tnc switch

TNC Switch is used in the RP/CP (Remote TNC Switch) and in the Breaker (Local TNC Switch) for Tripping and Closing the Breaker Manually.

What is a TNC Switch?

  1. It is a Three Position Switch, used for Closing or tripping the Breaker Manually from Control Panel/Relay Panel in the Control Room.
  2.  The Three Positions are called as Trip, Neutral and Close.
  3. The Switch is always at the Neutral Position.

Specifications:

  1. Contacts: Say, 2 N/O In each Position. This means that, there are total 4 Normally Open Contacts, for each position i.e. 2 Normally Open Contacts for Trip Position and 2 for close Position.
  2. Rating: Say, 110V DC, 15 A.

Working:

  1. To Trip the Circuit Breaker, the Switch needs to be turned from Neutral Position to Trip Position at which Positive is extended to Tripping Circuit of the Circuit Breaker and then due to the Spring Action, the Switch automatically comes back to Neutral Position.
  2. To Close the Circuit Breaker, the Switch needs to be turned from Neutral Position to Close Position at which Positive is extended to Closing Circuit of the Circuit Breaker and then due to the Spring Action, the Switch automatically comes back to Neutral Position.
  3. The Switch is loaded with Springs, which ensures that, the Switch returns to Neutral (N) Position after any operation (Close or Trip) . This is done to Isolate Closing/Trip Coils, which are short time rated, from Control Positive.

What happens when TNC Switch becomes Faulty?

  1. Lets say TNC Switch is turned to Close Postion, the Positive is extended to Closing Coil.
  2. Now the TNC Switch should return to Neutral Positin due to Spring Action.
  3. But, let say , at the same instant, the TNC Switch Spring Fails and due to which the TNC switch does not return to Neutral Position but gets stuck at the Close Position. In this case a continous supply is available at the Closing Coil.
  4. If its a Permanat Fault, Breaker Hunting will take place, damaging the Breaker Mechanism. For this purpose, Anti Pumping Relay is used in the Circuit Breaker.

Please note that same Switch is also availabe in Circuit Breaker for Local Closing and Tripping.

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