The Surge Suppressor, is a protection device, which prevents high voltage transients from damaging the main rectifier diodes. High Voltage transients are created by fault conditions in the
distribution system. The Voltage transient returns back to the Generator output terminals, enters the main stator windings, and by mutual inductance, is transferred to the main rotor windings, and the main rectifier assembly.
The Surge Suppressor can be tested with a Multimeter on Megohms range. A good Surge Suppressor should have a very high resistance, (more than 100 Megohms in either direction). A faulty Surge Suppressor will be either open circuit (usually showing signs of burning) or short circuit in both directions.