The selection of the cable voltage rating is based on the service conditions of 2.1, the electrical circuit frequency, phasing, and grounding configuration, and the steady-state and transient conductor voltages with respect to ground and other energized conductors.

A voltage rating has been assigned to each standard configuration of shield and insulation material and thickness in NEMA WC 3-1980, NEMA WC 5-1973, NEMA WC 7-1988, NEMA WC 8-1988, and in AEIC CS5-1987, AEIC CS6-1987, and AEIC CS7-1987.

The selected voltage rating must result in a cable insulation system that maintains the energized conductor voltage, without installation breakdown under normal operating conditions.

For high-voltage cables, it is usual practice to select an insulation system that has a voltage rating equal to or greater than the expected continuous phase-to-phase conductor voltage. The NEMA standards provide for a cable voltage rating that is only 95% of the actual continuous voltage.

For solidity grounded systems, it is usual to select the 100 Percent Insulation Level, but the 133 Percent Insulation Level is often selected where additional insulation thickness is desired. The 133 Percent Insulation Level is also applied on systems without automatic ground fault protection.

Distribution substations often utilize cable for the distribution circuits from the substation secondary switch-yard (substation getaways). The insulation system selected for this distribution cable may have a voltage rating that is a class above the minimum NEMA rating for the actual circuit voltage and ground fault protection, because it is believed that the additional insulation will result in a lower probability of insulation failure.

Research conducted by the Electric Power Research Institute has led to cable construction recommendations published in EPRI EL-6271 [B10].11 The EPRI recommendations for cable insulation systems have insulation thickness that are the same as those of the NEMA and AEIC standards.

For power and control cables applied at 600 V and below, some engineers use 1000 V-rated insulation because of past insulation failures caused by inductive voltage spikes from de-energizing electromechanical devices, e.g., relays, spring winding motors.

The improved dielectric strength of today's insulation materials prompted some utilities to return to using 600 V rated insulation for this application. Low voltage power and control cable rated 600 V and 1000 V is currently in use.

The selection of the power cable insulation system also includes consideration of cost and performance under normal and abnormal conditions. Dielectric losses, resistance to flame propagation, and gas generation when burned are the most common performance considerations.

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