• Purpose
• Unfavorable operating conditions
• Fault conditions
• Protection
• Single-device motor protection relays
• Summary of total motor protection
• Motor protection by thermistors
• Monitoring of a motor’s actual operating conditions
• Switchgears for LV motors
• Selection of main components
• Fuse-free system
• Switchgears for MV motors
• Microprocessor based devices (IEDs) and intelligent starters

APPENDIX Rule of thumb for everyday use

Purpose
An electric motor must be adequately protected against all unfavourable operating conditions and internal or external faults. We have classified these conditions into three categories to identify the most suitable protection:
1. Unfavourable operating conditions
2. Fault conditions
3. System disturbances and switching surges (for MV motors)

Unfavourable operating conditions
Operating conditions that may over-load a machine and raise its temperature beyond permissible limits may be called unfavourable. This over-heating, however, will be gradual (exponential), unlike rapid (adiabatic) heating as caused during a locked rotor condition. The machine now follows its own thermal curve and therefore a conventional thermal protection device can be used to protect it from such conditions. These conditions may arise due to one or more of the following: