Fuses Introductory Information

Transcription

1 Fuses Introductory Information Why a Fuse? There are many possible cases for overcurrents to occur due to a malfunction in appliances, including short-circuits: In low output power applications, miniature fuses are not used at all, or they may be simulated by PCB tracks with designed constrictions, or resettable fuses such as PPTCs and bimetal switches are used instead. However, the advantages of a miniature fuse are as follows: defined time/current characteristic; specified properties; enclosed disconnection point (to avoid destruction of adjacent components); electrical circuit isolation; standard terminations and dimensions; interchangeability. Miniature fuses can be effectively combined with other components for the purpose of overcurrent limitation. When series connected with a PTC, the miniature fuse is intended to provide short-circuit protection only, or, when combined with a varistor, it serves to limit overvoltages or overloads with the MOV suffering from extreme ageing. A miniature fuse can also be used to improve the interrupting behaviour of thermal cutoffs (extended range of maximum fault current). Spirit and Purpose of Approvals Approvals are certificates that prove conformity of special fuse properties with the requirements of a relevant standard. Current standards applicable to fuses are UL for the US-American area und IEC for the European area. Approvals granted by national testing bodies confirm that the tested fuses comply with the requirements of the relevant standard. The test mark of the respective certification body has to be indicated on the component and/or on the packaging. National testing bodies are, for example, VDE, SEMKO, BSI and IMQ. UL represents a special case in that it publishes standards and at the same time, as a service provider, performs testing. Using an approved fuse may facilitate product approval or acceptance by the relevant testing bodies depending on the application area. Fundamental differences between UL and IEC concern the rated current definition. The rated current of a fuse must be equal to or higher than the continuous operating current. Standard Voltage Admissible continuous current UL/CSA 250 V a.c. < 0,75 x I rat UL/CSA 125 V a.c. < 0,70 x I rat IEC 250 V a.c. < 1,00 x I N IEC (UMF) V a.c./d.c < 0,80 x I N

2 Properties of Fuses The most important point of a fuse specification is the time/current characteristic. This curve describes the breaking behaviour of fuses in the overload range which usually covers currents from 1 x I N or 1,5 x I N (depending on type and style) up to approximately 3 x I N. Above these currents, the shortcircuit range begins where specifically at 10 x I N the melting integral of a fuse (expressed in A 2 s) is calculated. This value is also used to determine whether a fuse is time-lag or quick-acting. The time/current characteristic is defined in different ways depending on the applicable standard: For UL types, there may be only one point where the fuse must operate within a specified time. In addition, a hold condition is specified where the voltage drop across the fuse under load (typically a constant current) is allowed to increase only by a defined amount within specified periods of time. This situation is equal to a kind of stabilization: The temperature of the fuse rises up to a certain level resulting in a thermally induced increase of the voltage drop across the fuse. If no stabilization occurs, it can be anticipated that the fuse will sooner or later operate; the respective UL requirement would not have been met. By contrast, the relevant IEC standard clearly defines time/current limits over the complete overcurrent and short-circuit range within which the fuse characteristic (a continuous line representing the geometric average values of the operating times for a range of test currents) must fall. The respective points are also known as hold gates and operating gates, respectively, and are indicated as black triangles that identify the time limits at multiples of the test current. In addition, and in particular at the hold gate, IEC compliant fuses must pass an endurance test of 125 h. The characteristic values measured in this test must also prove that there were no serious ageing effects over the test time. Time/current characteristics according to IEC specifications have the following appearance:

3 The curves vary depending on the differently scaled current axis: In the figure on the left it is scaled to multiples of the rated current, in the figure on the right it is log scaled (for comparison of different current ratings in one diagram). The second important point of a fuse specification is what is known as the breaking capacity, i.e. the maximum current that a fuse is capable of breaking at a stated voltage without impairing any fuseholder or adjacent components. The most common rated breaking capacities are (UL and IEC standards considered in combination): 250 V a.c.: 35 A, 100 A, 150 A, A (cos ϕ = 0,7); 125 V a.c.: 50 A, 100 A, A (cos ϕ = 0,7). With the exception of IEC (UMF standard), the breaking capacity is only specified for a.c. circuits. For IEC fuse types, a symbol denoting the rated breaking capacity is indicated directly on the component: L standing for 35 A or 10 x I N maximum, E for 150 A and H for A (cos ϕ = 0,7). Important parameters, particularly for densely packed applications with low operating voltages, are the maximum voltage drop of a fuse (generally measured at 1 x I N ) and the maximum sustained dissipation at 1,5 x I N. Fuse specifications are completed by supplementary information on environmental conditions such as: ambient temperature at normal operating conditions of the circuit, maximum vibration load, relative humidity. In the case of wired and chip fuses, mounting conditions such as soldering parameters (limits for solderability and resistance to soldering heat) must of course be considered. One point that often remains unconsidered is what is known as the rated current derating, given by the factor K T. This is not stated for a certain fuse type, but for classes of fuses having similar design features. Such derating describes a change of the rated current depending on ambient temperature variations, i.e. rated current reduction at higher temperatures and vice versa. It should be noted that this change of the rated current applies only to the overcurrent range up to approximately 3 x I N. Above this limit, the time/current characteristic does not show any change. Hence, an increased ambient temperature near the fuse does not affect its melting integral. A typical derating curve for fuse series 522.7XX is shown below:

4 For a fuse , 6,3 A, the rated current at an ambient temperature of 85 C would be reduced to approximately 5,6 A! Or, using a fuse , 5 A as an example, the time/current characteristic will change as follows: The green curve describes the original shape, while the red curve represents the time/current characteristic at an ambient temperature of 85 C. For special fuse types, it makes sense to state the cold resistance (measured at 1/10 I N ) for each current rating in addition to the basic parameters so that the voltage drop under pulse load conditions can be calculated. Hence, the core specification of a fuse covers: time/current characteristic; breaking capacity data and thus also the rated voltage; voltage drop;

5 sustained dissipation; melting integral and approvals, if applicable. A further issue is the marking of fuses (caps or enclosures): In addition to the rated voltage (e.g. 250) and the rated current in A or ma, symbols for the fuse characteristic (T, M, F etc.) and for the breaking capacity (L, E, H) as well as the most significant approval marks are indicated on a fuse. Thus, a fuse type is clearly described and can be replaced by a fuse of identical construction and equivalent specification, provided it is an interchangeable type. Construction Features, Fuse Characteristic The fuse characteristic has already been mentioned in the Properties section: it is measured and qualitatively classified by means of the melting integral. The relevant IEC standard specifies clearly defined classes to which the operating times at 10 x I N are allocated. For UL types, it is at the discretion of the manufacturer whether a fuse is classified as quick-acting or time-lag. However, these different characteristics clearly differ with regard to their melting integral. And, due to physical reasons, the curve shape in the overcurrent range is also different, i.e. a quick-acting fuse exhibits a much steeper slope compared to that of a time-lag fuse. The time/current characteristic of a certain fuse style is primarily determined by the fuse-element, i.e. by the core wire material, any applied electro-plated coatings and the way the fuse-element is mounted in the fuse enclosure, either as a straight wire or wound on a supporting material or as a wave shaped wire. The rated current is significantly determined by the diameter of the fuse-element. For enclosed fuse styles (glass, ceramic or plastic enclosures), the rated breaking capacity is controlled by filling media or by supplementary constructive means such as an additional enclosure of the fuseelement, outgassing holes etc. The style length has a significant influence on the rated voltage at which the rated breaking capacity is achieved. For telecommunication applications rated from 250 V up to 600 V elongated cylindrical fuses are therefore most commonly used to obtain a sufficiently long isolating distance for the arc during operation. Chip fuses owing to their design have a different structure where for time-lag types the rated current is adjusted by means of pastes and the fuse-element is arranged to fully utilize thermal coupling effects. Quick-acting types are provided with thin resistive layers or metallized surfaces that follow a given structure. The characteristic of chip types is significantly influenced by the substrate with regard to the coefficient of thermal conductivity. Standard fuse styles generally provide full-range protection, i.e. they are reliable in operation under any working conditions when used within the specified limits. Partial-range fuses are an exception to this rule and are primarily used for short-circuit protection only. Among these are also chip fuses of sizes 0805 to 0402, in particular those with high current ratings. When operated in the overcurrent range, these types may become desoldered on the circuit board due to excessive heat and this might result in an increased risk of danger or fire.

