Discs and assemblies

Metrosil varistor discs are used in a wide variety of applications.

This is due to their non-linear properties that enable them to act as as an “electrical safety valve”, protecting critical equipment and insulation from the effects of overvoltages.

Front of Large Diameter Metrosil Silicon Carbide Disc
Single Metrosil Varistor Tile
Front of Small Diameter Metrosil Silicon Carbide Disc


Our discs are available in sizes ranging from 25mm to 150mm diameter, they can be supplied as plain unmounted discs or they can be mounted on central studs (either as single discs or as multi-disc assemblies arranged in series or parallel). Varistor tiles, as used within the 9000 Series De-Excitation Unit, are available for space-saving installations.


Varying the composition of ingredients during manufacture enables us to achieve a broad spectrum of electrical characteristics. Consequently, a number of electrical specifications are available as standard, however, others can be manufactured on request according to the application.


When an inductive DC current is broken suddenly, there is a transient rise in voltage across the inductance, which can be 10 to 20 times the supply voltage and may damage insulation or circuit components. The source of this overvoltage is the energy that is stored in the magnetic field of the inductance at the moment of switching off.

It is therefore essential to provide some means for this energy to dissipate itself harmlessly. This can be achieved by connecting a discharge resistor across the inductive coil. At the instant when the circuit is broken, the current briefly held constant by the inductance of the coil is diverted through the resistor and rapidly decays to zero. The voltage across the coil is therefore limited to the voltage necessary to pass the coil current through the resistor, the value of which is chosen so that the voltage is a safe one.

Linear Vs. Non-linear resistors

If an ordinary linear resistor is used and it is permanently connected across the coil, there will be a continuous waste of power at normal voltage. If the resistor is inserted just before the circuit is broken, additional contacts and wiring will be required. A Metrosil resistor, on the other hand, can be permanently connected across the coil and yet will consume at normal voltages only a small fraction of the power taken by the ordinary resistor.

Benefits of Metrosil Assemblies

  • Able to handle currents from microamps to kiloamps
  • Absorb between 1kJ to 2,000kJ per assembly
  • Absorb 1A to 8,000A per assembly
  • Absorb 600V to 1,800V per assembly
  • Assemblies can be connected in series or parallel
  • Suitable for all AC and DC voltages
  • Can be used in conjunction with almost any inductive device

Why Metrosil?

With more than 80 years’ experience manufacturing surge protection devices, Metrosil can engineer intermediate characteristics and multiple disc assemblies to suit a customer’s individual requirements. To find out more, please click here.

If you are unable to find a specification that meets your high energy requirements, please contact a member of our dedicated team for bespoke solutions.

To find out if Metrosil is suitable for your application – Contact Us