getting in.

We explained that the compressor was healthy. The trips came from rainwater getting into the worn switch box and leaking to earth. The fix was to replace the whole box with a weather-rated outdoor enclosure. We fitted proper cable glands at both entry points to grip and seal around the cable, then added silicone sealant where each cable meets its gland as a second moisture barrier. With the new box installed, we ran the system at full cooling load and sprayed it with water for several minutes to mimic heavy rain. The breaker held steady throughout.