Electric cables are normally installed on the assumption of a safe working life of at least 20 years. Changes in the insulating material take place with the passing of time and these changes, which may eventually result in an electrical breakdown, are accelerated at higher temperatures. Thus, if the working life is fixed, the limiting factor is the temperature at which the cable is required to operate.
The number of electric wire contained in most common conductors is 3, 7, 19, 37, 61 or 91. Thus, 37/0·083 indicates that the conductor has 37 wires each having a diameter of 0·083 in.
Study of electric cable used for 18 years outdoors in Romania shows that only 2% of original quantity of di-(2-ethylhexyl) phthalate has been lost during service life. Formulation was stabilized with lead stabilizer. Twenty percent of original stabilizer was used and required replacement in recycling process.3
An area of rubber cable technology where much research and development work has been concentrated in recent years is that of the behaviour of cables in fires. Although they may overheat when subject to current overloads or mechanical damage, electric cables in themselves do not present a primary fire hazard. However, cables are frequently involved in outbreaks of fire from other causes which can eventually ignite the cables. The result can be the propagation of flames and production of noxious fumes and smoke. This result, added to the fact that cables can be carrying power control circuits which it is essential to protect during a fire to ensure an orderly shutdown of plant and equipment, has led to a large amount of development work by cablemakers. This work has included investigations on a wide range of materials and cable designs, together with the establishment of new test and assessment techniques.
A different requirement in many installations, such as in ships, aircraft, nuclear plant and the petrochemical industry (both on and off-shore), is that critical circuits should continue to function during and after a fire. Amongst the cables with excellent fire withstand performance, mineral insulated metal sheathed cables are particularly suited for use in emergency lighting systems and industrial installations where ‘fire survival’ is required. As fire survival requirements on oil rigs and petrochemical plants become more severe, new control cable designs have been developed to meet fire tests at 1000°C for 3h with impact and water spray also applied, and also to have low smoke and low toxic properties.
For splice kits, a splice is prepared in each of three sections of a MSHA-approved flame-resistant cable. The cable used is the type that the splice kit is designed to repair. The finished splice must not exceed 18 inches (45.7 cm) or be less than 6 inches (15.2 cm) in length for test purposes. The spliced cables are three feet in length with the midpoint of the splice located 14 inches (35.6 cm) from one end. Both ends of each of the spliced cables are prepared by removing five inches of jacket material and two inches of conductor insulation. The type, amperage, voltage rating, and construction of the power cable must be compatible with the splice kit design.