- Instantaneous Magnetic Trip-Only Circuit Breakers
As the name indicates, instantaneous magnetic-trip-only circuit breakers provide short circuit protection but do not provide overload protection. This type of circuit breaker is typically used in motor control applications where overload protection is provided by an the overload relay.
- Thermal-Magnetic Circuit Breakers
As described previously, a thermal-magnetic circuit breaker has a trip unit that senses heat to detect an overload and senses a magnetic field generated by current to detect a short circuit. This type of circuit breaker trips immediately when a short circuit occurs, but delays an appropriate amount of time before tripping in the event of an overload.
- Interchangeable Trip Circuit Breakers
The user cannot change the trip unit in many circuit breakers, but some circuit breakers have an interchangeable trip unit. This feature allows the user to change the continuous current rating of the breaker without replacing the breaker. This is done by replacing the trip unit with one of a different rating.
- Current Limiting Circuit Breakers
Many electrical power distribution systems can deliver large short circuit currents to electrical equipment. This high current can cause extensive damage. Current limiting circuit breakers protect equipment by significantly reducing the current flowing in the faulted circuit.
One way to accomplish current limiting is with an additional set of contacts that feature two moveable arms. These dual-pivot contacts separate even more quickly than the single-pivot contacts. The dual-pivot contacts are connected in series with the single-pivot contacts. As with the single-pivot design, current flows in opposite directions through the contact arms, creating a magnetic repulsion. As current increases, the magnetic repulsion force increases.
In an overload condition, where current may only be one to six times normal current, the dual-pivot contacts remain closed until the breaker trips. However, when a short circuit occurs, fault current is extremely high and both sets of contact arms open simultaneously, generating high impedance arcs. The contact gap of the dual-pivot contacts increases more rapidly, which increases arc impedance more rapidly. Once the arcs are extinguished, the dual-pivot contacts close due to spring tension. The single-pivot contacts are held open by the breaker mechanism, which tripped during the fault and must be manually reset.
The frame on current limiting circuit breakers of this design is extended to allow room for the dual-pivot set of contacts. Siemens current limiting breakers can handle fault currents of up to 200,000 amps.
Single.
- Solid-State Circuit Breakers
The brain of a solid-state trip unit is a microprocessor. Adjustments on the trip unit set numerical values that the microprocessor uses in performing protective functions. Current sensors connected to the trip unit monitor the load current. The trip unit continuously compares this current to trip unit settings. When current exceeds a preset value for the selected time, the trip unit triggers a magnetic latch. The magnetic latch opens the breaker’s contacts, disconnecting the protected circuit from the power source.
