The product standard only contains minimum requirements. Attention is drawn to the fact that appliance specifications might contain requirements additional to or deviating from those specified in the relevant product standards.

Please be aware that the specifications nominal value used in the German part of the Schurter catalogue and the data sheets, is synonymous with rated value.

The difference between these two values is a pure matter of definition. In order to avoid any unnecessary complications we will continue to use the specifications nominal value.

CE marking is the only marking which indicates that a product conforms to the relevant EU-directive.

This means that the CE-mark is no quality or standard conformity mark but only an administration mark.

SCHURTER products are covered by the low voltage directives 72/23/EEC and 93/68/EEC. Those are valid for equipment and appliances with rated voltage values between AC 50 V to AC 1000 V as well as DC 75 V to DC 1500 V.

The CE marking of SCHURTER parts will be found on the label of the smallest packing unit. On request we will submitt a CE conformity statement for each component. CE conformity statements and approvals can also be retrieved from the internet under www.schurter.com.

National testing institutions are testing according to national and international standards or other generally recognized rules of technology. Their certification/approval-marks confirm the observance of the safety requirements which electric appliances must fulfil.

Approvals

National approvals

In addition to the combined UL/CSA approvals, most of the SCHURTER components are also approved by one of the European certification bodies like VDE (Germany), Electrosuisse (Switzerland) or SEMKO (Sweden). The safety testing of all these European certification bodies are based on the commen European safety standards. With the harmonisation effort in Europe, the different national European certification bodies have lost their importance and SCHURTER has decided to maintain only one European approval (e.g. VDE, SEV or SEMKO) in future. The others will not be renewed once they have expired.

Because UL and CSA are not members of the CENELEC, the standards of UL and CSA are not harmonised yet with the European standards. However, UL and CSA are trying to harmonize their standards with each other. Where possible, SCHURTER will apply for the combined cULus or cURus approval.

Further to development in Asia, SCHURTER has obtained national approvals from China, Japan and Korea.

Information about approvals

SCHURTER products are certified according to EN / IEC standards and carry country specific approvals in Europe.

During the last few years European countries made much effort to reduce their approval marks to one generally accepted mark. The ENEC approval mark replaces (wherever possible) the previous approval mark. The ENEC mark is offered by all national certification bodies that signed for the European certification agreement (CCA)*.

SCHURTER decided to reduce the variety of European approval marks. For new approbations of SCHURTER parts only the ENEC will be mentioned in the future:

Approvals for the US and Canada are according to the UL and CSA standards:

As UL and CSA are not a member of CENELEC these two are not according to the European approval marks. Wherever possible SCHURTER want to acquire the combined cULus approval mark:

Since Aug. 1^st. 2003 the Chinese approval mark is required for a lot of products to import to China. SCHURTER strives to get the approvals for the concerned products. For not testable products we offer an import certificate (free of CCC).

Further information:

http://www.enec.com

Approval Industry Links

* members of ENEC agreement:

Standards IEC 60529; EN 60529 and DIN 40050

Scope

These standards apply to the classification of degrees of protection provided by enclosures for electrical equipment with a rated voltage not exceeding 72.5 kV.

Object

The object of these standards is to give:

a) Definitions for degrees of protection provided by enclosures of electrical equipment as regards:

1. Protection of persons against access to hazardous parts inside the enclosure

2. Protection of the equipment inside the enclosure against ingress of solid foreign objects

3. Protection of the equipment inside the enclosure against harmful effects due to the ingress of water.

b) Designations for these degrees of protection.

c) Requirements for each designation.

d) Tests to be performed to verify that the enclosure meets the requirements of these standards.

Designations

The degree of protection provided by an enclosure is indicated by the IP code.

Elements of the IP code and their meanings

A brief description of the IP code elements is given in the following table.

1. Protection against direct and indirect contact – general terms

The protection against electric shock on electric equipment as well as their components are divided into the following parts:

¹⁾ Accessible, conductive part, which is not conductive normally but which may be conductive due to a failure.

