insulation test

一、What is insulation testing

二、Selection of commonly used test voltages

三、Standard Qualification Determination (Universal for Low Voltage 500V)

1. New cable/new motor: ≥ 100M Ω
2. Low voltage equipment in operation: ≥ 0.5M Ω (minimum red line)
3. The insulation difference between phases and relative to ground of three-phase cables shall not exceed 2 times. If the difference is too large, it indicates local moisture damage
4. Double the standard for equipment in humid environments: ≥ 1M Ω

四、Standard test wiring (three terminal megohmmeter)

• L (Line): Connect the tested wire/winding
• E (Grounding): Connect to the metal casing of the equipment and the ground
• G (Shielding): When measuring length cables or wet insulation, connect the insulation surface to eliminate surface leakage current and ensure more accurate data

Common testing items

1. Phase to ground insulation (L-E)
2. Insulation between phase lines (L1-L2, L2-L3, L1-L3)
3. Cable shielding layer/neutral wire to ground

五、Operation steps (safety core)

1. Power outage verification: The equipment is completely powered off, tagged and locked, and residual charges are released; Capacitor devices should be fully discharged first
2. Remove all parallel components: frequency converters, sensors, surge protectors, meters, PLC modules, to avoid breakdown of weak electrical components
3. Clean the oil and water stains on the tested area to reduce surface leakage
4. The wiring is firm, shake the shaking table at a constant speed (120r/min) or turn on the machine and apply pressure. The reading should be stable for 60 seconds (absorption ratio test requires 15s/60s)
5. After completing the reading, disconnect the test line first, then stop shaking the meter and fully discharge again (cable and motor stored to prevent electric shock)

六、Absorption ratio/polarization index (required for motors and long cables)

• Absorption ratio K=R60s ÷ R15s
  Qualified: K ≥ 1.3, if it is lower than 1.3, it indicates that the insulation is damp
• Polarization index PI=R10min ÷ R1min, for large high-voltage equipment, ≥ 1.5 qualified

七、Common reasons for non-compliance

1. Line water ingress and moisture
2. Damaged insulation outer skin, broken skin grounding
3. Sealing failure of cable joints, accumulation of dust and oil stains
4. Aging of motor winding and phase to phase short circuit
5. The parallel weak current components have not been removed, causing a decrease in the overall resistance value

八、Safety precautions

1. Output high voltage during testing, do not touch the tested terminal, and do not allow anyone to work on the other end of the line
2. Outdoor high-voltage insulation testing is prohibited during thunderstorms
3. Large capacity cables and capacitors must be discharged for at least 3 minutes after testing
4. Prohibit live measurement of insulation resistance

九、Common supplements for industrial/energy storage (energy storage cabinets, wiring harnesses)