- Circuit Breaker Definition: A circuit breaker is defined as a device designed to protect an electrical circuit from damage caused by overcurrent or short circuits by interrupting the flow of electricity.
- Short Circuit Breaking Current: This is the maximum current a circuit breaker can handle during a short circuit, up to the point of clearing the fault, without suffering permanent damage.
- Short Circuit Making Capacity: Expressed in peak value, this is the highest current a circuit breaker can handle immediately after a fault occurs.
- Rated Operating Sequence: This refers to the specific sequence of operations (opening and closing) that a circuit breaker can perform under predefined conditions.
- Thermal and Mechanical Stresses: Circuit breakers must withstand significant thermal and mechanical stresses during a short circuit, which can affect their performance and durability.
For short-circuit duty, the rating of a circuit breaker includes:
- Rated short circuit breaking current.
- Rated short circuit making current.
- Rated operating sequence of circuit breaker.
- Rated short time current.
Short Circuit Breaking Current of Circuit Breaker
Rated short-circuit breaking current is the prospective current that a circuit breaker can interrupt under the voltage, frequency, asymmetry, transient recovery voltage and operating-sequence conditions specified by its standard.
Before interruption, short-circuit current creates heat in the conducting path and electromagnetic forces in contacts, conductors, supports and terminals. The breaker must carry the current until contact separation and interrupt it without damage that prevents the required subsequent operation.
Joule’s law of heating explains why thermal energy increases with current squared, circuit resistance and time. This I²t effect is one part of the duty. The peak asymmetrical current produces a separate mechanical-force duty.
Breaking capability also depends on the arc-interruption process and the transient recovery voltage across the opening contacts. It cannot be derived from a contact-temperature limit alone. For an electrical circuit breaker, the former claim that 160oC is a universal aluminium limit does not represent a standard rating.
The rated breaking current therefore states an interruption capability, not merely the current the closed breaker can survive before another device clears the fault.
Under the IEC convention, the rated short-circuit breaking current includes the RMS value of its AC component and the permitted DC component at contact separation. IEEE-rated equipment uses its applicable symmetrical-current and asymmetry rules. Use the same standard basis as the fault study and nameplate.
Mechanical forces are highest near the first asymmetrical peak, while interruption occurs later at contact separation. Both duties must remain within the breaker’s tested ratings, including terminal forces and support strength.
Breaker selection starts with the maximum prospective fault current at the installation point. It also checks the system X/R ratio, DC component, TRV, rated maximum voltage, continuous current, making duty, short-time withstand duration, operating sequence and any capacitor, reactor, out-of-phase or short-line-fault duty. Matching only one fault-current number is insufficient.
Rated Short Circuit Making Capacity
Rated short-circuit making current is the peak current a breaker can make when it closes onto a fault and then carry in the closed position. Rated breaking current is normally stated on an RMS value basis, so the two nameplate values must not be compared without their units and standard definitions.
Closing can occur at the point on the voltage wave that produces the largest DC offset. The first major peak of current can then be much higher than the symmetrical RMS fault current. The circuit breaker mechanism must latch, and its contacts and conducting structure must withstand the resulting electromagnetic force.
The making rating covers this close-and-latch duty. It does not mean the breaker is expected to interrupt at the first current peak. Opening time and the specified operating sequence determine when interruption follows.
IEC applications often use a making-current factor of 2.5 times the rated short-circuit current at 50 Hz and 2.6 at 60 Hz. A longer specified system time constant can require another factor. These values are standard and application dependent, not one universal 2.5 rule. The same electrical duty applies whether the operating command is local or uses a remote control circuit breaker scheme.
Rated Operating Sequence or Duty Cycle of Circuit Breaker
The rated operating sequence states the open and close-open operations that the breaker has demonstrated with defined recovery intervals. It covers the mechanism, interrupting units and stored-energy system under the specified test duty.
O denotes one opening operation.
CO denotes a closing operation followed by an opening operation with no intentional delay. The breaker’s inherent relay, release and mechanism times still apply.
The symbols t and t’ denote the stated time intervals between operations. They allow recovery of the operating mechanism and interrupting medium as required by the rated sequence. Their values are part of the sequence, not a universal thermal rule.
For a breaker rated for rapid auto-reclosing, one sequence can use t = 0.3 seconds. Other standard or product sequences use three minutes, 15 seconds or another declared interval. The intended protection and reclosing scheme must match the exact nameplate sequence.
For example, the rated sequence shown below represents a specific rapid-reclosing duty:
Read the symbols in order. The breaker opens, waits 0.3 seconds, performs a close-open operation, waits three minutes and performs another close-open operation. A CO operation does not mean zero physical operating time; it means there is no intentional delay between the closing and opening commands in that duty.
Rated Short Time Current
Rated short-time withstand current is the RMS current that a closed circuit breaker can carry for its rated short-circuit duration under specified conditions. The current and duration form one rating, such as 25 kA for one second or 40 kA for three seconds.
Fault-clearing time includes detection and operation of protection relays, breaker opening time and any intentional coordination delay in the power system protection scheme. Backup protection can expose an upstream closed breaker or switchgear section to current for longer than the primary clearing time.
The required duration must cover the maximum credible clearing time, including the specified backup case. One second is a common IEC standard value, with 0.5, 2 and 3 seconds also used. Some products declare four seconds. The page’s former rule that every breaker must carry fault current for at least three seconds is incorrect.
Short-circuit current creates two main withstand duties inside the closed breaker.
- I²t heating creates thermal stress in insulation, contacts and conducting parts.
- Peak current creates electromagnetic force in conductors, supports and contact systems.
The breaker must meet its rated short-time current and rated peak withstand current without unacceptable damage. On many products the RMS short-time withstand current equals the rated short-circuit breaking current, but this equality must be confirmed from the applicable standard and nameplate.
Rated Voltage of Circuit Breaker
Rated voltage is selected from the applicable circuit-breaker standard so the rating is not below the maximum system voltage at the installation. The former rule of adding 10% below 400 kV and 5% at or above 400 kV is not a general selection method.
Rated voltage also establishes or accompanies dielectric, TRV and switching duties. Normal operating variation in the power system is handled through the defined maximum system voltage and service conditions, rather than an informal percentage allowance.
The insulation rating includes separate withstand tests for the relevant voltage stresses and insulation paths.
- Rated short-duration power-frequency withstand voltage applies for the duration and test arrangement stated in the standard, often one minute. It checks insulation phase-to-earth, between phases and across open contacts as applicable.
The one-minute laboratory test is not a claim that all service-frequency overvoltages clear within one minute.
Temporary overvoltage protection and breaker insulation selection require their own system study. - A breaker connected to a power system can experience lightning and switching impulses.
Rated lightning-impulse and switching-impulse withstand values are specified crest test voltages. They must be checked for the applicable insulation path, equipment standard, altitude and service conditions. The incomplete table below has no cited standard edition or equipment scope, so it is retained only as legacy reference data and must not be used for procurement.
| Legacy Nominal System Voltage | Legacy Highest System Voltage | Legacy Power-Frequency Withstand | Legacy Impulse Level |
| 11 kV | 12 kV | Not listed | Not listed |
| 33 kV | 36 kV | 70 kV | 170 kV |
| 132 kV | 145 kV | 275 kV | 650 kV |
| 220 kV | 245 kV | 460 kV | 1050 kV |
| 400 kV | 420 kV | Not listed | Not listed |
