TNC switch stands for Trip Neutral Close switch. As its name, it is a three-position switch. TNC switch uses to control the Circuit Breaker of the electrical system. The trip position is used to trip the circuit breaker, the close position use for closing the circuit breaker, and the neutral position is the default position of the switch. No output contact is connected from the neutral position of the switch.
Why TNC switch use?
We know the circuit Breaker has the separate Trip and Close coil for Tripping and closing of the breaker. To energies the Trip and close coil, a DC pulse is required for less than 1 sec. After the energization of the coil, the DC supply must be disconnected from the coil immediately. For example, if the DC supply is not disconnected from the trip coil, after closing the breaker, it will automatically trip. Similarly, for the closing circuit, the Circuit breaker will close automatically after tripping the breaker.
To avoid the problem, DC pulse to be applied to the coil for a smaller time, it may around 1 sec or less than 1 sec. TNC switch has the facility to provide a similar facility for giving the short DC pulse to the coil.
TNC switch has 3 positions Trip, Neutral and Close. The neutral position is the default position of the switch. At the position, no DC pulse or command will go to the Trip or close coil. When it moves to the Trip position, the NO (Normally Open) contact which is used for the trip circuit will get NC and it will give the DC pulse to the Trip coil. When it moves to the close position, the NO (Normally Open) contact which is used for the close circuit will get NC and it will give the DC pulse to the close coil.
The spring is connected to the operating handle of the TNC switch to stay the switch at the Neutral position. In both the cases for tripping and closing, the operating handle of the TNC switch automatically gets back to the neutral position and NC contact becomes NO and it breaks the DC supply for the coil.