2. Protection against direct contact with live parts e.g. of a fuseholder

The data sheets of the relevant components inform about the taken measures.

3. Protection against indirect contact

Measures for the protection against indirect contact on electrical equipment are defined according to IEC 61140 by the 4 protection classes 0, I, II, III. Each protection class includes two protection measures. Even if one of these measures should fail, no electric shocks will occur.

Explanations, application notes

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.

1. Fuse

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.

2. Fuse-link (IEC 60127)

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.

3. Miniature fuse-link (IEC 60127)

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.

4. Sub-miniature fuse-link (IEC 60127)

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).

Standards for fuse-links

6. Rated voltage U_n

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.

7. Rated current I_n

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:

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:

8. Voltage drop

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.

9. Non fusing current I_nf

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:

UL fuse-links are normally divided into:

Application notes for the various characteristics:

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.

At direct current the breaking capacity of a fuse-link is lower than at alternating current. Values are given on request.

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.

12.1. Max. sustained power dissipation

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 I_nf 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:

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 · I_n on the outer surface of the fuse-link according to the UL standard.

12.2. Rated power dissipation

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)

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.

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:

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

14. Ambient air temperatures

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

Example: T¹⁾ 200 mA²⁾ L³⁾ 250 V^{4) ₅₎}

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.

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.

18. Quality assessment of fuse-links

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

More detailled information is available on request.

19. Reliability of fuse-link (MIL-HDBK-217F)

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.

Fuse-link selection guide

Telecommunication equipments serve for data exchange between a variety of subscribers. Communication takes place in various ways, e. g. per telephone, FAX etc.

This gives rise to the following classical network topology:

There can be extremely diverse distances between individual subscribers (man, machine). This means that network connections (overhead lines, signal cables) can be subject to various interference sources.

• Atmospheric interference, (lightning discharge, switching operations)

• Interference by power induction (equalizing currents, vicinity of power cables)

• Direct contact with energy network (short-circuits)

Atmospheric interference (Lightning Surge)

Interference through atmospheric discharge is very frequent. Occurring voltages are of the order of 100 kV with discharge currents up to 150 kA. Effects due to direct lightning stroke are principally to be expected on exposed signal lines (overhead lines).

Interference by induction (Power Induction)

Induction voltages occurring as interference on telecom lines are usually a result of circulating or equalizing currents in the earth or are produced by strong currents in adjacent power cables.

Direct contact with the power network (Power Contact)

The highest intensity and usually long duration influence on a telephone line (a few seconds to several minutes) is by direct contact with the power network, e.g. short-circuit with an adjacent power cable.

Regardless of which interference acts on the telecom equipment, it must be guaranteed at all times that no damage occurs, or only limited damage whose effects can be calculated.

As shown below, this requirement can be satsified by the use of appropriate protection circuits.

Protection circuits in the telecom branch are usually designed on the two-stage principle. They comprise a primary and secondary protection.

Primary protection

Primary protection frequently comprises a combination of resistors and surge arrestors and is usually located at the «building entry» interface.

The task of the illustrated primary protection circuit is to sufficiently reduce the high-energy interference distortion so that they can be safely absorbed by the following secondary protection.

The secondary protection

The secondary protection is normally located directly at the appliance entry of the telecom equipment and has two objectives.

1. It operates as a voltage limiter which ensures that interference up to a defined amplitude, not yet capable of activating the primary protection, is absorbed or reduced to a level harmless for the telecom equipment.

2. It effectively suppresses high energy level interferences, which can no longer be adequately absorbed by the primary protection (e.g. in case of direct contact between the signal lines and the power network), by galvanic decoupling of the circuit. This prevents the occurrence of serious damage, even fire, in the telecom equipment.

