VOLTAGE RATING AND ISULATION OF POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

Circuit breakers are built for voltage ratings as defined in IEEE C37.042 and IEEE C37.063. They have to be dimensioned to withstand the maximum voltages as specified. The rated maximum voltage is the upper limit for operation.

For circuit breakers rated in accordance with ANSI C37.06-19874 (or earlier), the range between upper and lower limit is defined by voltage range factor K. Current-interrupting capabilities vary within this range in inverse proportion to the operating voltage.

For circuit breakers rated in accordance with ANSI C37.06-19974 (or later), the current-interrupting capability is a constant Ka value at any voltage equal to or lower than the rated maximum voltage.

The insulation level is determined by the rated withstand test voltages specifying the low frequency voltage (kV, rms) and the impulse voltage (kV, crest). High-voltage breakers must essentially withstand switching surges and both full and chopped-wave lightning impulses.

For multiple-break circuit breakers, equal voltage distribution over the series breaks is achieved by grading capacitors paralleled to the interrupting chambers. Coordination between inner and outside insulation, as well as insulation coordination between interrupters and ground insulation, has to be properly designed to prevent flashover inside the breaker or over the open break.

Outdoor breakers are generally available with special bushings that provide increased creepage distance for installation sites with highly contaminated air. For heavily polluted atmospheres, spray washing of live or deenergized breakers may be an additional measure.

Because of the method of design with enclosed ground insulation, the GIS circuit breaker is not influenced by atmospheric pollution. For installation at altitudes above 3300 ft (1000 m), altitude correction factors have to be applied.

The values of rated maximum voltages and insulation levels are multiplied by these factors to obtain the values for the application. The altitude correction factors are as listed in ANSI/IEEE C37.04-1979.

Correction factors are under discussion in an IEEE Switchgear committee working group and are expected to change. These factors will be published in IEEE C37.100.15. Particular reference is made to the rating structures and preferred ratings for ac high-voltage circuit breakers per the latest standard revisions of IEEE C37.042 and IEEE C37.063.