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A fuse is device that that protects an electrical installation against the dangerous consequences of excessive overcurrents by breaking the current in a controlled and predictable way.

A fuse consists of several components, namely; a fuse-link, a fuse-base and a fuse-carrier.

The fuse-link is the component which breaks the current and requires replacement after operation, the fuse-base is the component which is connected into the installation to be protected, and the fuse-carrier is the component designed to hold the fuse-link for insertion in the fuse-base.

In some installations the fuse-carrier may be completely removed from the fuse-link after it is inserted in the fuse base. Other arrangements require the fuse-link to remain attached to the fuse-carrier after its insertion in the fuse–base.

The fuse-holder is the combination of a fuse-base and fuse-carrier.

A mechanical fusible link is a device consisting of two strips of metal soldered together with a fusible alloy that is designed to melt at a specific temperature, which allows the two pieces to separate.

A fuse-element is the part of the fuse-link designed to break the circuit under the action of excessive current.

The element is the heart of the fuse-link and determines its performance which must fulfil various roles;
1. It must safely interrupt very high current short-circuit faults.
2. It must safely interrupt very low current overload faults.
3. It must ensure that the fuse-link has a low power dissipation.

For fault interruption the element has two zones of operation; constrictions in the metal of the element and a region containing a low melting point alloy.

High fault currents flowing through the constrictions rapidly raise the temperature of the constrictions causing them to melt at about 1000 C and interrupt the fault current.

Lower fault currents raise the temperature of the low melting point alloy until it becomes molten and starts a diffusion process with the element metal at about 200 C. This process continues until all the element material in contact with alloy is diffused and the element is parted interrupting the fault current.

Efficient fault clearance requires narrow cross-sections of element material in both operation zones which leads to a high resistance element, however low power dissipation requires a low resistance element. It is the satisfactory reconciliation of these two conflicting requirements which results in optimal fuse-link performance

An overcurrent is a current exceeding the rated current of the circuit.

An overload is an overcurrent in an undamaged electrical circuit usually caused by excessive loading.

A short-circuit is an overcurrent in a damaged electrical circuit. The current does not follow its normal path, but may flow between phases and/or between phases and earth at the point of damage.

Fuses fall into three broad categories;

High Voltage (HV) Fuses have rated voltages exceeding 1000V a.c. or 1500V d.c.

Low Voltage (LV) Fuses have rated voltages not exceeding 1000V a.c. or 1500V d.c., and minimum breaking capacity of 6 kA a.c.

Miniature Fuses have a breaking capacity not exceeding 1500kA a.c. and one principal dimension not exceeding 10mm.

Each category complies with a suite of International Standards issued by the International Electrotechnical Commission (IEC). These IEC Standards form the basis for National Standards such as British Standards (BS) in the UK.

HV Fuses are covered by the IEC60282 series of Standards.

LV Fuses are covered by the IEC60269 series of standards.

Miniature Fuses are covered by the IEC60127 series of standards.

The principal national standards for HV, LV and miniature fuses are based on these IEC standards.

There may be additional National Standards for fuses which are not widely used outside an individual country.

The IEC Standards follow a similar arrangement with the Standard divided into several Parts.

Part 1 normally specifies general requirements for all fuses in the category, whilst subsequent parts specify supplementary requirements for different types of fuses in the category.

The supplementary parts must always be considered in conjunction with the general requirements of Part 1. For example; LV Fuses are covered by IEC60269.

Part 1 specifies General Requirements for all LV Fuses.

Part 2 specifies supplementary requirements for Industrial Fuses.

Part 3 specifies supplementary requirements for Household Fuses.

Part 4 specifies supplementary requirements for Semiconductor Protection Fuse-links.

Part 5 is an application guide for LV fuses.

Part 6 specifies supplementary requirements for fuse-links protecting PhotoVoltaic Arrays.

The equivalent UK Standard is BS88.

Not quite. BS88 omits supplementary requirements for fuse systems not used in the UK.

BS88-3:2010 has replaced BS1361:1971 as the standard for House Service Cut-Out Fuse-links.

BS88-3:2010 is the UK equivalent of the International Standard IEC60269-3:2010 which forms part of the restructured IEC60269 series of fuse standards.

BS88-3:2010 retains the requirements of BS1361:1971 though current ratings are now aligned with the preferred number series. Most current ratings are unchanged, however 15A is replaced by 16A, 30A is replaced by 32A, 60A is replaced by 63A

Lawson type ME&MF House Service fuse-links meet the requirements of both BS1361:1971and BS88-3:2010.

The design and performance of Lawson type ME&MF fuse-links is unchanged, therefore fuse-links marked and rated in accordance with BS1361:1971 may continue to be used in compliance with BS88-3:2010.

