Identification of fuses are by the printing on the side of them. The printing on them will identify the physical size, the type, and the ratings. There are four different systems used for identification of fuses . The systems are the old military designation, the new military designation, the old commercial designation, and the new com-mercial designation. All four systems are presented here, so you will be able to identify the type no matter which designation is printed on it.
You may have to replace bad device that is identified by one system with a good device that is identified by another system. The designation systems are fairly simple to understand and cross-reference once you are familiar with them.
OLD MILITARY DESIGNATION
The figure below shows a fuse with the old military designation. The tables in the lower part of the figure show the voltage and current codes used in this system. The upper portion of the figure is the explanation of the old military designation. The numbers and letters in parentheses are the coding for the fuse shown in the figure below.
Old type military fuse designation.
The old military designation always starts with "F," which stands for fuse. Next, the set of numbers (02) indicates the style. Style means the construction and dimensions (size) of the fuse. Following the style is a letter that represents the voltage rating of the fuse (G). The voltage code table in the figure above shows each voltage rating letter and its meaning in volts. In the example shown, the voltage ratings is G, which means the fuse should be used in a circuit where the voltage is 250 volts or less.
After this is a set of three numbers and the letter "R," which represent the current rating of the fuse. The "R" indicates the decimal point. In the example shown, the current rating is 1R00 or 1.00 ampere. Some other examples of the current rating are shown in the current code table of figure 2-8. The final letter in the old military designation (A) indicates the time delay rating of the fuse.
the old military designation is still found on some fuses, the voltage
and current ratings must be "translated," since they use letters to
represent numerical values. The military developed the new military
designations to make fuse identification easier.
NEW MILITARY DESIGNATION
The next figure below is an example of a fuse coded in the new military designation. The fuse identified in the example below is the same type as the fuse used as an example in the next figure above.
New type military fuse designation.
The new military designation always start with the letter "F," which stands for fuse. The set of numbers (02) next to this indicates the style. The style numbers are identical to the ones used in the old military designation and indicate the construction and dimensions of the fuse. Following the style designation is a single letter (A) that indicates the time delay rating of the fuse. This is the same time delay rating code as indicated in the old military designation, but the position of this letter in the coding is changed to avoid confusing the "A" for standard time delay with the "A" for ampere. Following the time delay rating is the voltage rating of the fuse (250) V.
In the old military designation, a letter was used to indicate the voltage rating. In the new military designation, the voltage is indicated by numbers followed by a "V," which stands for volts or less. After the voltage rating, the current rating is given by numbers followed by the letter "A."
The current rating may be a whole number (1A), a fraction (1/500 A), a whole number and a fraction (1 1/2A), a decimal (0.250A), or a whole number and a decimal (1.50A). If the ferrules of the fuse are silver-plated, the current rating will be followed by the letter "S." If any other plating is used, the current rating will be the last part of the fuse identification.
As you can see, the new military designation is much easier to understand than the old military designation.
You may find a fuse coded in one of the commercial designations. The commercial designations are fairly easy to understand and the next figure below shows the old and new commercial designations for the same type of fuse that was used in the first two figures above respectively.
Commercial designations for fuses.
OLD COMMERCIAL DESIGNATION
The figure above, view A, shows the old commercial designation for a fuse. The first part of the designation is a combination of letters and numbers (three in all) that indicates the style and time delay characteristics. This part of the designation (3AG) is the information contained in the style and time delay rating portions of military designations.
In the example shown, the code 3AG represents the same information as the underlined portions of F02 G 1R00 A from The first figure at the top of the tutorial (Old Military Designation) and F02A 250VIAS from the second figure (New Military Designation). The only way to know the time delay rating of this fuse is to look it up in the manufacturer’s catalog or in a cross-reference listing to find the military designation. The catalog will tell you the physical size, the material from which the fuse is constructed, and the time delay rating of the fuse. A 3AG fuse is a glass-bodied fuse, 1/4 inch x 1 1/4 inches (6.35 millimeters x 31.8 millimeters) and has a standard time delay rating.
the style designation is a number that is the current rating of the
fuse (1). This could be a whole number, a fraction, a whole number and a
fraction, a decimal, or a whole number and a decimal. Following the
current rating is the voltage rating; which, in turn, is followed by the
letter "V," which stands for volts or less (250V).
NEW COMMERCIAL DESIGNATION
The last figure above, view B, shows the new commercial designation for fuses. It is the same as the old commercial designation except for the style portion of the coding. In the old commercial system, the style was a combination of letters and numbers. In the new commercial system, only letters are used. In the example shown, 3AG in the old system becomes AGC in the new system. Since "C" is the third letter of the alphabet, it is used instead of the "3" used in the old system. Once again, the only way to find out the time delay rating is to look up this coding in the manufacturer’s catalog or to use a cross-reference listing. The remainder of the new commercial designation is exactly the same as the old commercial designation.