Fusistor^®

Construction Detail and Fault Interruption

Fusistor features unique dual fault-control-section construction.

High-Fault-Control Section

This section employs a helical coil of molybdenum wire wound on a ceramic core, embedded in silica sand. Molybdenum has comparatively high cold resistance, as well as a high coefficient of resistance and melting temperature (2620°C).

High fault interruption is achieved in three stages:

• The inherently high-resistance molybdenum coil initially limits the current.
• The resistance of the coil increases rapidly as it heats, further controlling the current before the melting point is reached — limiting peak current to 2000 amperes or less, and actually forcing the current to decrease appreciably below its peak value.
• When the coil melts there’s a further increase in resistance contributed by the high-resistance arc, which is confined and cooled by the surrounding silica sand. The current is smoothly forced to zero in 1/2 cycle or less, at which point interruption occurs and a positive isolating gap is established in the low-fault-control section.

Low-Fault-Control Section

This section utilizes the solid-material technique of circuit interruption, with a pre-tensioned nickel-chrome fusible element. Nickel-chrome has great tensile strength until its melting temperature is approached, at which point it loses strength abruptly. Because the element is tensioned, it separates at this shift in tensile strength, ensuring accurate time-current characteristics and precluding the possibility of damage from surges that heat the element nearly to the severing point.

Fast, positive low fault interruption is achieved by high-speed elongation of the arc within the arc-extinguishing chamber, and by the efficient deionizing gases liberated from the solid-material arc-extinguishing medium.

Positive Indication of Fusistor Operation

When the Fusistor operates, a high-dielectric gap is established in the low-fault-control section, preventing any possibility of reignition. Two spring-loaded indicating pins extend 3/8 inch beneath the lower ferrule to signal the operation.