Understanding Fuses: A Guide for Designers

Homi Ahmadi

Because so many fuses are available, and because they and their reference standards vary, designers should prepare a checklist of requirements to aid selection.

Engineers must consider many factors when selecting a fuse for a circuit.

A task that most design engineers face when designing a piece of electronic equipment is the selection or specification of a fuse for a particular application. With so many fuses on the market, selecting the appropriate fuse for a specific application can be a difficult, and sometimes even confusing, matter. This article is designed to clarify some of the issues surrounding fuses in the design of electronic equipment. It focuses on what needs to be considered during the design stages of a project to avoid possible problems later in the product development cycle.

The Protective Role

The purpose of a fuse or similar circuit protector is to protect electrical devices and components against overcurrents and short circuits. Specifically, the device prevents overheating or fire in the event of a fault. A fuse, therefore, can be considered as performing two functions: to establish the maximum continuous fault current and to open the circuit when the fault current exceeds the maximum continuous fault current.

It is important to note that the effect of overcurrent or short circuit on electrical circuits can be dramatic if no appropriate protection is provided. Fire, insulation damage, and distortion of conductors are some of the possible results of inadequate protection. Therefore, to control overheating, it is necessary to limit power dissipation.

Structure and Function

A fuse essentially consists of an internal element that is placed, typically, in an enclosure and connected to contact terminals. When a fault current occurs, the high current flowing through the fuse melts the element, causing the link to open. This happens because, when the link is cold, it has a low resistance, but, as it gets hot, its resistance increases until a known melting point is reached. That is, the element state changes from a solid to a liquid or a gas within a defined period and thus drops the current to zero.

Circuit breakers accomplish the same end, but they differ from fuses in a significant way. Both fuses and circuit breakers are designed to interrupt the power to a circuit when the current flow exceeds safe limits. However, circuit breakers are fairly complicated mechanical devices whose thermal element takes the form of a spring. The spring expands with heat and thereby trips open the circuit. Usually resettable, circuit breakers are typically used in houses or in situations where high current is present. Although a circuit breaker may be resettable, such a device should not be used in place of a switch unless it is specially designed for that purpose.

Fuses are available in a variety of sizes and shapes, including miniature, subminiature, cartridge or blade, axial/ferrule, and surface-mount device (SMD) or chip style. Although most are what are commonly known as one-shot fuses, some types are resettable. Some of these come with either a glass or a ceramic body.

A discussion of fuse parameters and terminology follows, after which the article considers fuse-related issues of selection, application, and regulatory compliance.

Technical Parameters

Voltage Rating. Fuses are not only current sensitive. They must also always be operated or used at a voltage that is less than the rated voltage. High-voltage fuses may be used even in circuits with much lower voltage; for example, a 250-V–rated fuse can be used in a 120-V circuit. But operating a fuse at a voltage above its rated value could result in its explosion, which might lead to fire.

Current Rating. Manufacturers recommend a current to which each of their fuses may be loaded. This steady-state current level is the current rating.

Ambient Temperature. Ambient temperature is the external air temperature immediately surrounding the fuse. Current-capacity tests of fuses are performed at 25°C, but fuse performance is affected by changes in ambient temperature. A fuse runs hotter as the normal operating current goes higher. Practical experience has indicated that, in order to prevent premature blowing of the fuse, the ambient temperature should be kept below 70°C.

Interruption Rating. Also commonly known as breaking capacity, a fuse’s interruption rating is the maximum approved current that the device can safely interrupt at rated voltage. Depending upon the type and design of the fuse, this current can range from a few amperes to several thousand. Typically, glass fuses have a low breaking capacity, while ceramic ones have a high breaking capacity.

If breaking capacity is exceeded, there is a risk that the fuse could blow apart like a firecracker, expelling molten material, damaging surrounding circuitry, or even harming the user.

