VFD Ground Fault & Short-Circuit Trips: Causes & Solutions

Estimated reading time: 11 minutes

Ground fault and short-circuit trips in VFDs are common but often confusing events for drive users. In a VFD, these faults appear suddenly and can stop the motor immediately. This often leaves operators puzzled about the cause.

Essentially, a ground fault means current is unintentionally flowing to ground. A short-circuit is a direct connection between two conductors (like phase-to-phase) that causes a sudden surge of current.

Both conditions trigger the drive’s protective circuitry to cut power in a split-second.

Understanding why these trips occur is crucial. It helps you clear the alarm. It also prevents damage to the VFD or motor and ensures safety.

When a VFD reports a “Ground Fault” or “Short-Circuit” fault code, it doesn’t necessarily mean the drive is broken. That fault simply indicates an electrical problem the drive has detected.

However, many users aren’t sure where to look first. Is the issue in the drive, the motor, or the wiring?

We’ll break down the causes of these faults below, walk through troubleshooting steps, and then discuss solutions to fix and prevent them.

By the end, you’ll know why these trips happen and how to address them with confidence.

Causes of VFD Ground Fault & Short-Circuit Trips

A VFD trips on these faults to protect itself and your equipment. But what actually causes them? Several underlying issues can create a path to ground or a short across phases.

1. Insulation Breakdown can cause a VFD ground fault:

As insulation on motor windings or cables degrades (from age or excessive voltage stress), it can eventually fail. Weak insulation may allow current to leak to ground or even arc between phases. This results in a sudden surge of current. In effect, it’s a ground fault or phase-to-phase short that forces the drive to trip.

2. Moisture or Contamination:

Water, condensation, oil, or conductive dust entering the motor or cable junctions can create unintended conductive paths. Moisture in particular can turn normally high-resistance insulation into a partial short to ground. Even a small amount of moisture tracking along a cable or inside a motor can cause intermittent VFD ground fault trips. This is especially true when the VFD’s high-frequency output is running.

3. Wiring Damage or Loose Connections can cause a VFD ground fault:

Physical damage to a cable (nicks in insulation, crushed sections, pinched wires) can expose conductors. This often leads to short circuits or leakage to ground. Similarly, a loose connection or frayed termination might intermittently touch a grounded panel or another phase. These wiring issues can be elusive. Faults may come and go until the damaged section is fixed.

4. Marginal Motor Insulation (Old or Non-Inverter-Duty Motors):

Older motors not designed for inverter use may have marginal insulation. High-speed voltage pulses (high dV/dt) from the drive put extra stress on weak insulation. This can lead to leakage currents (ground faults) or internal winding shorts at certain frequencies. For example, a motor that ran fine on regular AC power might start tripping a VFD ground fault. The stress of the VFD’s PWM output can further weaken marginal insulation.

5. Cable Capacitance & Long Leads can cause a VFD ground fault:

VFD output cables act like capacitors. This effect is especially strong over long distances or with shielded cable. The drive’s high-frequency pulses charge and discharge this cable capacitance, inducing leakage current to ground.

Over a long run, this capacitive current can fool the VFD into sensing a ground fault. Additionally, voltage reflections on very long leads create high peak voltages. Over time, these spikes can break down insulation and eventually cause a true short to ground.

In short, anything that compromises insulation or creates an unintended current path can trigger these trips. Most ground fault and short-circuit faults stem from problems in the motor or cabling, rather than an issue inside the drive. Next, we’ll look at how to identify the culprit through systematic troubleshooting.

How to Troubleshoot VFD Ground Fault and Short-Circuit Trips

When your VFD trips on a ground fault or short-circuit, troubleshoot it step-by-step. Don’t keep resetting the drive without finding the cause. That can lead to repeated trips or even permanent damage.

1. Note the VFD ground fault code and conditions:

Check exactly what code or message the VFD displayed (e.g. “GF”, “ShortCircuit”, or an error number). Different manufacturers use unique fault codes. For example, one brand might display “GF” or “Earth Fault” for a ground fault. Refer to your drive’s manual or a fault code guide for your model.

