
Fuses are a type of electrical service panel that acts as a control board for the entire electrical system of a house. Screw-in fuses are a type of fuse that was commonly used in older homes and buildings. They are screwed into threaded sockets in the fuse box, similar to how light bulbs are screwed into light fixtures. The two types of screw-in fuses are Edison base (Type T) and rejection base (Type S) fuses. Type T fuses have screw shell threads identical to those found on light bulb sockets, while Type S fuses have non-interchangeable porcelain threads that prevent the use of conductive objects like pennies from bypassing the fuse link.
What You'll Learn
Screw-in fuses vs screw-in bases
Fuses are basic safety devices that protect electrical circuits from excess current or overcurrent. They are designed to blow and break the circuit when a stronger-than-expected current surges through, thereby minimising heat damage and reducing the risk of electrocution or fire.
Homes and facilities built before the 1960s were equipped with fuse boxes that contained plug fuses. These fuse boxes have a series of threaded sockets into which the fuses are screwed in like light bulbs. Each circuit in the home is protected by a fuse, and it is imperative that the correct type of fuse with the appropriate amperage rating is used for its circuit. Using the wrong type of fuse for a circuit can pose a serious fire hazard.
There are two different types of bases and screw-in fuses: the Edison base (found on Type T fuses) and the rejection base (found on Type S fuses).
Edison Base (Type T)
The Edison base (Type T) looks like a light bulb base and fits the standard sockets found in old fuse boxes. Type T fuses can be interchanged regardless of amp ratings. The fuse threads are identical to a standard medium-base light bulb. This means that any amperage size fuse can be interchanged with the others in the "fuse cabinet".
Rejection Base (Type S)
Rejection bases are also known as "tamper-proof". They were developed to prevent homeowners from using the wrong type of fuse for a circuit. Each Type S fuse of a specific amperage rating has a matching base adapter with a specific size of thread that prevents mismatching the fuses. For example, it stops a person from putting a 20-amp fuse in a 15-amp circuit, a potentially serious mistake called "over-fusing". This condition can result in the fuse failing to blow before the circuit wiring overheats and potentially catches fire. A 15-amp Type S fits only a 15-amp base adapter.
Type-W Fuses
Type-W fuses are an older style of fuse that utilises an Edison base and are all but obsolete today. They are general-purpose plug fuses and are fast-acting, meaning they have no time-delay fuse element and quickly interrupt the circuit once the fuse's rated amperage is exceeded. Type-W fuses are designed for use in general lighting and power circuits that do not contain electric motors.
Type-SL and Type-TL Fuses
SL and TL fuses are medium-duty time-delay fuses and are now the most commonly used plug fuses found in home electrical systems. The only difference between SL and TL fuses is the type of base: the SL fuse has a rejection base, and the TL fuse has an Edison base. SL and TL fuses contain a plug of heat-absorbing solder that's attached to the centre of the fuse element, allowing the fuse to absorb a temporary circuit overload.
Type-S and Type-T Heavy-Duty Time-Delay Fuses
Heavy-duty time-delay fuses are used for circuits with critical or high motor loads or circuits serving motors that frequently cycle on and off (such as a sump pump motor). These fuses have a longer time-delay feature than the SL or TL fuses. The only difference between the S and the T heavy-duty fuses is the base: type-S has a rejection base, while type-T has an Edison base.
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Edison base (Type T)
The Edison base, also known as Type T, is a standard lightbulb socket for electric light bulbs. It was developed by Thomas Edison (1847–1931) and patented in 1881. The Edison base is characterised by its right-handed threaded metal base (cap), which screws into matching threaded sockets (lamp holders). The thread is generally connected to neutral for bulbs powered by AC current, while the contact on the bottom tip of the base is connected to the "live" phase.
