Dan Gilman, OEM Sales Manager
Littelfuse Inc.
Fuses are vital to equipment protection and overall safety, but there are so many types of fuses and so many rules for applying them that choosing the right one for a particular application may seem a daunting task. The job of specifying a fuse type can be simplified by breaking down the selection process into seven steps: voltage rating, current rating, time current characteristic, interrupting rating, current limitation, physical size and indication capability. This article covers each of these.
• 150 percent of the largest motor full load current plus,
• 100 percent of all other motors full load currents plus,
• 125 percent of the continuous non-motor load plus,
• 100 percent of non-continuous, non-motor load.
Voltage Rating
The most common point of confusion is voltage rating. The voltage rating of a fuse is the maximum AC RMS or DC voltage at which the fuse is designed to operate.
Insufficient voltage rating in a fuse can lead to an unsafe condition. The rating must equal or exceed the circuit voltage where the fuse will be installed. It’s OK to use fuses for system voltages lower than the fuse’s maximum rated design voltage. For example, a
250 VAC fuse can be used at 240, 120, 32, or 12 VAC, but not at 277 or 480 VAC.
Note that the voltage ratings for AC and DC are different, because of the way a fuse works when it clears a fault. When a fuse opens, an arc develops between the ends of the opening elements. It is the job of the filler inside the fuse (usually silica sand) to extinguish this arc. This is much easier with AC than with DC because in a 60Hz system
AC voltage and current go to zero 120 times per second, while DC voltage is constant.
Fuses intended for DC have additional arc quenching means beyond that present in AConly fuses. For that reason, never use an AC-only fuse in a DC circuit.
Figure 2. NEC Table 430.72(c) (1 through 5) and 450.3(b) show requirements for primary and secondary overcurrent devices for transformers.
Current Rating
It may seem simple, but many engineers are unsure how to select the current rating of a fuse. A fuse’s current rating is the AC RMS or DC current that it can carry continuously under specified conditions.
Fuses selected for a circuit must have ampere ratings that meet
NEC (National Electrical Code) or UL (Underwriters Laboratory) requirements. These requirements establish maximum ratings and, in some cases, minimum ratings. NEC Articles 240 and 430 contain specific requirements. Figures 1 and 2 show representative NEC tables.
A good rule of thumb is to calculate 125 percent of the maximum expected load, then round up to the next standard fuse current rating. For example, if the maximum expected load is
50 A, then 125 percent of that is
62.5 A, and the fuse would be
Figure 1. NEC Table 430.152 shows maximum allowable fuse ratings in terms of full load current.
chosen at the next standard rating, or 70 A.
The rule is a bit different when motors are involved. When motors are a significant part of the load, the recommended fuse current ratings are higher. For mains and feeders rated 1 to 600 A (combined motor and non-motor loads) the guidelines are:
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One more factor enters into the current rating selection: because fuses are thermal devices, fuse ratings change with temperature. The current carrying capacity of fuses is
110 percent of the fuse rating when installed in a standard UL test circuit and tested in open air at 25°C ambient temperature. This allows for derating to 100 percent of rating in an enclosure at 40°C. At higher ambient temperatures, the continuous current carrying capacity decreases, as shown in Fig. 3. This closely follows the derating tables for all electrical equipment, which means that fuses can help reduce equipment burnout due to high ambient conditions. Time-delay fuses derate more quickly in higher ambient conditions, so they, in effect, act as “self-protecting” devices and maintain their integrity until opening.
When considering the temperature to which the fuse will be exposed, it’s useful to remember that other equipment in the cabinet may be more temperature-sensitive than the fuse. As such, the entire cabinet may require some sort of cooling. Reducing the maximum temperature in the cabinet will make the choice of fuse a little simpler.
Despite any calculated temperature derating for the fuse, the
NEC and UL tables still apply.
Time Current Characteristic
A fuse’s time current characteristic defines how long it takes to open under specified overcurrent conditions. There are three basic types: time delay, fast acting, and very fast acting, also know as semiconductor, as shown in the
Fuse Characteristics table.
Time delay fuses typically have dual elements, and are designed to carry a specific overload for a specified amount of time before opening. They can protect against overloads and short circuits without nuisance blowing on startup or other surges, while offering the
Figure 3. The current carrying capacity of fuses is 110 percent of the fuse rating when installed in a standard
UL test circuit and tested in open air at 25°C ambient.
