The second guideline needs no explaining. Not only could this surge hurt you, it could also severely damage the system. Replacing the fuse with the system energized can result in surge of current. The first of three guidelines is designed to protect both you and the circuit. Never replace a fuse with the one that has a higher current rating! That is, use a fuse that is the same size and has the same current rating as the original fuse.ДЈ. For example, if youвЂ™re replacing the fuse for a central air conditioning unit, make sure that the unit is switched off at the thermostat.Дў. When replacing a fuse, these guidelines must be followed:ДЎ. When a fuse is blown, it must be replaced. The result is that the circuit is no longer protected. Because of this, the open fuse ionized and conducts. However, the applied voltage in Figure 4b is greater than the voltage rating of its fuse. In this case, the open fuse does not allow conduction, and the circuit is protected. In Figure 4a, the applied voltage is less than the voltage rating of the fuse. For example, consider the two circuits in Figure 4. If the voltage across an open fuse exceeds its voltage rating, the air in the fuse may ionize (charge), causing the open fuse to conduct again. The voltage rating of a fuse is basically the same thing. You may recall that the average breakdown voltage of an insulator is the difference of potential that will cause the insulator to break down and conduct. Once it has opened, the applied voltage (50 VDC) is felt across the fuse. As you can see, the fuse in the circuit has opened. The explanation of this rating starts with the circuit in Figure 3b. The voltage rating of the fuse is the maximum amount of voltage that an open fuse can block. This difference of potential is equal to the applied voltage, 50 VDC.ДЇIGURE 3 Circuit voltages with a good fuse and an open fuse. A difference of potential exists between points A and B. The applied voltage has a value of 50 VDC. For example, consider the circuit shown in Figure 3a. In an open circuit, the applied voltage is felt across the open. In order for you to understand the voltage rating of a fuse, we must establish one of the characteristics of an open circuit. If the current rating of a fuse is exceeded, the fuse will вЂњBlowвЂќ (open). The current rating of a fuse is the maximum allowable fuse current. Fuse RatingsДЇuses have a current rating and a voltage rating. This protects the circuit from excessive current.ДЇIGURE 2 Fuse position in a basic circuit.

However, if the problem develops that causes the circuit current to exceed the rating of the fuse, the component opens and the current path is broken. As you can see, the total circuit current provided by the power source in Figure 2 passes through the fuse.ДЄs long as the fuse is good, the current path is complete. For this reason, a fuse is normally placed between a power source and the circuit or system it is designed to protect, as shown in Figure 2. The purpose of the fuse is to protect a circuit or system from excessive current. When it melts, the connection between the ends of the fuse is broken, the fuse is destroyed, and it effectively acts as an open switch. If the current through the component reaches a specified level, the conductor heats to its melting point. It contains thick conductor that is designed to melt at relatively low temperatures.

The fuse in Figure 1 is called a Cartridge Fuse. That is, as current increases, so does the amount of heat it produces. The amount of heat varies directly with the value of current. The simplest type of fuse and its schematic symbols are shown in Figure 1.ДЇIGURE 1 A cartridge fuse and its schematic symbolДЄs current passes through a conductor, heat is produced. However, unlike a normally closed switch, a fuse is designed to open automatically if the current exceeds is specified value. A fuse is designed to allow current to pass under normal circumstances. In some ways, a fuse is like a normally closed switch.

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