For decades, users and specifiers of metal-enclosed power circuit breaker switchgear have used the phrase “bus bracing” to define a specific level of mechanical strength for the bus system of the switchgear. However, this term has not historically been used in any of the standards that contain the major requirements for metal-enclosed switchgear. Further, no specific rating for bus bracing is specified in the standards. This problem with ill-defined terms has been confusing for years. The purpose of this issue of TechTopics is to discuss the requirements for switchgear buses and to define what is meant when the term “bus bracing” is used.
The principle standard applicable to metal-enclosed power circuit breaker switchgear is ANSI/IEEE C37.20.1-2002. As it relates to the bus ratings, this standard defines several ratings:
- Rated short-circuit withstand current – This is the symmetrical current that the switchgear bus must withstand for a test duration of at least four cycles. During the test to verify this rating, the peak current must be at least 230 percent (for unfused circuit breakers) or 216 percent (for fused circuit breakers) of the smallest frame size circuit breaker used in the assembly.
- Rated peak-withstand current – This is not strictly a rating, but it is the peak current value (230 percent or 216 percent) that must be attained during the rated shortcircuit withstand current test described above.
- Rated short-time withstand current – This is the average symmetrical current that the switchgear must be able to withstand for two periods of 0.5 seconds, separated by a 15-second interval of zero current.
How do the above ratings relate to the ratings of the components used in the switchgear assembly? As one would expect, the ratings directly correlate to produce a coordinated system:
- Rated peak-withstand current – Historically, the standards have not stated the peak current-withstand rating as an explicit rating. Instead, this performance requirement has been embedded in the testing requirements for the circuit breakers and the switchgear, so that the user would not have be concerned with it. The peak withstand current required of the switchgear corresponds directly to the prospective peak current for the circuit breaker when it is tested in accordance with ANSI C37.50 (reference clause 126.96.36.199.3 for 230 percent peak current for unfused circuit breakers, and 188.8.131.52.4 for 216 percent peak requirement for fused circuit breakers).
- Rated short-time current – The switchgear requirement corresponds directly to the short-time test requirement for unfused circuit breakers in ANSI C37.50 (reference clause 4.10.4). Short-time ratings are not applicable to fused circuit breakers as the fuse will not allow the current to flow for the duration of a short-time test.
What are the equivalent ratings?
Finally, the “bus bracing” is most commonly considered to be short-circuit current rating of the switchgear, expressed as a symmetrical current. However, some users may wish to determine the equivalent values in other terms. The equivalent values are:
The values for the asymmetrical current conversion shown in the fourth column are from note 5 of ANSI/IEEE C37.20.1-2002 clause 5.4.4. The corresponding values for the X/R ratio and power factor (PF) are shown in the fifth column of the table above.
The difference between the peak current requirements with unfused circuit breakers (230%) and fused circuit breakers (216 percent) is due to the differing X/R requirements in UL standards (UL 248 series) for current limiting fuses as compared to those of unfused equipment in ANSI C37.50 and ANSI/IEEE C37.20.1.
Regardless of how the bus bracing requirement may be stated, the same performance is required of the switchgear buses and connections.
The required short-circuit withstand current for the switchgear assembly depends on the assortment of circuit breakers included in the assembly. The requirement is that the short-circuit capability of the switchgear assembly is determined by the short-circuit capability of the “poorest” circuit breaker included in the assembly (which is the “weak link”).
In more formal terms:
- For switchgear assemblies with no fused circuit breakers, the rated short-circuit withstand current is the short-circuit current rating of the circuit breaker with the lowest shortcircuit current rating in the assembly.
- For switchgear assemblies with only fused circuit breakers, the rated short-circuit withstand current is the short-circuit current rating of the fused circuit breakers used in the assembly.
- For switchgear assemblies with both fused and unfused circuit breakers, the rated short-circuit withstand current is the short-circuit current rating of the unfused circuit breaker with the lowest short-circuit current rating in the assembly.
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