Brochure

PowerPact H-, J-, and L-Frame Circuit Breakers
Motor Circuit Protection
49
02/2016
© 2011–2016 Schneider Electric
All Rights Reserved
Section 5—Motor Circuit Protection
General Information
The parameters to be considered for motor-feeder protection depend on:
• the application (type of machine driven, operating safety, frequency of operation, etc.)
• the level of continuity of service required by the load or the application
• the applicable standards for the protection of equipment.
The required electrical functions are:
• isolation
• switching, generally at high endurance levels
• protection against overloads and short-circuits, adapted to the motor
• additional special protection.
A motor branch circuit must comply with the requirements of standard UL 508 concerning contactors
and their protection:
• coordination of feeder components
• overload relay trip classes.
Motor Branch Circuit Protection Function
A motor branch circuit comprises a set of devices for motor protection and control, as well as for
protection of the branch circuit itself.
Switching
The purpose is to control the motor (ON / OFF), either manually, automatically or remotely, taking into
account overloads upon start-up and the long service life required. This function is provided by a
contactor. When the coil of the contactor's electromagnet is energized, the contactor closes and
establishes, through the poles, the circuit between the upstream supply and the motor, through the
circuit breaker.
Basic Protection
• Short-circuit protection
Detection and breaking, as quickly as possible, of high short-circuit currents to avoid damage to the
installation. This function is provided by a circuit breaker.
• Overload protection
Detection of overload currents and motor shutdown before temperature rise in the motor and
conductors damages insulation. This function is provided by a circuit breaker or a separate motor
overload relay.
• Phase unbalance or phase loss protection
Phase unbalance or phase loss can cause temperature rise and braking torques that can lead to
premature aging of the motor. These effects are even greater during starting, therefore protection
must be virtually immediate.