A typical rating in accordance with UL 248-14 is 10,000 A at 125 V ac. Using this as an example, if an overcurrent fault of less than 10,000 A occurs on a 125-V-ac line, the fuse will open safely. However, if the overcurrent is greater than 10,000 A, the risk increases for damage to occur.

IEC 60127-2 categorizes fuses into three groups in terms of their breaking capacities.¹ Low-breaking-capacity fuses are those tested to 35 A or to 10 times their rated current, whichever is greater. Industry uses the letter L to designate this type of fuse. A fuse said to exhibit enhanced breaking capacity is tested to 150 A and carries the letter E as a symbol of its capability. A fuse with high breaking capacity, designated with an H, is tested to 1500 A.

Figure 1. A typical time-current curve. An overload current to the right of the curve will open the fuse Table I. A comparison of North American and international fuse standards with respect to circuit-opening time under continuous overload.

Time-Current (T-I) Curve. The T-I curve is one of the graphs most widely used by engineers who deal with fuse characteristics. This curve is typically employed in calculating the melting time of the fuse at a target overload. It is typically drawn in a double logarithmic grid pattern. The vertical axis of the graph represents time in seconds, and the horizontal axis displays a range of fault currents in amperes (see Figure 1 as an example).

If the inrush current or the expected fault current of the equipment under test is known, the engineer locates the overload current on the x-axis of the graph and follows its line up to its intersection with the curve of the rated fuse. The time to interrupt can then be read on the y-axis.

Some manufacturers provide two curves on one sheet to represent a family of fuses, while others offer individual graphs for each fuse rating. In the first case, the upper curve represents the maximum fuse-blowing time and the lower curve, the minimum blowing time. The area between the curves indicates the range of blowing times for all ratings in a particular fuse series. A graph particularized for one fuse rating will give the typical opening time for the amperage rating selected. If the overload current falls to the right of the curve, the fuse will open, but if that current appears to the left of the curve, the fuse will not open.

Requirements for fuses used in equipment sold in Europe differ from those for fuses sold in North America.

The time-current curve is generally used as a design guide. The data are used to check whether a given fuse will cope with, for example, starting a motor or a power supply. These types of circuits produce a big inrush current. By knowing the value of that inrush current and consulting the T-I curve, the engineer can select a suitable fuse. If the device were to open too soon, it could be a nuisance for operation of the application. But if it did not open soon enough, the overcurrent could have an opportunity to damage downstream components. The T-I curve minimizes the likelihood of these undesirable outcomes.

I2t. This value, which signifies what is known as the melting integral or total clearing, is another very important fuse characteristic. The I2t determination is a measurement of the thermal energy necessary to melt the fusing element calculated by multiplying amperes squared by the time in seconds. This value denotes the energy to which the object to be protected by the fuse can be subjected before the fuse blows. It is used as a comparison between fuse links to judge the speed of operation. A fuse with a high I2t would let more energy through and take longer to fully operate (that is, blow) than a fuse with a lower I2t. It should be noted that the I2t is a constant value and is independent of temperature and voltage.

Fuse Selection

Table I. A comparison of North American and international fuse standards with respect to circuit-opening time under continuous overload (click to enlarge).

In addition to categorizing fuses by breaking capacity, IEC 60127-2 subdivides them according to their T-I curve. The fuse classifications identified are:

• Very-fast-acting (FF).
• Fast-acting/quick-blow (F).
• Medium-acting/semidelay (M).
• Slow-blow/time-lag/time-delay (T).
• Very-slow-acting/long-time-lag/super-time-lag (TT).

Choosing from among these types depends upon the circuit in which the fuse is being used and whether a high or low inrush current is present. For example, the application difference between a quick-acting and a time-lag fuse is that the former is typically used in circuits exhibiting little or no inrush current or in which high overcurrents or short-circuit currents must be quickly interrupted, whereas a time-lag fuse is used in applications characterized by high starting currents.

