2-16
Figure 2-13.—Open fuse indicators: Clip-type fuseholder with an indicating lamp.
Figure 2-13, view B, shows a plug-type fuseholder with an indicating lamp in the fuse cap. If the
fuse opens, the lamp in the fuse cap will light. Figure 2-13, view C, shows a clip-type fuseholder with an
indicating lamp.
Just as in visual checking, the indicator can show an open fuse. Since the indicator may not always
work, you cannot be sure a fuse is good just because there is no open-fuse indication.
Checking Fuses with a Meter
The only sure method of determining if a fuse is open is to use a meter. An ohmmeter can be used to
check for an open fuse by removing the fuse from the circuit and checking for continuity through the fuse
(0 ohms). If the fuse is not removed from the circuit, and the fuse is open, the ohmmeter may measure the
circuit resistance. This resistance reading might lead you to think the fuse is good. You must be careful
when you use an ohmmeter to check fuses with small current ratings (such as 1/32 ampere or less),
because the current from the ohmmeter may be larger than the current rating of the fuse. For most
practical uses, a small current capacity fuse can be checked out of the circuit through the use of a resistor.
The ohmic value of the resistor is first measured and then placed in series with the fuse. The continuity
reading on the ohmmeter should be of the same value, or close to it, as the original value of the resistor.
This method provides protection for the fuse by dropping the voltage across the resistor. This in turn
decreases the power in the form of heat at the fuse. Remember, it is heat which melts the fuse element.
A voltmeter can also be used to check for an open fuse. The measurement is taken between each end
of the fuse and the common or ground side of the line. If voltage is present on both sides of the fuse (from
the voltage source and to the load), the fuse is not open. Another method commonly used, is to measure
across the fuse with the voltmeter. If NO voltage is indicated on the meter, the fuse is good, (not open).
