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How will inrush current effect the thermal memory of the C440 overload relay

To see the effects of inrush we will use the curves attached below and based the trip times @ 6x FLA

For Class 10

  • Cold trip time = 10 seconds
  • Hot trip time = 2.5 seconds

Let’s assume the overload is set at Class 10 and the start current is at 6x FLA

Example #1:

  • If the 6x current is applied from a cold state then the following will occur:
    • If the 6x current is on for:
      • 2.5 seconds then the thermal memory will reach 25%.
      • 5.0 seconds then the thermal memory will reach 50%.
      • 7.5 seconds then the thermal memory will reach 75%.
      • 10 seconds then the thermal memory will reach 100% and a trip will occur

Example #2

  • Assume the motor has been running and the thermal memory is close to 100% when the motor is turned off:
    • The thermal memory will decay to 75% in 2 minutes
    • If the motor is restarted and the 6x current is present for > 2.5 seconds the overload will trip.
    • If less than 2.5 seconds then the thermal memory will level out below 100% based on the continuous motor current.
    • This process will repeat itself every time the motor starts if started every 2 minutes or more.

The conclusion is:

Based on the trip curves an estimated thermal memory can be calculated if the current and time functions are known.

The Trip curves are attached for your further review. 

How does the Thermal Memory work on the C440 Overload Relay

  1. C440 - Cold Trip Curves.pdf
  1. C440 - Hot Trip Curves.PDF