Thyrite arrester is most common and is mostly used for the protection against high dangerous voltages. It operates on the fact that thyrite, a dense inorganic compound of ceramic nature, has high resistance decreasing rapidly from high value to low value for currents of low value to those of high value.
The current increases 9.6 times on doubling the voltage. It consists of discs of 15 cm diameter and 19 mm thickness. Both the sides are metal sprayed so as to give electrical contact between consecutive discs. These discs are assembled inside the glazed porcelain container. It is used in conjunction with porcelain container.
When lightning takes place, voltage is raised and breakdown of the gap occurs, the resistance falls to a very low value and wave is discharged to earth. After the surge has passed the thyrite again comes back to its original position, there being no chemical change occurring simultaneously.
ADVERTISEMENTS:
The thyrite arrester discharges several thousand amperes without the slightest tendency to flashover on the edges. Most important of all the advantages of this type of arrester is that there is absolutely no time lag in its performance.
The current-voltage relation for thyrite is given as –
E = KIn …(9.17)
where E is the crest voltage, I is the crest current, K is a geometrical constant which depends on design, rating and dimensions of the element and the exponent n is typically in the range of 0.18 to 0.28
ADVERTISEMENTS:
As per design of other lightning arresters, the discs are stacked one upon the other usually 11 in number and they are further placed in a porcelain container with suitable arrangements of gaps between them. These gaps serve the purpose of preventing any current flow during normal operating conditions. In the case of occurrence of transients, these gaps get punctured.