Pole Discrepancy Protection

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.

Basically, Pole Discrepancy feature is incorporated through a Timer and a fixed time is set in the timer after which Pole Discrepancy is supposed to initiate. Suppose, the time is set at 2.5 sec in the Pole Discrepancy Timer, then if there is mismatch of more than 2.5 sec in Breaker opening / closing time then Pole Discrepancy shall operate and shall trip all the three poles of Circuit 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.

Feel free to ask if in doubt. Thank you!

17 thoughts on “Pole Discrepancy Protection”

  1. Sir, I think if the positive supply will be extended to te timer through contact S1, S4 and S6 therefor the timer will not initiate because if the timer will initite S2 must me closed. Is that right?

    Reply
    • Positive will extend through S1,S4,S6 because the timer connection are connected parallel to aux contact.

  2. 500KV has single pole Circuit Breaker while 132KV has three pole circuit breaker. So in which case there will be pole discrepancy in 500KV or 132KV?

    Reply
    • Pole Discrepancy Protection is generally provided where the operating mechanism for the three poles are different. This means individual operating mechanism is used for individual poles. In case of Single Pole three contact breaker, there is a single operating mechanism and therefore normally this protection is not provided. Your feedback is highly appreciated.

  3. I have done research and found that single pole circuit breaker is provided with pole discrepancy mechanism, and its not required in case of three pole circuit breaker. Though this article is very information I think you would add this information too. Thanks.

    Reply
    • I have seen and worked on three pole circuit breaker having pole discrepancy protection. It’s Siemens make circuit breaker, model: 3AP2

  4. If one pole of CB is completely stuck-up then which protection will operate?? Pole discrepancy or Breaker fail protection??
    How these two protections are co-ordinated??

    Reply
    • The logic for Breaker Failure protection and pole discrepancy is different. Assuming current is flowing through the stuck pole while lock out relay is operated, then breaker failure protection will operate as the time delay for pole discrepancy protection is kept higher as compared to LBB.

  5. What are the problems arising due to pole discrepancy. In Siemens make VCB 3AE1, pole discrepancy should not be more than 2 mSec. What happens if it is more ?

    Reply
  6. I am working in a 220 KV Sub-Station. On fault on a 220 KV Feeder, only one pole is tripping other two are not tripping. So what is the solution of this problem?

    Reply
  7. I am working in a 220 KV Sub-Station. On fault on a 220 KV Feeder, only one pole is tripping other two are not tripping. So what is the solution of this problem?

    So called single phase reclosing, pole discrepancy protection should not operate during single phase reclosing in progress.

    Reply
    • Kindly check if the output contact of lock out relay is wired to the trip circuit of all three poles or not? In this case, your breaker were tripping on pole descripancy?
      Since time delay of pole descripancy protection is more, it does not operate during singlr pole reclosing.

    • If the time delay of pole discrepancy protection is less than that of dead time in auto reclosure then definitely breaker three pole trip will occur. Ideally, the setting of pole discrepancy protection should be more than the dead time.

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