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Circuit Overload Protection for Hermetic Motors, Part 1

circuit-overload-protection-for-hermetic-motors-part-1Part VI of Article 440 provides the requirements for protecting the branch circuits of hermetic motors from overload. It specifies devices you can use for this purpose, and how to use them.

First, you need to understand what Article 440 means by “overload.” This is different from short-circuit and fault protection. Overload is a condition in which the motor is drawing too much current due to having too much load on it. Overload protection of hermetic motor systems protects against excessive heating from overload and/or failure to start [440.51]. But protects what? Only three things:

  1. Motor-compressor.
  2. Motor-control apparatus.
  3. Branch circuit conductors.

The application and selection of the protection for the motor-compressor is covered in 450.52(A), and for the other two items on the list above the application and selection of the protection are covered in 450.52(B).

You must use one of four methods to protect the motor-compressor against overload and failure to start. These are:

  1. A separate overload relay that’s responsive to motor current. It must trip at not more than 140% of the motor-compressor rated-load current.
  2. A thermal protector integral with the motor-compressor.
  3. A fuse or inverse time breaker that’s responsive to motor current.

A system that protects on the basis that it will prevent dangerous overheating of the motor-compressor due to overload and failure to start. It must be approved (by the authority having jurisdiction) for the use.

Part 2 » | Source: Mark Lamendola | Mindconnection