1、 EIA STANDARD USERS APPLICATION GUIDE TO FUSES EIA-772-A JUNE 2008 EIA Standards Electronic Components Association NOTICE EIA Engineering Standards and Publications are designed to serve the public interest through eliminating misunderstandings between manufacturers and purchasers, facilitating inte
2、rchangeability and improvement of products, and assisting the purchaser in selecting and obtaining with minimum delay the proper product for his particular need. Existence of such Standards and Publications shall not in any respect preclude any member or nonmember of ECA from manufacturing or sellin
3、g products not conforming to such Standards and Publications, nor shall the existence of such Standards and Publications preclude their voluntary use by those other than ECA members, whether the standard is to be used either domestically or internationally. Standards and Publications are adopted by
4、EIA in accordance with the American National Standards Institute (ANSI) patent policy. By such action, EIA does not assume any liability to any patent owner, nor does it assume any obligation whatever to parties adopting the Standard or Publication. This EIA Standard is considered to have Internatio
5、nal Standardization implication, but the International Electro-technical Commission activity has not progressed to the point where a valid comparison between the EIA Standard and the IEC document can be made. This Standard does not purport to address all safety problems associated with its use or al
6、l applicable regulatory requirements. It is the responsibility of the user of this Standard to establish appropriate safety and health practices and to determine the applicability of regulatory limitations before its use. (From Project Number 5074 formulated under the cognizance of the P-14 Committe
7、e on Overcurrent Protective Devices). Published by Electronic Components Association EIA Standards and Technology Department 2500 Wilson Boulevard, Suite 310 Arlington, VA 22201 PRICE: Global Engineering Documents, USA and Canada (1-800-854-7179) International (303-397-7956) All rights reserved Prin
8、ted in U.S.A. EIA-772-A Users application guide to fuses Page 1 1 Scope This guide has been prepared to aid the circuit and system designer in the selection of supplemental fuses. Traditionally, these have been fuses that interrupt the primary equipment power source when an overcurrent condition dev
9、elops in a device or product. This guide will cover only fuses intended for electronic applications. 1.1 Introduction Fuses that are covered in this Application Guide typically range in physical size up to 10.4 X 40 mm. These fuses fall within the ANCE-248 (Mexico)/CSA C22.2, 248 / UL 248, Supplemen
10、tal Fuse Standard for North American applications. Current ratings covered by this Standard range from milliamperes to as high as 60 amperes, with voltage ratings up to 600 volts AC or DC. These devices are available in a variety of mounting schemes including plug-in, cartridge, through-hole and sur
11、face mount. Fuses are quite complex despite their simple appearance. Each fuse type and current rating will have unique operating characteristics, maximum operating limits and internal construction. Subtle variations between manufacturers designs will affect equivalency even between two seemingly id
12、entical devices. Further, any given type of fuse from a single manufacturer will have electrical and mechanical parameter variations that result from slight production variations during processing within a given lot and from lot to lot. Safety agency approvals, e.g., ANCE, CSA, and UL, are related t
13、o parameters for fuses employed in worst case applications. EIA will set minimum, non-safety related, requirements for fuses that will not serve to diminish minimum requirements set by the safety agencies. These will be comprised of additional requirements not associated with safety-related paramete
14、rs. The goal of EIA is to standardize qualification performance and characteristics of equivalent fuses, manufactured by multiple sources in an effort to accommodate interchangeability. Careful investigation by the design engineer must be carried out to determine what type of fuse is best for a give
15、n application. It may be beneficial to use both a fuse and some other device to provide more complete circuit or component protection. Typically, an engineer designs a circuit to meet specified requirements. During the design phase it is important to consider circuit protection needs. The engineer s
16、hould always assume that some type of circuit protection will be required. The fuse may ultimately be determined to be unnecessary, resulting in cost savings. However, the proper spacing will have been made available to safely accommodate the fuse, if it is required, and the product development can
17、proceed on schedule within the initially budgeted costs. EIA-772-A Users application guide to fuses Page 2 1.2 Design parameter considerations to be addressed safety agency approvals; open circuit voltage; short circuit current potentially available (Interrupting current rating) steady state circuit