This will confirm what the code means. (For instance, “GF” indicates a ground issue.)

2. Inspect for obvious issues:

Before grabbing test instruments, power down and do a visual inspection. Look for clear signs of trouble. For example, check for water inside the motor terminal box. Also look for burned insulation, loose ground wires, or any debris in the drive cabinet.

Sometimes the cause of a short is visibly apparent. For example, you might notice scorch marks, condensation, or tools left inside the panel. Fix any issues you discover before further testing.

3. Isolate the VFD ground fault drive from the motor:

With power off, disconnect the VFD’s output wires at the motor. This completely separates the motor from the drive. Then power up the VFD and try running it with no motor connected. If the drive runs without tripping when nothing is attached, the fault is likely outside the VFD. If the VFD still trips with no load, its output section or sensors could be faulty.

4. Test the motor cables alone:

Reconnect the output cables to the VFD, but leave them disconnected from the motor terminals. Run the VFD again. If the fault reappears with only the cables connected, the cable is likely at fault. There may be a short to ground along that cable. If the drive does not trip now, the cables are probably okay.

5. Examine the motor:

Now reconnect the motor to the cables and drive, and attempt to run the VFD with the motor hooked up. If the fault only appears when the motor is connected, then the motor has an internal issue. Its windings may be shorted or grounded.

6. Perform insulation resistance tests:

Ensure all power is off and the motor leads are disconnected. Then use an insulation tester to measure resistance from each motor phase terminal to the motor’s ground (frame). Also measure between each pair of motor phase terminals. Healthy insulation will show very high resistance (tens or hundreds of megohms).

A reading of only a few megohms or less indicates a failed winding (short to ground). Similarly, test each output cable conductor to ground (or shield) with the megohmmeter. Any low insulation reading means that cable’s insulation is compromised.

7. Rule out the VFD ground fault hardware:

If the motor and cables pass the insulation tests and the fault still persists, consider the drive itself. Double-check that there are no installation mistakes. For example, make sure the motor leads are on the correct terminals and not touching anything else. In rare cases, the VFD’s own current sensors or output transistors could be malfunctioning and causing false trips. If everything external checks out, the drive may need to be inspected or swapped with a known-good unit to verify.

By systematically isolating the drive, cables, and motor, you can pinpoint where the fault lies. For example, if disconnecting the motor stops the trips, the motor or its wiring is the source. You won’t waste time pulling the drive apart unnecessarily.

Using a megohmmeter is crucial in this process. It can detect hidden insulation breakdowns that a regular multimeter might miss. Also, always double-check the fault code definitions. What one brand calls “Overcurrent” might actually be a short-circuit fault on another brand. The goal is to gather evidence at each step so you can apply the right fix.

Solutions and Preventive Measures for VFD Ground Fault Trips

Once you know the cause of a trip, implement the appropriate fix and take steps to prevent a repeat. Depending on the cause, the solution might involve replacing components or adding protective devices.

1. Repair or replace damaged components on a VFD with a VFD ground fault:

If a motor winding is grounded or a cable is shorted, fix or replace that part. A motor with badly damaged insulation should be rewound or replaced with a new one. Ideally, use an inverter-duty rated motor for the replacement. Likewise, replace any suspect power cable with a proper VFD-rated cable.

These cables have stronger insulation and shielding than standard ones. Often, investing in a fresh motor or cable is cheaper than dealing with repeated downtime. It’s certainly cheaper than replacing a blown drive. If you need assistance, our AC motor repair service can evaluate and restore faulty motors.

2. Upgrade insulation and hardware to address a VFD ground fault:

Prevent recurring faults by using components better suited for VFD operation. For instance, choose motors with a higher insulation class or an “inverter-duty” rating. They can withstand the voltage spikes from VFDs. Also, ensure all cable glands, conduit fittings, and junction boxes are sealed against moisture.

