Rated Voltage
The breaker's rated voltage must be at or above the system's nominal voltage, with enough margin for normal system voltage variation. This isn't just about the breaker surviving continuous operation — insulation clearances, creepage distances, and the vacuum interrupter's dielectric withstand are all designed around this rating, and it directly affects how the breaker performs during a fault, not just in steady-state operation.
Rated Current
Rated (continuous) current must exceed the maximum continuous load current the breaker will carry, including any planned future load growth — undersizing here means nuisance heating and accelerated contact wear even without a fault ever occurring. This is a straightforward load-based calculation, but it's worth building in reasonable headroom rather than specifying to the exact present-day load.
Rated Breaking (Interrupting) Capacity
This is the rating most often gotten wrong, because it depends on the system's available fault current, not the load. The breaker's rated short-circuit breaking capacity must exceed the maximum fault current the system can deliver at that point — determined by a proper fault study, not an estimate. A breaker sized correctly for continuous load current but undersized for breaking capacity can fail catastrophically when it actually needs to interrupt a real fault, which is the one moment its entire purpose depends on.
Other Ratings Worth Checking
- Rated short-time withstand current — how much fault current the breaker can carry (without necessarily interrupting) for a defined short duration, relevant to protection coordination schemes with time delays.
- Making capacity — the peak current the breaker can safely close onto, relevant if there's a chance the breaker could be closed onto an existing fault.
- Mechanical and electrical endurance class — how many operations the breaker is rated for before major maintenance, which matters more for breakers with frequent switching duty than for ones that operate rarely.
Practical Advice
All of this should come from an up-to-date system study (load flow and fault analysis), not from matching the rating on the breaker being replaced — system fault levels change over time as networks are reconfigured, transformers are upgraded, or generation sources change, and a like-for-like replacement can quietly leave a system under-protected if nobody re-checks the numbers.
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