leakage current
The current that flows through the grounding to earth.
leakage current: stray current finding ground paths
Leakage current is any unintended current that flows from a live conductor to ground or to the equipment frame, bypassing the normal load circuit. It travels through insulation that is degraded, contaminated, or simply imperfect, rather than through the designed path. In industrial equipment, leakage current typically ranges from microamperes in dry, well-maintained systems to milliamperes in older or damp installations.
The source of leakage varies widely. Capacitive coupling between the line and ground creates leakage in modern switched-mode power supplies and inverters, sometimes 50 to 500 milliamps per phase in equipment with large filter capacitors. Resistive leakage occurs through aged or moisture-saturated insulation, such as in motor windings exposed to humidity. Surface contamination, salt spray in marine environments, or dust buildup on insulators all increase the conduction path to ground.
Measurement and Standards
Leakage current is measured with a sensitive ammeter connected between the equipment frame and a grounding electrode, or by monitoring return current through the ground conductor during normal operation. Standards such as IEC 60610 and UL 60950 set limits for Class I equipment at 3.5 milliamps or less, and lower thresholds for double-insulated equipment. Portable tools and appliances are often held to tighter limits, typically under 5 milliamps.
High leakage current is a safety hazard. If grounding is poor or disconnected, dangerous voltage may appear on the equipment frame, creating shock risk. It also reduces the sensitivity of residual-current devices (RCDs or ground-fault circuit interrupters) that are meant to cut power in 30 milliseconds when ground current exceeds 30 milliamps. Nuisance tripping on circuits with multiple devices can result from cumulative leakage.
Troubleshooting leakage involves isolating the faulty component, checking for moisture ingress, and testing insulation resistance with a megohmmeter at rated voltage. Prevention relies on maintaining good insulation, controlling humidity in equipment enclosures, keeping surfaces clean, and ensuring reliable grounding conductors with resistance below 1 ohm.