Shophouse Trips After Rain: Sun-Cracked Cable on the Roof

A shophouse trips after rain and rooftop electrical work is proposed. Whether the parts have failed or water is reaching them.

By Team Snowflake | Updated 24 Aug 2026

Teaching case based on recurring Snowflake service patterns; identifying details are combined.

How this presents

Tripping during and after rain, on an installation whose cabling runs right across an exposed rooftop. Terminations at both ends are sound and tight. The enclosure itself is sealed and dry inside when it is opened.

Setup: Mitsubishi Heavy Industries Wall-mounted10-12 yearsShophouseRochor
Misread as: Faulty electrical parts inside the isolator, which will need replacing up on a rooftop.
Check first: The condition of the sheath along the exposed run, not at its ends

What else produces this

Sound terminations and a dry enclosure account for everything anyone routinely checks, which leaves only the run between them. A cable crossing an exposed roof takes direct sunlight for years, and sheath material hardens, chalks and eventually splits under it. Once the outer sheath is open, rain enters the cable itself and travels along inside it. Walking the run and looking at the sheath is the whole check, and it takes a few minutes.

Tight, sound terminations at both ends remove the parts anyone would normally inspect.
A dry, sealed enclosure rules out water arriving at the box from outside.
Sheath cracked and chalked along the exposed rooftop section lets rain into the cable.
Moisture tracking inside the cable toward the enclosure explains a trip with no visible water anywhere in the box.

Why it happens

Cable sheath is a plastic, and sunlight breaks plastics down over the years. On an exposed run the material loses its flexibility, chalks on the surface and eventually cracks, particularly where the cable bends or rests against an edge. Rain then enters through those cracks and travels inside the sheath, arriving at a termination that is itself perfectly sound. The fault sits in the middle of the run, which is the part nobody ever looks at.

What resolution involves

The damaged section is replaced and the new run protected from direct sunlight, either by conduit or by routing it out of the sun altogether. Protection is the part that matters most, since an unprotected replacement sits on the same timeline as the cable it replaced. On any rooftop or west-facing run, sheath condition is worth a look at every visit.

How to tell it worked

The sheath examined along the whole exposed length after the repair, rather than only where the old damage was found. Sunlight has been working on every metre of it for the same number of years.

Before you approve a quote

Sun destroys the sheath between the terminations

Terminations get inspected and the length between them does not. On an exposed run that is backwards.
Rain entering a cracked sheath travels inside the cable and arrives at a perfectly sound terminal.
Replacing without protecting puts the new cable on exactly the same timeline as the old one.

Questions this case answers

Breaker trips after rainy afternoons but the aircon is fine on dry days. Why?

Because rain is getting into a cable whose sheath sunlight has degraded over years. The plastic hardens and cracks, and water entering those cracks travels inside the cable to the termination, so the fault appears only during and after wet weather.

Can the outdoor cable itself be the problem rather than the isolator?

Yes. Walk the run and look at it: chalky, faded or hardened sheath is degrading, and any visible crack means rain is already getting in. The whole exposed length is worth checking, not just the ends.

Does a sudden power cut to the aircon mean the compressor is shorting?

Not necessarily. Water can enter the cable partway along and travel inside the sheath to the connection, so the enclosure looks dry while the fault sits upstream.

Which detail matters most in a first message about a tripping aircon?

Whether the tripping happens only during and after rain, and where the cable runs. On an exposed rooftop run, that pattern points at water entering a sun-degraded sheath rather than at the isolator, and it tells the technician to inspect the whole length.

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