Fuses

Fuses are used in everything from automobiles and computers to major appliances and home wiring systems. Their primary function is to monitor electrical currents and make sure they operate within safe limits for the type and size of wire transmitting the current. If unsafe levels are detected, whether due to overcurrents or short circuits, they break the connection to prevent wire damage from occurring and to reduce the risk of electrical fires. They are available in different styles and configurations, each designed to meet the needs of specific applications. Before you go shopping for a fuse, consider the following questions:

• What type of fuse do you need?
• Do you require a time-delay or fast-acting fuse?
• What are the benefits of tamper-proof fuses?
• What power rating does the application require?
• Are there any special features you would like?
  
  

Types, Ratings and Selection

While fuses are used in a large number of applications, the most familiar are those found right at home. In older houses where the electrical panel has not been upgraded to accommodate circuit breakers, fuses are used to monitor and regulate electrical currents. These simple yet highly important devices are installed in a central fuse box where all electricity enters your home and branches off to other areas. Fuses contain a thin wire that allows the current to flow across them uninterrupted. If this wire exceeds the specified current level, whether due to overcurrent or a short circuit, it will heat the wire beyond capacity and cause it to melt, which terminates the circuit. This basic protection helps diminish the risk of electrical fires. Each fuse has certain ratings that determine at what level and how quickly the wire will melt. Learning what each of these measurements means is crucial to selecting the right fuse.

Plug Fuses: Plug-type fuses have a threaded base that looks like the base of a light bulb. Standard plug fuses are interchangeable, so it is very important that you match the specifications of the existing fuse to the replacement fuse. Inserting a fuse with a higher rating than the slot is designed to handle will permit much higher, and consequently more dangerous, current levels. Tamper-proof fuses provide protection against operating your system outside safe parameters by incorporating special adapters into your fuse box. These adaptors simply screw into fuse openings and only accommodate one size, making it impossible to insert a device that is not rated appropriately.

• Some plug fuses have a glass top that allows you to see which wires are still intact
• Standard threaded bases on plug fuses are often referred to as “Edison” bases
• Standard plug fuses should be used for replacement only
• Tamper-proof fuses are also called Type-S fuses
• Tamper-proof fuses are the only type recommended by the NEC
  
  

Cartridge Fuses: These fuses have a cylindrical body, typically comprised of ceramic materials and ferrule contacts, or metal end caps. These metal caps connect with a wire inside the body of the cartridge and, when snapped into place on your electrical panel, complete the connection, enabling the electrical circuit to flow freely. These fuses are often used for more demanding applications, including running high-end household appliances such as your water heater, range or clothes dryer, all of which feature electrical motors that require more power upon startup. Some cartridge fuses are designed for medium-duty tasks such as lighting fixtures and other devices without motors.

• Cartridge fuses are often constructed to withstand larger current loads
• The main fuse on your electrical panel is often a cartridge-type fuse
• A special fuse-gripping tool helps you safely remove and replace cartridge fuses
• The element in a cartridge fuse is not usually visible and requires testing to determine if it is still functional
• Use a multimeter or continuity tester to determine if a fuse has been “blown”
  
  

Time-Delay and Dual-Element Fuses: With standard fuses, the internal wire is designed to handle only a specific level of electrical current. When that level is exceeded for any reason, the internal wire melts and the connection is broken. In order to restore the connection, you have to replace the fuse, which is costly and time consuming. This presents a particular problem for circuits that support motor-driven appliances, since motors tend to require an extra boost of power upon startup. Fuses with a built-in time delay are designed to handle these types of situations without breaking. Fuses that incorporate a dual element allow for even longer time delays by absorbing extraneous heat and taking pressure off the main wire.

• Time-delay fuses are rated according to how long they can withstand higher levels before breaking the current
• Always use time-delay and/or dual-element fuses in circuits that support motor-driven devices
• Both plug- and cartridge-type fuses are available with time-delay and dual-element features
  
  

Ratings and Selection: In order to ensure that you are selecting the right fuse, it is important to understand the significance of each rating associated with the device. Consult the chart below for a simple breakdown of important ratings, their meaning and key selection tips.

Rating	Indicates	Selection Tips
Amperage	The level of electrical current the fuse can withstand without melting	Referred to as “amps” Rating is stamped directly on the fuse Match replacement fuses to the existing fuse’s exact amperage rating
Interrupting Rate	Determines the highest current at the rated voltage that must be reached before a fuse will break the circuit	This rate is typically higher than the amperage rating Indicates the level of protection present in a fuse under normal conditions
Time-Delay	Determines how long the fuse can sustain an overload before breaking the circuit	Units with a dual element often allow for longer time delays Fast-acting units break more quickly and help protect sensitive devices
Voltage	The maximum voltage a fuse is designed to interrupt	Always choose a fuse that is equal to or higher than the voltage in the circuit Fuses not rated to handle incoming voltage levels may be unable to interrupt the circuit

Features

Miniature Circuit Breakers: These devices look very similar to plug fuses, but are actually circuit breakers that have been designed to screw into standard fuse slots. They feature an external “reset” button that allows you to reestablish the circuit with just one touch. They do not need to be replaced.

Circuit Breakers: Most current construction projects no longer utilize fuses to control home electrical systems. They use circuit breakers instead, which function in much the same way, only they do not need to be replaced, just reset. You may want to consider hiring a professional to rewire your system to accommodate circuit breakers for increased convenience.

Adaptors: Fit standard plug-type fuse receptacles with appropriately rated adaptors that can accommodate tamper-proof fuses. These adaptors limit the size fuse you can put in place, ensuring that your electrical system is always operating within safe limits.

Multiple Fuse Packages: For fuse-based home electrical systems, it is advisable to keep multiple fuses of the same type on hand for easy replacement in the event of a blown circuit. Many manufacturers sell fuses in packages of five, ten or more for added convenience.

Automotive Fuses: Automotive fuses help regulate the electrical system in your car, truck, van or other vehicle. They are often referred to as “blade” fuses. These fuses feature a transparent plastic covering that allows you to view the internal wire so you can easily determine if a fuse has been blown.