Circuit Breaker Rating Confusion

I put the following write-up together to help explain the ratings of medium voltage circuit breakers. You've got to recognize that the standards were written for the companies that make circuit breakers, not for the engineers who apply them. The relatively recent change in the momentary, or "close and latch" rating from an rms value to a peak value has been particularly confusing.

In C37.06-1979, Table 2, column 6 has the rated rms kA value for each breaker rating. For example, for a 15 kV breaker, you have the choices of 18, 28 and 37 kA (500, 750, and 1 000 MVA, even though the "MVA classes" are supposedly obsolete). These are the values to use for starters. They then apply the "K" factor to get the values in column 11. Then they take these values times 1.6 to get the value in column 13. The 1.6 comes from an equation once you plug in an X/R ratio of 25 and a time of 0.5 cycles. You have to find out your actual "K" factor based on the rated maximum voltage (15 kV) divided by your expected maximum system voltage (for example, maybe 13.8 kV x 1.1), which is Note 5 to the 1987 table. You can't use the "maximum" values from column 11 because you will not be applying the breakers at 11.5 kV (15 kV / max K). Column 6 is tricky because it is the ampere values at maximum voltage, so these are actually the minimum currents. Your current rating will be slightly higher if your breakers will never see 15 kV. The currents given in columns 11 and 13 are maximum values, so they represent the minimum allowable voltage of column 10.

There is a real interesting Note 9 to the 1979 table which states that "The current values in this column (13) are independent of operating voltage up to and including the maximum voltage". This tells me that the peak value in the chart does not need to be adjusted for the "K" factor, only the rms value needs to be adjusted. This same note on voltage insensitivity appears in the 1987 table as note 4d.

The 1987 standard changed the right hand column of this table, besides changing the table from Table 2 to Table 1. Instead of "1.6 K times rated short-circuit current in kA rms", it is "2.7 K times rated short circuit current, kA crest". This would just seem to be a simple change in definition, and you would think that it came from converting an rms value to crest value (peak), which would be a factor of the square root of 2. The problem is that 1.6 times the square root of 2 does not give you 2.7, but 2.26.

The only way this gets explained is to look in C37.09-1979, "Test Procedures for Circuit Breakers Rated on a Symmetrical Current Basis". Starting with Section there are a bunch of funky curves which allow manufacturers to convert their asymmetrical test oscillograms to an equivalent rms value. Section 7.1.5 has a table which gives the ratios of peak to rms for 1 00% - 0% asymmetry. A value of 0% asymmetry = 1.41, or the square root of 2; 50% asymmetry = 1.73; and 1 00% asymmetry = 1.63. It ends up that all the values in the table are relatively close, and that you can use a value of 1.69 without introducing a significant amount of error. 1.69 times 1.6 = 2.7, the new value in the chart. Note 10 to the 1979 table says "if currents are to be expressed in peak amperes, multiply values in this column by a factor of 1.69 which is a ratio of 2.7/1.6." This note by itself isn't much help in understanding the big picture since it doesn't explain where the heck the 1.69 comes from.

From the 1979 standard for 15 kV breakers, the ratings are

18 kA rms, 37 kA rms
28 kA rms, 58 kA rms
37 kA rms, 77 kA rms

From the 1987 standard for 15 kV breakers, the ratings are

18 kA rms, 62 kA crest
28 kA rms, 97 kA crest
37 kA rms, 130 kA crest

So to tell if you breaker interrupting rating is OK, you need to compare your calculated rms fault current to the adjusted rms rating of the breaker based on the actual "K" factor.

To tell if your breaker momentary rating is OK, what you need to do is take your calculated rms fault current value and multiply it by 2.7. If you're using a computer program, many will multiply the rms value by 1.6 to get the momentary asymmetrical value. Therefore first divide the given asymmetrical number by 1.6 and then multiply by 2.7. That's the value you can use to compare to the "momentary peak breaker rating" of either 62, 97 or 130 kA.

A comparison of ANSI and IEC ratings will be coming soon.