Fuse, commonly known as insurance, is one of the most simple protective electrical appliances. When the electrical equipment in the power grid or circuit overload or short circuit occurs, it can melt and break the circuit itself, avoid the power grid and electrical equipment damage due to the thermal effect of overcurrent and electric power, and prevent the spread of the accident.
One, model of fuse
The first letter R stands for fuse.
The second letter M means no packing closed tube type;
T means packed closed tube type;
L means spiral;
S stands for fast form;
C stands for porcelain insert;
Z stands for self-duplex.
The third is the design code of the fuse.
The fourth represents the rated current of the fuse.
Two, the classification of fuses
According to the structure, fuses can be divided into three categories: open type, semi-closed type and closed type.
1. Open type fuse
When the melt does not limit the arc flame and metal melting particles ejection device, only suitable for disconnecting short circuit current is not large occasions, this fuse is often used in combination with knife switch.
2. Semi-enclosed fuse
The fuse is installed in a tube, and one or both ends of the tube are opened. When the fuse is melted, the arc flame and metal melting particles are ejected in a certain direction, which reduces some injuries to personnel, but it is still not safe enough and the use is limited to a certain extent.
3. Enclosed fuse
The fuse is completely enclosed in the shell, without arc ejection, and will not cause danger to the nearby live part flying arc and nearby personnel.
Three, fuse structure
The fuse is mainly composed of the melt and the fuse tube or fuse holder on which the melt is installed.
1.Melt is an important part of the fuse, often made into silk or sheet. There are two kinds of melt materials, one is low melting point materials, such as lead, zinc, tin and tin-lead alloy; The other is high melting point materials, such as silver and copper.
2.The melt tube is the protective shell of melt, and has the effect of extinguishing arc when the melt is fused.
Four, fuse parameters
The parameters of the fuse refer to the parameters of the fuse or fuse holder, not the parameters of the melt.
1. Melt parameters
The melt has two parameters, the rated current and the fusing current. Rated current refers to the value of the current that passes through the fuse for a long time without breaking. The fuse current is usually twice the rated current, generally through the melt current is 1.3 times the rated current, should be fused in more than one hour; 1.6 times, should be fused within one hour; When the fuse current is reached, the fuse is broken after 30 ~ 40 seconds; When 9 ~ 10 times the rated current is reached, the melt should break instantaneously. The melt has the protection characteristic of inverse time, the larger the current flowing through the melt, the shorter the fusing time.
2. Welding pipe parameters
The fuse has three parameters, namely rated voltage, rated current and cut-off capacity.
1) The rated voltage is proposed from the Angle of arc extinguishing. When the working voltage of the fuse is greater than the rated voltage, there may be the danger that the arc cannot be extinguished when the melt is broken.
2) The rated current of the molten tube is the current value determined by the allowable temperature of the molten tube for a long time, so the molten tube can be loaded with different grades of rated current, but the rated current of the molten tube can not be greater than the rated current of the molten tube.
3) Cut-off capacity is the maximum current value that can be cut off when the fuse is disconnected from the circuit fault at the rated voltage.
Five, the working principle of the fuse
The fusing process of a fuse is roughly divided into four stages:
1. The melt is in series in the circuit, and the load current flows through the melt. Due to the thermal effect of the current will make the melt temperature rise, when the circuit overload or short circuit occurs, the overload current or short circuit current will make the melt excessive heat and reach the melting temperature. The higher the current, the faster the temperature rises.
2. The melt will melt and evaporate into metal vapor after reaching the melting temperature. The higher the current, the shorter the melting time.
3. The moment the melt melts, there is a small insulation gap in the circuit, and the current is interrupted suddenly. But this small gap is immediately broken down by the circuit voltage, and an electric arc is generated, which in turn connects the circuit.
4. After the arc occurs, if the energy decreases, it will self-extinguishes with the expansion of the fuse gap, but it must rely on the extinguishing measures of the fuse when the energy is large. In order to reduce the arc extinguishing time and increase the breaking ability, the large capacity fuses are equipped with perfect arc extinguishing measures. The larger the arc extinguishing capacity is, the faster the arc is extinguished, and the larger the short circuit current can be broken by the fuse.
Six, the selection of fuse
1. Choose fuses with corresponding voltage levels according to the power grid voltage;
2. Choose fuses with corresponding breaking ability according to the maximum fault current that may occur in the distribution system;
3, the fuse in the motor circuit for short circuit protection, in order to avoid the motor in the process of starting the fuse, for a single motor, the rated current of the melt should not be less than 1.5 ~ 2.5 times the rated current of the motor; For multiple motors, the total melt rated current shall not be less than 1.5~2.5 times the rated current of the maximum capacity motor plus the calculated load current of the rest of the motors.
4. For short-circuit protection of lighting or electric furnace and other loads, the rated current of melt should be equal to or slightly greater than the rated current of the load.
5. When using fuses to protect lines, fuses should be installed on each phase line. It is forbidden to install fuses on the neutral line in the two-phase three-wire or three-phase four-wire circuit, because the neutral line break will cause voltage imbalance, which may burn electrical equipment. On single-phase lines supplied by the public grid, fuses should be installed on neutral lines, excluding the total fuses of the grid.
6. All levels of fuses should cooperate with each other when used, and the rated current of melt should be smaller than that of the upper level.
Post time: Mar-14-2023