Time-delay fuses derate more quickly in higher ambient conditions, so they, in effect, act as “self-protecting” devices and maintain their integrity until opening. same short circuit protection and current limitation as fast-acting fuses. They’re used for protecting inductive and other loads that have an initial in-rush of current when energized. Typical inductive loads are motors, transformers and solenoids.
UL Standards require that a time-delay fuse must withstand 500 percent of rated current for a minimum of 10 or 12 seconds, depending on the UL class. Time delay characteristics vary among fuse types, so it is important in severe duty applications to consult the manufacturer’s time current curves for the particular fuse under consideration. It’s also important to remember that those curves are averages, and the opening currents, especially at the time limits of the curves, can be greater or less than expected by at least 10 percent.
Fast-Acting fuses don’t have any time-delay mechanism. At 500 percent of the fuse current rating, typical opening times range from 0.05 to approximately two seconds.
Fast-Acting fuses are used to protect non-inductive loads with very little or no initial inrush of current, such as incandescent lighting, resistance heaters, ovens, general-purpose feeders, or branch circuits with little or no motor load.
Semiconductor, or Very Fast Acting fuses, are used to protect extremely sensitive electronic equipment like rectifier banks, and they usually have better current limitation than time-delay and standard fast acting fuses. Their special characteristics, including quick overload response, very low I2t and Ipeak, provide protection for components that cannot withstand low value overloads or short-circuit currents. The idea is to make sure that the I2t let-through of the fuse is less than the I2t withstand rating of the device being protected.

Semiconductor fuses cost more than conventional fast acting fuses, and it is increasingly more common that the electronics being protected are cheaper than the semiconductor fuses being used to protect them. Additionally, traditional fast acting fuses are being rediscovered as having sufficient current limitation characteristics to prevent catastrophic failure of most devices being protected. As such, the use of semiconductor fuses is declining as the cost of electronics continues to fall, and users begin to specify standard fast acting fuses for inexpensive protection of devices like variable frequency drives.
Interrupting Rating
Interruption Rating, sometimes referred to as the “withstand rating”, is the maximum short circuit current that the fuse can safely interrupt without causing a hazard. The fuses and circuit breakers inside a cabinet are generally marked with an Interrupting Rating (IR) or Ampere Interrupting Capacity (AIC). It’s important to note that the interrupting capacity of a fuse in a DC circuit will usually be less than its rating in an AC circuit.
Standard UL fuse interrupting ratings are 10,000 A, 50,000 A, 100,000 A and 200,000
A. Fuses rated at 300,000 A are also available, although this value is not a UL standard, but “self-certified” by the manufacturer or “special purpose” listed by UL. In general, all modern fuses have Interrupting Ratings of 200,000 A RMS minimum, although there are some off-brand fuses with very low Interrupting Ratings that could be hazardous.
However, do not confuse Interrupting Rating with Short-Circuit Current Rating (SCCR).
SCCR is similar to Interrupting Rating except that the SCCR applies to all the components of a system not just the circuit protection devices. The 2005 edition of the
National Electrical Code (NEC) requires that all UL Listed industrial control panels must be marked and labeled with a Short-Circuit Current Rating (SCCR), which is defined as the maximum current that the control panel can safely withstand for a specified time.
That time is either three cycles of the incoming alternating current waveform (50 ms) or until the overcurrent protective device clears the fault. SCCR is usually expressed in RMS kiloamperes at the system voltage, for example 18 kA at 480 VAC. The interrupting capacity of the fuses in the control panel or cabinet determines the fuse’s SCCR, but the fuse’s current limiting capabilities help to determine the SCCR of the entire panel.
If the line feeding the panel can deliver more short-circuit current than the equipment inside can safely interrupt, then an electrical short can cause a fire or even an explosion.
The SCCR of the industrial control panel must be greater than the maximum possible fault current available at the input to the panel, but the SCCR of a panel is not necessarily equal to the interrupting rating of its main overcurrent protective device. In most cases the SCCR is less than the interrupting capacity of the overcurrent device and is determined by the SCCR of the “weakest link,” or lowest-rated device, within the panel with one important exception. If the panel has current-limiting devices in the feeder circuit, its SCCR may be greater than that of the lowest-rated component. Thus it’s often possible to increase the SCCR of a panel by replacing just a few components with current-limiting fuses.
