Fuse-Link

The design engineer of electrical equipment is responsible for its safety and functioning to humans, animals and real values. Above all, it is his task to make sure that the state of the art as well as the valid national and international standards and regulations be observed. 

The following information about fuse-links and their application are to be taken into consideration when selecting a fuse-link.

In view of the product liability of electrical equipment the selection of the most suitable fuse-link is of great importance. 

A fuse is a self-acting device that, by the fusing of one of its specially designed and proportioned components, opens the circuit in which it is inserted by breaking the current when this exceeds a given value for a sufficient time.

Definition according to IEC 60127:
The fuse comprises all the parts that form the complete device, that means fuseholder and fuse-link.

Definition according to UL 248-1:
A North American fuse is an IEC fuse-link. An IEC fuse is a North American fuse with a fuse-holder.

The part of a fuse including the fuse-element intended to be replaced after the fuse has operated. Fuse-links according to IEC 60127 relate to miniature fuses for the protection of electric appliances, electronic equipment and components thereof normally intended to be used indoors. These fuse-links are not permitted for equipment, which has to operate under special circumstances, e.g. in a corrosive or explosive environment.

An enclosed fuse-link of rated breaking capacity not exceeding 2 kA and which has at least one of its principal dimensions exceeding 10 mm.

A miniature fuse-link of which the case (body) has no principal dimensions exceeding 10 mm.

Sub-miniature fuse-links are especially suitable for printed circuit boards. They are available for the through hole technique and surface mounting technique (SMT).

The rated voltage is the voltage up to which the fuse-link correctly interrupts an overcurrent.

The rated voltage of a fuse-link must be greater than or equal to the operating voltage of the equipment which is to be protected.

The use during operating voltages below the rated voltage of the fuse-link is permitted only, when the instructions regarding voltage drop (pos. 8) are taken into consideration.

The fuse-links are on principle suitable for use at alternating and direct voltage. The breaking capacity at direct-voltage is however considerably lower than the one at alternating voltage. The performance of the fuse-link at direct-voltage mainly depends on the size of the time-constant T = L/R of the load circuit.

The rated current of the fuse-link corresponds to the operating current of the equipment to be protected. Basically there are two different rated current definitions:
a) On fuse-links according to IEC 60127 and EN 60127 the rated current corresponds to the current, which the fuse-link can be exposed to continually, according to the standardized regulations, without interrupting the fuse-link.
b) On fuse-links according to UL 248-14 however, the rated current corresponds to the current, which would interrupt the fuse-link already after a few hours. The current, which according to IEC, can flow constantly without interrupting the fuse-link, is approx. 0.7 · In.n

Regarding influences of ambient air temperatures > 23 °C on the rated current see pos. 1.

Correlation between the rated current of fuse-links according to IEC and UL:

The voltage drop across a fuse-link is measured at an ambient air temperature of 23 °C, when the fuse-link has carried its rated current for a time sufficient to reach temperature stability. Attention is drawn to the fact that problems can arise when fuse-links are used at operating voltages considerably lower than their rated voltage. Due to the increase of the voltage drop when the element of a fuse-link approaches its melting point, care should be taken to ensure that there is sufficient circuit voltage available to cause the fuselink to interrupt the current when an electrical fault occurs. Furthermore, fuse-links of the same type and rating may, due to difference in design or element material, have different voltage drops and may therefore not be interchangeable in practice when used in applications with low circuit voltages, especially in combination with fuse-links of lower rated currents.

A value of an over-current specified as that which the fuse-link is capable of carrying for a specified time (typical 1 hour) without melting.

The time-current-characteristic indicates the relation of the pre-arcing time (melting time) to the fault current.

The pre-arcing time is the interval of time between the beginning of a current large enough to cause a break in the fuse-element and the instant when an arc is initiated.

The arcing time is the interval of time between the instant of the initiation of the arc and the instant of final arc extinction. The arcing time is not considered in the time-current-characteristic.

The operating time (total clearing time) is the sum of the pre-arcing time and the arcing time. 

The time-current-characteristics are shown as an envelope for all mentioned rated currents.

