Voltage Dependent Resistors

Electrical systems and particularly electrical switchgear may be seriously endangered by overvoltage pulses. This overvoltage can destroy or impair the function of parts of the system. This may lead to the breakdown of the electrical system.

Varistors offer reliable and economical protection against internal and external overvoltages. They can can be used as protection against:

• overvoltage in the mains, caused by lightning, inductive or capacitive coupling.
• consumption in switch contacts with inductive load.
• too high switch-off voltages in transformers, motor and generator windings and magnet coils.

Series 820 SB

This special high performance series is used mainly in electromechanical engineering as protection against overvoltages in field windings of DC machines, chokes and hoisting magnets. The varistors must be selected and assembled to suit each individual application.

These varistors are voltage dependent resistors with symmetric voltage/current characteristics and are therefore suitable for use with direct and alternating current. As voltage increases, the resistance value of these voltage dependent resistors becomes inertia-free and exponentially lower, i.e. a slight increase in voltage effects a much greater increase in current. They have good voltage dependence and high energy absorption.

The varistors are composed of 90% silicon carbide and 10% ceramic bonding. The discs are 80mm in diameter, 10mm thick, with a 20mm hole for assembly purposes. They are mounted on steel bolts, 10mm diameter with ceramic insulation. Varistor columns may consist of 2 to 13 varistor discs connected in series or parallel. Standard values held in stock are:

• 110V 30-60mA
• 250V 15-30mA
• 400V 12-24mA
• 440V 8-16mA
• 550V 5-15ma

Please contact us for further details. Email us at info@powercontractors.co.za