The following schematic diagram shows a frequently used and extremely reliable protection circuit for this purpose. The circuit, which in its simplest form comprises two fuse-links and two varistors, is characterised by an extremely attractive cost-benefit ratio. The varistors limit the interference voltage peaks to a level compatible for the telephone exchange, respectively subscriber circuit. Under these normal conditions, the fuse-links remain intact.

Under worst-case conditions, e.g. direct contact with the power network, where both the telecom equipment components and the varistors in the protection circuit would be seriously damaged or destroyed, the fuse-links interrupt the circuit, thus effectively and reliably protecting the telecom equipment.

Standards, introduction

Several standards have been established for the telecom application field, all of which are aimed at combining the interference influences, lightning surge, power induction, power contact, previously described under the title “Application Note�? together with the associated safety aspects, and to derive suitable testing methods for the components in question.

Various kinds of loads have been defined and standardised as testing criteria. They can be simulated with the aid of an appropriate test circuit. This provides circuit designers with the facility for optimally adapting the stages of a protection circuit to one another.

The presently relevant standards are:

ITU-T K.20 International Telecommunication Union

UL 60950 UL Standard for Safety for Information

Technology Equipment

IEC 60950 IEC Standard for Safety for Information

Technology Equipment

Telcordia GR-1089 Telcordia Technologies

TIA-968-A Telecommunications Industry Association

(The list is not exhaustive)

Tests:

SCHURTER fuselinks have been tested according to the following standards and testing criteria:

1. ITU-T K.20

Lightning surge: Test circuit

Test:

1. The pulse amplitude (generator no-load) is set to 1000 V and the pulse shape to 10 μs / 700 μs.

2. The pulse current I_puls is set to the value I_puls max. stated in the

3. Test mode : 10 single pulses, at an interval of 60 sec. alternating polarity.

Requirement: The fuse shall not interrupt the circuit.

Note:

With a charge voltage of UC = 1000 V, the standardized pulse generator in Para. 1 supplies a maximum pulse current Ipuls = 67 A, providing the current limiting resistor is R_D = 0Ω. The shunt R_M for the current monitoring has a very low resistance and has therfore no notable influence to the current amplitude. This means that the data sheet current 67 A does not represent the maximum permissible pulse amplitude of the fuselink in question, but the maximum current amplitude which can be supplied by the pulse generator. If a max. current higher than 67 A is to be expected in a circuit, the I²t-values of the fuse-link can be calculated using the formula I²t = 0.72 x I² _peak x t², as a good approximation in order that the selected fuse-link can accept the expected current pulse without interrupting the circuit.

Power induction: Test circuit

Test: The fuse-link in the test circuit AC 300 V / 50 Hz is loaded 5 times with I_eff = 0.5 A for 200 ms at intervals of 60 sec.

Requirement: The fuse-link shall not interrupt the circuit.

Power contact: Test circuit

Test: The fuselink in the test circuit AC 250 V / 50 Hz is loaded with the current value I_SC stated in the data sheet. The supply voltage is maintained for 15 minutes.

Requirement: The fuse-link shall interrupt the circuit.

2. UL 60950/IEC 60950

Test circuit

Test 1

The fuse-link in the test current circuit is loaded with a test current of I_SC = 40 A .

The AC 600 V / 50 Hz source voltage is applied for a total of 1.5 sec.

Requirement: The fuse-link shall interrupt the circuit.

Test 2

The fuse-link in the test current circuit is loaded with a test current of I_SC = 7 A .

The AC 600 V / 50 Hz source voltage is applied for a total of 5 sec.

Requirement: The fuse-link shall interrupt the circuit.

Test 3

The fuse-link in the test current circuit is loaded with a test current of ISC = 2.2 A .

The AC 600 V / 50Hz source voltage is applied for at least 30 minutes, or until stable thermal conditions are achieved in the telecom unit or until the fuse-link interrupts the circuit. This test is performed together with the equipment in which the fuse-link is installed.

