This post describes the various parametric attributes used to characterize fuses, and their significance for component selection. It covers single-use fuses only; self-resetting types and similar protection components such as circuit breakers are discussed elsewhere.
Digi-Key’s listings of single-use fuse products are divided into two product families. The Fuses family contains product of a more “typical” sort, commonly used on printed circuit boards, in fuse holders on small appliances, in automotive applications, and so forth. The Electrical, Specialty Fuses family contains products of a less routine nature, and are distinguished by characteristics such as particularly high interrupt current ratings, intended use in potentially explosive environments, having a UL class designation, uncommon form factors or being intended for very specific use cases.
Mounting Type
The “Mounting Type” attribute describes the mechanism by which a fuse is mechanically retained and/or electrically connected in an application. It is a non-specific parameter that indicates a general style or form rather than one particular design; devices having the same mounting type may or may not be mechanically interchangeable.
Bolt Mount
Figure 1. A sampling of bolt-mount fuses. (not to scale)
Bolt-mounted fuses are designed to use threaded fasteners for mechanical fixation and electrical connection, and are most commonly found in high-current applications. Because this connection method does not lend itself conveniently to fuse replacement without touching the electrical connection, they tend to be reserved for applications where faults are expected to be very infrequent, particularly at higher voltage levels.
Free Hanging (In-Line)
Figure 2. A sampling of inline fuses. (not to scale)
Free hanging fuses are designed for permanent installation at some point along a conductor’s length, typically without any provisions for mechanical mounting aside from the electrical contacts. Common applications include solar panel installations and automotive applications. Because this installation method does not allow for convenient replacement, they are typically reserved for applications where faults are expected to extremely infrequent.
Holder
Figure 3. A sampling of fuses designed for use with a fuse holder. (not to scale)
Likely the most common fuse mounting type, these products are designed for use with a holder that provides mechanical retention, electrical contact, and in most cases provisions for allowing fuse replacement without need of touching the electrical contacts. This allows for relatively convenient fuse replacement while the electrical supply remains energized, making this mounting type popular for applications where faults may be expected with some degree of frequency.
Holder, Surface Mount
Figure 4. A sampling of fuses pre-inserted into a surface-mount holder. (not to scale)
A relatively unique fuse mounting type, these products are typically a small ceramic fuse pre-inserted into a surface-mountable fuse holder, allowing convenient placement of a replaceable fuse assembly onto a printed circuit board with a single manufacturing step. They are used at the circuit board level in applications where the chance of a fault occurring is significant enough to warrant the additional cost of a replaceable fuse, compared to a similar permanently-installed product.
Surface Mount
Figure 5. A sampling of surface mount fuses. (not to scale)
Surface Mount fuses are designed for direct, permanent attachment directly to a printed circuit board using the same soldering or adhesive mechanisms used to attach other surface mount components. As such attachments are generally considered permanent, they are usually used in applications where faults are expected to be very infrequent. Like other surface-mountable products, they tend to be used at lower voltage and current levels than through-hole, bolt mount, or other mounting types.
Through Hole
Figure 6. A sampling of through-hole fuses. (not to scale)
Fuses designed for through-hole mounting are often variants of products designed for use with a holder, that instead make provision for permanent mounting by solder attachment after the device has been inserted into holes formed in a printed circuit board. Because many such devices use solder at some point in their construction, it is quite possible to cause damage or change the fuse’s characteristics by applying excess heat during assembly. Paying careful attention to the manufacturer’s recommendations in this regard is recommended.
Current Rating
A fuse’s current rating characterizes the maximum current that can be carried without expectation of the fuse opening, and is measured according to procedures established by applicable regulatory bodies. Importantly, the UL/CSA (North American) and IEC (European) standards differ for several common fuse styles; in general fuses produced to UL/CSA specifications are expected to open in no more than an hour at 135% of rated current, whereas IEC-spec fuses are expected to hold for at least an hour at 150% of nominal value. The implication of this is that IEC-spec fuses can generally be used at up to their nominal current value, whereas designing for continuous operation at no more than 75% of nominal rating is recommend for UL-spec fuses. Checking the datasheet closely is recommended, as the spec to which a fuse is produced is not always obvious from the device markings; an IEC spec fuse may also be a UL recognized component for example, and marked as such. Among the two most common small appliance fuse sizes however (5x20mm and ¼ x 1 ¼”) the electrical specifications to which most are produced tend to be consistent with the units of measurement used to describe their size.
