Types of Circuit Breakers

Key Parameters of Circuit Breakers

(1) Rated Operating Voltage (Ue)

The rated operating voltage is the nominal voltage at which the circuit breaker can operate continuously under specified normal use and performance conditions.

In China, for voltage levels of 220kV and below, the highest operating voltage is 1.15 times the system rated voltage; for 330kV and above, it is 1.1 times the rated voltage. Circuit breakers must maintain insulation at the system’s highest operating voltage and operate under specified conditions.

(2) Rated Current (In)

The rated current is the current that the trip unit can continuously carry at an ambient temperature of 40°C or lower. For circuit breakers with adjustable trip units, it is the maximum current the trip unit can carry continuously.

When used at ambient temperatures above 40°C but not exceeding 60°C, the load can be reduced for continuous operation.

(3) Overload Trip Unit Current Setting (Ir)

When the current exceeds the trip unit current setting (Ir), the circuit breaker trips after a delay. It also represents the maximum current the circuit breaker can withstand without tripping. This value must be greater than the maximum load current (Ib) but less than the maximum current allowed by the circuit (Iz).

Thermal trip units typically adjust within 0.7-1.0In, while electronic devices offer a broader range, usually 0.4-1.0In. For non-adjustable overcurrent trip units, Ir=In.

(4) Short-Circuit Trip Unit Current Setting (Im)

Short-circuit trip units (instantaneous or short-time delay) trip the circuit breaker quickly when high fault currents occur. The trip threshold is Im.

(5) Rated Short-Time Withstand Current (Icw)

This is the current value the circuit breaker can carry for a specified time without causing conductor damage due to overheating.

(6) Breaking Capacity

The breaking capacity is the circuit breaker’s ability to safely interrupt fault currents, regardless of its rated current. Current specifications include 36KA, 50KA, etc. It is generally divided into ultimate short-circuit breaking capacity (Icu) and service short-circuit breaking capacity (Ics).

General Principles for Selecting Circuit Breakers

Firstly, choose the circuit breaker type and poles based on its application. Select the rated current based on the maximum working current. Choose the type of trip unit, accessories, and specifications as needed. Specific requirements include:

1. The circuit breaker’s rated operating voltage should be ≥ the line’s rated voltage.
2. The rated short-circuit breaking capacity should be ≥ the calculated load current of the line.
3. The rated short-circuit breaking capacity should be ≥ the maximum short-circuit current that may occur in the line (generally calculated as RMS).
4. The single-phase ground fault current at the line’s end should be ≥ 1.25 times the instantaneous (or short-time delay) trip current setting of the circuit breaker.
5. The undervoltage trip unit’s rated voltage should equal the line’s rated voltage.
6. The shunt trip unit’s rated voltage should equal the control power supply voltage.
7. The motor drive mechanism’s rated operating voltage should equal the control power supply voltage.
8. For lighting circuits, the instantaneous trip unit setting current is generally six times the load current.
9. For a single motor’s short-circuit protection, the instantaneous trip unit setting current should be 1.35 times (DW series) or 1.7 times (DZ series) the motor’s starting current.
10. For multiple motors’ short-circuit protection, the instantaneous trip unit setting current should be 1.3 times the largest motor’s starting current plus the working current of the other motors.
11. When used as the low-voltage side main switch for distribution transformers, the circuit breaker’s breaking capacity should exceed the short-circuit current on the transformer’s low-voltage side, the trip unit’s rated current should not be less than the transformer’s rated current, and the short-circuit protection setting current should generally be 6-10 times the transformer’s rated current. The overload protection setting current should equal the transformer’s rated current.
12. After initially selecting the circuit breaker type and rating, coordinate the protection characteristics with the upstream and downstream switches to prevent over-tripping and expand the fault range.

Circuit Breaker Selectivity

In distribution systems, circuit breakers are classified based on their protection performance into selective and non-selective types. Selective low-voltage circuit breakers offer two-stage and three-stage protection. Instantaneous and short-time delay characteristics suit short-circuit action, while long-time delay characteristics suit overload protection. Non-selective circuit breakers generally act instantaneously, providing only short-circuit protection, though some have long-time delay for overload protection. In distribution systems, if the upstream circuit breaker is selective, and the downstream breaker is non-selective or selective, the short-time delay trip unit’s delayed action or different delay times ensure selectivity.

When using an upstream selective circuit breaker, consider:

1. Whether the downstream breaker is selective or non-selective, the upstream circuit breaker’s instantaneous overcurrent trip setting should generally be no less than 1.1 times the downstream breaker outlet’s maximum three-phase short-circuit current.
2. If the downstream breaker is non-selective, prevent the upstream short-time delay overcurrent trip unit from acting first when a short-circuit current occurs in the downstream protected circuit due to insufficient instantaneous action sensitivity. The upstream breaker’s short-time delay overcurrent trip unit’s setting current should be no less than 1.2 times the downstream instantaneous overcurrent trip unit’s setting.
3. If the downstream breaker is also selective, ensure selectivity by setting the upstream breaker’s short-time delay action time at least 0.1s longer than the downstream breaker’s short-time delay action time. Generally, to ensure selective action between upstream and downstream low-voltage circuit breakers, the upstream breaker should have a short-time delay overcurrent trip unit, and its action current should be at least one level higher than the downstream trip unit’s action current, ensuring Iop.1 ≥ 1.2Iop.2.

Cascading Protection of Circuit Breakers

In distribution system design, ensuring selective coordination between upstream and downstream circuit breakers involves “selectivity, speed, and sensitivity.”

Selectivity relates to the coordination between upstream and downstream breakers, while speed and sensitivity depend on the protective device’s characteristics and the line’s operating mode.

Proper coordination between upstream and downstream breakers selectively isolates the fault circuit, ensuring other non-fault circuits in the distribution system continue operating normally. Improper coordination Types of Circuit Breakers