Low-voltage power distribution systems typically use low-voltage circuit breakers and fuses for short-circuit protection. Although both serve the same function of short-circuit protection, they also have significant differences in their application. Electrical engineers should understand and master the differences in their selection and use.
I. Low-voltage circuit breakers
Low-voltage circuit breakers, also known as automatic air switches or air circuit breakers, are low-voltage switching devices that can connect and disconnect normal load currents and overload currents, as well as short-circuit currents.
In addition to their control function, low-voltage circuit breakers also provide certain protection functions in circuits, such as overload, short circuit, and undervoltage protection. They can be used to distribute electrical energy, infrequently start motors, and protect power lines and motors. When a serious overload, short circuit, or undervoltage fault occurs in the power supply circuit, they can automatically disconnect the circuit, functioning similarly to a combination of a fuse switch and over/under-temperature relays. Low-voltage circuit breakers generally do not require replacement of internal components after interrupting a fault current, making them convenient to use. They are widely used in AC/DC low-voltage power distribution systems at all levels of feeders, power control of various mechanical equipment, and control and protection of electrical terminals, including for infrequent motor starting and switching circuits.
1. Classification of low-voltage circuit breakers
Low-voltage circuit breakers can be classified in many ways, but they are usually divided into the following categories:
(1) According to the different AC/DC systems, they are divided into low-voltage AC circuit breakers and low-voltage DC circuit breakers.
(2) According to the arc extinguishing medium, there are air type and vacuum type, but most of the low-voltage circuit breakers currently in use are air type.
(3) According to the usage category, there are two types: non-selective (protection device parameters cannot be adjusted) and selective (protection device parameters can be adjusted):
Class A: In the event of a short circuit, the circuit breaker does not explicitly specify selective protection for use as another short-circuit protection device connected in series on the load side. That is, in the event of a short circuit, there is no artificial short delay required for selective protection, and therefore no short-time withstand current requirement.
Class B: In the event of a short circuit, the circuit breaker is explicitly designated as a selective protection device used in series with another short circuit protection device on the load side. That is, in the event of a short circuit, selective protection can be achieved, with a human-made short delay (adjustable), and therefore a short-time withstand current requirement.
(4) Classified by structural type
Frame circuit breaker (ACB: Air)
Circuit Breakers (also known as universal circuit breakers) mount all components on an insulated or metal frame. They have more structural variations, more types of trip units, and more auxiliary contacts. Generally, selective circuit breakers, especially high-capacity circuit breakers, are often designed as frame circuit breakers.
Molded Case Circuit Breaker (MCCB): All parts are housed in an insulated enclosure. MCCBs are compact, small, lightweight, and inexpensive, making them suitable for stand-alone installations. Due to their lower rated current, non-selective circuit breakers are often designed in MCCB configurations.
Miniature Circuit Breakers (MCBs): Commonly used for short-circuit protection of small electrical equipment at the end of low-voltage power distribution systems. They have small rated current and breaking capacity and are the most widely used terminal protection devices in building electrical terminal distribution devices.
2. Basic characteristic parameters of low-voltage circuit breakers
(1) Rated voltage Ue: This is the voltage at which the circuit breaker operates under normal (uninterrupted) conditions;
(2) Rated current Ie: The rated current of a circuit breaker is the rated continuous current. That is, the current that the trip unit can carry continuously. For circuit breakers with adjustable trip units, it is the maximum current that can be carried continuously.
(3) Circuit breaker frame rated current: The rated current of the largest trip unit that can be installed in a frame or plastic housing of the same size and structure.
(4) Breaking capacity: There are two types of short-circuit breaking capacity: rated ultimate short-circuit breaking capacity Icu and rated operating short-circuit breaking capacity Ics. Icu is the breaking capacity under the conditions specified by the prescribed test procedure, excluding the circuit breaker's ability to continue carrying its rated current; Ics is the breaking capacity under the conditions specified by the prescribed test procedure, including the circuit breaker's ability to continue carrying its rated current.
(5) Tripping current setting range of overload protection (Ir or Irth) and short circuit protection (Im).
In addition to the parameters mentioned above, the technical parameters of low-voltage circuit breakers also include current limiting capacity, operating time, overcurrent protection characteristics (two-stage type with long overload delay and short circuit delay, and three-stage type with long overload delay, short circuit delay, and instantaneous operation for extreme short circuits) and service life (electrical life and mechanical life), etc.
3. Types of trip units for low-voltage circuit breakers
(1) Electromagnetic trip unit: only provides magnetic protection, that is, short circuit protection.
(2) Thermal-magnetic trip unit: Provides magnetic protection and thermal protection, which is also overload protection. Generally speaking, thermal-magnetic trip units are used in circuits to provide short-circuit and overload protection. Only in some special cases are electromagnetic trip units used to provide short-circuit protection, while other components (such as thermal relays) provide overload protection. However, it can only provide two-stage protection; the operating value error is relatively large and cannot be adjusted.
