Motors or Generators equipped with Surge Capacitors that have internal discharge resistors will have a lower insulation resistance reading during I/R testing. This is because of the internal discharge resistors being connected in parallel to the capacitor frame ground. The resistors provide a low resistance path to ground which causes the total motor or generator system resistance to be lowered to typically between 3 – 4 megohms. The discharge resistors are molded inside the capacitor assembly and therefore cannot be removed. Insulating the Surge Capacitor assembly itself from ground will correct the insulation readings but it is not recommended because the Surge Capacitors need to be grounded to operate properly. To eliminate the low insulation resistance readings a Surge Capacitor assembly must be used that does not have internal discharge resistors. These type of Surge Capacitors are available from manufacturers such as GE and ABB for all operating voltages, but need to be special ordered without the discharge resistors. Note: These type surge capacitors need to be manually discharged with a ground strap during the normal lock and tag out procedure before working on the equipment.

One thought on “Tech Note: Surge Capacitors

  1. Keith Tesch

    Can I just remove my old surge capacitors from the motor circuit to allow the Meg-alert system to work properly? Or do I need to install new surge capacitors without bleed resistors?

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