18、 current; space limitation; worst case current inrush or current spikes (peak current, time duration and multiple event time intervals must be anticipated); maximum permitted voltage drop across the fuse at the standard steady state circuit current; mounting method (clips, soldered leads, holders, s
19、urface mount, etc.); plating compatibility with clips, holders and solder; environmental issues: temperature, humidity, shock I2t limitations; processing requirements (wave solder, IR reflow, aqueous detergent cleaning); open circuit indication (local and/or remote); RoHS compliance; These are only
20、a few of the preliminary considerations before beginning circuit design. After a specific fuse is selected, the next step is to determine if the choice was the proper one. Fuses are somewhat unique in the realm of electronic components; if they function properly during stress testing, they are eithe
21、r weakened or made inoperable. Nondestructive test results, as well as tight production control, must be used to predict whether or not a given fuse will continue to perform as desired in a given application. This usually requires significant testing of a given fuse in the final product. Testing in
22、the actual application is essential in many cases even though it is both a time consuming and costly part of the development process. A critical task, often left unfinished when any fuse is finally selected, is adhering to approved parameters. This should be done using a specification that sets limi
23、ts on critical electrical and mechanical properties. EIA-772-A Users application guide to fuses Page 3 The Application Guide that follows will help in avoiding the pitfalls associated with erroneous fuse selection and sizing parameters. If problems should arise that have not been covered in this App
24、lication Guide, (fuseholder selection), the fuse manufacturers are available to assist in the selection and optimization process. EIA-772-A Users application guide to fuses Page 4 1.3 Standards organizations Fuse standards have been developed on a national, regional, and international level. However
25、, care must be taken in evaluating these documents as the rating systems used may vary. For example, the electrical current rating system used for IEC 60127, Part 2 and UL 248, Part 14 fuses are not the same. In fact, in many instances the systems are mutually exclusive. 1.3.1 International standard
26、s International Electrotechnical Commission (IEC) IEC 60127, Miniature Fuses 1.3.2 Regional standards Council for Harmonization of Electrotechnical Standardization of North America (CANENA)* 1.3.3 National standards IEC publication “Electrical Standards in World Trade“ lists most of the National Sta
27、ndards Organizations in the world. The following list identifies some of the organizations that are known to certify or list miniature and surface mounted fuses: Mexican Testing Agency (ANCE) ANCE 248, Low Voltage Fuses Canadian Standards Association (CSA) - C22.2 No. 248, Low Voltage Fuses Underwri
28、ters Laboratories Inc. (UL) - UL 248, Low Voltage Fuses * CANENA facilitates the development of standards but does not publish any standards. EIA-772-A Users application guide to fuses Page 5 2 Guide to fuse characteristics 2.1 Time-current characteristics A fuse must open at currents that exceed a
29、defined current value within a certain period of time. The higher the fault current in the circuit, the greater the dangerous effect of this current will be, so the shorter time this current will flow in the circuit the better. The time-current curve is used as a typical graphical representation of
30、the average pre-arcing (fusing) time as a function of fault current. Pre-arcing is defined as the period of time from the moment a current sufficient enough to melt a fuse element begins to flow until arcing begins. In reality, significant current often continues to flow after the element has melted
31、 in the form of arcing current when the fault current is high and circuit voltage approaches the rated voltage for the fuse. The melting time + the arcing time is referred to as the clearing time. Melting time for a fuse is relatively easy to predict and measure. Arcing time is not easily predicted
32、nor measured. This is the primary reason that manufacturers will provide melting time curves. While all manufacturers now provide time-current curves using melting time, it is important for the circuit design engineer be aware of - and understand - clearing time. The time-current (T-C) curve depicts
33、 this relationship in the following fashion. Typically pre-arcing time is expressed logarithmically in seconds along the vertical (or Y) axis. The current is expressed logarithmically in amperes along the horizontal (or X) axis. These parameters (e.g. time and current) can be reversed on the X and Y
