The basic concept of fuses, the type of fuses and how to choose the fuse model
What is a fuse? Fuse type
The fuse is mainly used for short-circuit protection in low-voltage distribution networks and can also be used for cable overload protection.
When the current through the fuse is greater than its specified value, it melts the melt with its own heat and automatically breaks the circuit.
The role of fuses in the circuit
The role of the fuse in the circuit is overcurrent protection, which can also be called short circuit protection.
When a short circuit occurs, the current in the circuit is the largest. This current value is definitely beyond the rated current of the equipment. If the circuit is not cut off in time, the power equipment will soon be burned because of the large current. Timely cutting off the circuit protection equipment from damage, using the fuse element, the principle is that it can generate large current in the circuit instantaneously, the fuse fuse fuses quickly before the equipment is not damaged, so as to achieve the purpose of cutting off the circuit.
Fuse switch disconnector symbol
The fuse-type isolation switch is a kind of current protector which is based on the principle that the current melts the melt after the current exceeds a predetermined value for a certain period of time, thereby breaking the circuit.
Fused disconnectors need to meet the following three points:
1. Protect the switch by using fuses
2. Under the normal circuit conditions (including the specified overload) can turn on, load and break the current, and in the specified abnormal circuit conditions (such as short circuit), the specified time, can carry the current isolation switch
3. The circuit can provide safe isolation. When it is in the opening position, it ensures that the circuit breaks and ensures safe operation.
Fuse-disconnector fuse disconnector (GB/T 2900.18-92 is a fuse isolator)
The fuse current and the fuse time
The relationship between the fuse time and the fuse current of the general fuse is shown in the table. In the table, IN is the rated current of the fuse in the fuse.
Fuses fuse current and fuse time relationship table
|Fusing current Is (A)||1. 25IN||1. 6IN||20. IN||2. 5IN||3. 0IN||4.0IN||8.0IN||10.0IN|
|Fusing time t (s)||∞|
It can be seen that the fuse is very insensitive to the overload reaction. When the electrical equipment is lightly overloaded, the fuse will only fuse for a long time and sometimes it will not even blow. Therefore, in addition to the lighting circuit, the fuse is generally not suitable for overload protection.
What are the types of fuses?
(1) According to the structural classification, it can be divided into three categories: semi-open type, non-filled closed tube type, and packed closed tube type.
(2) Classification according to the range of breaking, can be divided into full-range breaking capacity of the fuse – “g” fuse link and some of the scope of breaking capacity of the fuse – “a” two types of fuse.
(3) According to the classification of use, they can be divided into general-purpose fuse-links – “G” fuse-links and fuse-links that protect the motor circuit – “M” fuse-links
The fuses are lighting circuits that are used as overload and short-circuit protection and electrical circuits used as circuit protection in motor control circuits. It is connected in series in the line. When the circuit or electrical equipment is short-circuited or severely overloaded, the melt in the fuse is blown first, and the line or electrical equipment is disconnected from the power supply to protect it. It is a protective device. The utility model has the advantages of simple structure, low price, convenient use and maintenance, small size and the like.
The most important part of the fuse is the fuse (or fuse). The fuse is installed in the box or in the insulating tube. The specifications of the fuse are expressed by the rated current of the fuse, but the rated current of the fuse is not the fuse current of the fuse. The current of the fuse is more than 1.3 to 2.1 times the rated current. The maximum current that the fuse can cut off is called the current breaking capacity of the fuse. If the current is greater than this value, the arc will not extinguish when it is blown and it may cause an explosion or other accident.
A fuse is an overcurrent protector. The fuse is mainly composed of a melt, a fusion tube, and a filler. When used, the fuse is connected in series to the circuit to be protected. When the current of the circuit to be protected exceeds the specified value, and after a certain period of time, the heat generated by the melt melts the melt and disconnects the circuit, thereby protecting the circuit. effect. An electrical device that breaks a circuit by using a metal conductor as a melt is connected in series to the circuit. When an overload or a short-circuit current passes through the melt, the melt itself will generate heat and melt, thereby exerting power system, various electrical equipment, and household appliances. A certain degree of protection. With anti-delay characteristics, when the overload current is small, the fuse-off time is long; when the overload current is large, the fuse-off time is short. Therefore, within a certain overload current range until the current returns to normal, the fuse will not be blown and can continue to be used.
Conductor protection (gG/gL features):
Excessive current and short-circuit current in the circuit will cause the conductor and cable temperature to be too high, leading to insulation damage and even breakage of the conductor and cable. The fuses for the overload protection of the conductors and cables can be arranged at the incoming end or outgoing end of the conductors and cables. The rated current of the fuse is approximately 1.25 times of the line current; the fuses must be installed at the incoming end of the conductors and cables for short-circuit protection. The rated current of the fuse is approximately 1.45 times the trip current. <BR>Motor protection (aM/gM characteristics): <BR>A simple motor circuit is usually composed of fuses, contactors, thermal relays, motors, etc. According to experience, in this circuit, the rated current of the fuse is selected to be approximately 1.2 to 1.5 times the rated current of the motor.
