The Thermal magnetic element, as the name suggests, has two units viz. the thermal unit and the magnetic unit. The thermal unit is used to sense the current using the heating effect of current.
The Thermal unit consists of a bimetallic strip which bends due the heat produced by high current passing through it. The bimetallic strip takes time to bend. This enables the element to have a time delay feature. The time delay depends on the magnitude of the over current protection.
The Magnetic Trip element comes into play in case of severe faults. When the current is extremely high, say 400 %, the current causes the magnetic trip element to attract a trip element which causes the unit to trip.
Thus, the thermal element is used to trigger a delayed response to minor overcurrents while the magnetic element is used to swiftly respond to high overcurrents.