Every so often, your plant seems to lose motor drives in batches. Three weeks ago, four drives failed within a few hours of each other. Last summer, a similar incident happened on two different occasions with three drives the first time and two the second.

You contacted the drive manufacturer, who asked you to send in two of the failed drives. You just got the report back, stating that the surge protective devices (SPDs) in both drives were fried.

What could be causing this, and what should you do?

These internal SPDs are sized properly for protecting the motor drives at their nominal supply voltage. The fact they are "fried" means they have seen voltage levels that would not be present if your facility had proper surge protection.

Highlights of what to do include:

  • Ensure each motor and drive is properly bonded per NEC Art. 100 definitions and Art. 250, Part V.
  • Examine the facility envelope. Does your lightning protection system comply with NFPA 780? Do you have test results confirming this? Pay close attention to the bonding connections between all metallic objects on the roof and near the facility to ensure the system does not “build in” flashovers.
  • Do you have surge arrestors and SPDs installed in conformance to Art. 280 and Art. 285?
  • Have you calculated energy levels and the necessary SPDs for each distribution panel in cascading levels of protection? If you don't have the right cascading of SPDs between the service entrance and the drives, then you are asking those drive SPDs to function at a voltage for which they weren't designed.