5000A with no problem. Note, however, that UL requires that the contactor be tested under these conditions and rated ahead of time. The contactor manufacturer should have available a set of ratings showing SCCR with and without current-limiting fuses.
Case Size
Fuses have become smaller as their technology has improved; a fuse 5-1/2 inches long can be replaced by one 2-3/8 inches or even 1-7/8 inches long (and with correspondingly smaller diameter). Some people are unaware of this, and specify the old, larger sizes. The newer fuses tend to cost more, but their holders cost less, so the overall cost is similar while the smaller fuses save valuable panel space.
One way to keep users from installing over rated fuses is to choose a fuse type with physical size differences between ratings. Class G fuses, for example, come in four different lengths corresponding to amperage (<15A, 20A, 25-30A and >30A). A user is then prevented from installing a fuse with a higher rating than that which the equipment was originally designed to accept. This method is not suitable for all applications, of course, but is worth keeping in mind.
Along with case size, the designer must consider mounting requirements. There are a variety of ways to mount fuse blocks: screw mounting, DIN rail, panel mounts, custom mounting, directly to bus bars without a block, or with a fuse clip mounted to a bus bar.
Indication
Being able to spot a blown fuse by looking is more than a convenience; it can shorten downtime and reduce maintenance costs. In the early part of the 20th century, household fuses were generally plug type (they screwed in like light bulbs) and had a window that allowed the home owner to easily identify when a fuse blew. Similarly, tubular glass fuses
(3AG type, for example) are often easy to spot when blown. Cartridge and tubular fuses with fiber bodies generally give no such indication, but there are some fuses available that indicate when they are blown. Some have a clear window that changes color, and some have a pin that extends from the end of the fuse when the internal element opens.
The pin can then trip a switch to provide a remote indication. The difficulty with the latter is that this type of fuse cannot be used in a UL listed branch or feeder circuit as it can only be listed as a supplementary protector, requiring that branch circuit protection is provided upstream. Another approach is to use a fuseholder with a lamp that lights up to indicate a blown fuse.
Fuse Classes
UL sorts fuses into classes based on multiple factors. A handy one-page guide to UL fuse classes is available at www.littelfuse.com/data/en/ Catalog_Page/FuseClasses.pdf. A more complete explanation can be found on pages 192 and 193 of the last chapter of the Littelfuse POWR-GARD catalog. It’s available at www.littelfuse.com/data/en/
Product_Catalogs/Fuseology.169-198.pdf.
Current Limitation
Current limiting fuses are designed to clear a fault before the maximum available fault current can flow. A current limiting fuse is one that opens and clears a fault within the first half electrical cycle (0.00833 seconds), thus reducing the peak let-through current to a value substantially less than the potential peak current that would have occurred if the fuse were not used in the circuit, per NEC Article 240-2. The green shaded area in Fig. 4 is a measure of the total destructive heat energy (I2t) that passes through a fuse before it opens a short circuit. The peak current in figure 4 is a measure of the total destructive magnetic energy (I peak) that passes through the fuse before it opens the circuit.
As mentioned above, the use of current limiting fuses limits the stress on the components in an industrial control panel and make it possible to increase the rating of the panel to more than the rating of the components inside. Looking again at Fig. 4, if the maximum available current (the peak of the gray shaded area) was 20,000 A, but the maximum current allowed by current-limiting fuses was 2000 A
(the peak of the green-shaded area). As such, the system designer could use a contactor rated for
Summary
Fuse selection seems complicated, but taking it step by step can make it much easier.
One last note, after going through all of this work, do not forget to put a label near the fuse holder saying what type of replacement fuse should be used. This label should include the fuse class, voltage and amperage, and will help prevent a user from replacing a blown fuse with the wrong type.
About Littelfuse
As a leader in circuit protection products and solutions with annual sales of $535 million in 2006, the Littelfuse portfolio is backed by industry leading technical support, design and manufacturing expertise. Littelfuse products are vital components in virtually every product that uses electrical energy. For more information, please visit Littelfuse’s web site at www.littelfuse.com.
Figure 4. A current limiting fuse opens and clears a fault within the first half electrical cycle (0.00833
seconds), thus reducing the peak let-through current to a value substantially less than the potential peak current that would have occurred if the fuse were not used in the circuit, per NEC Article 240-2. The green shaded area shows the total destructive heat energy (I2t) that passes through a fuse before it opens a short circuit.
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