In North America and continental Europe, the Edison screw has become the standard connector for general lighting, replacing other socket types. The types of Edison screws are designated as "Exx", with "E" standing for "Edison" and "xx" indicating the diameter in millimetres of the socket. The four basic sizes of Edison screws are:
- Candelabra (E12 in North America, E11 in Europe): Used for very low wattage lamps such as night lights and chandeliers.
- Intermediate (E17 in North America, E14 in Europe): Used for appliance bulbs, such as those found in refrigerators or ovens.
- Medium or Standard (E26 in North America, E27 in Europe): The most common size for large mains voltage light fittings in the United States.
- Mogul (E39 in North America, E40 in Europe): Commonly used for street lights and high-wattage lamps rated 300W or higher.
It is important to note that the Edison base (Type T) is different from the rejection base (Type S). The rejection base was designed to prevent the use of incorrect fuses, which could lead to a serious fire hazard. Each Type S fuse has a matching base adapter with a specific thread size that prevents mismatching. On the other hand, the Type T fuse can fit into any Edison socket, regardless of the circuit's amperage.
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Rejection base (Type S)
Type S fuses have a rejection base, also known as a "tamper-proof" base, which was developed to prevent homeowners from using the wrong type of fuse for a circuit. Each Type S fuse of a specific amperage rating has a matching base adapter with a specific size of thread that prevents mismatching the fuses. For example, it stops a person from putting a 20-amp fuse in a 15-amp circuit, a potentially serious mistake called "over-fusing" that can result in the fuse failing to blow before the circuit wiring overheats and potentially catches fire.
Type S fuses are heavy-duty time-delay fuses that have a longer time-delay feature than medium-duty SL and TL fuses. They contain a spring-loaded metal fuse link attached to a solder plug. If the overloaded circuit condition continues for too long, the solder plug melts and the spring pulls the fuse link free, cutting power to the circuit. This allows the fuse to absorb a longer temporary circuit overload than with other time-delay fuses.
Type S fuses are not interchangeable with Edison Base (Type T) fuses. Type S fuses have a rejection base with porcelain threads, while Type T fuses have an Edison base with brass threads. Type S fuses are also slower to react to an excess current compared to Type T fuses.
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Type-W fuses
It is important to use the correct type and amperage rating of fuse for a circuit to avoid potential safety hazards. Using a fuse with the wrong amperage rating can pose a fire risk, as it may not blow in time to prevent the circuit wiring from overheating.
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Type-SL and Type-TL fuses
SL and TL fuses are the most commonly used plug fuses found in home electrical systems. They are medium-duty time-delay fuses, designed to prevent circuit overload and absorb temporary surges in power demand. For example, without a time-delay feature, simply starting a garbage disposer or refrigerator would cause a fuse to blow.
The difference between SL and TL fuses is the type of base: the SL fuse has a rejection base, while the TL fuse has an Edison base. The rejection base is also known as "tamper-proof" and was developed to prevent homeowners from using the wrong type of fuse for a circuit, which could pose a serious fire hazard. The SL and TL fuses are rated at 120 volts and up to 30 amps.
Type-S and Type-T fuses are heavy-duty time-delay fuses used for circuits with critical or high motor loads or circuits serving motors that frequently cycle on and off, such as a sump pump motor. They have a longer time-delay feature than SL or TL fuses. The difference between Type-S and Type-T fuses is also the type of base: Type-S has a rejection base, while Type-T has an Edison base. These fuses are rated at 120 volts and up to 30 amps.
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Frequently asked questions
The standard UK fuse sizes are 3A and 13A. 3A fuses are normally coloured red, and 13A fuses are brown.
Type S fuses have a rejection base, also known as a tamper-proof base, which prevents homeowners from using the wrong type of fuse for a circuit. Type T fuses have an Edison base, which looks like a lightbulb base and fits the standard sockets found in old fuse boxes.
Fuses are basic safety devices widely deployed in electrical circuits for excess current or overcurrent protection. If a stronger than expected current surges through, the fuse will blow and break the circuit, minimising heat damage and reducing the risk of electrocution or fire.