Overview

The IRMS (Megger) function measures insulation resistance between the series circuit and ground. The displayed resistance value reflects the condition of cable insulation, isolation transformers, field splices, connectors, and fixtures. Interpreting these readings correctly is essential for early detection of circuit degradation.

What the Reading Represents

The displayed value represents the total insulation resistance of the entire series loop to ground. Higher resistance values indicate better insulation condition. Lower resistance values indicate increased leakage current to ground.

High Resistance Readings

High insulation resistance generally indicates good cable condition, no significant ground leakage, and dry installation conditions. Stable, high readings over time suggest a healthy circuit.

Low Resistance Readings

Lower resistance values may indicate moisture intrusion, damaged cable insulation, contaminated connectors, failing isolation transformers, or ground faults. Sudden drops in resistance should be investigated immediately.

Gradual Decline in Resistance

A slow decrease in insulation resistance over time may indicate aging insulation, progressive moisture accumulation, or environmental exposure. Trend analysis is useful for preventative maintenance planning.

Extremely Low or Near-Zero Readings

Very low resistance readings may indicate direct ground fault, severe insulation failure, or shorted component in the series circuit. Immediate investigation is required.

Environmental Considerations

Insulation resistance may vary with temperature, humidity, recent rainfall, or underground water conditions. Compare readings to historical baseline values whenever possible.

Using IRMS with Troubleshooting

If a low reading is detected, review fault logs. Inspect recent maintenance areas. Sectionalize the circuit if necessary. Perform additional testing to isolate the fault location. IRMS identifies a condition but does not pinpoint the exact fault location.

Safety Reminder

Insulation resistance testing applies voltage to the series circuit. Always ensure the field circuit is clear of personnel before testing.

Airport Lighting Company | Type L-828, Type L-829 Ferroresonant Constant Current Regulator