Continued from first part:Â MV Circuit Breaker Or Vacuum Contactor? (Part 1)
When applied properly, both medium-voltage circuit breakers and medium-voltage fused contactors should provide decades of reliable service. Applied incorrectly, either can lead to major headaches.
Comparison Table
| Characteristic | Circuit Breaker | Contactor (NEMA E2 with fuses) |
| Continuous current | High (1200 A, 2000 A, 3000 A or 4000 A) | Moderate (400 A enclosed – NEMA size H3, or 720 A enclosed – NEMA size H6) |
| Switching capability | Switch currents from very low (magnetizing) values to full system short-circuit current | Switch currents from very low (magnetizing) values to interrupting capability of vacuum contactor without fuses (at least 10 x Continuous rating) Fuses operate for currents higher than the interrupting capability of the vacuum contactor alone, up to the interrupting capacity of the fuse |
| Endurance – Mechanical | High(typically 10,000 operations) (refer to ANSI/IEEE C37.06) | Very high, 750,000 operations for 400 A and 400,000 for 720A |
| Endurance – Electrical | High For vacuum, typically 10,000 operations at rated continuous current For vacuum, typically 30 to 100 operations at full short-circuit rating | Very high Switching continuous current, 400,000 operations for 400A or 200,000 operations for 720 A Switching short- circuit current, endurance data not established in NEMA or UL standards; short-circuit current interruption requires replacement of current-limiting fuses |
| Application limitations | Not appropriate for very high endurance applications | Well suited for very frequent switching operations |
| Operation | Electrically operated (manual operation for maintenance or emergency) | Electrically operated only |
| Control scheme | Mechanically latched – circuit breaker remains closed on loss of system voltage | Usually magnetically held – vacuum contactor opens on loss of system voltage; vacuum contactor will close automatically on system voltage return with two-wire control; manual restart required on system voltage return with three-wire control Latched contactors are available |
| Overcurrent/ short-circuit protection | Requires protective relays | Requires protective relays for overload protection and current- limiting fuses for short-circuit protection |
| Short-circuit let-through energy | High (three to five cycles or more of short-circuit current) | Low (current-limiting fuses interrupt in 1/4 cycle for highest short-circuit currents, and peak magnitude is limited) |
| Remote operation | Well suited | Well suited |
| Control power | Control power needed for protective relays, circuit breaker operation and space heaters (if present) | Control power usually provided by control power transformer (CPT) incorporated in the controller |
| Construction | Draw out, if metal-clad (ANSI/IEEE C 37.20.2) Stationary, if metal-enclosed (ANSI/IEEE C37.20.3) | Draw out or stationary |
| Space requirements | Larger enclosure; NEC® required workspace equal | Smaller enclosure; NEC® required workspace equal; Rear access not required |
| Purchase cost | Relatively high | Moderate |
| Maintenance | Medium (long maintenance intervals, need to clean insulation) | Low (simple mechanism, need to clean insulation, replace fuses) |
Reference: SIEMENS Tech articles
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Mohan Krishnasamy
Electrical Design Engineer having +3 Years Experience( Voltech Manufacturing Co., Ltd - Switchgear Division) in Medium Voltage & Low Voltage Switchgear Field. Freelancer Programmer Of Panel Design Calculation For Mechanical & Electrical As per IS,IEC StandardsProfile: Mohan Krishnasamy
What is the Normal KAic Rating of vacuum contactor rated capacity 400A with aux.voltage 120vac at 7.2kv ?
Looks pretty neat