Saturday, 15 October 2016

Rated Normal, Short Circuit Making & Breaking, Short Time Current Rating & Operating Duty Cycle of Circuit Breaker

Rated Normal Current:

The rated normal current of a circuit-breaker is the r.m.s. value of the current which the circuit breaker can carry continuously and with temperature rise of the various parts within specified limits.

The design of contacts and other current carrying parts in the interrupter of the circuit breaker are generally based on the limits of temperature rise. For a given cross section of the conductor and a certain value of current, the temperature rise depends upon the conductivity of the material. Hence, high conductivity material is preferred for current carrying parts. The cross-section of the conductors should be increased for materials with lower conductivity.

The use of magnetic materials in close circuits should be avoided to prevent heating due to hysteresis loss and eddy currents. The rated current of a circuit-breaker is verified by conducting temperature-rise tests.

Rated Short Circuit Breaking Current:

The rated short-circuit-breaking-current of a circuit-breaker is the highest value of short circuit current which a circuit-breaker is capable of breaking under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in kA r.m.s. at contact separation.

Rated Short Circuit Making Current:

It may so happen that circuit-breaker may close on an existing fault. In such cases the current increase to the maximum value at the peak of first current loop. The circuit breaker should be able to close without hesitation as contacts touch. The circuit breaker should be able to withstand the high mechanical forces during such a closure. These capabilities are proved by carrying out making current test. The rated short-circuit making current of a circuit-breaker is the peak value of first current loop of short-circuit current (Ipk) which the circuit-breaker is capable of making at its rated voltage.

Rated making current = 1.8 x √2 x Rated short circuit breaking

                                      = 2.5 x Rated short circuit breaking current.

In the above equation, the factor √2 converts the r.m.s. value to peak value. Factor 1.8 takes into account the doubling effect of short-circuit-current with considerations to slight drop in current during the first quarter cycle.

Rated Short Time Current:

The short time current of a circuit-breaker is the r.m.s. value of current that the circuit breaker can carry in a fully closed position during a specified time under prescribed conditions of use and behavior. It is normally expressed in terms of kA for a period of one second. Adjacent poles experience mechanical force during the test.

The rated duration of short circuit is generally 1 second and the circuit breaker should be able to carry short-circuit current equal to its rated breaking-current for one second. During the short-time current test, the contacts should not get damaged or welded.

The current carrying parts and insulation should not get deteriorated. Generally, the cross-section of conductors based on normal current rating requirements is quite adequate for carrying the rated short-circuit current for the duration of 1 second.

Operating Duty Cycle:

The operating sequence denotes the sequence of opening and closing operations which the circuit-breaker can perform under specified conditions. The operating mechanism experiences severe mechanical stresses during the auto reclosure duty. As per IEC, the circuit-breaker should be able to perform the operating sequence as per one of the following two alternatives:

(i) O-t-CO-T-CO

where, O=opening operation

C=closing opeartion

CO=closing followed by opening

t=3 mintes for circuit-breaker not to be used for rapid auto reclosure

t=0.3 second for circuti-breaker to be used for rapid auto reclosure.

T=3 minutes.

(ii) CO-t'-CO

where t'-15 second for circuit-breaker not to be used for rapid auto reclosure.

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