After an open fuse is found and the trouble that caused the fuse to open has been corrected, the fuse must be replaced. Before you replace the fuse, you must be certain the replacement fuse is the proper type and fits correctly.
Proper Type of Replacement Fuse
To be certain a fuse is the proper type, check the technical manual for the equipment. The parts list will show you the proper fuse identification for a replacement fuse. Obtain the exact fuse specified, if possible, and check the identification number of the replacement fuse against the parts list. If you cannot obtain a direct replacement, use the following guidelines:
Proper Fit of Replacement Fuses
When you have obtained a proper replacement fuse, you must make certain it will fit correctly in the fuseholder. If the fuseholder is corroded, the fuse will not fit properly. In addition, the corrosion can cause increased resistance or heating. Clean corroded terminals with fine sandpaper so that all corrosion is removed. Do NOT lubricate the terminals. If the terminals are badly pitted, replace the fuseholder. Be certain the replacement fuseholder is the correct size and type by checking the parts list in the technical manual for the equipment.
After you check for and correct any corrosion problems, be certain the fuse fits tightly in the fuseholder. When you insert the fuse in the cap of a plug-type fuseholder, the fuse should fit tightly. A small amount of pressure should be needed to insert the fuse and cap into the fuseholder body.
In clip-type fuseholders, the clips can be easily bent out of shape.
This causes an incorrect fit, which in time could cause an equipment malfunction.
Figure 2-14 shows examples of correct and incorrect fuse contacts for clip-type fuseholders used with knifeblade and ferrule cartridge fuses. The clips shown in the left picture of each row have the correct contact.
The three pictures on the right of each row show incorrect contact. Notice how the clips are not contacting completely with the knifeblade or ferrules.
This incomplete contact can. cause corrosion at the contacts, which in turn can create a high resistance and drop some of the circuit voltage at this point.
Figure 2-14. - Contact between clips and fuses.
If the fuse clips do not make complete contact with the fuse, try to bend the clips back into shape. If the clips cannot be repaired by bending, replace the fuseholder or use clip clamps. Clip clamps are shown in figure 2-15.
Figure 2-15. - Clip clamps.
Safety Precautions When Replacing Fuses
The following safety precautions will prevent injury to personnel and damage to equipment. These are the MINIMUM safety precautions for replacing fuses.
PREVENTIVE MAINTENANCE OF FUSES
Preventive maintenance of fuses consists of checking for the following conditions and correcting any discrepancies.
IMPROPER FUSE. Check the fuse installed against that recommended in the technical manual for the equipment. If an incorrect fuse is installed, replace it with the correct fuse.
CORROSION. Check for corrosion on the fuseholder terminals or the fuse itself. If corrosion is present, remove it with fine sandpaper.
IMPROPER FIT. Check for contact between the fuse and fuseholder. If a piece of paper will fit between the fuse and the clips on a clip-type fuseholder, there is improper contact. If the fuse is not held in the cap of a plug-type fuseholder, the contacts are too loose.
OPEN FUSES. Check fuses for opens. If any fuse is open, repair the trouble that caused the open fuse and replace the fuse.
Q.26 You have removed an open fuse from a fuseholder and repaired the cause of the fuse opening. The parts list specifies a fuse coded F02BI25V½A. There are no fuses available with that identification.
In the following list, indicate if the fuse is a direct replacement, a good substitute, or not acceptable. For the fuses that are good substitutes, number them in order of preference and explain why they are numbered that way. If the fuse is not acceptable, explain why.
Q.27 What two things should you check before replacing a fuse?
Q.28 List the safety precautions to be observed when replacing a fuse.
Q.29 What conditions should you check for when conducting preventive maintenance on fuses?