Step Six: Troubleshooting

You can visually inspect the integrity of your wire or cable during regular maintenance schedules or after a catastrophic event. The white cable on the left has been damaged by fire and needs to be replaced, while the discolored cable in the middle, which has been subjected to a high heat application, will continue to function well. On the right is a pristine cable for reference. The same holds true for the brown cables with Kevlar* braid on the left. Although the binder tape of the wire in the middle has turned brown, it is simply a sign of heat aging and not an indication that the wire should be replaced.

How do you evaluate whether or not a cable has been damaged in a catastrophic incident? Because you cannot perform destructive testing, you must rely on visual inspection and a knowledge of what causes insulation materials to age, degrade, or combust.

First you need to be aware of what happens to the wire materials over time. After long-term exposure to heat, wire and cable can lose flexibility, which can damage the cable if it is subjected to vibrations or flexing. To check for loss of flexibility, de-energize the circuit and gently bend the insulated conductor in question. You may see brittleness, discoloration, tiny cracks or a checked appearance. All these characteristics are indication of age and degradation.

Some materials have special characteristics that indicate age or degradation. In the case of treated glass braids, a bright white or silvery appearance usually indicates that the braid treatment has been damaged or degraded, which may be a concern in damp or even humid areas. For plastic materials, signs of melted or dripping material is indicative of severe damage. If samples may be obtained for laboratory testing, tensile strength and elongation may be measured and compared with typical values for the particular insulation.

Prior knowledge of how certain insulation materials will behave under catastrophic accidents, such as fires, may prevent extensive replacement costs. In the case of a fire that exceeds 500 F, thermoplastic insulation materials will melt inside a conduit, forcing you to replace the entire system. But thermoset insulation materials will char inside a conduit, allowing you to easily extract and replace damaged cable without replacing conduits or cable trays.

Of course, electrical tests are most effective in exposing defects that occur from aging, but these are often destructive, such as voltage withstand tests. Insulation resistance or megger testing will expose existing failures, but are not necessarily good indicators of near-term potential failures. The best approach is a combination of all of these measures. In this way, outages may be planned and downtime minimized.