Application variables (temperature in particular) will influence the actual level of current flow at which a fuse will open. A quality fuse datasheet will provide information on how the nominal current rating (usually given for a 25°C temperature) should be adjusted to account for this effect. An example of such information is shown in figure 6. The note mentioning a 25% standard derating suggests these to be UL-spec products, which they are.
Figure 7. Temperature re-rating excerpt from the Littelfuse 452-series datasheet.
These recommendations for de-rating are intended to avoid unwanted opening due to a phenomenon known as fuse fatigue. Because a fuse element will heat up significantly prior to melting, approaching a fuse’s rated current too closely on a routine basis will tend to weaken it, eventually causing the fuse to open under conditions where such opening is unwanted.
Fuse Type
The Fuse Type attribute describes the general form factor and construction style of a product in informal terms, often in reference to a particular application environment or distinguishing product feature. “Automotive” fuses for example are those of styles commonly used in automotive applications, “indicating fuses” are designed to provide a particularly prominent visual indication of having opened, and so forth.
Because they are commonly available in similar sizes with similar ratings, the distinctions between glass and ceramic cartridge style fuses are often a matter of question. In general, ceramic fuses offer higher breaking capacities and less risk of rupture during a serious fault than a glass fuse of equivalent size and nominal rating. In exchange, glass fuses offer the convenience of being able to determine (usually) whether or not the fuse has opened through a simple visual inspection. These distinctions are not trivial; rupture of a glass fuse during a severe fault can create additional circuit faults and lead to significant equipment damage. Glass fuses should not be substituted for ceramic types for this reason.
Voltage Rating (-AC and -DC)
A fuse’s voltage ratings indicate the maximum AC or DC voltage of the circuit in which the fuse is rated for use. AC ratings refer to nominal RMS voltage values, DC ratings to DC values. For devices offering both AC and DC voltage ratings, the two figures will often differ; sometimes they will be the same. Interrupting a fault in a DC circuit is typically a more severe case than in a comparable AC circuit, because the reversing current flow in an AC circuit helps quench arcs within the fuse that form as the fuse elements open.
Ultimately however, fuse voltage ratings are less a measure of a given device’s particular characteristics than a description of the limits to which they are tested and certified for use. As safety-related protection components, fuses are subject to standards established by various regulatory bodies around the world, with different agencies having various geographic and application jurisdictions. Use of a fuse in a circuit at voltages beyond its rated value creates a risk of a fuse failing to open properly, due to formation of an arc between its terminals (or what remains thereof) after the fuse element melts. When such arcs do form their effective resistance can be quite low, allowing fault current to flow almost as readily as if the fuse was still intact. For this reason, fuse voltage ratings are NOT additive; two 250V fuses in series for example, are not appropriate protection for a 480V circuit.
Response Time
A fuse’s response time attribute qualitatively describes how quickly a fuse opens in response to an overcurrent condition. Inrush currents, motor startup loads, and similar phenomena often cause brief periods of high current flow in excess of what would be permissible on a long-term basis. For such applications, slow-blow fuses are used, to avoid unwanted opening of the fuse in response to routine, safe conditions or nuisance opening due to fuse fatigue. In many electronic or appliance applications where such momentary current surges are not expected, fast-blow types may be used to provide a faster response time to over-current conditions. As an example figure 7 shows the characteristics for Littelfuse 312 and 313 series fuse products, both a 3AG size glass fuses having fast and slow response times respectively. At a current of 2A, the fast 1A fuse would be expected to open in approximately 0.3 seconds, while the slow fuse would require nearly 20 seconds to open.