34、 axis and still provide the same ease of evaluation. The relationship forms the T-C curve for a given fuse rating and is used as a design aid. The curves can be shown either as a single line, or as a band. The band is the truest representation of the fusing function, since the fuses will have minor
35、variations due to the manufacturing process. These variations are greatest at the low level current overloads and become smaller as the current overload increases. They should be used to compare circuit requirements to the fuses performance by plotting the known fault currents and required opening t
36、imes on the fuses T-C curve and reviewing the results. If the fault condition falls to the left of the fuse T-C curve, it will not open. If the fault condition falls either on or to the right of the curve the fuse will open. Care should be taken if the overcurrent condition lies close to the T-C cur
37、ve. Product testing may be necessary to ensure proper coordination. T-C curves for IEC fuses must be assessed differently than T-C curves for fuses for use in North America. IEC curves usually represent the actual steady state current that can be used for safe continuous operation for a fuse. A UL o
38、r CSA (North American) fuse curve will typically represent the 100% rated current point, with the understanding that a fuse will never be used at above 75% rated current to obtain normal fuse life. For EIA-772-A Users application guide to fuses Page 6 example, an IEC 5A fuse can be used to continuou
39、sly carry 5A without degradation. The UL or CSA 5A fuse will degrade rapidly at 100% rated current, but will provide a long life if the steady state current does not exceed 75% of rated current. 2.2 Fuse characteristics A fuse can be characterized by its opening time relative to an overload. General
40、ly, fuses for use in North America are split into two categories: fast-acting or time delay. As can be assumed, fast-acting fuses open quickly when subjected to overload currents while time delay fuses allow a certain predefined minimum delay before opening. Other terms, such as “surge withstand”, “
41、medium time delay”, etc. will be tested by the safety agency as a fast acting or normal fuse. Similar terms used for IEC fuses include quick acting and time lag. IEC actually provides for five discrete types of fuse speed. 2.2.1 Rated current (IN) The rated current of the fuse is determined using sa
42、fety agency standards. The North American standards differ from those of IEC as follows: North AmericanNorth American fuses should not be used at a level greater than 75% of the fuses rating. Above this level they are susceptible to nuisance openings. IEC - in contrast, fuses rated in accordance to
43、IEC 60127 may be used at 100% of their rated current. In general, North American and IEC fuse rating are not interchangeable. In the case of using an IEC fuse as a substitute for a North American fuse, the circuit may not open as expected. This is because North American fuses are designed with lower
44、 fusing factors. If the situation is reversed the circuit may experience nuisance opening. Because of the specification differences no single fuse can comply completely with both standards. We often refer to IEC and North American fuses being “mutually exclusive.” The only way to obtain some form of
45、 approval from all the major safety agencies is to have an IEC approved fuse component “recognized” in North America. However, be aware that IEC fuses are typically designed for use in a 230 V environment, while the North American fuses are typically designed for use in a 125 V environment. If I cut
46、 the voltage in a circuit by nearly half, I will normally double the current to provide the same power to the circuit components. NOTEThere are significant differences that can exist between “listed” and “recognized” fuses in North America. “Listed” fuses are defined as those fuses that meet all of
47、the requirements of the appropriate North American standard. While, by contrast, “recognized” fuses need only meet the EIA-772-A Users application guide to fuses Page 7 requirements as defined by the manufacturer. Any circuit using a “recognized” fuse will be evaluated for the fuses ability to safel
48、y protect in the actual circuit application by the safety agency. 2.2.2 Temperature rerating Ambient temperature refers to the temperature immediately surrounding the fuse and is not to be confused with room temperature. The fuse ambient temperature is higher in many cases, because it is mounted or
49、enclosed near other heat generating devices. Since fuses are thermally responsive devices (they require heat to open) special care must be taken when locating a fuse in the circuit. The fusing element can be a relatively low melting temperature conductor , such as tin or bismuth, or it can be of a relatively high melting temperature conductor such as silver or stainless steel. The effect of ambient temperature will be greater on the low melting temperature element than on the higher melting temperature element. If the fuses ambient temperature diff
