What is the difference between thermal magnetic & electronic trip unit?

Question

What is the difference between thermal magnetic & electronic trip unit?

Answer 1

Both are associated with circuit protective devices (like circuit breakers), and both 'measure' the current in some fashion to determine if the circuit should be interrupted.

A thermal magnetic trip unit consists of 2 main parts: the thermal element, and the magnetic element.

The thermal element is a bimetal spring that heats up when the current exceeds a selected level (the pickup current).  When the spring reaches a preset level of total energy input (i.e. the spring stretches enough), the spring activates a breaker release which trips the breaker.  As you can imagine, higher currents cause the spring to operate faster than lower currents.  This behavior approximates the thermal overload characteristics of the protected circuit (wires, motors, etc.).

At the same time, there is a solenoid that monitors the current magnitude.  If the current ever exceeds the preset level, it immediately trips the breaker.  Since this solenoid is an electromagnetic, the trip element is called a 'magnetic trip'.

The electronic trip unit is a microprocessor-based module that uses current sensors to measure the current, then implements the same or similar protection characteristics as the thermal-magnetic units.

The microprocessor trip units are generally more adjustable and may offer more features, including online monitoring, meter readouts, more complex protection settings, etc.