Voltage Limiting Devices for Electric Traction

Voltage limiting devices are used in Electric Traction systems, to prevent dangerous voltages from appearing the insulated tracks and the earthed components of the installation. 

Overvoltages can occur due to lightning or due to short circuits.  Voltage limiting devices typically use a MOV (metal oxide varistor) and and air gap mechanism to conduct the high voltage impulse to the ground.

If case of minor overvoltages, the MOV operates and diverts the surge.  It then returns to its normal non-conducting state.  In case of severe overvoltages, a permanent short circuit occurs between the protective electrodes and the device has to be replaced.