Siemens is often asked to discuss the application considerations that favor the use of medium-voltage circuit breakers, and those that favor the use of medium-voltage, fused, load-interrupter switches. This is one of those age-old questions, so this issue of TechTopics is devoted to it.
To compare the application of medium-voltage circuit breakers and of load-interrupter switches, we must understand the basic characteristics of each switching technology.
The table on the next page shows the major characteristics of medium-voltage circuit breakers and load-interrupter switches that influence the application. Of course, the table entries are generalized, and the information varies by the voltage and current ratings of the equipment. This is particularly true for load-interrupter switches, where the endurance capabilities vary greatly according to the ratings. However, the table is valid for an overall understanding.
From the data in the table, Siemens makes these observations:
- Medium-voltage circuit breakers are favored when:
- Continuous load current is high
- Central monitoring/control is needed; remote control is necessary
- Switching is frequent; high endurance (1,000s of operations) is needed
- Process continuity is critical
- Loads are concentrated.
- Medium-voltage, fused, load-interrupter switches are favored when:
- Continuous load current is low
- Central monitoring/control is not needed; manual control is acceptable or electrical operation is infrequent
- Switching is infrequent; low endurance (150 to 500 operations, depending on rating) is acceptable
- Process continuity is not critical
- Loads are widely dispersed.
In industrial applications, a medium-voltage, fused, loadinterrupter switch is most frequently used as the primary disconnect for a load center substation transformer. A loadinterrupter switch is well suited to this application, as load currents are low, switching is infrequent and remote control is not ordinarily needed.
Medium-voltage, metal-clad switchgear with drawout circuit breakers is suitable for main power distribution in a large installation, with each feeder circuit breaker feeding several load center substations, each having a fused, load-interrupter switch on the primary of the transformer. The circuit breakers are suitable for switching high currents, and for control from a remote location. Since the circuit breakers can switch shortcircuit currents and be ready for immediate closing, they are suitable where process continuity is critical.
When applied and maintained properly, both medium-voltage circuit breakers and load-interrupter switches should provide decades of reliable service. Applied incorrectly, either can lead to major headaches.
Should you have any questions about this issue of TechTopics or any of our products, solutions, or services, please contact your local Siemens sales representative for more information.