Leakage during motor startup (often causing RCD trips or tingling sensations when touching the housing) is a critical safety concern. Root causes fall into these categories:

A. Winding Insulation Damage/Deterioration

Causes: Long-term operation, overheating, moisture, contamination, vibration, or voltage spikes degrading insulation between windings and core/housing.

Startup Effect: High inrush current (5-7× rated) increases voltage stress and heat, potentially puncturing compromised insulation and creating leakage paths.

B. Internal Wiring/Connection Insulation Failure

Causes: Loose terminals (arcing), chafed wires from vibration, or poor insulation at junctions.

Startup Effect: Vibration may momentarily contact exposed conductors with housing.

C. Moisture Ingress

Causes: Humid environments reducing insulation resistance.

Startup Effect: Heat from inrush current evaporates moisture, forming conductive paths. Symptoms may fade post-startup but indicate permanent damage.

D. Terminal Box Issues

Causes: Failed seals (admitting moisture), contaminated terminals, or damaged cable insulation contacting housing.

Startup Effect: Vibration or electrical stress creates temporary leakage paths.

Aging units, incorrect sensitivity, or type mismatch (e.g., using AC-RCDs for VFD applications) causing false trips.

Troubleshooting Protocol

⚠️ Safety First: De-energize and lock out power before inspection!

1. Visual Inspection

Check for: Physical damage, moisture, burns, loose terminals, cable integrity, and ground connection corrosion.

2. Insulation Resistance Test

Use a megohmmeter (500-1000V) to measure:

Phase-to-ground resistance (>1 MΩ, ideally >5 MΩ)

Phase-to-phase resistance (>1 MΩ)

Critical for identifying insulation breakdown

3. Winding Resistance Check

Measure DC resistance across phases. <2% imbalance suggests shorts/connection issues.

4. Ground/Power Verification

Confirm ground resistance ≤4 Ω using earth tester.

Validate correct voltage and wiring.

5. RCD Assessment

Test functionality with trip button.

Ensure type compatibility (A/B-type for VFDs) and appropriate sensitivity (e.g., 30mA).

6. VFD-Specific Measures

Install dv/dt or common-mode filters.

Shorten motor cables to reduce capacitance.

Adjust RCD settings per safety standards.