Pole Discrepancy Protection is an inherent feature of Circuit Breaker which ensures that all the three poles of Circuit Breaker is tripped / closed within a predefined time period. If there is a mismatch of Breaker opening / closing time then Pole Discrepancy Protection will operate and will trip all the three poles of the Breaker. Suppose due a particular fault, a trip command is issued to the Breaker but due to some problem only two poles of Breaker could open simultaneously and the one pole of Breaker opened after some time more than the set time. In this case Pole Discrepancy Protection will operate and will trip all the three pole of the Breaker.
For better understanding of Pole Discrepancy, carefully observe the figure below.
In the figure above, S1, S2, S3, S4, S5 and S6 are Breaker Auxiliary contacts. Mind that S1, S3 and S5 are Normally Close contacts while S2, S4 and S6 are Normally open contacts. Breaker Auxiliary contacts are those contacts which are associated with Breaker physical opening and closing.
If the Circuit Breaker is Open, it means that S1, S3 and S5 will be close and S2, S4 and S6 will be open. But as soon as Circuit Breaker closes, the configuration of contacts S1, S2, S3, S4, S5 and S6 will change i.e. S1, S3 and S5 will open while S2, S4 and S6 will close.
Now, we will examine the operation of Pole Discrepancy. If the Circuit Breaker trips and all the three phase open then as shown in figure above, the status of contacts will be as below.
S1 – Close
S3 – Close
S5 – Close
S2 – Open
S4 – Open
S6 – Open
Thus we see that 220 V DC supply will not be extended to the Timer and timer will therefore, not initiate.
But in case, say one phase (say R phase) Circuit Breaker fails to close, and then in that case the status of contacts will be as below.
S1 – Close
S3 – Open
S5 – Open
S2 – Open
S4 – Close
S6 – Close
Carefully observe in the figure above that, in this configuration of contacts, the positive supply will be extended to the Timer through contact S1, S4 and S6 and therefore the Timer will initiate. After the set time period if Circuit Breaker still fails to close, Timer will actuate its contacts (Normally Open to Close) to give trip command to the three poles of the circuit breaker. In this way Pole Discrepancy Protection trips the Circuit Breaker if there is a mismatch in timing of Circuit Breaker operation.
The operation of Pole Discrepancy will be same as described above for tripping of Circuit Breaker also.