Q: Testing on a humid, rainy day shows extremely low IR. What is the Guard terminal?
A: The Guard (G) terminal acts like a bypass valve to eliminate current leaking across the dirty, humid outer surface of the cable termination.
The Surface Leakage Problem:
• True Bulk Current (I_bulk): Flows from conductor, through internal insulation, to screen (ground).
• Surface Leakage Current (I_leakage): Leaks across the wet/dirty insulation outer surface.
• Without Guard: Megger measures the total sum. A wet termination makes a dry cable look completely shorted.
How the Guard (G) Terminal Works:
• Wrap bare copper wire tightly around the exposed insulation between the conductor and the screen.
• Connect this wire to the G terminal on the megger.
• G is held at the same potential as Positive, but its current path bypasses the internal measurement shunt.
• Result: Zero surface leakage flows into the measurement circuit. True internal IR is revealed!