A Complete Guide to Fuses

Our fuses guide explains what they are, what they are used for, how they work, and the different types available.

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What is a Fuse?

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. This is called tripping a circuit. The point at which a fuse blows is called its breaking capacity and once it has blown, it interrupts current flow and becomes an open fuse.

Overcurrent can be caused by short circuits (in which current travels down the wrong path), configuration faults, electrical arcs, and similar issues. The resulting heat can be sufficient to melt cable insulation.

What is the Difference Between a Fuse and a Circuit Breaker?

Fuses and circuit breakers serve a very similar function - both break electrical circuits in the event of a current surge or short circuit. However, there is a crucial difference. Circuit breakers are switches, and like all switches, they can be reset once they have been flipped. Most fuses, though, can only be used a single time. Once they have blown, they must be replaced. They are, by definition, a weak link - entirely by design.

Fuses are inexpensive and can be replaced (pulled) quickly and easily for minimum downtime. It is important to ensure that you fit a fully compatible model with the correct current rating for your device. This figure indicates the level of current the fuse will accept before blowing.

There are other relevant metrics. The speed rating indicates how quickly the fuse will blow in response to excess current; a more sensitive fuse is a safer option. Voltage drop is the voltage lost through cables due to impedance - essentially when the voltage at the end of a run of cable is lower than at the beginning. If too much is lost, some low equipment may not function correctly.

It is also sensible to check the amount of energy released by the fuse when blowing. Too much could cause damage to the device and even be hazardous.

What is a Fuse Box?

Fuse boxes can essentially be explained as control panels for the electrical system in a particular location. They are the location at which incoming power is divided into separate circuits. Typically, they feature a central switch controlling:

  • The electricity supply to the entire property

  • A number of residual current devices (RCD) which instantly disconnect electrical circuits to prevent shock or injury

  • Fuses or circuit breakers for each individual electrical circuit

Fuse boxes have a number of alternative names, including control board panels, distribution boards, breaker panels, fuse boards, and fuse panels. Domestic fuse boxes are frequently referred to as consumer units.

What is a Switch Fuse Unit?

Switch fuse units distribute current while shielding cables and devices from the damage that would be caused by power fluctuations along a particular electrical circuit. Switch fuse units are used in industrial and commercial buildings as well as domestic properties.

There are three principal varieties:

  1. Rewirable fuse units feature a bank of fuses or circuit breakers in a durable steel enclosure, along with highly conductive nickel or silver contacts. Different amp ratings are available, including 16, 32, 63, 100 & 200A

  2. Porcelain fuse units provide first-rate protection from electrical overloads and short circuits. A range of fuses are available to suit different power ratings

  3. Double Break Combination Fuse Switch Units are a higher capacity model used to ensure an uninterrupted power supply to machinery and appliances. A double break, also known as a double switch, is a safety measure which requires two poles or switches to be activated to open or close a circuit

How Does a Fuse Work?

Fuses are very simple components. Inside each is a thin metal strip or wire called a resistor that forms a link in an electrical circuit. A number of different metals are used to make resistors. Tinned copper wire is a popular choice. This is copper which has been coated in a thin layer of tin to protect against corrosion.

This resistor will melt in response to the heat generated by the excess current, breaking the circuit and stopping the flow of potentially hazardous voltage. Fuses do not produce sparks, gas or electrical discharge.

Fuse

Different Types of Fuses

Unsurprisingly for such a widely used component, fuses are available in a multitude of models and voltage capacities for use in different industrial, manufacturing and electronic settings.

The time taken for a particular type of fuse to blow is called the fuse speed. The three principal fuse speeds available are:

  1. Ultra-rapid fuses: typically used in sensitive circuitry and transistors to protect against short circuits, reacting almost instantaneously. These are also known as Very Fast Acting, Super Rapid and High-Speed fuses

  2. Fast Acting Fuses: these are a more general-purpose fuse typically used with cabling and sturdier components. They are also known as Fast Blow or F-Type fuses

  3. Slow Acting fuses include a delay mechanism which allows short, lower power electrical surges to pass through the circuit without the fuse blowing. They are also known as Slow Blow or Time Delay fuses. Dual Element fuses feature a higher performance delay mechanism

Circuit breakers have mostly replaced the fuse in domestic electrical systems.

