How To Coordinate Earth Leakage Protection for VFDs in Mining MCCs

Why do VFDs and long cable runs increase earth leakage and trip risk in mining MCCs?

VFDs generate high switching frequencies that travel through the motor cable insulation and return through earth. This creates steady state and transient leakage that does not represent a fault but can exceed the trip threshold of standard earth leakage devices. The effect becomes more pronounced in mining installations because MCCs often sit 100 to 200 metres away from field motors.

Key contributors to leakage

• PWM switching creates high frequency components that flow through parasitic capacitance.

• Long cable runs add capacitance to earth and amplify leakage.

• Multiple VFDs connected to a shared upstream EL device create cumulative leakage.

• Incorrect grounding or shared neutrals increase unwanted earth currents.

• EMC noise from nearby conveyors and crushers can appear as residual current.

Practical example:  If a 110 kW conveyor VFD sits 150 metres from its motor, the combined capacitance of the cable, drive filter, and motor can easily produce more than 150 to 250 mA of leakage even when running normally.

Engineering implication: Standard 30 mA or 100 mA devices are guaranteed to nuisance trip. Sites need VFD compatible earth leakage relays, wider current ranges, programmable curves, and commissioning tests to measure real leakage before setting thresholds.

What settings should engineers use for earth leakage thresholds and delays on VFD driven feeders?

Mining sites require sensitive protection, but not at the cost of plant shutdowns. The goal is simple. Set the trip level high enough to ignore harmless VFD leakage yet low enough to detect genuine insulation faults.

Indicative settings for VFD feeders

• Trip threshold typically begins around 100 to 300 mA depending on drive size and cable length. Always measure each feeder’s steady state leakage during commissioning.

• Time delay should ride through the drive’s transient leakage during acceleration. Typical delays are tens to a few hundred milliseconds but must be coordinated with upstream relays.

• Ramp coordination increases delay as circuits increase in rating to ensure upstream selectivity.

• Verification always confirm settings with injection tests and a simulated start under load.

Recommended devices: Use Type B or VFD compatible earth leakage relays capable of detecting DC and high frequency leakage.

Why Zoning Prevents Cascading Trips

If 15 feeders share one upstream earth leakage device, the combined steady leakage can exceed 150 to 300 mA even when everything is healthy. One transient can trip the entire bank. Zoning isolates the problem and protects uptime. See how start supervision interacts with EL delays in the MA Relay Product Guide.

How do long cable runs, EMC noise, and grounding errors affect EL coordination?

Long cable runs are unavoidable in mining conveyors, dewatering pumps, ventilation fans, and crushers. These distances increase cable capacitance, which increases harmless leakage. They also introduce EMC noise that interacts with the EL relay.

Engineering Considerations

• Use screened or armoured cables with symmetrical earth returns.

• Terminate shields correctly at both ends unless site standards specify single point bonding.

• Install EMC filters such as common mode chokes or sine filters on the VFD output.

• Ensure earthing points are low resistance and inspected regularly.

• Route signal cables away from VFD output cables to prevent induced current.

• Avoid mixed cable trays for noisy and sensitive circuits.

Risk from EMI

EMI can mimic leakage and trigger a false trip if the EL relay cannot distinguish between high frequency noise and true residual current. VFD compatible devices with filtering and programmable curves are essential.

Commissioning checklist

• Verify cable lengths and insulation resistance for each feeder.

• Confirm filters, reactors, or chokes are installed as designed.

• Measure steady state leakage during run and acceleration.

• Set EL thresholds at least fifty percent above measured leakage.

• Coordinate time delays by zone.

• Perform current injection tests for each relay.

• Validate CT orientation for differential EL devices.

• Record baseline leakage values for future maintenance comparison.

• Confirm PQ stability under load and during simultaneous starts.