What is a low voltage circuit breaker?

Low-voltage circuit breaker (formerly known as automatic switch) is a switching appliance that can not only connect and break normal load current and overload current, but also can connect and break short-circuit current. In addition to playing a control role in the circuit, low-voltage circuit breakers also have certain protection functions, such as overload, short circuit, undervoltage and leakage protection.

There are many classification methods for low-voltage circuit breakers. According to the use category, there are selective type (protection device parameters are adjustable) and non-selective type (protection device parameters are not adjustable). According to the arc extinguishing medium, there are air type and vacuum type (currently domestically produced Mostly air type).

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

(1) Characteristics and technical parameters of low-voltage circuit breaker

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

1) Type:

The circuit breaker type includes the number of phases, number of poles, rated frequency, arc extinguishing medium, closing method and breaking method.

2) Main circuit rating:

Main circuit ratings are:

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

The rated current of the universal circuit breaker is also divided into the rated current of the main circuit and the rated current of the frame level.

3) Rated working system:

The rated working system of the circuit breaker can be divided into two types: 8h working system and long-term working system.

4) Auxiliary circuit parameters:

The auxiliary circuit parameters of the circuit breaker are mainly the auxiliary contact characteristic parameters. Universal circuit breakers generally have three pairs of normally open contacts and three pairs of normally closed contacts for signaling devices and control circuits; plastic case circuit breakers generally do not have auxiliary contacts.

In addition to the above items, the circuit breaker characteristic parameters also include: tripper type and characteristics, usage category, etc.

(2) Working principle of low voltage circuit breaker

The introduction of low-voltage circuit breakers into homes has great advantages over blade switches: First, it can break larger currents and has an arc extinguishing device, which is suitable for situations where the load of household circuits increases. In addition, its thermal stability and lifespan are better than those of knife switches. Second, it can automatically cut off the circuit when a fault occurs. There are two releases inside the circuit breaker. One is a bimetallic thermal release, which is used for overload delay protection; the other is an electromagnetic release, which is used for short-circuit instantaneous protection.

In household circuits, fuses are no longer installed after using circuit breakers. When a fault occurs, the tripper automatically trips (commonly known as tripping) to cut off the circuit. After the fault is eliminated, it can be reused simply by re-closing it without replacing parts.

The circuit breaker has three basic components: contacts and arc extinguishing system, tripper, transmission and operating mechanism. Its working principle can be explained by the schematic diagram attached. The contacts shown in the drawing are closed, and the lock catches the closed state with a hook to make it stable.

When an overload occurs, the overload release operates, and a small multiple of the overload current causes the bimetallic sheet to bend to a certain extent after a certain period of time. At this time, the screw (adjustable) at one end pushes the connecting rod upward to cause it to trip, causing instability. The contacts are separated to achieve overload delay protection.

When a short circuit occurs, the short-circuit release operates. The large-scale short-circuit current quickly attracts the armature through the magnetic force generated by the electromagnet, causing it to move downward. The screw (adjustable) installed at the other end pushes the connecting rod upward, causing it to trip and cause failure. Stable, the contacts quickly separate, thereby achieving instantaneous short-circuit protection.

(3) Five steps to correctly select low-voltage circuit breakers

1) The rated current of the circuit breaker is determined by the calculated current of the line.

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

3) Verify the breaking capacity of the low-voltage circuit breaker according to the maximum short-circuit current of the line.

4) Verify the sensitivity of the circuit breaker action according to 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 of the circuit breaker (maximum expected peak current) according to the short-circuit impulse current on the line (i.e., the maximum instantaneous value of the full short-circuit current), that is, the latter should be greater than the former.

(4) Wiring method of low-voltage circuit breaker

The wiring methods of circuit breakers include front panel, rear panel, plug-in type, and drawer type. If the user has no special requirements, the circuit breaker will be supplied as front panel. Front panel wiring is a common wiring method.

1) Post-board wiring method:

The biggest feature of rear-board wiring is that the circuit breaker can be replaced or repaired without rewiring. Just disconnect the front-end power supply. Due to the special structure, the product has been equipped with a special mounting plate, mounting screws and wiring screws according to the design requirements when it leaves the factory. Special attention should be paid to the fact that the reliability of the contact of the large-capacity circuit breaker will directly affect the normal use of the circuit breaker. Therefore, during installation Attention must be paid to installation in strict accordance with the manufacturer's requirements.

2) Plug-in wiring:

On the mounting plate of the complete device, first install a circuit breaker mounting base with 6 plugs on the mounting base and 6 sockets on the circuit breaker connecting plate. There is a connecting plate on the surface of the mounting base or bolts behind the mounting base. The power cord and load line are pre-connected to the mounting base. When in use, insert the circuit breaker directly into the mounting base.

If a circuit breaker goes bad, just pull out the bad one and replace it with a good one. Its replacement time is shorter and more convenient than front-board and rear-board wiring. Because plugging and unplugging requires a certain amount of manpower. Therefore, the current frame current of plug-in products in my country is limited to a maximum of 400A.

This saves repair and replacement time. When installing a plug-in circuit breaker, you should check whether the plug of the circuit breaker is pressed tightly, and the circuit breaker should be securely fastened to reduce contact resistance and improve reliability.

3) Drawer type wiring:

The in and out drawer of the circuit breaker is rotated clockwise or counterclockwise by the rocker. Both the main circuit and the secondary circuit adopt a plug-in structure, omitting the isolator necessary for the fixed type, achieving two uses in one machine, improving It improves the economy of use, brings great convenience to operation and maintenance, and increases safety and reliability. In particular, the main circuit contact holder of the drawer base can be used in common with the NT-type fuse contact holder, so that the fuse can be directly inserted for power supply in an emergency.

(5) Summary

Since the shunt release, under-voltage release, electric operating mechanism and locking electromagnet have different voltage levels and different AC and DC power supplies, the user must indicate this when ordering. At the same time, it is impossible for the user to use a single Attachment, if two circuit breakers are required to be electrically interlocked (when one is closed, the other must be opened), auxiliary contacts and shunt releases or electric operating mechanisms can be used, and wiring before and after the board is carried out. Be sure to tighten the screws to avoid burning out the circuit breaker.

Precautions for installation and use of low-voltage circuit breakers:

1) The low-voltage circuit breaker should be installed vertically according to regulations, and its upper and lower connecting conductors must use conductors (or busbars) of specified cross-sections and must not be too small.

2) The tripper setting current and other characteristic parameters and selection parameters of the low-voltage circuit breaker are not allowed to be changed at will once they are adjusted. After using it for a long time, check whether the spring is rusty and stuck to prevent it from affecting the correct action.

3) After maintenance, it should be closed and closed several times without power, and the operation should be verified to be accurate and reliable before being put into operation.

XZT 7111 High Voltage Circuit Breaker Tester