Capacitor switchgear protection:
In the capacitor switchgear, the fuses are stored for short-circuit protection and the rated current of the selected fuse is not to be less than 1.6 times the rated current of the capacitor.
Semiconductor device protection fuse selection
The rated voltage UN of the fast-acting fuse should be slightly larger than the applied AC voltage of the faulty circuit appearing at both ends of the fast-acting fuse. If the load of a semiconductor device is an inverter load such as an active inverter or an inverter braking motor, the possibility of a short-circuit on the DC side of the device due to loss of control of the semiconductor device should be considered. When the quick fuse blows, the fuse is melted. The superimposition of AC voltage and DC voltage at both ends of the chip, the rated voltage of the fast fuse shall be calculated as follows: UN ≥ Uac + Udo × 1 / √ 2 Where: Uac: the quick fuse is applied after the fuse is applied AC voltage; Udo: Semiconductor Equipment load side inverter DC voltage.
The rated current INF of the fuse is based on the effective current of the actual current flowing through the fuse in the circuit and is calculated based on factors such as ambient temperature, cooling conditions, and current margin. INF ≥ K × IF In formula: K value is generally desirable 1.5~2. For the self-cooling fuse K to take a larger value, especially for the short circuit of the fuse connected at both ends of the circuit, you need to take the maximum value; take the smaller value of the water-cooled fuse K. The selection of the rated current of the fast-acting fuse increases the I2tF of the fuse, which is harmful to the protection of the semiconductor device.
When a semiconductor device is operated in series with a fast-acting fuse, the I2tD value allowed to pass through the semiconductor device should be greater than the I2tF value of the flash fuse. Otherwise, the device is also burned when the fuse is blown. The relationship between the two should meet: I2tF≤0.9I2tD.
In the process of arc suppression, the excessive voltage generated in the circuit during the arc-reduction process may cause reverse breakdown of the semiconductor device. Therefore, the breaking overvoltage must be less than or equal to the allowable reverse peak voltage of the semiconductor device. The over-voltage (peak value) generated when the quick fuse is blown is generally about 2 to 2.5 times higher than the fault voltage (square root value).
Rated breaking capacity:
The rated breaking capacity of the fast-acting fuse should be greater than the peak value of the fault current flowing when the fast-acting fuse is disconnected in the semiconductor device, and generally should include the short-circuit current value of the transformer valve side and the short-circuit current value of the DC side in the semiconductor device, otherwise it will cause rapid Fuse burst, string arc and other accidents.
Selection of fuse types and rated parameters
The selection of fuses can usually be considered from two aspects of fuse types and rated parameters:
(1) Selection of types. The type of fuses should be selected according to the application, line requirements and installation conditions. In factory electrical equipment, semi-enclosed plug-in fuses and filled screw fuses are generally used; in power supply and distribution systems, there are usually used closed-filled-type and non-filled closed-tube type fuses; In electronic circuits, fast fuses are usually used for short circuit protection.
(2) Selection of rated parameters. To select the rated parameters of a fuse, the following aspects should generally be considered:
1) Rated voltage. Should be greater than or equal to the operating voltage of the line.
2) Rated current. That is, the rated current of the support is selected, and its rated current must be greater than or equal to the rated current of the loaded melt.
3) Selection of the melt rated current. According to different fuse protection objects, the selection method of the melt rated current is not the same. among them:
a. Resistive load. Such as resistance furnaces, lighting, etc., the melt rated current is equal to or slightly greater than the operating current of the circuit.
b. Single motor. When using a fuse to protect a motor, the motor must be blown off due to the start-up current of the motor due to the impact of the starting current. Therefore, the rated current of this type of fuse can be selected according to the following requirements
IRN ≥ (1.5 to 2.5) IN
In the case of IN-motor rated current, when the light-load start-up or start-up time is short, a smaller factor may be taken; if the overload start-up or start-up time is long, a larger factor may be taken.
c. Multiple motors. When fuses are used to protect multiple motors, the fuse breaking current can be selected according to the following requirements IRN ≥ (1.5 ~ 2.5) IMN + ∑ IN
In the type IMN – the largest motor rated current;
∑IN – the sum of the rated currents of the remaining motors;
1.5 to 2.5-coefficient. Light load and startup time are short. It is desirable to use a small coefficient of 1.5, a heavy starting load, a long starting time, and a large number of starting times.