The fuse provides protection by melting away a thin metal link in the faulted circuit. The metal link may be of silver, copper, or nickel, silver being more common for long-term performance stability.
The fuse body is generally filled with a sand-type filler (Figure 1) to suppress sparks when the fuse link melts and interrupts the inductive energy in the load and wire loops of lead and return conductors. The fuse life is determined primarily by aging of the metal link under thermoelastic cycles of load on and off.
The ground insulation requirements on a fuse vary with the voltage rating. For example, the 120 V fuse may typically require:
- Dielectric withstand capability >500 V measured by megger.
- Ground leakage current at rated circuit voltage < 1mA.
- Withstanding full recovery voltage equal to the open circuit voltage in steady state and up to twice the circuit voltage during transient that appears across the fuse terminals after clearing the fault.
- Resistance after fuse clearing >1 MΩ to 10 MΩ, depending on the rated current and the maximum fault current it can interrupt without exploding.
- After fuse clearing, the body and terminal surface temperatures must remain below 50°C for fuses up to 100 A. For larger fuses up to 500 A, the body temperature must remain below 50°C, but the terminal temperature may rise up to 75°C.
Fuse Selection Criteria
Three key ratings to consider in the fuse selection are:
- rated amperes it must carry continuously,
- rated circuit voltage it must support, and
- the maximum current it must interrupt without exploding, which must be greater than the prospective fault current at the fuse location.
The fuse clears (melts or blows) at higher than rated current as per its fault current versus clearing time characteristic (known as i-t curve). Due to inherently large manufacturing variations, the i-t curve is typically given by a band as shown in Figure 2.
For example, at 5 times the rated current, the clearing time may be from 0.01 to 1.0 sec. This must be accounted for in the system design to properly protect the circuit so that it clears when required and does not clear when not required. The fuse ampere rating must be carefully selected because both a conservatively or liberally sized fuse is bad for the circuit protection. General criteria for selecting the current rating of a fuse are as follows:
- Carry 110% of rated current for at least 4 h without clearing
- Clear within one hour at 135% of rated current
- Clear within 2 minutes at 200% of rated current
- Clear within 1 ms at 1000% (10×) rated current
- Voltage drop below 200 mV at rated current
The selected fuse rating is typically the circuit voltage and 1.2 to 1.3 times the rated current in the load circuit it protects, rounded upward to the next standard available rating.
When the fuse clears, the break in the circuit current causes the full rated voltage to appear across the blown fuse. If the load is inductive, the transient voltage across the fuse may be substantially higher—up to twice the rated line voltage due to the inductive energy kick (analogous to water hammer in water pipes).
Under this overvoltage, a destructive arc may be formed across the fused element, and may continue to grow. The resulting heat and pressure may cause the fuse to explode in the worst case. The voltage rating of the fuse is therefore selected such that the fuse can interrupt a dead short without shattering or emitting flame or expelling molten metal.
In installations, where explosive vapors may be present, the general purpose nonsealed fuse poses a safety concern due to possible arcing when the fuse clear. The hermetically sealed fuse may be used in such applications. It has been developed for safe operation in explosive mixtures of chemical vapors. The hermetically sealed fuse eliminates the explosion possibility by containing the arcing inside. Moreover, it is fast and more predictable in clearing time, since there is no arcing in open air and the associated plasma, which can linger on for a long time.
Types of Fuse
Three types of fuses are available in the industry for different applications:
Standard (single-element) fuse, which is a general purpose fuse used in lighting and small power circuits. It has one element that melts when accumulated heat brings it to the melting temperature. Its i-t characteristic has a rather wide band as was shown in Figure 2.
Time-delay (slow-blow or dual-element) fuse, which is designed to ride through the starting inrush current drawn by certain load equipment— such as motor, transformer, capacitor, heater, etc. for a short time immediately after turn-on.
This fuse has two elements in series, one heavy bead that takes time to heat up under moderate currents under overloads, and the other thin strip with a large dissipation area which melts only when the current rises rapidly to a very high value under faults. The i-t characteristic of such a dual-element fuse also has a large manufacturing tolerance band.
Current limiting (fast-acting) fuse, which clears before a large prospective fault current builds up to the first peak. Its fusible link has brief arcing and melting time such that it clears the fault in about a quarter cycle, resulting in the let-through current much less than the prospective peak fault current that a normal fuse would see.
Such fuse is used in delicate heat sensitive power electronics circuits with diodes, thyristors, transistors, etc. Figure 3 shows a typical clearing time characteristic of a 200 A, 600 V current-limiting fuse in a branch circuit with the prospective symmetrical fault current of 10,000 Arms (25,000 A first peak). It will interrupt the fault at a peak let-through value of 8000 A (instead of 25,000 A prospective first peak).
Table 1 lists typical applications of the three fuse types, and Figure 4 compares the current versus average clearing time characteristics. For each type, significant manufacturing variations exist around the average clearing time, as was seen in Figure 2 for the standard general purpose fuse.
The fuse rating is based on factory tests conducted at room temperature. The ambient air temperature in actual operation has an influence on the actual rating of the fuse.
The fuse rating must be lowered if the ambient air temperature is higher than standard room temperature, and vice versa. Tests suggest that the fuse must be derated or up-rated with the ambient temperature as shown in Figure 5. As a rule of thumb, the nominal current rating of fuse operating in high ambient air temperature must be derated by:
- 0.5% per °C ambient above 25°C for time-delay fuse
- 0.2% per °C ambient above 25°C for standard general purpose fuse
Thanks for reading about “fuse selection criteria.”