Magnet Protection Using ZNR Surge Suppressors

In the Fermilab Main Ring power supply system, we have added ZNR type (zinc oxide nonlinear resistor) suppressors to each power supply terminal to limit the magnet voltage-to-ground to 2.1 kV. The ZNR´s conduct 50 A at the 2.1 kV clipping voltage, and leak less than 1 mA at the 1.5 kV maximum operating voltage. Their repetitive surge rating is 1.4 kJ. Their main advantage, in comparison to spark gaps, is that the ZNR´s do not break down and cause transients and high voltages at neighboring points on the magnet bus.