Auxiliary Switch in Circuit Breaker with Wiring Diagram

 

What is Auxiliary Switch in Circuit Breaker?

Auxiliary Switch indicates the Position (Breaker On/OFF) of the Power/ Main Contacts of the Breaker.Auxiliary Switch is mechanically interlocked with the Circuit Breaker Operating Mechanism. When we Switch ON/OFF the Breaker, Power (Moving) Contact changes its Position, and at the same time, Auxiliary Switch changes it state, when a predefined Distance is reached between the Power Contacts.

When, Auxiliary Switch changes its state, its contacts called as Auxiliary Contacts also change its state, i.e. NO (52a) Conatcts become Close and NC (52b) contacts become open.

What is the Function of Auxiliary Contacts in Circuit Breaker Control Circuit?

1. Auxiliary contacts disconnect Control Circuits.

 

i) Protects the Closing/Tripping Coil

A continuous Positive from a stuck TNC Switch will burn the Closing Coil and hence the Auxiliary Contacts, breaking the Circuit, protects the Closing Coil in this case.

ii) Protects the Trip Relay Contact, used for Tripping

Lets say the Breaker is ON and there is a Fault, Trip Relay Operates and its Trip Contact Closes. Positive DC is extended to the Tripping Coil, Tripping Coil is Energised and the Breaker Trips.But, during this process, for a very short time, there is a flow of current in the Tripping Circuit.Now, the Trip Relay contacts are lightweight, so as to keep the weight of the Moving Parts low and also, to keep the Power consumption by the Relay low. And so, the Trip Relay Contacts are not designed to interrupt the current in the Trip Circuit.Auxiliary Contact 52a, are designed to interrupt the current in the tip circuit. And so, we place Auxiliary Contact 52a in the Trip Circuit, the current in the Trip circuit is interrupted by the auxiliary switch and not by the protective device.

Now, while opening the trip circuit, an inductive circuit is also opened. This type of opening needs the robust switching device. Auxiliary switch is designed for such a duty. 

2. Breaker ON/OFF Indication:

DC Negative is given to one end of Green (ON) and Red (OFF) LED’s and the DC Positive comes through NO & NC Auxiliary contacts of the Breaker. The NO contact is used for Breaker ON (Green LED) Indication and the NC Contact is used for Breaker OFF (Red LED) Indication.

3. For Trip Circuit Supervision Relay:

The Auxiliary Contacts are used in the Trip Circuit Supervison Circuit for Monitoring the Tripping Circuit and the same will be explained in details in the Article of Trip Circuit Supervision.

4. Multiplier Relay:

Breaker NO NC Contacts are used tp Pick up Multiplier Relays, whose contacts can then be used to Breaker ON/OFF Status to Busbar Relay, SCADA, etc.