Answer / rajesh t k

The NAT and NAC contacts are provided on TNC switch to facilitate additional functionalities from the TNC switch than mere Tripping or Closing.


These contacts are also known as Lost Motion Contacts. They help identify the last operation (Trip/Close) performed by the operator through the TNC switch.

The NAC contact will be closed whenever the operator turns the TNC lever to Close position and the contacts stays closed even after the lever is returned to the Neutral position by the operator, until the lever is turned to Trip side.

Hence, the 'Auto Trip' indication can be given via the NAC contact(instead of giving through an NO contact of the master trip relay), in series with the CB's 52b contact. That is, this is just an alternate way to wire up the 'Auto Trip' indication.

The NAT contact, likewise will be through whenever the CB has tripped via the TNC lever switch by the operator, until the lever is turned to Close position again.

Answer / k.prakashchandra

NAT- normal after trip and NAC normal after close.These are
'lost motion contacts' which indicate last operated position
of breaker .With NAC contact we get 'Auto-trip' indication
when the breaker is tripped by relay.

Just a silly question but just to make my curious mind bac to normal - Why was not 67 and 67N relays built into a single relay as i have noticed that the R,Y,B CT wiring end up into the 67 relay while the common of the R,Y,B CT wiring end up into the 67N relay. Then it was better if a single relay was manufactured just like the distance relay (21). Another question i would like to ask is - how do we know where the 67 and 67 N relay has to be placed.why is it not placed at the transformer bay? In the substaion that i am working -i have noticed that it is placed between a breaker and a common double bus bar and this line bay is connected to another substation at a remote place which is furthur connected to a load. B U S______CT for 67 & 67N ______Breaker _____ remot s/s. B A R

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