SOLUTION

Thyrite Lightning Arrester

Thyrite is a mixture of a certain type of clay and carborundum. It has a non-linear property which at lower voltages acts as a non-conducting or insulating material and at higher voltages acts as a good conductor. The resistance offered by thyrite is voltage-dependent. Whenever the voltage is doubled, the resistance decreases so as to pass more current through it. Hence, during a lightning surge, it allows the current through it to the earth. Once the surge has passed away, the thyrite regains its original resistance value at normal voltages without any permanent chemical changes. The basic cells of thyrite are used inside this type of lightning Arrester.

A thyrite lightning arrester consists of a number of discs stacked one above the other and electrically in series with air gaps in the series-gap unit. The discs and series-gap units are assembled inside a wet-process porcelain container. At the top and bottom, there are two aluminum castings. These castings are cemented to the porcelain container. The discs are kept in position by the spring underneath the top cap casting. The cap of the casting is connected to the line by a terminal at the top. The bottom case is connected to the earth.

During the surge, the gap spark-over and the thyrite discs offer relatively low resistance to the flow of surge current. After the surge disappears, the discs regain their original high resistance, and the series gap together with this high resistance does not allow any flow of current under the normal operating conditions.

Thyrite is used widely in lightning arresters. It does not follow Ohm’s law, for each time the voltage is doubled the current increases 12.6 times. This means the resistance decreases as the current and voltage increase