sheath

A sheath is the tough outer jacket that wraps around the conductors and insulation of an electrical cable. It sits as the final barrier between the live copper or aluminum inside and whatever environment the cable will face, whether that is damp soil, corrosive air, mechanical abuse, or direct sunlight. Without it, a cable fails fast.

The sheath material depends on the cable's duty. Most general-purpose power cables use polyvinyl chloride (PVC), which resists moisture and many chemicals but softens above 60°C. Oil-resistant applications call for chloroprene rubber, which handles mineral oils and some synthetic fluids. Cables run in cable trays or conduit indoors may use thinner PVC. Buried cables or those exposed to UV need cross-linked polyethylene (XLPE) or ethylene propylene rubber (EPR), both of which tolerate higher temperatures and resist degradation. Marine and subsea cables use lead or copper sheaths under an outer plastic or rubber layer for mechanical strength and corrosion protection.

Thickness and markings matter

Sheath thickness is typically 1 to 4 mm, specified in standards like IEC 60502 or UL 1001 depending on cable type and voltage. The sheath must be thick enough to resist puncture and abrasion during installation and service, but not so thick that bending radius becomes impractical. Most industrial power cables have the sheath marked with the cable type, voltage rating, gauge, and country of origin printed along its length at regular intervals, either embossed or printed directly onto the surface.

Damage to the sheath is the most common cause of cable failure in the field. Cuts from installation tools, crushing under heavy loads, thermal degradation near heat sources, and UV embrittlement all degrade the sheath and expose inner insulation. Once moisture or contaminants reach the conductor insulation, failure accelerates. Sheath breaches are often hard to spot until tracking or corrosion has already begun, which is why cable routes are planned carefully and why armored cable or conduit is used in high-risk areas.

The term sheath reflects its function: it clothes and protects. In older British usage it sometimes appears as covering, but sheath is the standard industrial term across IEC and North American standards. Some regions distinguish between the overall outer layer (outer sheath) and inner protective layers (inner sheath), though this usage has largely given way to clearer terms like bedding and inner covering in modern cable construction documentation.