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When disconnecting an 11,000 volt Elecrtrical motor, should the HV supply cable be earthed or not (at the substation) when the HV VCB is racked to the isolated position? I have concerns if a HV fault occures & the HV supply cable is earthed the potential at the disconnected end of the HV cable will rise to an unsafe level.

Hi Daryl,

Thank you for your question regarding proper grounding procedures for high-voltage and high-current motors.  It is our pleasure to help.

While it would be impossible to give accurate advice to anyone without seeing schematics and the motor specifications, you are correct in be concerned about hazardous voltages forming even after the 11,000-volt motor is isolated. 

Generally speaking, there are two major concerns when dealing with high-voltage and high-current motors such as those found at a smelting plant.  The first concern is in regards to capacitive discharge.  An 11,000-volt motor can hold and store a tremendous amount of electrical energy in its windings.  When the power is shut off that energy remains in the motor and must be discharged to make it safe for maintenance workers.  This is technically a Touch-Voltage hazard as the current and voltage levels can easily exceed safe fibrillation current levels.  The second concern comes from inductive currents.  The feed wires for the inactive motor are often within close proximity to other active high-voltage motors.  The electrical energy from the active wires feeding other 11,000-volt motors can easily induce hazardous energy into the inactive wires of your isolated motor, charging the motor and causing another unsafe Touch-Voltage hazard.

When maintenance is scheduled on one of your 11,000-volt motors, the other motors in the line are typically left in operation.  Once you open the primary disconnect and lock it out (lockout/tagout) in the disconnected position; before starting your work on the motor you need to discharge all three phases on the motor side to ground by attaching a grounded cable to each phase.  During the work you also need to keep these leads grounded.  If the isolated motor leads are running in proximity to the other energized cables, you will get a voltage potential rise on your inactive cables due to induced currents from the adjacent active cables.  This is especially true it the adjacent motors were to carry heavy loads, were started, or (worse case) were to fault.

Often, plants such as yours have what are called “Grounding Cart’s (or Buggy’s)”.  These carts have a special adapter on the end of the cable that is attached directly to the motors disconnect switch.  The adapter has stabs that are positioned to ensure that the motor leads below the disconnect switch are grounded.  The adapter also generally works as a lockout/tagout device and shields the hot-side of the disconnect switch from accidental contact (as often the cover must be left opened).  The cart is then attached to a  ground source, and not only safely discharges the residual energy left in the system when the motor is shut off, but discharges any induced voltages generated by close proximity to active high-voltage lines.

We highly recommend that you consult with the manufacturer of the motor before implementing any new procedure and get a consultant who can review the schematics for you and make appropriate recommendations.   E&S Grounding Solutions is of course more than happy to help you with these life-saving procedures.  But if not us, please get someone with the grounding experience to assist you.  Feel free to contact one of our engineers and they will be happy to speak with you about your project, free of charge.

Best regards,

The Engineering Team at E&S Grounding Solutions


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2 Responses

  1. Karo

    E&S response to this question is simple, direct and professional. I have conducted several HV load grounding as a safety measure and this certainly defines the critical reason for this practice.

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