Effects of moisture on PVC insulated and sheathed cables

The question is often asked: Are PVC cables suitable for use in an environment where they are expected to be exposed to water (e.g. in underground conduit flooded with water). A definitive answer is often difficult since there are many factors that can affect the cable life. The time of exposure, e.g. from occasional contact to complete submersion. Other factors include; the cleanliness of the water, e.g. if polluted with chemicals - what types of chemicals are present.

Unlike HV XLPE cables, which suffer from a form of insulation degradation due to the presence of water, called water tress, PVC insulation does not suffer the same degradation process.

PVC has been used as an insulating material on electrical cables for many decades due to its low cost, ease of processing and of course its good electrical insulating properties.

However PVC is not impervious to the ingress of moisture and if exposed to water in the long term, the moisture will pass through the material and can be contained within the PVC. This will not be detrimental to the electrical properties of the PVC, although some reduction of its insulation resistance is expected. The
moisture (assuming the water is relatively clean) will not cause any chemical reaction with any of the compounds used in our PVC and when the source of the water is taken away, the PVC will eventually “dry out” and return virtually to its original state.

The reduction in the insulation resistance of PVC due to moisture will not be significant enough to cause failure of the cable at LV levels (0.6/1kV).

If water is allowed to migrate to the conductor via insulation breaks or splits or joints, or via gradual
migration through the PVC some corrosion of the conductors will occur.

With copper conductors only minor surface oxidation is expected. Clean water does not corrode copper and corrosion will only occur with salts and minerals getting into contact with the conductor. With the insulation and cable sheath intact, oxidation of the conductor surface takes place due to moisture permeation over extended periods of time (some years).

Aluminium conductors, however, will experience high levels of corrosion when water has entered the
conductor through cuts or holes in the insulation.

This is evident in the form of white powder, aluminium hydroxide and aluminium oxide, which swells and shows as a bulge under the insulation. The corrosion eats away at the conductor and the reduced cross section leads to increased heating which enhances the chemistry of corrosion. Should salts and minerals enter the conductor with the water, then corrosion will take place at a faster rate.

LV PVC copper cables are entirely suitable for use in installations where short or long term exposure to moisture is expected provided that the source of moisture is relatively free from contaminants and pollutants.

LV PVC aluminium cables are deemed suitable where additional precautions are taken to prevent water ingress to the conductor. However, greatly reduced cable life is expected if water is allowed to enter the conductor.