Current relay: 

The relay that connects or breaks the circuit magnitude of current in the coil is called current relay, which is a kind of electromagnetic relay, so it is sometimes collectively referred to as electromagnetic relay. 

The wire of the current relay coil is thick, its number of turns is less, is connected in series in the main circuit. The relay that acts when the coil current is higher than the set current value is called over-current relay; the relay that acts when the coil current is lower than the set current is called undercurrent relay. The over-current relay is often used for motor short circuit protection; The time delay type over-current relay is mainly used for frequently and heavy-load starting occasions, and used as short circuit and overload protection for motor or main circuit. The relays commonly used are over-current relays of Series JT4, JT12 and JT14. The undercurrent relays are often used for excitation-loss protection of DC motor and magnetic chuck. 

When the over current relay protection is selected, for the DC motor with small capacity and the wound rotor asynchronous motor, the rated current of the coil generally is selected according to the rated current for the motor to work in long-term . For the motor that starts frequently, in consideration of the heating effect of starting current in the relay, the rated current of the coil shall be selected one size bigger. The setting value of over-current relay is generally 1.7 ~ 2.5 times the rated current. 

Voltage relay: The relay that connects or breaks the circuit according to the voltage magnitude at both ends of relay internal coil is called voltage relay. The wire of the current relay coil is thin, its number of turns is more and is connected in parallel in the main circuit. 

There are overvoltage relays or under voltage relay. Generally the overvoltage relay acts when the voltage is 1.1 ~ 1.5 times above the rated voltage; The under voltage relay acts when the voltage is 0.4 ~ 0.7 of the rated voltage; The zero pressure relay acts when the voltage drops to  0.05 ~ 0.25 times the rated voltage, in order to protect the circuit. The commonly used relay is JT4P series.