The primary technical parameter of a thermal relay is its setting current. This refers to the maximum current that can flow continuously through the heating element without causing the relay to trip. When the current flowing through the heating element exceeds the setting current by 20%, the thermal relay should trip within 20 minutes. The setting current can be adjusted using a dedicated knob. When selecting and configuring a thermal relay, it is essential to match the setting current to the motor's rated current.
Because thermal relays operate based on heat, they possess significant thermal inertia; consequently, they do not trip immediately even if the current through the heating element briefly exceeds the setting current by several times. This characteristic ensures the relay does not trip due to the high inrush current during motor startup-which would otherwise prevent the motor from starting. Conversely, the relay will trip if the current exceeds the setting value for a prolonged period, even if the magnitude of the excess is small. Thus, the functions of thermal relays and fuses differ: thermal relays provide overload protection but not short-circuit protection, whereas fuses provide short-circuit protection but not overload protection. A comprehensive control circuit-particularly one for a high-capacity motor-should incorporate both types of protection.
