Types of fuses explained: markings, speed codes & ratings

Fuses are the simplest and cheapest form of overcurrent protection, and they are everywhere: in mains plugs, inside power supplies, soldered to circuit boards and bolted into industrial distribution panels. This guide explains the main fuse types, how to read the markings printed on them, and how to select a replacement with confidence.

What are fuses used for?

Fuses protect circuits and the people using them. If a fault causes a device to draw more current than intended — a short circuit, a failed component, a damaged cable — the fuse element heats up and melts, interrupting the flow before it can start a fire or destroy downstream components. You will find them protecting individual appliances at the plug, protecting equipment internally at the point where mains power enters, and protecting individual sections of circuitry on a PCB so that one fault cannot cascade through a whole design.

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

Both interrupt current when a fault is detected, but a fuse works once and must be replaced, while a circuit breaker trips mechanically and can be reset. Fuses respond faster to large fault currents and cost pennies, which is why they still dominate equipment-level protection even though breakers have largely replaced them in domestic consumer units.

What are the main types of fuse?

Physical format is usually the first thing that identifies a fuse:

• Cartridge — a cylindrical body, glass or ceramic, with a metal end cap at each side. The most common format in equipment and instrumentation, usually held in a fuse holder for easy replacement.
• Blade (automotive) — the flat, colour-coded plug-in fuses used throughout vehicle electrics. Rapid stocks automotive fuses in maxi and miniature blade formats.
• PCB mount and surface mount — compact fuses soldered directly to the board, either through-hole (PCB mount fuses) or as SMD fuses for modern high-density designs.
• Tag and bolted (HRC) — high rupturing capacity fuses with blades or tags for bolted connections, used in industrial power distribution where fault currents are large.
• Bottle — a threaded, screw-in format found in older European domestic installations.
• Resettable (PTC) — polymer devices that jump to high resistance under overcurrent, then recover once the fault clears. Resettable fuses suit circuits where occasional, recoverable faults are expected and physical access for replacement is difficult.

Beyond format, fuses divide by technology: AC fuses rely on the current's zero crossings to help extinguish the arc when the element melts, while DC fuses are built to quench a sustained arc and should never be substituted with an AC-only part. Striker fuses release a spring-loaded pin when they blow, giving a visual fault indication or mechanically tripping an isolator — a detail worth knowing in industrial control panels.

How do cartridge fuses work?

Cartridge fuses contain a short length of wire or strip that melts when too much current flows — which is why blowing a fuse is also called burning out. Once the element has melted, the circuit is broken and stays broken; the fuse must be replaced after the fault has been found and fixed. Glass-bodied cartridge fuses let you inspect the element visually, so a blown fuse is obvious at a glance. Ceramic bodies hide the element but offer a much higher breaking capacity — they can safely interrupt larger fault currents without shattering — which is why ceramic is specified for mains-side and industrial positions.

What do fuse markings and speed codes mean?

A typical cartridge fuse marking such as T2A 250V reads in three parts: speed code, current rating, voltage rating. The speed codes come from German terms and run from fastest to slowest:

• FF — very fast acting (flink flink) — blows almost instantaneously; used to protect sensitive semiconductor devices where even a brief overcurrent causes damage.
• F — fast acting (flink) — the common "quick blow" fuse for general-purpose protection of circuits without high inrush currents.
• M — medium acting (mittelträge) — a moderate delay for circuits with minor, temporary surges.
• T — slow acting (träge) — the "time delay" or "anti-surge" fuse, built to ride through the harmless inrush current drawn when motors, transformers or capacitive loads switch on.
• TT — very slow acting (träge träge) — an extended delay for prolonged inrush.

The speed rating exists to solve one problem: protecting the device without nuisance-blowing under normal conditions. A device that draws a large switch-on surge but modest running current needs a T fuse; a sensitive measurement circuit needs an F or FF. The voltage rating is the maximum circuit voltage the fuse can safely interrupt — a fuse rated 250V must not be used at 400V, though the reverse is fine. Size matters too: the two most common cartridge sizes are 5×20mm and 6.3×32mm, and a replacement must match the holder exactly.

Which fuse goes in a UK plug?

UK mains plugs take a BS 1362 ceramic cartridge fuse, and the everyday ratings are 3A and 13A. The working rule: appliances rated up to about 700W take a 3A fuse; anything above 700W takes a 13A fuse. Always check the appliance's rating plate or manual first — some manufacturers specify 5A. The plug fuse protects the flexible cable to the appliance, not the appliance itself, which is why fitting a 13A fuse to a lamp with thin flex defeats the point. Rapid's domestic mains plugs and adaptors range covers rewireable plugs and replacement BS 1362 fuses.

What current rating should you choose?

Choose a rating slightly above the device's normal operating current, with enough margin that expected surges pass without blowing. Never fit a higher rating to stop a fuse blowing repeatedly — a fuse that keeps blowing is reporting a fault, and uprating it removes the protection while the fault remains. When replacing, match all three parameters on the original marking: speed code, current and voltage, plus the physical size. If the original is ceramic, replace with ceramic; the breaking capacity was part of the design decision.

Replacing and choosing fuses

How can you tell if a fuse has blown?

In a glass fuse, look for a visible break or discolouration in the element. Ceramic fuses hide the element, so test with a multimeter on continuity or resistance: a good fuse reads near zero ohms, a blown one reads open circuit. Always test with the fuse removed from the circuit.

Can you replace a fuse with a higher rating?

No. The rating protects the cable and components downstream, so uprating removes that protection while the original fault remains. Replace like for like — same speed code, current, voltage and size — and investigate why the fuse blew before re-energising.

What does T2A mean on a fuse?

T2A is a time-delay (träge) fuse rated at 2 amps. The T means it tolerates brief inrush surges without blowing, making it suitable for circuits with motors, transformers or switch-mode supplies. A voltage figure usually follows, such as T2A 250V.

Are resettable fuses reusable?

Yes. Resettable (PTC) fuses are polymer devices that switch to high resistance under overcurrent, then recover once power is removed and the fault clears. They suit recoverable faults in hard-to-access circuits, but respond more slowly than a conventional fuse and are not a substitute where fast disconnection matters.

Can Rapid supply fuses for education and industrial use?

Yes. Rapid stocks glass and ceramic cartridge fuses, PCB and SMD fuses, automotive blades, resettable fuses and HRC industrial fuses from ESKA, Schurter, Siba, Bussmann and Mersen, plus fuse holders.