Current Limiting MCCBs - Cascading 1

[NickParker]

How does current limiting MCCBs help reducing the interrupting capacity of downstream breakers? Is the current limiting MCCBs limits the current or just trip instantaneously?

Say, if the upstream current limiting MCCB trips before the peak of the sine wave, for the fault down of the downstream MCCB, then there is no coordination.

Is there some inbuilt impedance in the current limiting MCCBs which reduces the fault current?

How is the cascading/discrimination achieved?

Is the current Limiting MCCBs comes under Category A or Category B breakers?

 

[che12345]

Dear Mr NickParker

Q!. How does current limiting MCCBs help reducing the interrupting capacity of downstream breakers? Is the current limiting MCCBs limits the current or just trip instantaneously?
A1. a) The main characteristics of a current-limiting circuit-breaker (including MCCBs) is the circuit-breaker with a [breaking-time short enough] to prevent the short-circuit current reaching its otherwise attainable peak value.
b) it does [not] help reducing the [interrupting capacity] of the downstream breakers. It helps by the way that a [lower interrupting capacity breaker] can be used in the downstream.
INFO: a) a current-limiting breaker, the breaking-time is < 1/4 of a cycle.
b) a non current-limiting breaker, the [instantaneous trip] time is > 0.01s .

Q2. Say, if the upstream current limiting MCCB trips before the peak of the sine wave, for the fault down of the downstream MCCB, then there is no coordination.
A2. When the upstream CB trips faster than the downstream, [selectivity failed] (=no coordination).

Q3. Is there some inbuilt impedance in the current limiting MCCBs which reduces the fault current?
A3. No, there is [no inbuilt impedance]. It is by its [short breaking-time (mechanism)], see also above A1.

Q4. How is the cascading/discrimination achieved?
A4. a) cascading is achieved when the upstream CB's [current peak value) is lower than the downstream CB can withstand. i.e. due to the upstream CB fast opening which limits the [current peak] that the downstream CB is not subjected to.
b) when cascading technique is utilized, discrimination [failed], is not achieved.

Q5. Is the current Limiting MCCBs comes under Category A or Category B breakers?
A5. Reference: 60947-2. A current Limiting MCCBs comes under utilization Category A.

Che Kuan Yau (Singapore)