Step One: Define the Environment

Several factors influence the correct selection of both high- and low-temperature wire and cable - voltage ratings, current or kva loading, ambient temperature, dielectric strength, and voltage regulation permissible, all of which are regulated by the National Electrical Code (NEC). But for high-temperature wire and cable, it's important to have a clear understanding of what environmental condition or combination of conditions can affect your wire and/or cable installation and how important each condition, or combination of conditions, is to overall cable performance. Here's a checklist of environmental conditions and the reasons why you need to consider them before selecting high-temperature wire or cable:

• Heat - Exposure to excessive heat over time can cause accelerated aging of the conductor and insulation materials, resulting in the loss of insulating properties due to decomposition, combustion, embrittlement, or cracking. Elevated ambient temperature also contributes to the aging of adjacent wires or cables, which must be carefully matched to ensure that a wire with the capacity to withstand high temperature does not transfer heat to an adjacent wire or component that might be severely damaged by this heat.

• Moisture - An insulation material's ability to resist moisture absorption is vital in wet, humid applications. Water absorbed by insulation materials produces electrical leakage through the insulation wall, creating an increased demand for power to energize the circuit, greater resistance values throughout the length of the wire or cable, and choked output at the demand side of the circuit.

• Open flame/fire - Non-propagation is a must for wire and cable, but time to circuit failure is another necessity of flame resistance. In many high-temperature applications, there is a need to maintain circuit integrity during an accident, such as a fire, so that the process may be shutdown safely and critical circuits, such as fire pumps, may continue to operate.

• Mechanical fatigue: flexing, impact, abrasions - Every cable suffers some abrasion and torsional stress when installed, but mechanical fatigue of all kinds can best be met with composites of different materials. Some insulation materials provide more mechanical and environmental protection than others and can be used as a sacrificial layer while the inner layer continues to provide the necessary electrical insulation.

• Hot material spills - These are usually catastrophic accident conditions which may subject the wire and cable to extreme, concentrated heat usually resulting in severe damage or destruction of the wire and cable.

• Exposure to oils, acids, solvents, and chemicals - Many insulation materials may be damaged by contact with these reagents, even in diluted form. This may be evidenced by swelling, decomposition, hardening, or a gummy appearance. Some insulation materials are naturally more resistant to attack by particular reagents. In other cases, a jacket of resistant material may be added to prevent or retard the penetration of the oil or chemical into the insulation.

• Ozone resistance - Ozone is a naturally occurring "free radical" which can chemically bond with many insulation materials and cause significant damage in the process. Ozone may also be produced by an electrical or carbon arc. Some insulating materials, like silicone rubber, are resistant to attack by this gaseous contaminant.

• Low smoke - Safety considerations in public buildings and aboard ships, trains, and other modes of transportation have resulted in construction materials, including wire and cable, being rated for smoke generation. When burned, Teflon*, fiberglass, or other inorganic materials produce much less smoke than other insulation materials and are ideal for such wire and cable applications.

• Radiation - This environmental factor can cause decomposition of the chemical structure of many insulation or jacket materials. It may be present in the environment (such as in outer space) or be associated with conditions resulting from an accident. There are now materials which can withstand large amounts of radiation without negative impact to their insulating properties.

• Cryogenic temperatures - The most common manifestation of extreme low temperatures upon insulation materials is embrittlement. This, together with mechanical stress such as vibration, can cause cracking and the onset of insulation failure. One such material, FEP Teflon, resists cracking at temperatures as low as negative 166 C.

• Character of installation: underground, aerial, submerged in water - For each installation scenario there may be a particular attribute that needs to be enhanced. For example, crush-resistant cables for underground applications.

Knowing the environmental conditions for a specific installation helps to clearly define the problem and thus leads to an effective solution. With a clear understanding of the intended application environment, the next requirement is to have a sure knowledge of cable construction materials. Again, the key to performance success lies in matching the most appropriate wire and/or cable construction to your specific application.

NOTE: * Teflon is a registered trademark of E.I. DuPont de Nemours & Company.