In older residential electrical services, individual circuits are often protected with fuses rather than the circuit breakers used in more modern service panels. All the fuses protecting the household circuits are found in the main fuse box, usually located in a utility area in the home.
Fuses are relatively simple devices, containing a metal filament that melts through and stops the flow of electricity if the power draw exceeds the safe capacity of the circuit wires. In residential use, the fuses protecting the circuits are of two types: plug fuses, which generally protect 120-volt circuits, and cartridge fuses, which protect 240-volt circuits, as well as being used to protect the entire electrical service.
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Screw-In Plug Fuses: The Basics
Screw-in plug fuses are the most common type used on 120-volt household circuits. As the name hints, a screw-in plug fuse inserts into a threaded socket in a fuse panel, much the way a lightbulb screws into a light fixture socket. For this reason, these fuses are sometimes known as "Edison-base fuses," after the design popularized by Thomas Edison.
The body of a screw-in fuse is made from ceramic material, with threaded metal around the base and a small glass face window, through which you can see a metal-alloy filament inside. All current being drawn by the circuit passes through this metal filament, and if the power draw becomes too great, the filament melts ("blows) and the power ceases to flow.
For safety, the fuse must be rated at an amperage appropriate to the wire gauge of the circuit. Installing a fuse that is too large can damage the circuit wires and potentially cause a fire.
- A 15-amp screw-in fuse is typically used for household lighting and receptacle circuits wired with 14-gauge wire.
- A 20-amp screw-in fuse is generally used for outlet and appliance circuits wired with 12-gauge wire.
- A 30-amp screw-in fuse is most commonly used for circuits supplying electric clothes dryers or air conditioners. These circuits are wired with 10-gauge wire.
Because it is very easy to mismatch screw-in fuses, innovations were developed to safeguard against using the wrong fuse sizes.
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Type T Fuses
Ordinary screw-in fuses that fit directly into the threaded Edison sockets on a fuse panel are known as Type T fuses. These fuses can fit into any socket in the fuse panel, so there is no means of safeguarding against inserting the wrong-sized fuse into a socket.
These fuses will have "Type T" printed on the front face. A variation, "Type TL" is designed for use on circuits that feed motor-driven appliances, such as a refrigerator or dishwasher. Type TL fuses have a time-delay feature that prevents the momentary power surge that occurs during motor start-up from blowing the fuse. Another variation, "Type TC," has even more time-delay; it is used mostly for industrial equipment.
There is nothing inherently dangerous about Type T fuses, provided you take care to use the correct size when you replace a blown fuse.
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Type S (Tamper-Proof ) Fuses and Adapters
Type S fuses are sometimes called "tamper-proof" fuses. They are the current standard and the most commonly sold type of fuse. Each fuse size will fit only into a complementary adapter base that is preinstalled in one of the fuse panel's Edison sockets. These bases are sometimes known as "rejection bases," since they will reject the wrong-sized fuse. Once installed, the adapter bases are quite hard to remove. Thus, you are prevented from mismatching the fuse size when replacing a blown fuse. For example, a 20-amp fuse will not fit into an Edison fuse socket fitted with a 15-amp adapter base.
These fuses will have "Type S" printed on the face of the fuse. The "Type SL" variation is for use with circuits that power motor-driven appliances, such as a refrigerator or dishwasher. The SL type has a time-delay feature that prevents the fuse from blowing under the momentary surge that occurs when a motor comes up to speed.
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Circuit Breaker Fuses
Something of an oddity, a circuit-breaker fuse screws into an Edison socket on the fuse panel, but rather than having a metal-alloy filament, this fuse has a mechanism that can be reset by pushing a small button on the face of the fuse. Circuit breaker fuses are designed as permanent replacements for Type T fuses; there are no Type S circuit breaker fuses. Circuit breaker fuses are widely available for 15-amp and 20-amp circuits.