Thermal relay
A thermal overload relay is a device to protect an electrical device like a motor/generator from catching fire/burning out due to excessive heat produced during operation. The relay has a bi-metallic lamella that operates with temperature. When the temperature of the system goes above a pre-set value, the lamella is moving and, by mechanic action, opens contact.
The coefficient of expansion is one of the basis properties of any material. Two different metals always have different degree of linear expansion. A bimetallic strip always bends when it heated up, due to this inequality of linear expansion of two different metals.
Class
There are four classes of thermal overload relay: • 10A • 10 • 20 • 30 |  |
 | These class numbers indicate the trip characteristics from cold state, and should be selected according to the starting characteristics of the motor and its application. Below is a table giving the characteristics for each class Ir = Current setting of overload relay. This should be the Full Load Current (FLC or FLA) shown on motor rating plate. For the vast majority of applications 10A will be the first choice. (Schneider electric) |
Overload Thermal Relay Class 10
