It usually needs to work according to the current signal, and also determines the contact action according to the current of the coil. When installed and used, the current relay coil needs to form a series connection with the load circuit. It can be divided into AC and DC according to the coil current, and over-current and under-current according to the operating current.
The undercurrent relay is based on the principle that when the current in the coil reaches or exceeds the operating current value, the armature will perform a pull-in action. On the contrary, the armature will be released immediately when the coil current is less than the operating current value. When working under normal conditions, the armature will pull in if the load current exceeds the coil operating current, and the armature will be released when the load current drops below the coil release current value. The under-current relay plays an under-current protection role in the circuit. The symbol of the current relay is shown in the following figure (the left is overcurrent and the right is undercurrent).