3. Telcordia GR-1089

3.1 Lightning surge

Test circuit

Test:

Note: With a charge voltage of U_C = 1000 V, the standardized pulse generator in Para. 3.1 supplies a maximum pulse current I_puls = 14 A, providing the current limiting resistor is R_D = 0Ω . The shunt R_M for the current monitoring has a very low resistance and has no notable influence to the current amplitude. This means that the data sheet current 14 A does not represent the maximum permissible pulse amplitude of the fuse-link in question, but the maximum current amplitude which can be supplied by the pulse generator. If a max. current higher than 14 A is to be expected in a circuit, the I²t- values of the fuse-link can be calculated using the formula I²t =0.72 x I² _peak x t², as a good approximation in order that the selected fuse-link can accept the expected current pulse without interrupting the circuit.

3.2 Power cross

Test circuit

see UL 60950/IEC 60950

Test 2, Second Level (only TF 600)

The fuse-link in the test current circuit is loaded with a test current of ISC = 60 A .

The AC 600 V / 50 Hz source voltage is applied for a total of 5 sec.

Requirement: The fuse-link shall interrupt the circuit.

When it comes to Polymeric Positive Temperature Coefficient (PPTC) circuit protection, you now have a choice. If you need a reliable source, look to SCHURTER resettable overcurrent protectors.

SCHURTER’S PTC products are made from a conductive plastic formed into thin sheets, with electrodes attached to either side. The conductive plastic is manufactured from a nonconductive crystalline polymer and a highly conductive carbon black. The electrodes ensure even distribution of power through the device, and provide a surface for leads to be attached or for custom mounting.

The phenomenon that allows conductive plastic materials to be used for resettable overcurrent protection devices is that they exhibit a very large non-linear Positive Temperature Coefficient (PTC) effect when heated. PTC is a characteristic that many materials exhibit whereby resistance increases with temperature. What makes the SCHURTER PTC conductive plastic material unique is the magnitude of its resistance increase. At a specific transition temperature, the increase in resistance is so great that it is typically expressed on a log scale.

How SCHURTER resettable overcurrent protectors work

The conductive carbon black filler material in the PTC fuse device is dispersed in a polymer that has a crystalline structure. The crystalline structure densely packs the carbon particles into its crystalline boundary so they are close enough together to allow current to flow through the polymer insulator via these carbon “chains�?.

When the conductive plastic material is at normal room temperature, there are numerous carbon chains forming conductive paths through the material.

Under fault conditions, excessive current flows through the PTC fuse device. I²R heating causes the conductive plastic material's temperature to rise. As this self heating continues, the material's temperature continues to rise until it exceeds its phase transformation temperature.

As the material passes through this phase transformation temperature, the densely packed crystalline polymer matrix changes to an amorphous structure. This phase change is accompanied by a small expansion. As the conductive particles move apart from each other, most of them no longer conduct current and the resistance of the device increases sharply.

The material will stay “hot", remaining in this high resistance state as long as the power is applied. The device will remain latched, providing continuous protection, until the fault is cleared and the power is removed. Reversing the phase transformation allows the carbon chains to re-form as the polymer re-crystallizes. The resistance quickly returns to its original value.

To select the correct SCHURTER PTC circuit protection device, complete the information listed below for the application and then refer to the resettable overcurrent protector data sheets.

The benefits of SCHURTER resettable overcurrent protectors are being recognized by more and more design engineers and new applications are being discovered every day.

The use of polymeric fuses has been widely accepted in the following applications and industries:

• Personal computers

• Laptop computers

• Personal digital assistants

• Transformers

• Small and medium electric motor

• Audio equipment and speakers

• Test and measurement equipment

• Security and fire alarm systems

• Medical electronic

• Personal care products

• Point-of-sale equipment

• Industrial controls

• Automotive electronics and harness protection

• Marine electronic

• Battery-operated toys

• Telecom e lectronics

Protection against electric shock (against direct contact with live parts), for fuseholders

The assessment of the protection against electric shock assumes that the fuseholder is properly assembled, installed and operated as in normal use, e.g. on the front panel of the equipment.

