What are the working principles and selection criteria for low-voltage circuit breakers?

What is a Low-Voltage Circuit Breaker?

A low-voltage circuit breaker (formerly known as an automatic switch) is a switching device that can connect and disconnect not only normal load current and overload current, but also short-circuit current. In addition to its control function, a low-voltage circuit breaker also provides certain protection functions in a circuit, such as overload, short-circuit, undervoltage, and leakage current protection.

Low-voltage circuit breakers can be classified in many ways. By application type, they can be selective (with adjustable protection device parameters) and non-selective (with non-adjustable protection device parameters). By arc-extinguishing medium, they can be air-cooled or vacuum-cooled (currently, most domestically produced ones are air-cooled).

Low-voltage circuit breakers have a wide capacity range, from a minimum of 4A to a maximum of 5000A. They are widely used in low-voltage power distribution systems at all levels of feeders, power control of various mechanical equipment, and control and protection of electrical terminals.

Low-voltage circuit breaker characteristics and technical parameters

 

my country's low-voltage electrical appliance standards stipulate that low-voltage circuit breakers should have the following characteristic parameters:

(1) Type:

Circuit breaker type includes number of phases, number of poles, rated frequency, arc-extinguishing medium, closing method, and breaking method.

(2) Main circuit ratings:

Main circuit ratings include:

①Rated operating voltage; ②Rated current; ③Rated short-time making capacity; ④Rated short-time withstand current.

The rated current of universal circuit breakers is further divided into the rated current of the main circuit and the rated current of the frame level.

(3) Rated duty cycle:

The rated duty cycle of circuit breakers can be divided into two types: 8-hour duty cycle and continuous duty cycle.

(4) Auxiliary Circuit Parameters:

The auxiliary circuit parameters of a circuit breaker mainly consist of the characteristics of its auxiliary contacts. Universal circuit breakers generally have three pairs each of normally open and normally closed contacts for signaling devices and control circuits; molded case circuit breakers generally do not have auxiliary contacts.

In addition to the above items, the characteristic parameters of a circuit breaker also include: the type and characteristics of the trip unit, and its application category.

 

Working Principle of Low-Voltage Circuit Breakers

 

Low-voltage circuit breakers offer significant advantages over knife switches in household applications: First, they can interrupt larger currents and have arc-extinguishing devices, making them ideal for increasing household circuit loads. Furthermore, their thermal stability and lifespan are better than knife switches. Second, they automatically disconnect the circuit in the event of a fault. Internally, the circuit breaker contains two trip units: a bimetallic thermal trip unit for overload delay protection and an electromagnetic trip unit for short-circuit instantaneous protection.

In household circuits, fuses are no longer installed after using circuit breakers. In the event of a fault, the trip unit automatically trips (commonly known as breaker tripping) to disconnect the circuit. After the fault is cleared, the circuit breaker can be reused simply by resetting it; no parts need to be replaced.

A circuit breaker has three basic components: contacts and arc-extinguishing system, trip unit, and transmission and operating mechanism. Its working principle can be illustrated by the attached diagram. In the diagram, the contacts are closed, and the latch locks this closed state with a hook to stabilize it. In the event of an overload, the overload trip unit operates. A small multiple of the overload current, after a certain period, causes the bimetallic strip to bend to a certain extent. At this point, the screw (adjustable) at one end pushes the connecting rod upwards, causing tripping and instability, allowing the contacts to separate, thus achieving overload delay protection. In the event of a short circuit, the short circuit trip unit operates. A large multiple of the short circuit current, through the magnetic force generated by the electromagnet, rapidly attracts the armature, causing it to move downwards. The screw (adjustable) at the other end pushes the connecting rod upwards, causing tripping and instability, rapidly separating the contacts, thus achieving instantaneous short circuit protection.

 

Five Steps to Correctly Select a Low-Voltage Circuit Breaker

 

(1) Determine the rated current of the circuit breaker based on the calculated current of the line.

(2) The short-circuit setting current of the circuit breaker should exceed the normal operating starting current of the line.

(3) Verify the breaking capacity of the low-voltage circuit breaker based on the maximum short-circuit current of the line.

(4) Verify the sensitivity of the circuit breaker's operation based on the minimum short-circuit current of the line; that is, the minimum short-circuit current of the line should not be less than 1.3 times the short-circuit setting current of the circuit breaker.

(5) Verify the rated short-circuit making capacity (maximum expected peak current) of the circuit breaker based on the short-circuit impulse current (i.e., the maximum instantaneous value of the total short-circuit current); that is, the latter should be greater than the former.

 

Wiring Methods for Low-Voltage Circuit Breakers

 

Circuit breakers can be wired in several ways: front-panel, rear-panel, plug-in, and drawer-type. Unless otherwise specified by the user, front-panel wiring is the most common method.

(1) Rear-panel wiring: The biggest advantage of rear-panel wiring is that it eliminates the need for rewiring when replacing or repairing the circuit breaker; only the upstream power supply needs to be disconnected. Due to its unique structure, the product is shipped with a dedicated mounting plate, mounting screws, and wiring screws as per design requirements. It is crucial to note that the reliability of the contacts in a high-capacity circuit breaker directly affects its normal operation; therefore, installation must be carefully performed according to the manufacturer's specifications.

(2) Plug-in wiring: A circuit breaker mounting base is installed on the mounting plate of the complete unit. The mounting base has six plugs, and the circuit breaker's connection plate has six sockets. The mounting base has a connection plate on its surface or bolts on its rear. The mounting base is pre-connected with power and load lines. During use, the circuit breaker is directly plugged into the mounting base. If a circuit breaker fails, simply remove the faulty one and replace it with a good one. This replacement time is shorter and more convenient than front-panel or back-panel wiring. Since plugging and unplugging require some manpower, the current rating of plug-in circuit breakers in my country is currently limited to a maximum of 400A. This saves maintenance and replacement time. When installing a plug-in circuit breaker, check that the plug is securely pressed and tighten it securely to reduce contact resistance and improve reliability.

(3) Drawer-type wiring: The circuit breaker's drawer is accessed by rotating a rocker arm clockwise or counterclockwise. A plug-in structure is used in both the main and secondary circuits, eliminating the isolator required for fixed circuits, achieving dual functionality, improving economic efficiency, and greatly facilitating operation and maintenance, while increasing safety and reliability. In particular, the main circuit contact base of the drawer base is compatible with the NT-type fuse contact base, allowing direct insertion of a fuse for power supply in emergency situations.

 

Conclusion

 

Since shunt trip units, undervoltage trip units, electric operating mechanisms, and interlocking electromagnets have different voltage levels and AC/DC power supplies, users should specify this when ordering. Furthermore, users cannot use a single accessory. If electrical interlocking of two circuit breakers is required (when one is closed, the other must be open), auxiliary contacts and shunt trip units or electric operating mechanisms can be selected. When wiring before and after the circuit breaker, screws must be tightened securely to prevent damage to the circuit breaker.

 

Shaanxi Huadian low-voltage circuit breakers feature overload, short-circuit, and leakage fault type indication for rapid troubleshooting and reduced downtime. Utilizing high-precision trip units and silver alloy contacts, they offer enhanced breaking capacity, extend electrical life by 30%, and eliminate false tripping and failure to trip. Contact us:pannie@hdswitchgear.com.