According to the "General Electrical Equipment Distribution Design Specifications" (GB50055-2011), low-voltage AC motors should be equipped with short-circuit protection and ground fault protection. Additionally, overload protection, phase failure protection, and low-voltage protection should be installed based on the motor's intended use.

I. Short-Circuit Protection

Installation Conditions Each motor should have individual phase-to-phase short-circuit protection. However, multiple motors may share a protection device if:

(1) The total calculated current does not exceed 20A, and disconnection without selectivity is permitted.

(2) A group of motors that must start and stop simultaneously for safety reasons.

Installation Requirements

(1) Short-circuit protection should be fast-acting to quickly interrupt current during a fault.

(2) Protection devices should not malfunction during normal starting, braking, or self-starting of the motor.

Common Protection Components 

Short-circuit protection for AC motors is best achieved using fuses or low-voltage circuit breakers with instantaneous overcurrent trip devices, or overcurrent relays with instantaneous components.

Selection of Low-Voltage Circuit Breaker Instantaneous Trip Device 

The setting value of the instantaneous trip (Ist) should exceed the peak current: Icst>KIst

where Ist is the motor's starting current (maximum effective value of the periodic component); K is the reliability coefficient, typically taken as 2-2.5. For short-time delayed overcurrent trip devices, the setting current should avoid the maximum effective value of the starting current, with a delay of no less than 0.1 seconds.

II. Ground Fault Protection

Each motor should have separate ground fault protection. Motors sharing a short-circuit protection device can also share ground fault protection components.

If the short-circuit protection device meets the ground fault protection requirements, it can serve as both.

III. Overload Protection

Motors prone to overload during operation or those requiring limited starting time due to difficult starting conditions should be equipped with overload protection.

Motors operating continuously should have overload protection that disconnects the power supply. If the loss from disconnection exceeds that from overload, the protection should trigger an alarm instead.

Motors with short-time or intermittent duty cycles may not require overload protection; however, if there is a risk of the motor stalling, overload protection for stalling should be installed.

IV. Phase Failure Protection

Continuous three-phase motors should have phase failure protection when using fuses; it is advisable to have it when using low-voltage circuit breakers.

Motors with short-time or intermittent duty cycles may not require phase failure protection.

V. Low-Voltage Protection

Purpose

(1) To protect personal and equipment safety and prevent unintentional motor startup.

(2) To ensure critical motors can self-start while disconnecting a sufficient number of secondary motors.

Installation Conditions

(1) Motors that should not self-start for process or safety reasons should have low-voltage protection.

(2) Secondary motors that must be disconnected to ensure the self-start of critical motors should also have low-voltage protection.