Notes on Mega Insulation Resistance Measurement

Overview

• Mega is an instrument used for measuring insulation resistance.

Components of Mega

• Generator: Hand-driven cranked lever rotates the armature.
• Clutch Mechanism: Slips at a predetermined speed to maintain constant voltage during testing.
• Coils:
  • Coil A: Functions as a voltage coil in a moving coil voltmeter.
  • Coil B: Functions as a deflecting coil in an ammeter.

Connection Setup

• Testing Terminal X: Connects to the hot terminal of the equipment being tested.
• Terminal Y: Connects to the body of the equipment, usually grounded.

Operation

1. Crank Handle Rotation:
   • Generates voltage in the generator.
   • Voltage is applied to coil A through resistance R1.
2. Initial State:
   • When X and Y are free, no current flows through coil B.
   • Torque from coil A indicates infinity (no resistance).
3. During Measurement:
   • Terminals X and Y connect across the machine's terminal and body.
   • Current flows through coil B.
   • Deflecting torque from coil B interacts with coil A's torque to indicate resistance value.

Voltage Levels

• Generated voltage typically around 500 volts.
• Available voltage levels: 1000 volts, 2500 volts, and 5000 volts.

Types of High Voltage Megas

• High voltage megas can be:
  • Motor operated
  • Power operated