Generally speaking, the appropriate MF to account for the DC component of the short circuit current waveform depends on the short circuit X/R ratio at the fault location and the rating structure of the interrupting device. Complicating the selection of the appropriate MF for medium-voltage circuit breakers is the fact that there is a huge installed base of medium-voltage circuit breakers whose short circuit ratings are based on older standards that differ significantly from the latest standards. Specifically, the historic constant MVA rating structure pre-dates the latest constant kA rating structure that was initiated around 1999. [See Siemens TechTopics Nos. 4 and 23 (available at www.energy.siemens.com/us/en/services/power-transmission-distribution/te...) for a well-written summary of these rating structures.] Consequently, the procedures summarized below to select the appropriate MFs to calculate the short circuit current duties of medium-voltage circuit breakers are classified according to rating structure. [The reader is directed to Chapters 9 and 10 of the IEEE Violet Book (IEEE Std 551-2006) for detailed coverage of these procedures.]

First-Cycle Duty (Constant MVA Rated Breakers)

First-cycle (momentary) duty of Bus 2 medium-voltage breakers = MF × Bus 2 first-cycle (momentary) symmetrical rms current = 1.6 × 8.920kA = 14.272kA (asymmetrical rms)

The closing and latching (momentary) withstand capabilities of constant MVA rated medium-voltage circuit breakers in rms kA must exceed the first-cycle duty in asymmetrical rms kA.

Note: MF = 1.6 is applicable whenever the first-cycle short circuit X/R ratio is less than or equal to 25. If the first-cycle short circuit X/R ratio exceeds 25, the following formula can be used to calculate the MF.

First-Cycle Duty (Constant kA Rated Breakers):

The closing and latching (momentary) withstand capabilities of constant kA rated medium-voltage circuit breakers in peak kA must exceed the first-cycle duty in asymmetrical peak kA.

First-cycle (momentary) duty of Bus 2 medium-voltage breakers = MF × Bus 2 first-cycle (momentary) symmetrical rms current = 2.6 × 8.920kA = 23.192kA (asymmetrical peak)

Note: MF = 2.6 is applicable whenever the first-cycle short circuit X/R ratio is less than or equal to 17. If the first-cycle short circuit X/R ratio exceeds 17, the following formula can be used to calculate the MF.

Interrupting Duty (Simplified Method)

The simplified method to find the contact-parting (interrupting) short circuit current duty applies to constant MVA or constant kA rated medium-voltage circuit breakers and uses a conservative MF of 1.25, rather than calculating the MF from a formula based on the contact-parting short circuit X/R ratio.

Contact-parting (interrupting) duty of Bus 2 medium-voltage breakers = 1.25 × Bus 2 contact-parting (interrupting) symmetrical rms current = 1.25 × 8.399kA = 10.499kA (asymmetrical rms)

The required symmetrical interrupting capability of the medium-voltage circuit breaker in rms kA must exceed the contact-parting (interrupting) duty in asymmetrical rms kA, where the required symmetrical interrupting capability depends on the rating structure as follows.

Constant kA rated breakers: For operating voltage less than rated maximum design voltage, the required symmetrical interrupting capability = rated short circuit current, where the rated maximum design voltage and rated short circuit current are listed in the manufacturer’s table for constant kA rated medium-voltage circuit breakers.

Constant MVA rated breakers: For operating voltage between (1/K) × rated maximum design voltage and rated maximum design voltage, the required symmetrical interrupting capability = rated short circuit current × (rated maximum design voltage ÷ operating voltage).

For operating voltage less than (1/K) × rated maximum design voltage, the required symmetrical interrupting capability = K × rated short circuit current = maximum symmetrical interrupting capability, where voltage range factor K, rated maximum design voltage, rated short circuit current, and maximum symmetrical interrupting capability are listed in the manufacturer’s table for constant MVA rated medium-voltage circuit breakers.

Interrupting Duty (Only Remote Sources)

A less conservative method than the simplified method takes into account the actual decay of the DC component of the short circuit current waveform, but assumes no decay in the envelope of the symmetrical (AC) component. The assumption of no AC decay is characteristic of a power system without local (in-house) generation. Note the following procedures differ by rating structure. [For further details, the reader is directed to ANSI/IEEE C37.010-1979 for the older constant MVA rating structure and IEEE Std C37.010-1999 (R2005) for the newer constant kA rating structure.]

Constant MVA rated breakers: Contact-parting (interrupting) duty of Bus 2 medium-voltage breakers = MF × Bus 2 contact-parting (interrupting) symmetrical rms current = 1.0 × 8.399kA = 8.399kA (asymmetrical rms), where MF = 1.0 was found from the curve in Fig. 10 of ANSI/IEEE C37.010-1979 for a 5-cycle breaker with 3-cycle minimum contact-parting time and Bus 2 contact-parting (interrupting) short circuit X/R ratio of 16.2 from the Table. (Generally speaking, MF = 1.0 for a 5-cycle breaker with 3-cycle minimum contact-parting time if the contact-parting (interrupting) short circuit X/R ratio is 15 or less, because a certain degree of asymmetry is built into the rating structure.)

The required symmetrical interrupting capability of the medium-voltage circuit breaker in rms kA must exceed the contact-parting (interrupting) duty in asymmetrical rms kA. (The required symmetrical interrupting capability of constant MVA rated medium-voltage circuit breakers was described in the previous section.)

Constant kA rated breakers: Contact-parting (interrupting) duty of Bus 2 medium-voltage breakers = MF × Bus 2 contact-parting (interrupting) symmetrical rms current = 1.0 × 8.399kA = 8.399kA (asymmetrical rms), where MF = 1.0 was found from the curve of Fig. 10 of IEEE Std C37.010-1999 (R2005) for a 5-cycle breaker with 3-cycle minimum contact-parting time and bus 2 contact-parting (interrupting) short circuit X /R ratio of 16.2 from the Table. [Generally speaking, MF = 1.0 for a 5-cycle breaker with 3-cycle minimum contact-parting time if the contact-parting (interrupting) short circuit X/R ratio is 17 or less, because a certain degree of asymmetry is built into the rating structure.]

The required symmetrical interrupting capability of the medium-voltage circuit breaker in rms kA must exceed the contact-parting (interrupting) duty in asymmetrical rms kA. (The required symmetrical interrupting capability of constant kA rated medium-voltage circuit breakers was described in the previous section.)

Interrupting Duty (Local and Remote Sources) 

The least conservative procedure to find the contact-parting (interrupting) short circuit current duty is applicable if the fault is fed by local and remote sources of short circuit current. Remote sources include the electric utility and those in-house synchronous machines that are electrically remote from the fault location. Local sources include those in-house synchronous machines that are electrically local to the fault location. [Induction motors local to the fault location may be classified as remote sources of short circuit current, because the AC decay effect in the induction motor short circuit current waveform has already been taken into account in the induction motor reactance multipliers for the contact-parting (interrupting) network. See Table 9-1 in the IEEE Violet Book (IEEE Std 551-2006) for details.]

The duty calculation makes use of both remote and local multiplying factors in a weighting process. The duty is calculated as the sum of the remote MF multiplied by the portion of the contact-parting (interrupting) symmetrical rms current from remote sources and the local MF multiplied by the remainder (local portion) of the contact-parting (interrupting) symmetrical rms current. [Another formula to calculate the duty, involving the so-called “no AC decay” ratio, yields an identical result. The reader is directed to Chapters 9 and 10 of the IEEE Violet Book (IEEE Std 551-2006) for details.]