Thyrite Type Lightning Arrester

Thyrite is material obtained by mixing a special type of clay with carborundum (silicon carbide). Thyrite act as a nonlinear resistance. Resistance of Thyrite depends on voltage. It has high resistance at low voltage and low resistance at high voltage.

Construction :

It consists of an spark gap in series with a set of thyrite disks. The spacing of the series gaps is such that it will withstand normal circuit voltage. Each disk is of 16 cm diameter and 1.75 cm thick. These disk are stacked one upon other. This arrangement is placed in a porcelain container. The non linear resistor discs are made of an inorganic compounds such as thyrite or Metrosil.

Working:  

Under normal working, the normal system voltage is insufficient to cause the break  down of air gap assembly. When lightning occurs line voltage is raised to dangerously high voltage which results in the breakdown of electrode gap. Thus the high voltage reaches thyrite disks and resistance of thyrite reduces to a very low value. As a result the high voltage surge is discharged to earth. After the discharge thyrite comes back to its original condition without any permanent chemical change.