Figure 8. Comparison of Littelfuse 312 (fast) and 313 (slow) series fuse characteristics.
Package/Case
A Fuse’s Package/Case attribute describes the physical form factor of the device, and can be used in conjunction with the “mounting type” and “fuse type” parameters to help identify mechanically compatible replacement fuses for an existing application. The 3AG (1/4” x 1 ¼”) and slightly smaller 5x20mm sizes are the most common in AC-powered appliance applications, with ATO/ATC style blade fuses most common in automotive use.
Breaking Capacity @ Rated Voltage
This parameter characterizes the amount of current that a fuse can safely interrupt, at its maximum rated operating voltage. Current flows may be many times their normal values under short-circuit fault conditions. In order to ensure that a fuse can safely interrupt such a fault, it is important to select one with a breaking capacity greater than the maximum amount of current available from the source that the fuse must interrupt, taking into account the characteristics of the source, cabling and all elements of the circuit.
Breaking capacities for a given device may differ depending on whether the AC or DC voltage rating is referenced; listed values do not specifically refer to either. Particularly for devices listed with both, the datasheet should be consulted to verify which voltage rating the listed breaking capacity figures apply to.
Melting I2t
The melting I2t parameter characterizes the amount of energy needed to melt the fuse element in a particular device, and is used in estimating whether a fuse can safely carry a current surge of a specified magnitude and duration, how much energy a fuse will allow to pass through to a faulty circuit before opening, and for similar purposes. It is a quantitative counterpart to the Response Time attribute (fast fuses have a lower I2t value than slow fuses for the same current rating) that is more useful for engineering purposes and less convenient for quickly selecting a desired product. It is commonly measured by applying progressively larger current pulses to a fuse of a given design until the amount of current flow required to melt the fuse element in a short period of time (around 8ms, roughly ½ cycle at 60 Hz) is established.
Approval Agencies
This attribute enumerates the various certification bodies from which a given fuse has received approval. Safety standards and certification processes vary across global jurisdictions, and policies may vary with regard to which approvals are required or accepted if a product incorporating it is to be lawfully sold in a given region.
Applications
Electrical and specialty fuses are often designed for use in applications with unique or uncommon requirements, such as in handheld multimeters or operation in potentially explosive environments. Where such usage is mentioned in product literature, it is reflected in the Applications attribute in order to help quickly identify these specialilzed products. Because this data is based on marketing copy rather than generally recognized technical specifications, it should be understood as loose guidance and not relied upon too heavily. For example, fuses marked for some particular application may also be quite suitable for other purposes, and not all fuses suitable for use in that application will necessarily be so marked.
Color
A number of fuse styles (mostly those intended for automotive use) are produced in differing colors, in an effort to aid identification of a fuse’s nominal current rating. Though useful as a guide, reference to labeled current ratings should be considered more reliable as color coding systems are not necessarily consistent across all fuse styles, and ambiguities exist which can lead to lead to inconsistencies in the information itself.
Class
UL standards establish a number of fuse classes that describe characteristics of devices suitable for application in different contexts. These standards cover topics such as physical form factor, interrupt ratings, time delay characteristics, and similar matters in an effort to facilitate availability of products suitable for different purposes, while minimizing risks arising from improper fuse replacement.
Products listed with a UL class designation are commonly (though not exclusively) of a current limiting type and used to protect feeder or branch circuits of local power distribution networks operating at 600V or less, with current ratings extending into the thousands of amps. Though addressed within the same bodies of standards, the fuses found in small appliances, automobiles, and similar applications with which the average consumer is most familiar are classified as providing supplementary protection, and not typically listed with a UL class designator.
The term “current limiting” in this context refers to a fuse behavior that limits worst-case current flow as the fuse is in the process of opening during a short-circuit fault event. This is typically achieved by packing the space around the current-carrying element of a fuse with a specialized sand, in order to cool and quench the electric arc formed between the remnants of the fuse element as it melts.