kAIC Rating Assessment of an Electrical Recloser

Electrical reclosers are widely used in overhead electrical distribution systems to detect and interrupt momentary faults mostly due to vegetation, and wild animals. This study assesses the kAIC rating of an existing recloser installed in an overhead distribution system due to an increase in fault levels as a result of major system reconfiguration.

Electrical Recloser Specifications

Maximum Available Short Circuit Current

The 0.5 cycles and 1.5 – 4 cycle short circuit currents are the same for this application since the electrical distribution source is mostly from the transmission grid.

The maximum available short circuit current resulting from the major system reconfiguration is

while the X/R ratio is

Calculation of Close and Latch Duties

The calculation of the asymmetrical RMS and peak close and latch duties is done by introducing multiplying factors to the short circuit current, respectively.

Asymmetrical Peak

The asymmetrical peak close and latch duties are calculated as shown below.

Asymmetrical RMS

The asymmetrical RMS close and latch duties are calculated as shown below.

To verify your calculation, you can use the circuit breaker short-circuit duty calculator available on our website.

Top Menu > Tools > Circuit Breaker Short-Circuit Duty Calculator

or you may click on the link below.

Circuit Breaker Short-Circuit Duty Calculator

Calculation of Interrupting Duty

The calculation of the interrupting duty of an electrical recloser is very important in order to assess the kAIC rating. The calculation is the same as that of a High Voltage Circuit Breaker (HVCB) and is dependent on the identification of fault source as either local or remote. For simplicity, all sources to the fault were assumed to be remote sources.

Going back to our discussion on remote and local sources based on the ANSI standards, the identification of generators as local or remote is important in determining the correct multiplying factors used in the calculation of circuit breaker, in this case, electrical recloser interrupting duty.  The generator AC decay is conditional on its proximity to the fault. AC decay is considered only for local generators while remote generators are assumed to feature no AC decay.

Click HERE to learn more about how remote and local generators affect the magnitude of the circuit breaker interrupting duty.

The above formula shows the local and remote contributions considered into the calculation of the interrupting duty of the electrical recloser. The assumption of remote sources only greatly simplifies the calculation in that it reduces the Adjusted Multiplying Factor (AMFi) to just the remote multiplying factor over the circuit breaker rating structure.

The calculation of the remote multiplying factor is a very straightforward process and depends only on the circuit breaker contact parting time (CPT) and the fault X/R ratio.

The circuit breaker contact parting can be derived from its interrupting time. For the electrical recloser assessed, the interrupting time is 3 cycles giving us the contact parting time of 2 cycles. To know more about contact parting times and other related stuff, follow this link.

After calculating the adjusted multiplying factor, calculating the interrupting duty requires the identification of whether the circuit breaker is Totally or Symmetrically rated. Totally rated circuit breakers reflect an earlier breaker rating structure while symmetrically rated circuit breaker reflect a more recent rating structure. Our previous discussion on Circuit Breaker Interrupting Rating Calculations further discusses the difference between these two rating structures.

For the electrical recloser under assessment, the rating structure is symmetrically rated. This requires us to select the applicable standard.

If C37.010-1999 is selected, the S-factor should be taken from the manufacturer. If not available, choose the C37.010-1979 and older. This allows us to select the S-factor based on CPT. For a 2 cycle CPT, the S-factor is equal to 1.2. For the list of S-factors from C37.010-1979, you may refer to this link.

The next step is to calculate the adjusted multiplying factor to reflect the rating structure of the electrical recloser.

Since the resulting adjusted multiplying factor is less than 1, we set it equal to 1 little to the conservative side.

From this, the interrupting duty of the electrical recloser is

Using our on-site circuit breaker short-circuit duty calculator, the same values can be obtained.

The short-circuit duties of 23.055kA close and latch peak and 9.3kA interrupting RMS as a result the increased system fault levels are below the electrical recloser close and latch, and kAIC rating of 32.5kA peak and 12.5kA RMS, respectively. Therefore, we can conclude that the electrical recloser rating is sufficient to withstand and safely interrupt the electrical distribution fault level at its location.

References

IEEE Std 551-2006 [The Violet Book]: Recommended Practice for Calculating AC Short-Circuit Currents in Industrial and Commercial Power Systems. (2006). S.I.: IEEE.

IEEE Std C37.5-1979: ANSI Guide for Calculation of Fault Currents for Application of AC High-Voltage Circuit Breakers Rated on a Total Current Basis. (1979). S.I.: IEEE.

IEEE Std C37.010-2016: IEEE Application Guide for AC High-Voltage Circuit Breakers > 1000 Vac Rated on a Symmetrical Current Basis. (2016). S.I.: IEEE.

IEEE Std C37.04-2018: IEEE Standard for Ratings and Requirements for AC High-Voltage Circuit Breakers with Rated Maximum Voltage Above 1000 V. (2018). S.I.: IEEE.

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