In some applications of high impedance relays for protection and control duties, the maximum internal fault current can develop high voltages that can damage the relay insulation.
A range of Metrosil non-linear resistors, mainly based on 150mm diameter discs, has been developed to limit this voltage to a safe level.
Correct Metrosil selection ensures that the best protection is obtained while maintaining a Metrosil leakage current low enough to have negligible affect on the accuracy of the relay at its maximum setting voltage.
The Metrosil chosen depends on the relay setting voltage and the maximum secondary internal fault current. Single disc Metrosils are suitable for secondary internal fault currents up to 50A rms. Metrosils with multiple discs connected in parallel are used for larger fault currents.