What To Do When Bearing Currents Bypass Grounding Devices

You installed the grounding rings, followed the specs, and double-checked everything. Yet the bearings still failed in six months, succumbing to the same fluting damage you were trying to prevent.

This plays out in facilities running VFD-driven motors everywhere. The grounding device gets blamed, but that’s rarely the problem. The real issue? Bearing currents found another way around.

Electricity takes every available path, not just the one you want

Grounding devices create a low-impedance path for shaft voltage to dissipate before it arcs through bearing races. The ring diverts current away from the bearings (in theory). But current flows through every available path simultaneously, splitting based on relative impedance.

A grounding ring lowers impedance on one route — preferentially, not exclusively. If alternate paths exist, current still flows through bearings. Common bypass routes include:

• Coupling guards touching both motor and driven equipment shafts
• Motor mounting feet with inadequate isolation
• Auxiliary equipment connections (encoders, brakes, cooling fans)
• Conduit or cable shields grounded at both ends

Installing a grounding device doesn’t eliminate bearing current; it reduces it — sometimes dramatically, sometimes marginally. It depends on what other paths exist.

Picture a 100 HP motor driving a gearbox through a metallic coupling. You install a shaft grounding ring. While currents flow through the ring, they also flow through the coupling into the gearbox, through its bearings, and back to ground. The motor bearings see less current. The gearbox bearings take the hit. And the motor bearings still carry enough current to fail because the coupling bypasses your grounding device.

You just added one more path. The system uses them all.

Failed bearings indicate the grounding device isn’t sufficient

When bearings fail despite grounding devices, troubleshooting focuses on the device. Is it making good contact? Is the brush worn? Those checks matter, but they miss the larger question: Are currents bypassing the device?

Bearing damage patterns reveal the answer. With a functioning grounding device and no bypass paths, damage should decrease significantly. With bypass paths present, you’ll still see fluting — sometimes asymmetric patterns depending on which bearing carries the dominant current.

Testing for bypass requires more than visual inspection:

• Measure insulation resistance between motor and driven equipment frames.
• Check shaft voltage under load.
• Use current probes at mounting points and coupling guards.

A grounding ring showing wear doesn’t prove effectiveness. Remember, wear indicates current flow, not bearing protection.

Fixing bypass paths means finding them first

Addressing bearing current problems requires system-level thinking. The grounding device is one component in a strategy that must account for every possible current path. Start with these steps:

• Isolate the motor frame from driven equipment using insulated couplings and feet.
• Replace metallic coupling guards that touch both shafts with non-conductive versions.
• Verify that encoders, fans, and brake assemblies aren’t creating ground loops.
• Consider bearing insulation for the non-drive end (ceramic balls or insulated outer races).
• Test shaft voltage and bearing current before and after modifications for effectiveness.

Isolation integrity degrades over time, paint wears off, and vibration creates metal-to-metal contact. What worked during commissioning may fail months later.

Trace the paths, solve the problem

Grounding devices work when the system supports them. This means controlling every current path intentionally, not just adding one low-impedance route and hoping. Bearing protection accounts for how current actually flows. The facilities that solve bearing current problems stop assuming a single device will fix a system-level issue. Find the bypass paths and eliminate them. Then, the grounding device can do its job.