Q: This is the most famous safety rule. What is the physical reason behind it?
A: Swapping their operating states turns their source characteristics into destructive forces.
1. Why CT must never be open-circuited:
• CT is a current source. It forces rated current (e.g., 5A).
• Open circuit → impedance becomes infinite → voltage jumps to kilovolts (V = I × Z).
• Result: Flashover, arcing, lethal shock, and core destruction.
2. Why PT must never be short-circuited:
• PT is a voltage source (maintains a constant voltage ratio).
• Short circuit → impedance becomes near-zero → current jumps to hundreds of Amperes.
• Result: Winding insulation melts instantly, oil vaporizes, explosive tank failure.