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Figure 2-12. - Fuseholder identification.

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A fuse, if properly used, should not open unless something is wrong in the circuit the fuse is protecting. When a fuse is found to be open, you must determine the reason the fuse is open. Replacing the fuse is not enough.

Before you look for the cause of an open fuse, you must be able to determine if the fuse is open.


There are several ways of checking for an open fuse. Some fuses and fuseholders have indicators built in to help you find an open fuse; also, a multimeter can be used to check fuses. The simplest way to check glass-bodied fuses, and the method you should use first, is visual inspection.

Visual Inspection

An open glass-bodied fuse can usually be found by visual inspection.

Earlier in this chapter, figures 2-4 and 2-5 showed you how an open plug-type and an open glass-bodied cartridge-type fuse would look. If the fuse element is not complete, or if the element has been melted onto the glass tube, the fuse is open.

It is not always possible to tell if a fuse is open by visual inspection.

Fuses with low current ratings have elements that are so small, it is sometimes not possible to know if the fuse link is complete simply by looking at it.

If the fuse is not glass-bodied, it will not be possible to check the fuse visually. Also, sometimes a fuse will look good, but will, in fact, be open.

Therefore, while it is sometimes possible to know if a fuse is open by visual inspection, it is not possible to be sure a fuse is good just by looking at it.

Fuse Indicators

Some fuses and fuseholders have built-in indicators to show when a fuse is open. Examples of these open-fuse indicators are shown in figure

2-13. Figure 2-13, view A, shows a cartridge-type fuse with an open-fuse indicator. The indicator is spring loaded and held by the fuse link. If the fuse link opens, the spring forces the indicator out. Some manufacturers color the indicator so it is easier to see in the open-fuse position.

Figure 2-13. - Open fuse indicators: Clip-type fuseholder with an indicating lamp.

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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). Remember there is no voltage drop across a straight piece of wire. Some plug-type fuseholders have test points built in to allow you to check the voltage. To check for voltage on a clip-type fuseholder, check each of the clips. The advantage of using a voltmeter to check for an open fuse is that the circuit does not have to be deenergized and the fuse does not have to be removed.


Safety Precautions When Checking a Fuse

Since a fuse has current through it, you must be very careful when checking for an open fuse to avoid being shocked or damaging the circuit. The following safety precautions will protect you and the equipment you are using.

  • Turn the power off and discharge the circuit before removing a fuse.
  • Use a fusepuller (an insulated tool) when you remove a fuse from a clip-type fuseholder.
  • When you check a fuse with a voltmeter, be careful to avoid shocks and short circuits.
  • When you use an ohmmeter to check fuses with low current ratings, be careful to avoid opening the fuse by excessive current from the ohmmeter.

Q.21 What are three methods for determining if a fuse is open? answer.gif (214 bytes)
Q.22 You have just checked a fuse with an ohmmeter and find that the fuse is shorted. What should you do?
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Q.23 You have just checked a 1/500-ampere fuse with an ohmmeter and find it is open. Checking the replacement fuse shows the replacement fuse is open also. Why would the replacement fuse indicate open?
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Q.24 How could you check a 1/500-ampere fuse with an ohmmeter?
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Q.25 List the safety precautions to be observed when checking fuses.
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