What Are the Causes of Insulation Breakdown in Wiring?

Historic Materials

• Early insulation was made up of fabric or spun glass. Heavy-duty cable was often clad in rubber. Some fabric-insulated wire is still in use, but the sight of it in an old house will give an insurance inspector goose bumps. Fabric is susceptible to separation, which can lead to short circuits when wires touch. It can stretch or fall apart with age. Woven glass is still being used but can break down in applications where it is subject to continual movement and pressure. Rubber, a rugged, nonflammable material, becomes dry and brittle with age or heat.

Modern Materials

• Modern insulation, much of which is polyvinylchloride-based, covers more completely and is more rugged than historical materials. Thermoset insulation is either rubber or synthetic rubber. Some insulation is sheathed in nylon. The variety of materials responds to higher voltages and more extreme conditions than Thomas Edison and early consumers worked with. Materials are made to withstand temperatures between 140 degrees and 197 degrees Fahrenheit. Special materials withstand higher temperatures. Materials also carry ratings that specify use in dry or wet locations.

Heat

• Heat originates from resistance as electricity travels through the wire, and insulation insufficient for the electrical load may fail. Other sources of heat also contribute to failure, too. Installations in hot sunlight, overloading extensions, or adjoining cables or equipment also contribute to failures due to heat. Proximity to fire or appliances may not cause immediate failure but can dry materials and create brittle patches likely to fail.

Moisture

• Insulation not formulated for moisture can fail if it is used where humidity softens the material. Color on cords used for fish pond pumps or buried underground as outdoor extensions fade as moisture works its way into the material. Insulation designed for use in wet locations can become brittle and crack in very dry locations; cord to be used in indoor applications lacks the extra layers of protection necessary for use outdoors and can crack or break easily.

Other Causes

• Excessive, repetitive movement can weaken insulation, eventually causing a crack. Romex, a cable used for interior wiring, is protected inside building walls but is vulnerable to nails put through finished walls, so in some parts of the wall, cable may be inserted in conduit for added security. Homeowner inspection of cords can catch potential failure caused by pet rabbits, new puppies or stiff, aging coatings. Periodic replacement of supply cords on high-resistance appliances such as cookers not only avoid failure but may actually improve the performance of the appliance.