Which Fuse types can be used in AC circuits?

It will depend on which protection object this Fuse need to protect.

Generally, The fuse rating is affected by the following factors:

1, Ambient temperature

Fuses protecting semiconductors may need derating for ambient temperatures above or below 21°C (70°F). Using derating graphs, you will need the adjusted fuse ratings at different ambient temperatures. Factors affecting ambient temperature include poor fuse mounting, enclosure type, and proximity to other heat-generating devices and fuses. We should determine The maximum high-speed fuse rating for each application using the ambient temperature of the Fuse’s installed location as described in the section on selecting the current rating.

2, Fuse operating temperatures

Operating temperatures vary by fuse construction and materials. Fiber tube fuses tend to run hotter than ceramic body fuses. Generally, for fuses with a ceramic body that are fully loaded under IEC conditions, the temperature rise lies from 70-110°C (158-230°F) on the terminals and 90-130°C (194-266°F) on the ceramic body. The fuse load constant for porcelain body fuses usually is 1.0, and with fiber body fuses, the factor usually is 0.8. Remember that temperature measurements can be misleading when determining whether a particular fuse is suitable for a given application. 

3, Forced cooling

Many installations need to maximize ratings; diodes or thyristors are force cooled by an air stream. Fuses can be similarly uprated if placed in an air stream. However, air velocities above five m/s (16.5 ft/s) do not provide any substantial increase in the ratings. For further information, see the sections on selecting rated current and datasheets.

4, Mean, peak, and RMS currents

We must take care in coordinating fuse currents with the circuit currents. Fuse currents are usually expressed in “Root-Mean Square” (RMS) values, while diodes and thyristors currents are described in “mean” matters.

5, Time-current characteristics

These are the time and current levels needed for a fuse element to melt and open. They are derived using the same test arrangement as the temperature rise test, with the Fuse at ambient temperature before each test. The nominal melting times were plotted against RMS current values down to 10 ms for branch circuits and supplemental fuses. The virtual melting time (tv) for high-speed fuses is used and plotted down to 0.1 ms. We can find The formula for determining virtual melting time in the glossary. The melting time plus arcing time is called total clearing time, and the arcing time is negligible for long melting times.

6, Cyclic loading/surges

Effects of cyclic loading, or transient surges, can be considered by coordinating the practical RMS current values and surge durations with the time-current characteristics. The following conditions should be accounted for when using published characteristics: • They are subject to a 10 percent (10%) tolerance on current • For times below one second, circuit constants and instants of fault occurrence affect the time-current characteristics. Minimum nominal times are published according to symmetrical RMS currents. • Pre-loading at maximum current rating reduces the actual melting time. Cyclic conditions are detailed in the section on selecting rated current.

7, Short-circuit performance

The Fuse’s short-circuit operation zone is usually taken as operating times less than 10 ms (1/2 cycle on 60 Hz supply in AC circuits). It’s in this short-circuit operation zone that high-speed fuses are current limiting. Since most high-speed fuse applications are on AC circuits, their performance data are usually given for AC operation. Where applicable, prospective RMS symmetrical currents are used.

8, I²t ratings

The pre-arcing (melting) I²t tends to be a minimum value when the Fuse is subjected to high currents (this value is shown in the datasheet). The total clearing I²t varies with applied voltage, available fault current, power factor, and the point on the AC wave when the short circuit initiates. The entire clearing I²t values shown are for the worst of these conditions. The majority of power semiconductor manufacturers give I²t ratings that should not be exceeded for their product during fusing at all times below 10 ms. These are statistically the lowest values the device has been tested to. For adequate device protection, the total I²t value of the Fuse must be less than the I²t capability of the device.

9, Peak fuse currents

Under short-circuit conditions, high-speed fuses are inherently current limiting (the peak let-through current through the Fuse is less than the peak short-circuit current). The “cut-off” characteristic (the peak let-through current against prospective RMS symmetrical current) is shown in the datasheets. Peak let-through currents should be coordinated with diode or thyristor data in addition to I²t values.

10, Arc voltage

The arc voltage produced during fuse opening varies with the applied system voltage. Curves showing variations of arc voltage versus system voltage are included in the datasheets. You must coordinate the peak arc voltage of the Fuse with the semiconductor device’s peak transient voltage limit.

11, Conductor size

The RMS current ratings assigned to Bussmann series fuses are based on standard-sized conductors at each Fuse end during rating tests. These are based on a current density between 1 and 1.6 A/mm². Using smaller or larger conductors will affect the Fuse’s current rating.

12, Package protection

Some semiconductor devices are so sensitive to overcurrents and overvoltages that high-speed fuses may not operate fast enough to prevent some or complete damage to the protected device. Regardless, high-speed fuses are still employed in such cases to minimize the effects of overcurrent events when the silicon or small connection wires melt. Without using high-speed fuses, the packaging surrounding the silicon may open, potentially damaging equipment or injuring personnel.

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