IEC 60127-6 and EN 60127-6 divides into three categories:

a) Closed fuseholder

Remarks on PC 3

Influencing factors

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.

In view of the safety of electrical equipment the selection of the most suitable fuseholder is of great importance. Among other parameters, one has to make sure that the maximum admissible power acceptances and temperatures defined by the manufacturer are followed. Differing definitions and requirements in the most important standards for fuse-links and fuseholders are time and again origin for the incorrect selection of fuseholders.

To equate the rated current of a fuse-link with the rated current of the fuseholder, may, especially at higher currents, cause high, not admissible temperatures, when the influence of the power dissipation in the contacts of the fuseholder was not taken into consideration.

For a correct selection the follwing influence factors depending on the application and mounting method, have to be taken into consideration.

It is recommended testing the fuseholder with the choosen fuse-link in the worst case operating condition.

Rated current of a fuseholder

The value of current assigned by the manufacturer of the fuseholder and to which the rated power acceptance is referred.

Rated power dissipation of the fuse-link

(power dissipation at rated current)

Rated power acceptance and admissible temperatures of a fuseholder.

The rated power acceptance of a fuseholder is determined by a standardised testing procedure according to IEC 60127-6. It is intended to be the power dissipation caused by the inserted dummy fuse-link at the rated current of the fuseholder and at an ambient air temperature of T_A1= T_A2 = 23 °C (over a long period). During this test the following temperatures must not be exceeded on the surface of the fuseholder:

Illustration of temperatures experienced in practice

Correlation between operating current I, ambient air temperature T_A1 and the power acceptance P_h of the fuseholder.

This correlation is demonstrated by derating curves.

Example of a derating curve

I = operating current of the fuseholder

I_n = rated current of the fuseholder

The derating curves demonstrate the admissible power acceptance of a fuseholder depending on the ambient air temperature T_A1 for the following fuseholder operating currents: I << I_n, I = 0.7 · I_n and I = 1.0 · I_n. This power acceptance corresponds to the max. admissible power dissipation of a fuse-link.

The corresponding values for other operating currents can be interpolated between the existing curves or calculated as follows:

P _h = P _o - P _c = P _o - (R _c · I ² )

Selection of a suitable fuseholder with respect to the power acceptance at the corresponding ambient air temperature.

Summary

The adherence to the limits, indicated by SCHURTER, in particular the power acceptance limits at the corresponding ambient air temperatures and mounting conditions of the fuseholder, is important for the safety of the product. It is therefore necessary to observe the following two steps:

Step 1

Selection of the fuseholder based on the power acceptance

P_h at operating current I and maximum ambient air temperature T_A1.

Step 2

The reduction of the power acceptance of the fuseholder (from step 1) based on the different conditions at the mounting place etc. have to be determined by the design engineer responsible.

Examples:

Therefore, temperature measurements on the appliance under normal and faulty conditions are absolutely necessary.

What's given?

P_h at I << I_n = 2.5W

I = 0.7 · I_n = 7 A = 2.2W

I = 1.0 · I_n = 10 A = 2 W

What is the admissible power acceptance P_h of the fuseholder?

The result of the interpolation for the rated current I = 5 A

is a P_h of approx. 2,4 W.

The result of the calculation is

P_h = P_o – (R_c · I2) = 2.5 – (0.005 · 52) = 2.37 W P≈2.4 W.

Derating curves of the fuseholder, type FEF, rated current I_n = 10 A

Verification of the thermal requirements

Step 1

Conclusion (without consideration of step 2)

IEC: International Electrotechnical Commission

UL: Underwriters Laboratories Inc. USA

CSA: Canadian Standards Association

NF: French Standard

As mentioned in section 2, the most relevant standards define rated current and rated power acceptance differently. This lead in the past often to confusion or even to a wrong fuseholder design-in.

