Fuse common faults, disassembly and repair practice manuals

Short circuit and overload protection - Industrial/Marine standards - Safe operation

I. Structure and characteristics of the fuse

1. Plug-in fuses

Plug-in fuses are used to realize low voltage circuits for short circuit protection or overload protection of electrical equipment. The fuse is equipped with a fuse or fusible piece, the rated current of the fuse when in use must not exceed the rated current indicated on the porcelain, otherwise the arc generated when the fuse burns out is extremely strong and will burn out the fuse.

2. Screw-in fuses

Spiral fuses are equipped with a fuse and quartz sand inside the fuse tube, which has a colored point at one end, and when the fuse is blown, the colored point jumps out, indicating that the fuse is broken.

3. Packed closed tube fuses

By the fuse tube, fuse, indicator, packing and contact base and other components. Fuse with silver tape made of V-shaped narrow cross-section or mesh form, with rapidity, can be used as a semiconductor rectifier components of the short-circuit protection and overload protection.

Second, the selection of fuses

Fuses are mainly selected according to the load and the size of the circuit short-circuit current. For the protection of lighting lines or motors with small capacity, semi-closed tube fuses or closed tube fuses without packing can be used; for circuits with considerable short-circuit currents or places with flammable gases, closed tube fuses with packing should be used; for the protection of thyristors and silicon components, fast fuses should be used.

Core Computational Principles:

Due to a variety of electrical equipment have a certain overload capacity, when the overload capacity is weak, can be allowed to run for a longer period of time, and more than a certain overload multiplier, the melt is required to fuse within a certain period of time. There are also some equipment start-up current is very large, such as three-phase asynchronous motor start-up current is 5 to 7 times the rated current, therefore, the choice of melt must be considered when the characteristics of the equipment.

(1) Lighting branch circuits:
Rated fuse current ≥ sum of the operating currents of all lamps on the branch circuit

(2) Single direct start motor:
Melt rated current ≥ (1.5~2.5) × motor rated current

(3) The low voltage side of the distribution transformer:
Rated current of the fuse = (1~1.2) × rated current of the low voltage side of the transformer

The fuse should fuse reliably under the action of short-circuit current. If it is selected on the large side, it will not fuse in time; if it is selected on the small side, it will fuse under normal load.

Third, the fuse when using the precautions

(1) When replacing a fuse, replace it with one of the same type, material, size, and current rating as specified.
(2) The rated voltage of the fuse shall not be less than the rated voltage of the line.
(3) Fuses with appropriate breaking capacity shall be selected to cope with the maximum possible short-circuit current.
(4) Cooperation at all levels:It is usually required that the former melt is 2 to 3 times larger than the latter melt to avoid transgressive action.
(5) A fuse cannot be loaded for continued use with a single fuse lapped over the ends of the fused tube.
(6) Fuses can only do the short circuit protection of the main circuit of the motor, can not do overload protection.
(7) In grounded lines, three-phase four-wire neutral lines, DC motor excitation circuitsimpermissibleTap into the fuse.

Fourth, the common failure of the fuse

1. Fuse blowing
Cause: circuit overload, short circuit, improper selection or aging. Phenomenon: Circuit breaks, fuses are disconnected or burned black.

2. Poor contact
Cause: Oxidized, loose and dirty base contacts. Phenomenon: Heat, circuit on and off, arc.

3. Cosmetic damage
Cause: Mechanical impact, high temperature ablation. Phenomenon: Cracked, deformed or carbonized shell.

4. Malfunction or failure to fuse
Cause: Poor quality, current mismatch. Phenomenon: Circuit abnormal but not disconnected, vulnerable equipment.

V. Fuse disassembly steps

(1) Disconnect and test the power:Turn off the power switch and use a power test pen to confirm that the circuit is free of electricity.
(2) Remove fuses:Use an insulated screwdriver or special tool to loosen the fixing screws. Pull out vertically (insertion type) or unscrew (screw type).
(3) Check the base:Observe contacts for oxidation, ablation or looseness and clean or replace as necessary.
(4) Select matching fuses:Make sure the old and new specifications (current and voltage) are the same.
(5) Fixed fuses:Make sure the contact is tight and tighten the screws to avoid false connections.
(6) Energized test:Close the gate and observe if it is normal and there is no abnormal heat or spark.

VI. Repair and maintenance of fuses

1. Handling after fusing

direct replacement of the fuse with a new one.Prohibition of substitution of copper and iron wiresThe following is an example of how to replace an overload or short-circuit fault. Always troubleshoot an overload or short circuit before replacing.

2. Contact cleaning and repair

Polish the oxidized contacts with fine sandpaper. If severely deformed or burnt, the entire base must be replaced.

3. Fuse housing repair

Minor damage can be temporarily repaired with insulating tape. Severe carbonization or cracking must be scrapped and replaced.

4. Melt replacement

Only for replaceable melts such as knife type. Install the new melt according to the original specifications, making sure that the connection is secure.

VII. Security considerations

(1) Power-off operation: power must be disconnected before disassembly or repair to avoid electric shock or arc burns.
(2) Protective measures: wear insulated gloves and goggles, use insulated tools.
(3) Prohibition of electrically charged replacement: Do not directly electrically replace the fuse after it has blown.
(4) Environmental requirements: Avoid operation in humid and flammable environments.
(5) Periodic inspection: clean dust, tighten screws.