6 Selection Criteria for Suitable Fuses The following steps are intended to facilitate selection of a suitable fuse. Basic parameters are the rated voltage and the breaking capacity, i.e. the continuous operating voltage must not exceed the rated voltage or, respectively, a possible short-circuit current must generally be less than the rated breaking capacity. Intermittent voltage peaks of short duration (in the range of µs!) that exceed the rated voltage are admissible. For all a.c. applications, it should be noted that the values to be compared are r.m.s values! This does, however, not mean that a fuse with U N = 250 V a.c. is suitable for d.c. applications up to 350 V. Separate d.c. specifications are required in this case. If not available, the following rule of thumb applies: U DC = U N /2 must not be exceeded. For the maximum current that a fuse type is capable of breaking at such voltages the manufacturer must be contacted. On the other hand, a fuse needs a sufficient amount of voltage to meet the operating times according to the typical characteristic shape so that a voltage limitation imposed by the source does not lead to a decrease of the fault current and thus to an extension of the melting time, or cause the fuse not to operate. As a general benchmark, the following formula applies: U min = U f *I f /I N *X where X U min U f I f I N = 3 for time-lag fuses and 2 for quick-acting fuses; is the minimum voltage required for proper operation of the fuse; is the voltage drop across the fuse; is the fault current in the application; is the rated current of the fuse. A fuse is a current-controlled component. The current produces a voltage drop across the fuse-element and due to the resulting sustained dissipation the temperature of the fuse-element rises until the wire melts. Therefore, consideration of the continuous operating current of the respective application in relation to the rated current of the fuse is of great importance. An electric current may be composed of different components: a) pure d.c. component; b) a.c. current of known r.m.s. value; c) a.c. current with d.c. offset; d) a.c. current containing harmonic components (overtone spectrum); e) a.c. or d.c. current with pulse repetitive components, phase-synchronous or freewheeling in the first case; f) inrush current surge with large mark to space ratio tp/tcycle, or current surge caused by voltage limiting devices. For a.c. currents, a rough estimate can be given by just using the amplitude of the signal or rather the peak values. This applies to sine wave and triangular periodic current shapes. The r.m.s value can be calculated from a given signal shape I(τ) as follows: I eff t 2 = I ()d t t 0 t where I(t) = I 0 sin(ωt) + I 1 sin(2ωt) + I 2 sin(3ωt) + + I n sin((n+1)ωt).

7 I 1 to I n describe possible harmonic frequency components. If the signal shape cannot be described, discrete measuring points X n can be used according to the following formula: I eff n 1 1 x x x x... x n = n i1 = i n where the time intervals within a signal period must be constant: Δ t 1 1 = T = T n Δt The easier way, however, is to measure the currents listed in items a) to e) above. After the continuous operating current has been determined, for increased ambient temperatures the derating factor specified for the given device must be considered to adjust the current rating of the fuse type accordingly. In addition, the current rating has to be corrected according to the relevant standard. Thus, if for example I operation = 1,6 A, T amb = 85 C and the derating factor K T65 = 0,88 (example!), the properly selected IEC fuse type would be I operation *1/K T = 1,82 A, hence: 2 A. For UL fuse types, the factor of 1/0,75, for example, would additionally have to be considered due to the different rated current definition compared to IEC: I operation *1/K T *1/0,75 = 2,42 A, hence: 2,5 A. In all cases, the next higher rated current of the fuse type must be chosen. In a next step, any fault situations have to be verified, i.e. whether the operating time of the selected fuse is sufficiently short under overcurrent conditions. For inrush pulses (particularly in the case of switching power supplies without limitation of starting current), it has to be verified, whether the I 2 t value of the selected fuse is sufficiently high. In addition, the expected number of inrush pulses (n) per life cycle of the appliance must be considered. Assuming that the pulse is of exponential decay, the I 2 t value for the respective time interval can be estimated using the following formula: I 2 t pulse = I t 2 /2, with the following conditions: total pulse decay time = 5t, I(t) = 0,37*I peak. For proper selection of the melting integral of the fuse, the following relationship should be considered: I 2 t fuse 0,97*I 2 t pulse *log(n), where n 100. Finally, the following should be noted: If an appliance approval is planned to be simplified with the target market area being defined, attention should be paid to the fuse approval. Special Types, Application-Specific Solutions

8 There are many additional requirements, which cannot be addressed in detail here. Hence, only recommendations will be provided on what fuse type would be suitable for a given application (soldering processes such as infrared/wave soldering of surface mount components or wave soldering of wired components will not be considered, but will restrict selection of possible fuse types). The following list does not include typical fuse types for high-voltage, automotive or indicator applications: High pulse resistance at rated voltage of 250 V (protection of switching power supplies with parallel MOV at input, test with 8/20 µs wave at 2 kv with 2 Ω): 521.5XX 2,5 A, UL522.2XX 1,25 A, 522.4XX 1 A Circuits of high efficiency with transducers working in the khz range: 522.7XX, 522.3XX, 522.0XX Applications with enhanced continuous operating current (cooling and heating systems, fans): 521.0XX, 632.1XX, 632.5XX, 632.0XX, 632.2XX, XX, XX Low internal resistance of the circuit in short-circuit cases: 521.0XX 1,6 A, 522.7XX High packing density at moderate short-circuit currents (consumer area): 885.0XX, 887.0XX, 883.0XX Low signal distortion in data transmission etc.: SMD Chip Semiconductor protection (diodes, thyristors, etc.): 520.1XX, 520.5XX, 520.0XX, XX Industrial control systems SPS: ES-MSXX, 888.4XX, 220.0XX, 222.0XX Short-circuit protection for batteries: 823.6XX, 888.3XX, 888.2XX Telecommunication applications according to ITU-K20/21, Telcordia, UL 1950: 522.5XX 1,25 A, 530.6XX, ES-SMP 1,25 A Fuses for lamp ballasts: 515.3XX 1,25 A Universal UMF approval (approval to both UL and IEC standards): 900.0XX, 910.0XX