Here are some of the principal types of fuse in use:

AC Fuses

AC fuses are specifically designed for use with alternating current circuits. They are more resistant to electric arcs when the fuse blows than DC fuses and more compatible with standard voltage power supplies. They can usually cope with higher voltages, too.

DC Fuses

DC fuses, by contrast, are used with direct current circuits. Quoted voltage ratings for these fuses are maximums which should not be exceeded.

Cylindrical Fuse Types

Cylindrical fuses are a basic design and many of the most widely used and familiar models fall within this category.

HRC Fuse Types

HRC Fuse Types High rupturing capacity (HRC) fuses feature a transparent capsule around the central resistor.

This capsule is typically made from a type of rugged ceramic material called steatite, although epoxy resins are now also used.

HRC Fuse

Car Fuses

Fuses for cars function as protective devices for the many electrical components in modern vehicles.

There are different types of automotive fuse. The most widely used are blade fuses. These feature one or more blades which are inserted into a plug, making them easy to remove and reinsert. Six different sizes are available.

Car Fuse
These are:
  • Micro2
  • Micro3
  • Low profile mini
  • Mini
  • ATO - also known as ATC or regular
  • Maxi - also known as Bosch or glass tube fuses
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In addition to blade type fuses, car electronics also sometimes make use of:

  • Bolt-down fuses, also known as bolt-on fuses, for high current circuits

  • Cartridge fuses

Vehicles typically feature two fuse boxes - one under the dashboard and the other under the hood. If a fuse blows, check both boxes to identify the blown fuse. It is normally possible to pull it manually but if not, use a fuse puller. Plug-in and replace the blown fuse with one that has exactly the same specification. Fuses normally display their amp ratings on the top so this should be clearly visible once you open the fuse box. Colour coding varies by size - so a violet mini blade fuse is 3A but a violet maxi-fuse will be 100A. Note that there is no reliable link between the size of an auto fuse and its amp rating.

Bolted Tag Fuses

Bolted Tag Fuse

Bolted tag fuses are part of a broad category of fuses with protruding metal tags. They have a cylindrical shape. The tags are attached to each end and are used to bolt the fuses into a fuseholder. These fuses are used in a range of industrial and electrical equipment. They are available in different sizes and with varying current ratings, although typically used in low voltage circuits.

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Bottle Fuses

Bottle Fuse

Bottle fuses are bottle-shaped, with ends in two different sizes. The lower end is precisely aligned with the screw holes in the circuit adaptor, so no other type of fuse can be fitted into that slot. These fuses also feature an indicator in their head which drops out when the fuse blows, to show that there has been a problem in the circuit. Once this occurs, the fuse must be replaced.

They are also known as diazed fuses. Like other fuses, they are available in different sizes and voltages.

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Cartridge Fuses

Cartridge Fuse

Cartridge fuses are perhaps the most familiar, general-purpose form of fuse. The cartridge can be made from a variety of conductive materials, including glass, ceramic and porcelain. The internal resistor wire is typically made from copper, silver, aluminium or zinc.

Cartridge fuses have a contact point on each end and are compatible with 240 volts settings. Like other fuses, they are available in different sizes, including smaller ferrule fuses with a 60-amp rating and larger knife-blade cartridges with a 600 amp rating. They have a very high fuse speed, are inexpensive and, thanks to the cylindrical design, easy to replace.

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Centred Tag Fuses

Centred Tag Fuse

Centred tag fuses feature a tag which protrudes from their centre. Available models include:

  • Centre blade tag fuses
  • Centre bolted tag fuses
  • Centre slotted tag fuses
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Fuse Kits

Fuse Kit

Fuse kits are convenient, portable boxes containing a useful selection of fuses with varying ratings, sizes and designs, all clearly labelled and in separate compartments for maximum clarity and ease of access. They are designed for seamless inclusion in the broader toolkits of professional electricians, mechanics and technicians.

