Fuse Markings

Fuse Markings

A sample of the many markings that can be found on a fuse.

Surface Mount Fuses on 8 mm tape. Each fuse measures 1.6 mm x 0.79 mm and has no markings.
Most fuses are marked on the body, or end caps to markings[1] show their ratings. Surface mount technology"chip type" fuses feature little or no markings making identification very difficult.
When replacing a fuse, it is important to interpret these markings correctly as fuses that may look the same, could be designed for very different applications. Fuse markings will generally convey the following information;
Ampere rating of the fuse
Voltage rating of the fuse
Time-current characteristic ie. element speed
Approvals
Manufacturer / Part Number / Series
Breaking capacity

Fuse approvals
The majority of fuse manufacturers[2] build products that comply with a set of guidelines and standards, based upon the application of the fuse. These requirements are devised by many different Government agencies and certification authorities[3]. Once a fuse has been tested and proven to meet the required standard, it may then carry the approval marking of the certifying agency.
Fuse Packages
Fuses come in a vast array of sizes & styles[4] to cater for the immense number of applications in which they are used. Whilst many are manufactured in standardised package layouts to make them easily interchangeable, a large number of new styles are released into the marketplace every year. In terms of fuse body construction, ceramic is the most commonly used material. Glass & plastic are also used in lower voltage applications.
Cartridge (ferrule) fuses feature a cylindrical body terminated with metal end caps. Whilst most cartridge fuses are symmetrical, some cartridge fuses are manufactured with differing body proportions to reduce the possibility of inserting an incorrect fuse into the holder (circuit). An example of such a fuse range is the 'bottle fuse', which in appearance resembles the shape of a bottle.
Fuses designed for soldering to a printed circuit board traditionally featured wire leads that originated from the fuse body in a radial or axial configuration, however with the advancement of surface-mount technology, manufacturers have vastly reduced fuse body size and replaced the leads with solder pads.
Fuses used in higher voltage/ampere circuits as required by industrial applications, commonly feature metal tags or blades located on each end of the fuse. Tags allow the fuse to be bolted into the fuse holder whilst blades slot into metal pressure clamps located on the fuse holder. Blade type fuses often require the use of a special purpose extractor tool to remove them from the fuse holder.

Glass vs. Ceramic Construction
Whilst glass fuses have the advantage of a visible fuse element for inspection purposes, they have a low breaking capacity which generally restricts them to applications of 15 A or less at 250 VAC. Ceramic fuses have the advantage of a higher breaking capacity facilitating their use in higher voltage/ampere circuits. Filling a fuse body with sand provides additional protection against arcing in an overcurrent situation.

Measurements
Cartridge fuses[5] are generally measured as the overall length & diameter of the fuse. Due to the large variety of cartridge fuses available, fuse identification relies on accurate measurements as fuses can differ by only a few millimeters between types. 'Bottle style' cartridge fuses also require the measurement of the cap diameter as this varies between ampere ratings.
Other fuse packages can require a variety of measurements such as;
body (width x height x depth)
blade or tag (width x height x depth)
overall length of the fuse (when the fuse features blades or tags)
overall width of the fuse (when the fuse features 2 bodies)
width of the mounting holes (when the fuse features tags)
distance between blades (when radially configured)
fixing centre[6] (when the fuse features tags - see below)
Fuses fitted with tags require the fixing centre measurement. This measurement is the distance between the tag mounting holes on either end of the fuse as measured from the centre of each mounting hole.

Special Features
As visual identification of a blown fuse is only possible when the element is visible ie. glass body fuses, manufacturers have designed a variety of methods to indicate whether the fuse element is intact or blown such as;
Indicating pin: extends out of the fuse cap when the element is blown.
Indicating disc: a coloured disc (flush mounted in the end cap of the fuse) falls out when the element is blown.
Element window: a small window built into the fuse body to provide visual indication of a blown element.
Striker pin: similar to an indicating pin, but extends with more force to trip a switch when the element is blown.
Flag: an external sprung arm that is released to an extended position once the element is blown.
External trip indicator: similar function to striker pin, but can be externally attached (using clips) to a compatible fuse.
Some fuses allow a special purpose microswitch[7] or relay unit to be fixed to the fuse body. When the fuse element blows, the indicating pin extends to activate the micro switch or relay which in turn triggers an event.