Even a simple step like replacing a worn gasket on the motor’s terminal box can keep water and dust out. This helps avoid ground faults caused by the environment.

3. Add an output reactor for long leads might help with a VFD ground fault:

An output load reactor installed between the VFD and motor will smooth the drive’s output waveform. This reduces high-frequency stress on the motor and cable. It also prevents the voltage spikes that contribute to ground fault trips.

4. Use an input line reactor on a VFD with a VFD ground fault:

An input line reactor on the VFD’s power supply buffers the drive from line-side surges and harmonics. A line reactor reduces inrush current. It also helps mitigate nuisance trips coming from the power source. Additionally, it protects the drive’s rectifier from voltage spikes. In turn, this improves the VFD’s overall fault tolerance.

5. Verify drive settings and grounding to address a VFD ground fault:

Check that the VFD is configured correctly for your system. Make sure all ground connections are solid. The drive, motor frame, and cable shields should all connect to a common ground point. If you have multiple motors on one drive or very long cables, consult the drive manual.

There may be special parameter adjustments for those situations. Some drives allow lowering the PWM carrier frequency or adjusting trip sensitivity in such cases. Be sure to follow the manufacturer’s guidelines when making those changes. Proper grounding and correct drive settings will help prevent false error triggers.

6. Ensure stable input power on the VFD with a VFD ground fault:

Unstable or imbalanced incoming power can trigger fault sensors erroneously. If your incoming power has significant voltage unbalance, correct that issue. For example, you might install a line reactor to help balance the phases. If you’re running a VFD on single-phase power by derating it, consider using a dedicated VFD phase converter instead.

Clean, balanced power helps prevent nuisance tripping. It ensures the drive’s protection circuits aren’t seeing “ghost” faults. Always adhere to the drive’s input power requirements.

7. Replace or repair the VFD with a VFD ground fault if needed:

In some cases, a severe short-circuit may have damaged the VFD’s output stage. Alternatively, the drive’s current sensors could be faulty. If the motor and cables check out but the drive still trips, the VFD itself likely needs repair or replacement. Don’t keep resetting a drive that persistently trips. Doing so risks a complete failure. Instead, find a suitable replacement in our AC Variable Frequency Drives catalog. Or have the unit professionally inspected and repaired.

Apply the appropriate fix to restore reliable operation. This could mean replacing a burnt cable, upgrading a motor, or adding the right reactor. The key is to address the root cause, not just reset the fault repeatedly.

For example, if cable capacitance was causing nuisance trips, simply swapping out the drive wouldn’t help. Installing an output reactor or shortening the cable would solve the real issue. Think of these measures as part of a holistic approach to VFD maintenance and safety.

Still Have A VFD Ground Fault? Get Expert Help & Keep Your VFD Running

Troubleshooting VFD ground fault and short-circuit trips can be challenging, especially in complex systems. The good news is you don’t have to tackle it alone. Contact Precision Electric for professional VFD support, troubleshooting, and repair. You can reach us at 574-256-1000 or through our website’s contact page.

We’ll gladly help diagnose tricky fault issues. Our team can also guide you to the right replacement drive or component from our inventory. With the proper approach and quality equipment, those confusing fault trips will be a thing of the past. Your VFD system will run smoothly and safely.

For more information, check out our in-depth guide on VFD Overcurrent Faults & Motor Overload (Causes & Fixes) for additional tips on handling common drive fault codes. Regular VFD preventive maintenance and proper installation practices go a long way toward avoiding surprises like ground fault trips.

Read The Full VFD Ground Fault Trip Guide:

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• VFD Overcurrent Fault & Motor Overload – Causes & Fixes
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• Lenze SMV Training Lesson 1: Basic Input Voltage & Motor Wiring – Variable Frequency Drives
• Lenze SMV Training Lesson 11: Diagnostic Fault Monitoring, History And Troubleshooting – VFDs