Usual time-current-characteristic and their symbols: 
FF: denoting very quick acting
F: denoting quick acting
M: denoting medium time-lag
T: denoting time-lag
TT: denoting long time-lag 

UL fuse-links are normally divided into:

• Non time delay fuse-links. These fuse-links are sometimes also referred to as normal blow or quick acting types.

• Time delay fuse-links. These fuse-links are sometimes also refered to as slow blow or surge proof types.

Application notes for the various characteristics:
FF: Super-quick-acting fuse-links
Protection of semiconductors (thyristors, triacs, diodes).
This fuse type tolerates small overcurrents only during a short period of time and limits the current at small short circuit currents. Current limiting even with low short circuit currents.

F: Quick-acting fuse-links
Protection of semiconductors and of an equipment with no current surge when operating or switching on and also for such devices where high overcurrent or high short-circuit current must be interrupted quickly.

M: Medium time lag fuse-links
Protection devices subjected to moderate in-rush currents and/or overcurrent peaks for a short time. Low voltage drop.

T:  Time-lag fuse-links
Protection of devices subjected to high in – rush currents and/or overcurrent peaks which decrease only slowly (e.g. transformers and motors).

TT: Super time-lag fuse-links
Protection of devices subjected to longer lasting in-rush currents and/or high overcurrent peaks.

A value (r.m.s. for alternating current) of prospective current that a fuse-link is capable of breaking at a stated voltage under prescribed conditions of use and behaviour.

The max. short-circuit current, which can occur in electric circuit of an equipment, due to fault conditions, may not exceed the breaking capacity of the fuse-link. Non-compliance of this rule can cause the danger of explosions and fire.

IEC 60127 miniature fuse-links are classified into two categories (for sub-miniature fuse-links other breaking capacities are defined).

Fuse-links with low breaking capacity, symbol L:
Typically, the fuse-element of this type of fuse-link is visible. The insulation tube consists of transparent material, normally glass. There is no extinguishing medium, the arc is quenched in air. 

The breaking capacity is:
250 VAC/35A or 10.In p.f.1 whichever is greater.

Fuse-links with high breaking capacity, symbol H: 
Typically, the fuse-element of this type of fuse-link is not visible. The insulation tube normally is of ceramic material or glass. To quench the arc, there is often an extinguishing medium.

The breaking capacity is:
250 VAC 1500A p.f. 0.7 to 0.8 

UL's and CSA's short circuit requirements (interrupting rating IR) are different as relates to IEC.

Interrupting ratings at 125 VAC = 10’000 A } p.f. 0.7-0.8
250 VAC = 35 to 1500 A, depending on rated current of the fuse-link.

a) Fuse-links according to IEC 60127:
The test is carried out according to a standardised test procedure (open fuse-holder, room temperature). The power dissipation produced by the non fusing current Inf after one hour is determined. Non fusing currents are different and depend on the fuse-link type. In the SCHURTER catalogue you will usually find two values of sustained power dissipation, namely:

• The maximum sustained power dissipation i.e. according to IEC 60127.

• The typical sustained power dissipation of the SCHURTER fuselinks.

These values are mostly lower than the standardised ones. 

b) Fuse-links according to UL 248-14:
UL does not, like IEC, determine the sustained power dissipation, but measures the maximum permissible temperature increase from 75 °C at 1 · In on the outer surface of the fuse-link according to the UL standard.

The power dissipation caused by the rated current (over a long period). With respect to the power acceptance for the selection of a suitable fuseholder this rated power dissipation is considered.

I2t-value (joule integral)2
The integral of the square of the current over a given time interval. The I2t-value is a measure of the energy required to disrupt the fuselink. That means for heating up the fuse-element to its melting temperature, for melting the fuse-element and for interruption of the current via an arcing period. Normally, distinction is made between:

The pre-arcing I2t (or fusing I2t)22
is the I2t integral extended over the pre-arcing time of the fuse-link. It represents the energy for heating up and melting the fuseelement. At high over-currents with melting times <10 ms the prearcing l2t remains constant (adiabatic conditions). Sometimes the pre-arcing I2t is determined by 10.times the rated current, based on the time-current-characteristic. The pre-arcing I2t is a characteristic value of a fuse-link and informs about his resistance to pulses and in-rush-currents.2222

The arcing I2t2
is the I2t integral extended over the arcing time of the fuse-links. It represents the arc-energy. The arcing I2t depends on the electrical circuit parameters (e.g. operation voltage, power factor, closing angle etc.) of an electrical circuit.22

The operating I2t (or: total I2t)22
is the sum of pre-arcing and arcing I2t. This value is an important parameter for the application of a fuse-link. It characterises the energy exposed to the object (let-through-energy) to be protected by the fuse-link in case of a fault current.2

Application notes:
In order to choose the right fuse-link, the permitted I2t-value of the component or component group to be protected, has to be known.