Some kits may be specialised, offering different fuses for specialist work - for example in automotive engineering or plant maintenance. They may also include a range of both standard and heavy-duty options. Many include fuse pullers - these are a handheld tool for removing fuses from their housing.

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Hole Mounted Tag Fuses

Hole Mounted Tag Fuse

Hole mounted tag fuses are so-called because they feature a threaded hole in their casing which allows them to be easily screwed into place.

Typically, hole mounted tag fuses are used for semiconductor circuit protection. Semiconductors are substances with a medium conductivity - between insulators, which block current, and metallic conductors which allow free current flow.

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Neutral Links

Neutral Links

Neutral links are a metal connector which provides a terminal in an electric circuit. They are typically made from aluminium or brass and are often attached to one or more fuse holders. They act as terminating points for one or more neutral wires in an electrical circuit and can be used to isolate it from the power supply. When used to terminate two or more neutral wires, they are sometimes referred to as neutral bars.

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Offset Tag Fuses

Offset Tag Fuse

Offset tag fuses are a category of fuse which feature a metal tag on each end of a cylindrical body. The tags, which may or may not be symmetrical, slot into a fuse holder. Three different tag designs are used - slotted, bolted and blade.

Different designs, current ratings and speeds are available.

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Slotted Tag Fuses

Slotted Tag Fuse

Slotted tag fuses are a category of fuse which includes centred tag and offset tag fuses. They feature a similar design to bottle fuses but are distinguished by a slot in the protruding tag, which slides into a connector or fuse holder.

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Solid Links

Solid Links

Solid links are a type of fuse accessory. These metal strips are connected to switches which are activated when a fuse blows, creating an electrical isolator and freezing the circuit. This is an important safety measure if a fault occurs and repairs are needed.

A number of shapes and sizes are available for compatibility with different fuses.

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Thermal Fuses

Thermal Fuse

Thermal fuses are used in settings which produce heat. The internal resistor blows when the surrounding temperature reaches a certain level and the fuse must then be replaced with a fully compatible new one.

Thermal fuses have both a holding temperature rating and functioning temperature rating, both measured in celsius. The holding temperature is the range in which the fuse will hold without blowing. Meanwhile, the functioning temperature designates the lowest temperature at which the fuse will blow and break the circuit.

Resettable thermal fuses are also available but these are typically more expensive. Thermal fuses can be found in domestic devices like coffee machines, tumble dryers and hair dryers.

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SMD Fuses

Surface mount devices (SMDs) are electrical components that sit on the surface of printed circuit boards (PCBs). SMD fuses are available in both resettable and non-resettable forms.

Non-resettable SMD fuses function like other types. The internal resistor melts, breaking a circuit if over-current or short circuits occur. Once they have blown, they must be replaced. Different sizes, current and temperature ratings are available and they are made from various materials including ceramic and thermoplastics.

Resettable SMD fuses, by contrast, can be used multiple times. They function in a similar way to resettable thermal fuses and are available in multiple sizes and capacities.

Striker Fuses

Striker fuses are fuses fitted with a spring-loaded striking device which can trip adjacent switches and also indicate that a fuse has blown.

Type C Fuses

A Type C fuse is actually a type of miniature circuit breaker (MCB). MCBs are a type of circuit breaker used with lower-powered circuits to cut off current in the event of an overload. Type C fuses trip circuits when the current exceeds the rated one by between five and ten times - i.e. at 100-500 amps for a 20amp device.

Type D Fuses

The definition of a D-type fuse is a miniature circuit breaker that trips when the current exceeds the rated level by between ten and 20 times. Therefore, a 16 amp Type D breaker will trip if the current reaches between 160 and 320 amps.

Type F Fuses

F type fuses are also known as quick blow or fast-blow fuses. These are highly responsive to excess current. They typically feature a glass tube with metal caps.

Standard UK Fuse Sizes

Fuses are distinguished by their design and by the power rating within the application. This indicates the maximum amount of current, measured in amps, which the fuse wire will accept before blowing. Amps, or amperes, are the standard international unit of electromagnetic force.