For example the standard UL 512 does not define a maximum power acceptance value, but sets a certain value of temperature rise for the fuseholder. For this reason the marked amperage values on the fuseholder, defined by UL and CSA, are not suggested to be used except in special cases.

In order to eliminate such confusion, SCHURTER new decided to define the rated current and rated power acceptance values according to IEC 60127-6 and EN 60127-6.

The most important definitions are to be found in section 2.

Conclusion

• The high UL and CSA current ratings are replaced by more realistic rated currents defined by SCHURTER.

Your advantages:

Varistors (Variable Resistors) are voltage-dependent, nonlinear resistors with a symmetrical current-voltage characteristic curve (Figure 1) where the impedance decreases when the voltage decreases. Varistors are often also referred to as MOV (Metal Oxide Varistors), ZnO (Zinc Oxide Varistors), VDR (Voltage Dependent Resistors) and TVSS (Transient Voltage Surge Suppressors).

Figure 1: Voltage/current curve, green: Normal operation, yellow: Conductive in the event of high voltages

Varistors come into operation when a power surge occurs, which may be caused by a lightning strike or by inductive load switching. Parallel-connected to the protective circuit, they keep the surge from exceeding a predefined level and thus prevent the downstream-connected components from being exposed to large voltage spikes (figure 2).

Figure 2: Voltage peak, red: Absorbed by the varistor, blue: Absorbed by downstream-connected components

A varistor's current flow during normal operation (high-impedance state) and its current flow when a malfunction caused by voltage transient occurs (low-impedance state) are shown in figures 3 and 4.

Figure 3: Current flow during normal operation (high impedance)

Figure 4: Current flow when a malfunction caused by voltage transient occurs (low impedance)

Varistors are made from a polycrystalline ceramic material, mainly zinc oxide and a number of metal oxides, sintered at a temperature of approx. 1250 °C. Their impedance changes depending on the voltage applied. The dependency is not linear, but symmetrical, and the impedance, within a small voltage range, changes abruptly from a high-impedance (MΩ) to a low-impedance (a few Ω) state

Operating voltage:

The catalogue data sheet's table of variants specifies a maximal AC and DC rated voltage which should not be exceeded when in operation; hence the voltage supply's tolerance range needs also to be taken into account when selecting a component.

Max. clamping voltage when transients/surges occurs:

This parameter defines the maximal voltage when a spike occurs. The voltage/current curves (figure 5) show the maximal clamp voltage on a standard surge (8/20 μs) according to IEC 60060. The downstream-connected components must be capable of withstanding this voltage level with a tolerance of between 10 and 20%.

Figure 5: Voltage/current curves for maximal clamping voltage, shown on the example of the AVTP, 20 mm disc size

Permissible peak current:

The number of peak currents (surges, transients) that a varistor can absorb over its lifespan depends on the amplitude and the surge duration. The catalogue data sheet's table of variants specifies how many one-time and two-time non-repeated standard surges (8/20 μs) according to IEC 60060 a varistor can withstand. Longer and/or repeated surges require an appropriate derating, as shown in figure 6.

Figure 6: Derating curve for quantity of surges and pulse duration

Energy absorption capability:

Energy absorption correlates to the permissible peak current. This parameter is stated in the catalogue data sheet using the standard surge waveform (10/1000 μs) according to IEC 60060. This value supports the selection process, when e.g. for a protection of a choke a varistor needs to be choosen. The energy of a choke can be directly calculated by the inductance and the current.

Minimal leackage current:

Figure 7 represents the varistor's behavior during normal operation (high-impedance state), showing the minimal leakage current at a given voltage.

Figure 7: Voltage/current curves for leakage voltage, shown on the example of the AVTP, 20 mm disc size

Varistor voltage at the 1 mA point:

The catalogue data sheet states minimal and maximal voltage values for when a 1 mA current is flowing through the varistor. This parameter, a standard by now, permits the comparison of varistors. If it deviates by more than 10% from the original value, the varistor is deemed defective. The term used in such cases is degradation.