An engineer must consider many factors when selecting a fuse for a circuit. To help ensure that the selection is appropriate, designers should prepare a checklist of requirements before specifying a fuse. Some of the checklist items would be derived from systems specifications, whereas others would have to be calculated or taken from measurements. Some key items on a likely checklist include the following:

• Maximum operating voltage.
• Maximum operating current.
• Maximum fault current.
• Mechanical considerations.
• Ambient operating temperature.
• Melting integral.
• Intended market of the end product.
• Safety agency considerations.
• Type of fuse.

Application

Fuses can be used in series or in parallel in a circuit, but extreme care should be taken in designing the system. If two or more fuses are employed in series, then each fuse must be able to withstand the maximum operating voltage and maximum breaking capacity expected in the circuit. Using fuses in series layout is not a common practice.

However, if two or more fuses are to be used in parallel, they must be identical in every respect. It is also highly recommended that a derating factor be taken into consideration when paralleling fuses. Loading them to around 80–90% of the maximum allowable operating current leaves a safe margin.

Basically, when two fuses are used in parallel, the current rating is doubled. Two 8-A fuses in parallel would be essentially equivalent to one 16-A fuse, for example. Taking into account the recommended safety margin, the two fuses would actually be equivalent to around 14 A. The voltage rating and the interruption rating (breaking capacity) are, however, only as they would be for one fuse. For instance, if a fuse has an interruption rating of 1500 A, then two of them in parallel would still have only a 1500-A rating. A typical application calling for the use of two fuses in parallel would be one in which a single fuse with a particular value appropriate for the design is not widely available.

Regulatory Compliance

The next important fuse-related issue that must be considered is regulatory compliance. Requirements for North America and Europe differ. Most significant, the respective standards are based on very different conceptions of operating conditions.
North America. The fuse standard applicable in the United States is UL 248-14.² In Canada, it is CSA 22.2 No. 248-14.³ These two standards are practically identical.

Fuses are either UL listed or UL recognized. It is extremely important to know the difference between these two designations. A UL-listed fuse meets all of the requirements of the UL/CSA 248-14 standard. A UL-recognized fuse, however, does not necessarily meet all of its requirements. UL will test a fuse to a specification requested by the manufacturer and, finding that it meets the specification, recognize it as such. When using a UL-recognized fuse, or any other UL-recognized component, engineers should study the condition of acceptability provided by the certifier.

One of the advantages of using a listed fuse is that, if an engineer decides to use some other manufacturer’s fuse after the product has been approved and certified, this can be accommodated without any additional testing of the component in the end equipment, provided that the fuse rating and type are the same. A UL listing indicates that the fuse meets the UL 248-14 specifications, whereas recognition implies that it does not meet this standard for one reason or another; the latter indicates only that Underwriters Laboratories has tested the part and has confirmed that it meets the specifications laid out by the manufacturer. The two options allow companies to offer a wider variety of products, yet still have UL approval without being restricted to the UL 248-14 standard. For example, because of their small size, most surface-mount fuses cannot meet UL 248-14. However, UL will still approve them to a manufacturer’s specifications for safe operation.

Because a fuse is classified as a critical component by safety agencies, fuses are controlled and noted in the end-product safety reports produced through the CB Scheme or by UL. Therefore, it is recommended that equipment manufacturers, when getting a product approved at a test house, provide the agency with fuse samples from two or more manufacturers at the time of testing. This will prevent the necessity of recertifying the product if the manufacturer decides to use more than one tested component vendor.

Europe. Compliance requirements for fuses used in equipment sold in Europe and most of the rest of the world are governed by IEC 60127. This standard, like its North American counterpart, consists of many parts; the part of concern to this article is Part 2.

Test requirements in UL 248-14 and in IEC 60127-2 are not the same. Hence, a fuse carrying a UL or CSA mark cannot qualify for incorporation in a product intended for use outside North America without first undergoing additional testing. The converse is also true. Differences between UL and IEC fuses with respect to pre-arcing time versus the current characteristics of fuse activation (that is, the T-I curve) are significant. These two standards also rate overload in different ways.

