A simple guide to fuses

Fuses are safety devices used in an electric plug or circuit. Their function is to prevent damage to the device, or to prevent resultant damage, such as fires by interrupting the electrical flow.

What are fuses used for?

Fuses are commonly used to protect everyday appliances, such as hairdryers. If the device draws more than the specified amount of current, the fuse will break and prevent further current flow, therefore preventing further damage, fires or risk of electrocution. Fuses are also an essential of every household's electrical system, in the form of control board panels (also called fuse boxes or consumer units) which protect the circuits within our homes and shut them down if fault conditions are detected.

What is the difference between a fuse and a circuit breaker?

Fuses and circuit breakers are both used for over-current protection. Whilst they both interrupt the electrical flow when a fault is detected, fuses only work once and thus have to be replaced, circuit breakers can be reset without the need for replacement.

Different types of fuses

There are a number of physical fuse types available including:

• Cartridge - A cylindrical fuse with a metallic end cap on each side. It's often used in domestic, commercial, and industrial electrical applications.
• Tag (e.g Bolted, Centred. Slotted, Offset) - A high-rupturing capacity (HRC) fuse with protruding tags or blades at its ends for secure bolted connections, commonly used in industrial power distribution.
• Square Body - This fuse has a square or rectangular ceramic body and is designed for high-current applications, such as protecting semiconductor devices like rectifiers and thyristors.
• Bottle - Named for its shape, this fuse has a threaded metal cap that screws into a holder and is typically used in older European domestic wiring systems.
• Automotive - A low-voltage fuse designed specifically for vehicles, it comes in various types like blade, glass, and Lucas fuses, protecting the vehicle's electrical circuits.
• Surface Mount - A small, compact fuse designed to be mounted directly onto a printed circuit board (PCB), commonly used in modern electronic devices.

Fuses also have different technologies. For example:

• DC - Designed specifically for direct current circuits, this fuse is built to quickly extinguish the sustained arc that forms when it blows, which is more difficult to suppress than in an AC circuit.
• AC - Designed for alternating current circuits, this fuse relies on the current's periodic zero crossings to help extinguish the arc when the fuse element melts, making it simpler in construction than a DC fuse.
• Resettable - A fuse made from a polymer that, when subjected to overcurrent, heats up and increases its resistance to a high level, effectively stopping the current. Once the fault is removed, it cools down and resets to its low-resistance state.
• Striker - This fuse features a spring-loaded striker pin that is released when the fuse blows. The pin can either serve as a visual fault indicator or mechanically activate a switch or other device to de-energize the circuit.
• Over voltage - A device that protects against excessively high voltage by triggering a component to short-circuit the line, which then causes a separate overcurrent fuse to blow and clear the fault.
• Drop out - A type of high-voltage fuse used on utility poles where the fuse element is housed in a hinged tube. When the fuse blows, the tube drops out from its holder, providing a clear visual indication of the fault and an air gap to prevent arcing.
• Over current - The most common type of fuse technology, it protects a circuit by interrupting the flow of electricity when the current exceeds a predetermined safe level, preventing damage to downstream components.

Cartridge type fuses

Cartridge fuses are small low cost electrical safety devices used for overload protection. They are available in a number of standard sizes and are typically made from glass or ceramic. Cartridge fuses are cylindrical in shape with contact points at each end. They are usually used with a fuseholder to make them relatively easy to change in the event of a fault causing the fuse to blow, although they can also be used with solder leads, which require more involved service to change.

How do cartridge fuses work?

Cartridge fuses contain a short length of wire that melts when too much current flows through the circuit. They are therefore known as Over Current Protection devices. When the wire melts, it breaks the circuit which prevents any damage. This is commonly known as burning out or blowing. Put simply, when a fuse blows, the circuit is broken. Once a fuse has blown, it will need replacing after the fault has been identified in order to make the circuit complete and allow the flow of current.

What are the different types of fuse markings and speed codes?

Cylindrical cartridge fuses are used in a range of industrial, commercial, and domestic applications. Selecting the correct fuse speed is essential for the protection of your device. Here are some of the most common markings you will find on a cartridge fuse.

• FF = Very Fast Acting (Flink Flink) - Designed to protect highly sensitive semiconductor devices, these fuses blow almost instantaneously to prevent even brief overcurrents from causing damage.
• F = Fast Acting (Flink) Often referred to as ‘Quick blow’ - Often called "quick-blow" fuses, they are the most common type and are used for general-purpose protection of standard electrical circuits and components that do not have high inrush currents.
• M = Medium Acting (Mitteltrage) - These fuses have a moderate time delay, making them suitable for circuits that experience minor, temporary current surges without needing to be protected against short, harmless overcurrents.
• T = Slow Acting (Trage) Often referred to as ‘Time delay’ - Commonly known as "time-delay" fuses, they are engineered to withstand harmless, temporary overcurrents, like those experienced when starting a motor or other inductive loads.
• TT = Very Slow Acting (Trage Trage) - These fuses have an extended time-delay characteristic, making them ideal for circuits with significant and prolonged inrush currents where a less tolerant fuse would blow prematurely.

The speed rating is used to ensure the device is adequately protected, but does not cause the fuse to blow under normal operating conditions. For example, a particular device may draw a large current when it is switched on, but much less in normal operation. In this case, the designer may select a time delay fuse to prevent the fuse blowing during normal start up. Fast acting fuses may be used to provide protection where fault conditions can very quickly cause further damage.

Why use cartridge fuses?

Cartridge fuses are a relatively cheap way of protecting electrical equipment from short circuits and overcurrent. They are incredibly safe as they do not produce flames, arcs, or gas when they burn out and break the circuit. They can be used at different points in a circuit to isolate sections of circuitry that may be at risk of different fault conditions. For example, audio amplifiers often use separate fuses for the mains electricity supply, but also for the audio output.

Why use glass or ceramic fuses?

Ceramic fuses offer several advantages over other fuse types, primarily due to their robust construction and ability to handle high currents and temperatures. They are known for their high breaking capacity, meaning they can safely interrupt large fault currents without damage.

Glass fuses also have the advantage of being able to inspect the wire element and therefore provide a visual method to check if the fuse has blown.

What current rating should you choose?

Cartridge fuses are available in a range of standard amperages. Generally, fuses are chosen with a slightly higher current rating than the standard operating current of the device to be protected. Consideration needs to be given to ensure any normal surges in current do not cause the fuse to blow.