A17.9 | Schneider Electric Thermal Unit, Type S, Type A

The A17.9 is a thermal unit characterized by Type S and Type A configurations, employing a melting alloy construction and classified under Class 20. Thermal units are designed to safeguard motors, controllers, and branch circuit conductors against excessive heating caused by prolonged motor overcurrents, including locked rotor currents. Protection against higher currents resulting from short circuits or grounds is primarily facilitated by branch circuit fuses or circuit breakers. 

Class 20 thermal units respond to overload current conditions based on trip curves, tripping in 20 seconds or less during overload situations. These thermal units are constructed as a single piece, allowing the motor current to pass through a small heater winding within the unit. In the event of overload conditions, the generated heat causes a special solder within the unit to melt, enabling a ratchet wheel to spin freely. Consequently, this action opens the control circuit contacts and subsequently the motor contacts, effectively mitigating the risk of overheating and potential damage to the motor and associated circuitry. 

Overall, the A17.9 thermal unit serves as a crucial component in motor protection systems, providing reliable and efficient protection against overload conditions. Its Class 20 classification ensures prompt response to overcurrent events, contributing to the safe and uninterrupted operation of motors and associated equipment. The melting alloy construction enhances durability and reliability, while the one-piece design simplifies installation and maintenance procedures.