Average power dissipation:

This parameter is negligible under normal operating conditions. Power dissipation becomes meaningful where there is not enough time for the varistor to cool down between two surges. When temperature rises, the varistor voltage at the 1 mA point decreases and the power dissipation is getting more. When this is the case, the varistor becomes defective.

Operation at high temperatures:

If a varistor operates at more than 85 °C, voltage, current and energy must be derated as shown in figure 8.

Figure 8: Temperature derating curve

Response time:

Radial leaded varistors typically feature response times of 50 ns or less.

Varistor voltage tolerance range:

All SCHURTER varistors have a tolerance range of 10%.

Intrinsic capacitance:

The catalogue data sheet shows information on the capacitance at 1 kHz, which increases along with disc size, while it decreases in proportion to the rise in nominal voltage. This show, that varistors are not suitable for protection in high frequency circuits. Varistors work in a very efficient way at line frequency of 50/60 Hz.

The following component standards apply to zinc oxide varistors:

Figure 9: AC or DC circuit

Figure 10: AC or DC circuit with thermal fuse

Figure 11: AC 3-phase circuit

Figure 12: AC 3-phase circuit with thermal fuse

Figure 13: Data lines

Figure 14: Switch-off protection

Figure 15: Switch protection (arc suppression)

Figure 16: Semiconductor protection

Varistores are typically used to enable appliance to pass the surge test according to IEC 61000-4-5. To give an example: An appliance must withstand ten 2 kV surges at 2 Ω line impedance and meet the required criterion B, i.e. the appliance may be temporarily impaired in terms of its purpose and/or properties as a result of the test, but it must reset itself autonomously. No permanent or temporary shutoff is permitted. Varistors make it possible, for instance, to reduce the 2 kV surges to a lower level (figure 17), which in turn allows manufacturers to include less expensive components offering lower surge voltage resistance in their circuits.

Figure 17: Test circuit for surge test according to IEC 61000-4-5.

Figure 18: Standard waveform (8/20 μs) according to IEC 60060

Step 1: Establishing the varistor's voltage

The lowest varistor voltage depends on the highest AC or DC voltage applied. Usually, the tolerance range is assumed to be between +/- 10 and 20%, by which the voltage supply may fluctuate. The following varistor voltages are commonly used:

- 24 VDC secondary voltage

- 120 VAC line voltage (USA

- 230 VAC line voltage (EU)

- 400 VAC 3-phase line voltage (EU)

- 480 VAC 3-phase line voltage (USA)

Another approach leading to the same result is to check whether the minimal voltage at the 1 mA point exceeds the peak value of the applied voltage including tolerance, such as is the case in Europe, where the line voltage is 230 V * 1.1 * √2 = 358 VAC. The 270 VAC (AVTP, 0071.27xx.xx) version, according to the catalogue data sheet, has a minimal voltage at the 1 mA point of 382 VAC and therefore meets the requirement.

If a varistor operates at temperatures of up to 85 °C, a voltage temperature derating of -0.05% / °C is to be taken into account in addition to the above. The catalogue data sheet parameters apply to an operating temperature of 25 °C. Ambient temperatures above 85 C and up to 125 °C necessitate the use of the temperature derating curve (figure 8).

Once the varistor voltage has been established, the maximal clamping voltage, which depends on the expected peak current, can be determined using the voltage/current curve (figure 6) as a reference. The circuit to be protected will require components capable of withstanding this level of interference voltage. The relevant data are usually available from the manufacturer. And again, a safety margin of between 10 and 20% needs to be maintained here as well.

Step 2: Establishing the maximum permissible peak current

Step 3: Establishing energy absorption

Step 4: Establishing power dissipation

Step 5: Testing the protective performance in practice

All important information regarding steps 2 to 4 can be found in the section on "Parameters".