Fuse-links are categorised by breaking capacity and Utilisation Category under a two letter code. For example “gG”.

The first letter indicates the breaking range; 

“g” is a fuse-link with a full range breaking capacity from its conventional fusing current to its rated breaking capacity.

“a” is a fuse-link with a partial range breaking capacity from approximately 5 times its current rating to its rated breaking capacity.

The second letter indicates the utilisation category which is a combination of requirements specified for each utilisation category;

“G” specifies requirements for fuse-links for general purpose usage.

“M” specifies requirements for fuse-links for the protection of motor circuits.

“U” specifies requirements for fuse-links for the protection of electricity supply networks.

The overwhelming majority of fuse-links used in the UK are category “gG”.
However, there is a small but significant usage of category “gM” fuselinks in the UK.“

gU” fuse-links are not normally found in general usage, but are installed in electricity supply networks operated by public utilities.

“aM” fuse-links are not usually used in the UK.

Dimensions, voltage rating; current rating, breaking capacity; utilisation category, power dissipation; time-current characteristic, time-current zones; I²t values.

The lowest voltage (a.c. or d.c.) at which any of the standardised tests were carried out.

The highest value of prospective current that the fuse has interrupted at a specific voltage under standardised test conditions.

IEC60269 and BS88 define various requirements for each individual current rating (In) of a fuse. The requirements include;

1. Ability to continuously carry its rated current.
2. Have a power dissipation (watts loss) at rated current below a specified maximum value.
3. Operation at the conventional fusing current (If) of 1.6 times In within the conventional time.
4. Non-operation under the conventional non-fusing current (Inf) of 1.25 times In within the conventional time.
5. Compliance with specified operating currents at specified times of 5 seconds and 0.1 seconds.
6. Compliance with specified non-operating currents at specified times of 10 seconds and 0.1 seconds.
7. Compliance with specified pre-arcing I2t values at 0.01 seconds.

The tests for operating and non-operating currents together, with the requirements for pre-arcing I²t, comprise a series of gates which form an envelope called the time-current zone. Each rating has a unique time current zone. The values of the gates define the limits of each zone such that zones for currents in a ratio of 1.6:1 do not overlap.

The current that would flow in the circuit if the fuse-link was replaced by a link of negligible impedence. It is expressed as an ‘rms’ value.

I²t is also known as the Joule Integral and is a measure of energy. It is relevant to the performance of fuse-links under adiabatic conditions where there is negligible heat loss from the fuse-element. This occurs at times below 100ms.

Under adiabatic conditions a definite amount of energy is required to raise the fuse-element to its melting point; this amount of energy is called the pre-arcing I²t. As the fault current falls to zero, additional energy flows in the faulty circuit. This additional energy is called the arcing I²t. The total amount of energy flowing into the faulty circuit is the sum of the pre-arcing I²t and the arcing I²t. This amount of energy is called the total operating I²t.

Where the arcing I²t is reached before the prospective fault current has reached its maximum value, the element breaks the circuit at this lower value of current which is called the cut-off current. It is expressed as an instantaneous value.

Fuses are rated for use at frequencies from 45 to 60 Hertz without any noticeable effect on performance.

This is the maximum value of the earth loop impedance of a circuit to ensure that the fuse-link will operate in 5 seconds or 0.4 seconds. Each fuse-links current rating is assigned a ZS value to ensure that this requirement is met. The Zs values for individual fuse ratings can be found in Tables 41.2 and 41.4 of BS7671 (formerly the IEE Wiring Regulations)

The current CE mark indicates compliance of a product with the appropriate directives of the European Union (EU) and is marked on the product or its immediate packaging. It signifies that the product may be used in all EU countries without any restrictions.

If the product is made in the EU, the responsibility for applying the mark lies with the manufacturer.

If the product is imported into the EU, the responsibility for applying the mark lies with the original importer. The CE mark is not a Quality Mark, it is a mark of compliance.

Since the United Kingdom has left the European Union, the CE mark will be changed to UKCA mark. This will come in place in 2022 and will mean the product has been made in the United Kingdom.

ASTA is a certification body with a longstanding worldwide reputation for the testing of fuses. ASTA is now a specialist part of the Intertek Certification Company.

Since a fuse-link cannot be tested, calibrated or modified after manufacture, the satisfactory operation of the product relies on the continuing integrity of its design and manufacture. ASTA therefore operates a special scheme, known as the ASTA 20 Scheme. Under this scheme, the fuse-link is certified by ASTA, its design and construction details are registered with ASTA;  its production must be carried out under an ISO9001 scheme accredited by ASTA, who carry out regular surveillance to ensure continued adherence to the rules of the ASTA 20 scheme.

An ASTA Authorisation is issued for the fuse-link authorising it to be marked “ASTA 20 CERT”.