The rating, or voltage class, of a cable is based on the phase-to-phase voltage of the system even though the cable is single, two, or three phase. For example, a 15 kV rated cable (or a higher value) must be specified on a system that operates at 7,200 or 7,620 volts to ground on a grounded wye 12,500 or 13,200 volt system.
This is based on the fact that the phase-to-phase voltage on a wye system is 1.732 (the square root of 3) times the phase-to-ground voltage. Another example is that a cable for operation at 14.4 kV to ground must be rated at 25 kV or higher since 14.4 times 1.732 is 24.94 kV.
The wye systems described above are usually protected by fuses or fast-acting relays. This is generally known as the 100 % voltage level and was previously known as a “grounded” circuit.
Additional insulation thickness is required for systems that are not grounded, such as found in some delta systems, impedance or resistance grounded systems, or systems that have slow-acting isolation schemes.
The following voltage levels are found in AEIC specifications:
100 Percent Level
Cables in this category may be applied where the system is provided with relay protection such that ground faults will be cleared as rapidly as possible, but in any case within 1 minute. While these cables are applicable to the great majority of cable instaltations that are on grounded systems, they may also be used on other systems for which the application of cables is acceptable provided the above clearing requirements are met in completely de-energizing the faulted section.
133 Percent Level
This insulation level corresponds to that formerly designated for “ungrounded” systems. Cables in this category may be applied in situations where the clearing time requirements of the 100 percent category cannot be met and yet there is adequate assurance that the faulted section will be de-energized in a time not exceeding one hour. Also, they may be used when additional insulation strength over the 100 percent level category is desirable.
173 Percent Level
Cables in this category should be applied on systems where the time required to de-energize a section is indefinite. Their use is recommended also for resonant grounded systems. Consult the (cable) manufacturer for insulation thickness.
Cables Not Recommended
Cables are not recommended for use on systems where the ratio of the zero to positive phase reactance of the system at the point of cable application lies between - 1 and -40 since excessively high voltages may be encountered in the case of ground faults.
Ratings of Low Voltage Cables
Low voltage cable ratings follow the same general rules as for the medium voltage cables previously discussed in that they are also based on phase-tophase operation. The practical point here is that a cable that operates at say 480 volts from phase-to-ground on a grounded wye system requires an insulation thickness applicable to 480 x 1.732 or 831.38 volts phase-to-phase. This, of course, means that a 1,000 volt level of insulation thickness should be selected.