The Standards Compared. Table I compares how the UL and IEC standards rate overload for a typical fast-acting fuse. The table makes clear that at the same overload current it will take the IEC fuse longer to open. Even though the UL fuse is rated to last at least four hours at rated current, experience has shown that it will eventually open at some point after that, if 100% of rated current is applied. Therefore, fuse manufacturers recommend that current at only 75% of the rating be applied continuously for fuses designed in accordance with UL 248-14. But with its different opening characteristics, a fuse designed in accordance with IEC 60127-2 can be run at a rated current continuously. The oversizing factor in that case is intended to simulate real-world conditions, by contrast with the tightly controlled laboratory conditions underlying the UL rating.

Another major difference between the North American and international standards is the current ratings. UL and CSA certify fuses to as high as 6000 A, depending on the classification according to UL 248-14, without there being any specific test increments; it is up to the individual manufacturer to submit and certify various current ratings. On the other hand, IEC 60127-2 lists individually specified current ratings— 1 A, 2 A, 3.15 A, and so on—for which fuses can be built and tested for approval. The maximum current rating to which a fuse can be approved according to IEC 60127 is 6.3 A. If one decides to use a fuse of greater than 6.3 A for the European market, then the fuse will be tested during end-product
testing.

The two standards differ significantly also on what is known as a stress test. IEC 60127 requires an endurance test whereby a fuse is subjected to 120% of its rated current for 100 cycles followed by one hour at 150% of the rated current. The test is said to be successful if the fuse still conducts and has a maximum voltage-drop increase of 10%. The UL and CSA standards do not require this stress test.

Sleeves and Holders

Sometimes the fuse in a product design is sleeved. It is vitally important to ensure that the sleeve has an adequate and correct temperature and flammability rating. All relevant safety standards require that the fuse’s rating (voltage, current, and type of fuse) be clearly marked adjacent to the fuse or fuse holder. Both IEC 60127 and UL 248-14 specifically require that the fuse casting be marked with the manufacturer’s name or trademark, the rated current, the rated voltage, and the breaking capacity.

In the majority of designs, a fuse is used with a fuse holder. Fuse holders are available in a variety of shapes and forms, the ones most commonly used being totally enclosed. Because they are made of plastic materials, careful consideration should be given to fuse holders when choosing one for an application. The principal matters of concern are:

• Ambient temperature close to the fuse holder.
• The cross-sectional area of the conductors that will be
connected to the fuse holder.
• The maximum power dissipation of the fuse.
• The fuse holder material, particularly its flammability
rating.

Conclusion

Fuses are small but critical electronic equipment design components. Proper fuse selection is important, but it can be confusing unless undertaken systematically and with full awareness of applicable regulatory requirements.

Use of a prepared checklist of device parameters and characteristics is recommended. Among other things, the checklist should include maximum operating voltage, normal oper-ating current, ambient temperature near the fuse, overload or inrush current and interval within which the fuse must open, dimensions and mounting method, the intended market of the end product, and required regulatory approvals. Designers must address these issues early to avoid problems later.

References

1. IEC 60127-2, “Miniature fuses—Part 2: Cartridge fuse-links,” Ed. 2.0 (Geneva: International Electrotechnical Commission, 2003).
2. UL 248-14, “Low-Voltage Fuses—Part 14: Supplemental Fuses,” (Northbrook, IL: Underwriters Laboratories, 2004).
3. CSA 22.2 No. 248-14, “Low-Voltage Fuses—Part 14: Supplemental Fuses,” (Mississauga, ON, Canada: Canadian Standards Association, 2004).

Homi Ahmadi is senior compliance engineer for Zebra Technologies Corp. (Camarillo, CA). He can be reached at hahmadi@zebra.com.