A Complete Guide to Fuses

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 that requires two poles or switches to be activated to open or close a circuit

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 that 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 that 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.

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 that 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.

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:

• Eaton
• Littelfuse
• Mersen
• RS Pro
• Siemens
• Schurter
• SIBA

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.