Water Gets In Through The Cable Entry, Not The Case

Outdoor and wet-location equipment is rarely defeated by water beating on its casing. It is defeated at the small hole where a cable enters, which is the hardest part of the enclosure to seal and the part that moves.

Sealing a moving object is the hard problem

A lid can be clamped against a gasket with an even pressure all the way round, which is a straightforward mechanical task. A cable cannot, because it flexes, twists and is pulled during installation and use.

The gland has to grip a compliant jacket tightly enough to seal without crushing the conductors inside, which is a narrow band of clamping force and easily missed during assembly.

Cables are also not perfectly round, and their jackets vary in diameter along their length, so the seal is fighting a shape that does not match the circular hole it is sitting in.

The cable itself is a pipe

A stranded conductor has continuous air gaps running between the strands, and the space between the conductors and the outer jacket is another continuous path along the length of the cable.

Water entering a damaged section or an unsealed far end migrates along those paths by capillary action and by pressure differences, and it emerges inside an enclosure that is otherwise perfectly sealed.

This is why sealing the enclosure achieves nothing if the other end of the cable sits in a puddle, and why the failure often appears at the dry, protected end of the installation.

Breathing draws moisture in

A sealed box heated by sunlight or by its own electronics expands the air inside and pushes some of it out past the seals. Cooling in the evening lowers the internal pressure again.

The air drawn back in comes from outside and carries whatever humidity is present, and the water it contains condenses on the coldest internal surface as the box continues to cool.

Repeated daily, this pumps moisture into an enclosure that never leaked, which is why vented membranes exist to equalise pressure while blocking liquid water.

Drip loops and orientation do real work

Water running down a vertical cable arrives at the gland with gravity behind it and finds every imperfection in the seal on the way past.

A loop below the entry gives the water a lower point to leave from, so it drips clear instead of collecting at the joint, and this costs nothing at installation time.

Similarly, entries on the underside of an enclosure are far less exposed than entries on the top, and a product installed the other way up from its intended orientation loses that protection entirely.

Corrosion is the delayed consequence

Small amounts of moisture inside an enclosure rarely cause an immediate short. They condense, evaporate and condense again, concentrating dissolved salts and flux residues on the board surface.

Those concentrated deposits corrode component legs and allow tiny leakage currents between adjacent tracks, producing intermittent behaviour long before there is any visible water inside.

By the time the enclosure is opened and found damp, the damage is chemical rather than electrical, and cleaning and drying rarely restores the affected joints.