When selecting a new fuse, it is essential to find one to suit your power rating. It will need to react to current surges quickly enough to prevent damage or fires. Fuses should have a current rating that is slightly higher than the standard operating current of the device. This means that the fuse will not blow and trip the circuit in the event of a low, harmless surge.

3A is one of the two standard UK fuse sizes and these are normally coloured red. They are suitable for a wide range of devices, including lamps, laptops, desktop computers, photocopiers, printers, soldering irons and electric drills.

Fuses

The second size for use with wall sockets is the 13A fuse, distinguished by a brown casing. You will find these deployed in higher wattage devices, up to a maximum of 3,000 watts, including:

  • Microwaves

  • Kettles

  • Computer servers

  • Test equipment

  • Heaters

5-amp fuses are still found in some older equipment, but this is no longer a standard size.

Electrical Fuse Manufacturers

As a basic electrical component, fuses are produced by many manufacturers in a huge variety of sizes, designs and ratings. Such abundance means you can be sure of finding exactly the right fuse for your device.

Here are just a few of the leading fuse manufacturers:

FAQs

How Do You Change a Fuse in an Old Fuse Box?

Despite their everyday name, domestic fuse boxes now typically contain circuit breakers rather than fuses, and they are more formally known as consumer panels. However, if your property still contains a traditional model, here is how to go about changing the wire in a fuse if it blows and cuts the power to one of the circuits that run through the building.

  1. Obtain the tools you will need - typically fuse wire, wire cutters, a screwdriver and a torch so you can see what you are doing

  2. Inspect the electrical appliances in the affected area. Make sure that the fault was not triggered by a cartridge fuse blowing in one or more plugs or wall sockets. If you replace a fuse in the fuse box without first replacing the blown fuse in the plug, the former will simply blow again

  3. Once you have replaced any necessary powerpoint fuses, locate your fuse box, using your torch if necessary. Ensure that the master switch is in the off position. It should be following a trip, but you should always make sure that this is the case

  4. Identify the blown fuse. If in doubt, remove each fuse and inspect the fuse wire

  5. Ensure you have the correct replacement fuse wire. It must have exactly the same amp rating as the wire you are replacing. A typical rating for a light circuit is 5 amps and for wall sockets, 30 amps. Replacing the blown fuse wire with one that has a higher rating could cause a fire

  6. Loosen the screws at each end of the affected fuse and carefully remove the damaged wire. Insert the new wire, threading this through the central component of the fuse (normally made from porcelain). Wrap the wire around both screws, removing any excess, then tighten them back into place with your screwdriver

  7. Reinsert the fuse

  8. Turn the master switch back on

What Should the Fuse Rating in a Plug Be?

The rating of fuse fitted within a plug depends on the power rating of the device to which it is attached. The fuse must be compatible with currents slightly higher than those used to operate the device.

Most UK plugs are fitted with a 3 amp or 13 amp fuse. 3 amp fuses are used with a range of lower power domestic goods and tools, including lamps, TVs, and drills. Meanwhile, higher power goods such as washing machines, kettles and toasters normally require a 13 amp fuse.

What Happens When a Fuse Blows?

When a fuse blows, the electrical supply to a particular circuit is immediately cut off. This lowers the risk of fire and prevents damage to the wiring. The blown fuse can safely be replaced without the risk of electrocution.

Are Old Fuse Boxes Illegal?

No. Older fuse boxes featuring fuses rather than circuit breakers or simply aged components are not illegal but may be unsafe. Have your fuse box assessed by a professional electrician and be prepared to replace it if necessary.

How Do You Calculate Fuse Ratings?

You can use Ohm's Law to calculate the required rating for a fuse. Divide the power of the attached device (measured in watts) by the voltage of the power supply. If the result is a fraction, round this up or down.

For example, if an electric drill requiring a new fuse has a 700-watt rating, divide this by 230 (the standard UK mains voltage). The result is 3.04, so you will require a 3A amp fuse for the plug.

Where Would a Type D Fuse Be Installed?

Type D fuses are a good choice for lighting and heating systems and other equipment not prone to temporary power surges.

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