(3) Electronic trip unit: It can have all the above functions and can be easily adjusted. It can provide three or even four protections, and its action is relatively accurate and adjustable.
II. Low-voltage fuses
Low-voltage fuses are electrical devices that provide safety protection in low-voltage power distribution systems. They are widely used for power grid protection and equipment protection. When a short circuit or overload occurs in the power grid or equipment, fuses can quickly and automatically disconnect the circuit, preventing damage to electrical equipment and preventing the spread of accidents. Low-voltage fuses have advantages such as simple structure, ease of use, and low cost, therefore, they are widely used in low-voltage systems.
1. Structure of low-voltage fuses
2. Types of low-voltage fuses
Low-voltage fuses are commonly classified into three main types: plug-in type, tube type, and screw type. They can also be divided into three types: open type, semi-enclosed type, and enclosed type.
R - Fuse; C - Insert type; L - Screw type; M - Sealed tube type; S - Quick-connect type; T - Filled tube type. For example, RC1 and RC1A are insert type; RM - Unfilled tube type; RT0, RL1, and RLS are filled tube type and filled screw type, respectively.
Low-voltage fuses have three different types of fuse elements: one is used for overload and short-circuit protection of electrical circuits (gG); another is used for short-circuit protection of semiconductor devices and their complete sets of equipment (aR); and it can also be derived for short-circuit protection of motors (aM).
3. Selection of low-voltage fuses
The selection of low-voltage fuses mainly includes choosing the fuse type and determining the rated current of the fuse element. The selection requirements for fuses are: the fuse should not blow during normal operation of the electrical equipment; it should blow immediately in the event of a short circuit; it should not blow during normal current fluctuations (such as during motor starting); and it should blow after a delay when the electrical equipment is under continuous overload. The rated voltage of the fuse must be greater than or equal to the rated voltage of the circuit. The basic principles for selecting low-voltage fuses are as follows:
(1) Select the fuse model according to the circuit requirements and installation conditions. For circuits with small capacity, select semi-enclosed or unfilled enclosed fuses; for circuits with large short-circuit current, select filled enclosed fuses; for semiconductor component protection, select fast-acting fuses.
(2) Select the rated current of the fuse according to the load characteristics.
(3) The selection of each level of fuse needs to be coordinated. The next level should be smaller than the previous level. The current on the incoming line (main switch) and each branch line is different, so the selection of fuses is also different.
(4) Select the rated voltage of the fuse according to the line voltage.
(5) The current of the protective fuse for AC asynchronous motor should not be too small (usually 2 to 2.5 times the rated current of the motor). If it is too small, the motor may burn out due to a single phase fuse blowing. A thermal relay must be used for overload protection.
Note: The fuse current includes two aspects: one is the rated current of the fuse holder and frame, and the other is the rated current of the fuse element. These two should not be confused. This is similar to the circuit breaker.
III. Comparison of Low-Voltage Circuit Breakers and Fuses
Low-voltage circuit breakers and fuses differ significantly in their application, as summarized below:
1. Non-selective circuit breaker
(1) Main advantages and features
① After the fault is disconnected, it can be manually reset without replacing components, unless maintenance is required after interrupting a large short-circuit current.
② It has two protection functions: a long-delay trip unit with inverse time-limit characteristics and an instantaneous current trip unit, which are used for overload and short-circuit protection, respectively.
③ Remote control is possible when the electric operating mechanism is in operation.
(2) Main disadvantages
① It is difficult to achieve selective disconnection between non-selective circuit breakers at different levels. When the fault current is large, it is easy for both circuit breakers at different levels to disconnect instantaneously, causing malfunctions and expanding the power outage range.
② Some circuit breakers have a relatively small breaking capacity. For example, if a circuit breaker with a small rated current is installed near a large-capacity transformer, its breaking capacity will be insufficient.
③The price is relatively high.
2. Selective circuit breaker
3. Fuse
(1) Main advantages and features
① Good selectivity. As long as the rated current of the fuse elements of the upper and lower level fuses meets the requirement of an overcurrent selectivity ratio of 1.6:1 specified in the national standard and IEC standard, that is, the rated current of the upper level fuse element is not less than 1.6 times the value of the lower level fuse element, the upper and lower level fuses are considered to be able to selectively interrupt fault current.
② It has good current limiting characteristics, high breaking capacity, and short action time.
③ Relatively small size
④ The price is relatively cheap
(2) Main disadvantages
① The fuse element must be replaced after a faulty fuse blows.
② It has a single protection function, with only one overcurrent inverse time characteristic. It is used for overload, short circuit and ground fault protection.
③ When one phase of a three-phase motor blows, it will result in two phases operating, which is an undesirable consequence. Of course, this can be compensated for by using a fuse with an alarm signal. A single-phase blowout can disconnect all three phases.
④ Remote control is not possible; it needs to be combined with an electric knife switch or switch to achieve this.
The above is a comparison of the differences between low-voltage circuit breakers and fuses. Both have different advantages and disadvantages, and a reasonable and cost-effective choice should be made based on the specific circumstances when designing and selecting them.
