Rated normal current:
The Rated normal current (sometimes referred to as Rated current, nominal current or rated continuous current) is the maximum continuous current the equipment is allowed to carry.
The current is expressed in A rms. (3150 Amp RMS)
The rated normal current is based on a maximum ambient temperature of +40 °C.
Rated short-time withstands current:
The Rated short-time withstand current is the maximum current (expressed in kA rms) which the equipment shall be able to carry in closed position for a specified time duration.
The rated short-time withstand current is equal to the rated short circuit breaking current.
Standard values for duration are 1 or 3 s.
Rated short-circuit breaking current:
The Rated short-circuit (breaking) current is the maximum symmetrical short-circuit current in kA rms, which a circuit breaker shall be capable of breaking. (50 kA)
Two values are related to the rated short-circuit current:
- The RMS value of the AC component
- The percentage DC component (depending on the minimum opening time of the circuit breaker and the time constant τ)
Rated short-circuit making current:
The Rated short-circuit making current is the maximum peak current the circuit breaker shall be able to close and latch against. This is also referred to in IEEE as closing and latching capability.
Rated short-circuit making current is equal to Rated peak withstand current.
The peak value is related to the rms value of the rated shortcircuit breaking current, frequency and time constant (τ). Specified values are:
- 2.5 x rated short-time withstand current at 50 Hz at τ = 45 ms (127 kA)
Line charging breaking current:
When the circuit breaker is closed to energize a section of the power line which could be 100 km long, there is a surge current to charge the capacitance of the power line.
This is called the charging current and it can be much larger than the regular load current.
If due to a fault of some sort, the circuit breaker is closed and then opened immediately, it has to switch this charging current off.
The maximum that the circuit breaker can safely switch off under these conditions is called the “charging breaking current”. (900 Amp)
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