Fuses- Electricities Safety Valve

This is a very brief overview.

Yes, the simple fuse. So small, and yet it can save your life or prevent a fire.

They are designed to blow with a current overload.

They come in:
 Fast Blow & Slow Blow.
 They come in many different types & styles also.
 Some are in a tubular form, some round, some square, and some industrial types have a replaceable link inside.

They have changed the designations of the fuses a couple of times too, and it seems different mfg. had their own designations for each style of fuse.

 They used to just be Fast Blow- AGC & Slow Blow-3AG-SB, C,  N, Indicating, and a very few others.
 Now they have a plethora of types and styles.
 TDL, GMA, T, MDL, and way to many more letters to mention here, and many different physical sizes too.

 They all have a purpose- to protect the equipment and YOU!

Always replace the defective fuse with one of the same type and ratings. They have a specific purpose for a specific type.

Now they will tell you that "Something caused the fuse to fail, because they don't fail unless there is a reason, and they can't be intermittent". Well, I have seen intermittent fuses!

 They can fail because of a "non-electrical" problem.  The fuse can mechanically fail from  excessive heat & vibration too. 

  Closely look at a low amperage fast blow fuse (Easiest to see the element flex) to see it move when the equipment is turned on and you will actually see the element in it expand or contract. Like anything else mechanical, the material that is the element can just break from mechanical fatigue.

 Always test a fuse with a meter. Just looking at it is not enough. Some open fuses look good when they are not. The element fails a fraction of an inch from the end cap, where you can't see it.

 If you have a fuse that is ceramic shelled,(Type ABC) do not replace it with any other type.
Those are sand filled, high temperature fuses. Typically used in microwave ovens and some military / industrial equipment.

NEVER REPLACE A FUSE WITH A NAIL, BOLT, METAL ROD, OR ANYTHING ELSE!
 Replace with the EXACT fuse specified.

Voltage Ratings:
 Look at any quality fuse and for example you may see: "1A 120V." The voltage rating is the "flashover" voltage. It means that the fuse is designed not to arc over when it opens at the specified voltage. You can always use a higher rated voltage fuse for a lower rated one, but not a lower rated one for a higher voltage.