Selection criteria:
The electric circuit to be protected contains:

• Components, which can cause in-rush currents, e.g. transformers. In this case, a fuse-link has to be chosen with a pre-arcing I2t-value which is higher than the one of the in-rush-current.2

• Components, which are sensitive to current impulses, e.g. semiconductors. In this case a fuse-link has to be chosen, with an operating I2t-value which is lower than the one of the components to be protected.2

Shift of the operating current as a function of ambient air temperature

The standardised current carrying capacity tests (IEC and UL) of fuse-links are performed at 23 °C and 25 °C respectively. In practical applications, the fuse-link’s ambient temperature may be significantly higher, especially if the fuse-link is used in an unexposed fuseholder or mounted near other heat generating components. For such applications, the shift of the operating current according to the following diagram has to be considered.

Marking according to IEC 127

Additional marking: the respective approval marks
1) symbol, denoting the relative pre-arcing time-current-characteristic
2) rated current in mA or A
3) symbol, denoting the rated breaking capacity
4) rated voltage in V
5) SCHURTER Logo

Fuse-links according to IEC und UL have different features and are on principle not interchangeable. However, after a thorough check of the technical data it may be possible to interchange, when the following, most important requirements are met.

• The rated currents must be adapted (see pos.7)

• The breaking capacity must be compatible.

• The time-current characteristic and voltage drop must be roughly the same.

A fuseholder with an installed fuse-link shall not be used as a «switch» for turning power “on” and “off”.

An opening and closing of electric-circuits may cause current- and voltage surges, depending on the dimension of the electric circuit. Such current or voltage peaks produce an arc between the contact points, which causes an increase of the contact resistance. In order to prevent the fuseholder from permanent damage, a fuselink shall only be exchanged when power in an electric circuit is switched off.

SCHURTER fuse-links meet with the requirements according to IEC 60127-5 and EN 60127-5.

More detailled information is available on request.

The reliability modeling of fuses presents a unique problem. Unlike most other components, there is very little correlation between the number of fuse replacements and actual fuse failures. Generally when a fuse opens, or “blows” something else in the circuit has created an overload condition and the fuse is simply functioning as designed.

1. The operating voltage UB of the equipment to be protected defines the rated voltage UN of the fuse-link (see pos. 6) UN ≥ UB For UB << UN please refer to the remarks regarding voltage drop (see pos. 8).BNNBBN

2. The max. operating current of the equipment to be protected defines the rated current of the fuse-link. The different definitions for rated current according to IEC or UL as well as the influence of higher ambient temperatures are to be taken into consideration (see pos. 6 and 14). 

3. The possible fault current as well as its permitted operating times in the electric circuit of the equipment to be protected define the time-current-characteristic of the fuse-link (see pos. 10).

4. The necessary breaking capacity of the fuse-link depends on the max. short-circuit current, which can occur under fault conditions in the electric circuit of the equipment to be protected. It must be lower than the max. current which can be interrupted by the fuselink (see pos. 11). 

5. The rated power dissipation of the fuse-link is of importance for the selection of the suitable fuseholder (see pos. 12.2).

6. If current impulses occur in the electric circuit of the equipment to be protected, which may not interrupt the fuse-link under any circumstances or if the let-through-energy of the fuse-link may only reach a certain value (eg. protection of semi-conductors) the I2t values have to be taken into consideration accordingly (see pos. 13).2 

7. The necessary approvals are mostly defined by national and international standards for equipment. SCHURTER fuse-links are according to international standards and were approved by the different agencies (refer to data sheets for the individual fuse-links).

8. It is essential that the selected fuse-links/fuse-holders that are fitted to the equipment to be protected, are being tested under normal and fault conditions, even if all relevant criteria for selection have been taken into consideration.