Fuse and melt at work in the low voltage load switch is concatenated in the circuit, the circuit and electric equipment overload and short circuit protection function. In the replacement of the melt, what are the principles to follow? Now the points mentioned below:

　　1. Copper and aluminum wire not a high-voltage drop type fuse fuse inside.  

　　High pressure drop type fuse fuse inside generally made from materials such as copper and silver, because the resistivity of copper and silver is very small, conductive performance is strong, so some wires can do fine, this is advantageous to the arcing, but their high melting point, can make the fusion of fuse tube overheating, easy to damage. So made of silver and copper fuse, adopt artificial way to reduce the melting point of fuse, the fuse on bead welding on small beads or lead. When the fuse is heated to the tin (232 ℃) or lead melting point (327 ℃), melt ball bead, to disrupt fuse, formed by the breakpoints arc melt fuse at the both sides, so as to protect the circuit or electrical equipment with large current of fever and damage.

　　2. 10 kv voltage transformer primary side blown fuse. Can't use ordinary fuse instead.  

　　L0kV voltage transformer RN2 or RN4 model is often used to fuse protection. The rated current of the fuse is 0. 5 a, 1 min for o the fusing current. 6-1. 8 a. both kind of fuse fuse tube filled with quartz sand, thus has good arc extinguishing performance and big breaking capacity (not less than 1000 mva). Due to it is made of nichrome wire of fuse, total resistance is about 90 Ω, thus has the short circuit current limiting function. If replace with common fuse, when voltage transformer for failures or other reasons make the blown fuse, both neither can limit short circuit current, and cannot be extinguished arc, is likely to fire equipment, and even lead to system outage. So when blown fuse, voltage transformer should be switch to the original specification of the fuse and cannot be replaced with ordinary fuse.

　　3. Filled with quartz sand of high-voltage fuse can only be used with rated voltage on the same grid.  

　　Fuse filled with quartz sand, when the melt fuse, arc narrow ditch in the quartz sand burning, according to the principle of slit arcing, arc closely contact with packing around cools down and out, its arc ability is very strong, can be in before goes out to the peak arc current, current limiting function. But it will produce overvoltage, the overvoltage conditions related to use the location of the voltage, if use in the lower than the rated voltage of the power grid, overvoltage may reach 3.5 4 times the phase voltage, corona will make the power grid, and even damage the equipment in the grid. If used in higher than the rated voltage of power grid, the fuse of overvoltage may cause arc reigniting, unable to go out again, will cause the fuse shell burn out: with the same rated voltage of power grid, the fuse overvoltage was only about 2-2. Five times the phase voltage of power grid, a little higher than the line voltage equipment. So there is no danger.

　4. Replacing the melt (fuse) cannot melt the enlarge or reduce the rated current and rated voltage level. 　 

　　Can replace and the original capacity of the same voltage grade and fusion, otherwise loses the fuse protection.

　　5. The rated current of the melt must and fuse to cooperate.  

　　Because of the same kind of fuse with rated current different melt, but is only able to melt the rated current is smaller than rated current of the fuse, as much as equal, must not be exceeded. Because the rated current of the fuse is according to the contact part and terminals, such as fever situation to decide. In the same way, made up by isolating switch and melt the isolating switch, load switch, fuse type rated current of the melt is smaller than rated current of the switch, as much as equal, must not be exceeded.

　　6. Master switch and separate clearance melt the size of the rated current must not be upside down, also cannot be the same.  

　　Distribution circuit commonly, it is combined with isolation switch and fuse to as control and protection equipment, the main switch and separate clearance within the fuse melts rated current is different, the main switch is big, separate close to smaller.

　　7. Cannot load switch after replace the fuse melts.  

　　Both because of the short circuit current and overload current, the fuse, or other reasons, cannot be rushed to replace the melt, need to find out the reasons, troubleshooting after the replacement. At the same time can't break a phase change of a phase, but the melt were replaced with load switch. Due to its fusing also passes through the hot melt through at the end of continue to use in normal working days after fusing, so old and the new melt some similar characteristics, but the burn-out properties are different, cross-sectional area is not the same size.

　　8. The same load switch and fuse type isolating switch.  

　　Must exchange the same species of molten. Can't mix change capacity rather than the same kind with fuse.

　　Because of the different types of fuse fusing features, such as lead tin alloy fuse fusing current is rated current of 1. 5 times; The copper melt is 2 times; Made of zinc melting is 1. 3 to 2. 1 times the left and right sides.

　　9. When changing the melt, must cut off power supply.  

　　Under the isolation switch, absolute band electric operation, to avoid the body or tool touched the live part and cause accident getting an electric shock, but also avoids the possible loading pull out fuse of melting a accident.

　　10．Can't let the fuse into the shell.  

　　Changing lead tin alloy fuse, fuse with 60 to 85 mm, the length of the assembly time all can't keep the fuse and protection of polonium shell contact directly.

　　Copper melt assembly time can't keep the shell and melt direct contact, and enough length, wire diameter in 0. 46 mm below, need from 60 to 85 mm long, bigger may not be less than 100 mm in diameter. Change the small and medium-sized melt when the amount of asynchronous motor, rated current of the melt can't in 1 protected the rated current of the motor. 5 times, the general should be 2.5 times the left and right sides.

　　11．Changing porcelain plug fuse melts, not at liberty to take off, and throw away the porcelain bottom of asbestos placemat.  

　　More than 30 a current levels of fuse in arcing to pad have asbestos placemat for vibration reduction, insulation, and help arc.

　　12. Can't change the workings of a spiral fuse 。 

　　Spiral fuse (RL1) has a cut-out ability, small volume, convenient to use, and the advantages of safe and reliable, widely used in electrical equipment in the main circuit, control circuit and the lighting circuit of short circuit protection.

　　When the fuse tube blown fuse, should according to the rated current of load switch to a new fusion tube 。

　　13. In the incandescent lamp lighting the way. Cannot use the heat capacity of small fast fuse protection 。 

Because incandescent lamp under the cold light of instantaneous current is very big, can be achieved from the blazing light bulb 12 ~ 16 times the rated current, the impact current duration (time) to normal to zero. 05 ~ 0. 23 seconds (bulb power grows big time long). And quick fuse protection feature: breaking 4 ~ 6 times the rated current, the melting time less than 0. 2-0. 02 seconds. Incandescent light bulbs, it shows that the impact current duration, a quick fuse fuse will rong.

14.Correctly, reasonably choose fuse, to ensure the safety of the electrical equipment and line running relationship is very big, the principle of selection are: 1. When the current exceeds the equipment normal after a certain time fuse should fuse (2) in electrical equipment during normal short-term over-current (such as motor start-up) fuse should not fuse.

Selection method is (in in addition to the above is not strict with conditions) :

1. Lighting and electric equipment lines

(1) in the main circuit fuse rated current is equal to 0.9-1 times of the rated current meter;

(2) on the branch fuse rated current is equal to all load on the branch of the sum of the rated current 1 -- 1.1 times.

2. Ac motor circuit

(1) a single ac motor fuse rated current of the circuit is equal to 1.5-2.5 times rated current of the motor;

(2) sets of ac motor fuse rated current of the circuit to be equal to the largest one on the line 1.5 -- 2.5 times rated current of the motor, plus the sum of other rated current of the motor.

3. The ac arc welder in communication

A single fuse on the ac welding lines available the following simple method to estimate:

(1) when the supply voltage 220 v, rated current of the fuse is equal to the welding machine power (kW) numerical 6 times.

(2) the power supply voltage 380 v, rated current of the fuse is equal to the welding machine power (KW) values of 4 times.