Installing Circuit
Protection Devices

[lesson title]

INSTALLING FUSES

Fuses become part of the circuit when installed in series with the hot wire of the circuit. This is possible because a fuse contains a thin metal strip called a fuse element. This strip provides protection against overcurrent and overload. The element has a low melting point. The size of the element determines how much current it can carry before heating to the melting point.

This capacity is the ampere rating of the fuse. The rated current can flow through the element indefinitely. When a greater amount of current passes through the element, it becomes hot and melts. This opens the circuit and prevents a possible fire or other overcurrent damage.

Types of Fuses

Fuses are divided into two general categories-plug and cartridge. Each category has many types of fuses (Figure 1-2).

Figure 1-2.   Types fuses

Plug fuses. Types of plug fuses that will be discussed in this lesson are standard, time-delay, type-S, and circuit-breaker.

• Standard. The standard fuse has an element designed to melt when the current through the fuse exceeds its rated amperage. Therefore, the standard fuse can only be used one time. Standard fuses are only rated up to 30 amperes ( Figure 1-3).
• Time-delay. The time-delay fuse is a dual-element fuse ( Figure 1-4). It offers the protection of a standard fuse for shorted circuits. It also provides protection against heating caused by light overloads. This secondary protection prevents nuisance tripping, which is sometimes caused when starting motor-driven appliances such as refrigerators and air conditioners. The cutaway of Figure 1-5, shows both elements of a time-delay fuse. Note how the fuse responds to shorts and overloads.

Figure 1-3.   Standard fuse

Figure 1-4.   Time-delay fuse

Figure 1-5.   Cutaway of a time-delay fuse

• Type-S. The type-S fuse has the operating characteristics of a time-delay fuse, and also the added advantage of being nontamperable. The type-S fuse is considered nontamperable because the fuse will not fit into the base adapter unless it is the correct ampere rating for the circuit (Figure 1-6). For example, a 20-ampere fuse cannot be put into a 15-ampere base. The adapter base is designed to stay in place once it is inserted into the panel.
• Circuit-breaker. The circuit-breaker fuse is a devise that can be screwed into a fuse panel, but it has the operating characteristics of a circuit breaker. This device can be rest after an overload (Figure 1-7).

Cartridge fuses. Cartridge fuses are the only type available for circuits rated over 30 amperes.

• Ferrule-contact. Cartridge fuses for 30 to 60 amperes have ferrule contacts (Figure 1-8).

Figure 1-6.   Circuit-breaker fuse

Figure 1-7.   Type-S fuse and fuse base Figure 1-8.   Ferrule-contact cartridge fuse

• Knife-blade-contact. Cartridge fuses for 60 amperes and above have knife-blade contacts (Figure 1-9).
• Time-delay. Cartridge fuses with a time-delay feature are available at all ampere ratings (Figure 1-10).

Figure 1-9.   Knife-blade-contact cartridge fuse

Figure 1-10.   Time-delay cartridge fuses

Replacing Fuses

The length and diameter of cartridge fuses increase in steps with the ampere rating. This limits, but does not eliminate, the possibility of replacing a fuse with one of the wrong size. Blown fuses must be replaced with fuses of the proper size and ampere value.

Testing Fuses

The cartridge fuses used in residential wiring provide no visible evidence of being blown, as plug fuses usually do. The only way to tell if a cartridge fuse is blown is to perform a continuity test on it (Figure 1-11).

Replacing the Element

Some blown cartridge fuses can be reused by installing a new fuse element in the fuse cylinder. To replace the element unscrew the end cape, remove the blown element, and insert the new element. It is important to tighten the end cape firmly when the new element is in place (Figure 1-12).

Figure 1-11.   Checking a cartridge fuse

Figure 1-12.   Replaceable element cartridge fuse

Using Spare Fuses

Fuses are a simple, highly reliable, and inexpensive way of providing overcurrent protection. Fuses have no mechanical parts to fail. When fuses blow, there is often no visible evidence. Therefore, it can be time-consuming to locate, test, and replace blown fuses. Always keep spare fuses of the proper sizes near the panel box.

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