Its basic function is to interrupt current flow after a fault is detected. These typically require a separate power source, such as a , although some high-voltage circuit breakers are self-contained with , , and an internal control power source. Three-pole common trip breakers are typically used to supply to large motors or further distribution boards. The maximum current value and let-through energy determine the quality of the circuit breakers. Typically, the heating or magnetic effects of electric current are employed. Trip characteristics are often fully adjustable including configurable trip thresholds and delays.
These are used on overhead systems, to prevent short duration faults from causing sustained outages. These may form part of an system for standby power. Its purpose was to protect lighting circuit wiring from accidental short circuits and overloads. With very large over-currents during a short-circuit, the magnetic element trips the circuit breaker with no intentional additional delay. Medium-voltage circuit breakers in switchgear line-ups are often built with draw-out construction, allowing breaker removal without disturbing power circuit connections, using a motor-operated or hand-cranked mechanism to separate the breaker from its enclosure. This breaker has a 2 A rating. A spring restrains the core until the current exceeds the breaker rating.
Interconnection of multiple generator sources into an electrical grid required the development of circuit breakers with increasing voltage ratings and increased ability to safely interrupt the increasing short-circuit currents produced by networks. Short-circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Small circuit breakers typically have a manual control lever to switch off the load or reset a tripped breaker, while larger units use to trip the mechanism, and to restore energy to the springs. High-voltage breakers are nearly always -operated, with current sensing operated through. Medium-voltage circuit breakers nearly always use separate current sensors and , instead of relying on built-in thermal or magnetic overcurrent sensors. The current-carrying parts near the contacts provide easy deflection of the arc into the arc chutes by a magnetic force of a current path, although coils or could also deflect the arc into the arc chute used on circuit breakers for higher ratings.
When supplying a branch circuit with more than one live conductor, each live conductor must be protected by a breaker pole. Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers up to about 40. Trip current may be adjustable in larger ratings. They are no longer seen in new installations as they cannot detect any dangerous condition where the current is returning to earth by another route - such as via a person on the ground or via plumbing. A direct current circuit breaker has blow-out coils that generate a magnetic field that rapidly stretches the arc.
During an overload, the speed of the solenoid motion is restricted by the fluid. Under short-circuit conditions, the calculated or measured maximum may be many times the normal, rated current of the circuit. Large low-voltage molded case and power circuit breakers may have electric motor operators so they can open and close under remote control. Power Circuit Breaker Theory and Design Second ed. Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit. The number of plates in the arc chute is dependent on the short-circuit rating and nominal voltage of the circuit breaker.
As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch, which lets the contacts open by spring action. This allows short current spikes such as are produced when a motor or other non-resistive load is switched on. The circuit breaker must first detect a fault condition. Small circuit breakers are either installed directly in equipment, or are arranged in a. These circuit breakers are often installed in draw-out enclosures that allow removal and interchange without dismantling the switchgear. Utility companies in the United States have been reviewing use of the technology to turn on and off appliances, as well as potentially turning off charging of electric cars during periods of high electrical grid load.
They are most often used for transmission-level voltages and may be incorporated into compact gas-insulated switchgear. Z Above 2 I n up to and including 3 I n for periods in the order of tens of seconds. Electrical accessories — Circuit breakers for over-current protection for household and similar installations. Electric Power Systems Second ed. The definition of high voltage varies but in power transmission work is usually thought to be 72.
When electrical contacts open to interrupt a large current, there is a tendency for an to form between the opened contacts, which would allow the current to continue. The length of the arc is generally proportional to the voltage while the intensity or heat is proportional to the current. Breakers illustrated above are in this category. Circuit breakers are usually able to terminate all current very quickly: typically the arc is extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending upon age and construction of the device. Vacuum circuit breakers have longer life expectancy between overhaul than do other circuit breakers.
Circuit breakers are also rated by the maximum fault current that they can interrupt; this allows use of more economical devices on systems unlikely to develop the high short-circuit current found on, for example, a large commercial building distribution system. . Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault condition and to operate the opening mechanism. The circuit breaker contacts are held closed by a latch. Many classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features.
Miniature circuit breakers have a fixed trip setting; changing the operating current value requires changing the whole circuit breaker. These devices under research and testing would have wireless capability to monitor the electrical in a house via a smartphone app or other means. Larger circuit breakers can have adjustable trip settings, allowing standardized elements to be applied but with a setting intended to improve protection. The circuit breaker is labeled with the rated current in , but excluding the unit symbol, A. Miniature and molded-case circuit breakers are usually discarded when the contacts have worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts. Hydraulic energy may be supplied by a pump, or stored in accumulators. Stotz's invention was the forerunner of the modern thermal-magnetic breaker commonly used in household load centers to this day.