A protective relay is a device for sending a cutting signal to a Circuit Breaker when a power failure is detected. So what is the protection relay? what is its operation principle? Let’s find out in this article.
What is the protection relay?
Protection relays are electromagnetic devices, based on the operation of the electromagnetic coil acting on the drive parts to detect abnormal activities such as Overcurrent, overvoltage, reverse power line or too low frequency.
Operation principle of the protection relay?
When there is an electric current flowing through the relay, this current will flow through the inner coil and create a suction magnetic field. This suction magnetic field acts on an internal lever that closes or unlocks electrical contacts and thus changes the state of the relay. The number of electrical contacts changed maybe 1 or more, depending on the design.
Relay has 2 independent operating circuits. A circuit that controls the coil of the relay in the ON or OFF state. A current control circuit based on the ON or OFF status of the relay.
The line runs through the coil to control the relay ON or OFF which is usually about 30mA with 12V voltage or up to 100mA. Most chips cannot provide this line. Now we need to have a BJT to amplify the small current in the IC output into a larger current for the relay.
Note: However, the IC 555 has an output current of up to 200mA, so the 555 IC does not need a BJT to amplify the current.
The Symbols on the Protection relay
On the relay, there are 3 symbols: NO, NC and COM.
+ COM (common): is the common pin, it is always connected to one of the other 2 pins. It connects to which pin it depends on the operating status of the relay.
+ NC (Normally Closed): It means that it is normally closed. This means that when the relay is in the OFF state, the COM pin will be connected to this pin.
+ NO (Normally Open): When the relay is ON, the COM pin will be connected to this pin.
=> Connect COM and NC when you want to have current-controlled when the relay is in OFF state. And when the relay is ON, this line is disconnected.
=> Otherwise, connect COM and NO.