You can determine the physical size and type of a fuse by looking at it, but you must know other
things about a fuse to use it properly. Fuses are rated by current, voltage, and time-delay characteristics to
aid in the proper use of the fuse. To select the proper fuse, you must understand the meaning of each of
the fuse ratings.
The current rating of a fuse is a value expressed in amperes that represents the current the fuse will
allow without opening. The current rating of a fuse is always indicated on the fuse.
To select the proper fuse, you must know the normal operating current of the circuit. If you wish to
protect the circuit from overloads (excessive current), select a fuse rated at 125 percent of the normal
circuit current. In other words, if a circuit has a normal current of 10 amperes, a 12.5-ampere fuse will
provide overload protection. If you wish to protect against direct shorts only, select a fuse rated at 150
percent of the normal circuit current. In the case of a circuit with 10 amperes of current, a 15 ampere fuse
will protect against direct shorts, but will not be adequate protection against excessive current.
The voltage rating of a fuse is NOT an indication of the voltage the fuse is designed to withstand
while carrying current. The voltage rating indicates the ability of the fuse to quickly extinguish the arc
after the fuse element melts and the maximum voltage the open fuse will block. In other words, once the
fuse has opened, any voltage less than the voltage rating of the fuse will not be able to "jump" the gap of
the fuse. Because of the way the voltage rating is used, it is a maximum rms voltage value. You must
always select a fuse with a voltage rating equal to or higher than the voltage in the circuit you wish to
TIME DELAY RATING
There are many kinds of electrical and electronic circuits that require protection. In some of these
circuits, it is important to protect against temporary or transient current increases. Sometimes the device
being protected is very sensitive to current and cannot withstand an increase in current. In these cases, a
fuse must open very quickly if the current increases.
Some other circuits and devices have a large current for short periods and a normal (smaller) current
most of the time. An electric motor, for instance, will draw a large current when the motor starts, but
normal operating current for the motor will be much smaller. A fuse used to protect a motor would have
to allow for this large temporary current, but would open if the large current were to continue.
Fuses are time delay rated to indicate the relationship between the current through the fuse and the
time it takes for the fuse to open. The three time delay ratings are delay, standard, and fast.
A delay, or slow-blowing, fuse has a built-in delay that is activated when the current through the fuse
is greater than the current rating of the fuse. This fuse will allow temporary increases in current (surge)
without opening. Some delay fuses have two elements; this allows a very long time delay. If the over-
current condition continues, a delay fuse will open, but it will take longer to open than a standard or a fast
Delay fuses are used for circuits with high surge or starting currents, such as motors, solenoids, and