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. 1 INTRODUCTION
Fuses (fusible wire links) are one of the oldest and most universally used overload protection methods. However, the fuse sometimes does not get the close attention it deserves for a thorough understanding of its characteristics.
Modern fuse technology is an advanced science; new and better fuses are continually being developed to meet the more demanding requirements for protection of semiconductor circuitry. To obtain the most reliable long-term performance and best protection, a fuse must be knowledgeably chosen to suit the application.
2 FUSE PARAMETERS
From an electrical standpoint, fuses are categorized by three major parameters: current rating, voltage rating, and most important, "let-through" current, or I^2 t rating.
Current Rating
It is common knowledge that a fuse has a current rating and that this must exceed the maxi mum DC or rms current demanded by the protected circuit. However, there are two other ratings that are equally important for the selection of the correct fuse.
Voltage Rating
The voltage rating of a fuse is not necessarily linked to the supply voltage. Rather, the fuse voltage rating is an indication of the fuse's ability to extinguish the arc that is generated as the fuse element melts under fault conditions. The voltage across the fuse element under these conditions depends on the supply voltage and the type of circuit. For example, a fuse in series with an inductive circuit may see voltages several times greater than the supply voltage during the clearance transient.
Failure to select a fuse of appropriate voltage rating may result in excessive arcing during a fault, which will increase the "let-through" energy during the fuse clearance. In particularly severe circumstances, the fuse cartridge may explode, causing a fire hazard.
Special methods of arc extinction are utilized in high-voltage fuses. These include sand filling and spring-loaded fuse elements.
"Let-Through" Current (I^2t Rating)
This characteristic of the fuse is defined by the amount of energy that must be absorbed by the fuse element to cause it to melt. This is sometimes referred to as the pre-arcing let-through current. To melt the fuse element, heat energy must be absorbed by the element more rapidly than it can be conducted away. This requires a defined current and time product.
For very short time periods (less than 10 ms), very little heat is conducted away from the fuse element, and the amount of energy necessary to melt the fuse is a function of the fuse element's specific heat, its mass, and type of alloy used. The heat energy absorbed by the fuse element has units of watt-seconds (joules), and is calculated as I^2 R t for a particular fuse. As the fuse resistance is a constant, this is proportional to I^2 t, normally referred to as the I^2 t rating for a particular fuse or the pre-arcing energy.
For longer periods, the energy required to melt the fuse element will vary according to the element material and the thermal conduction properties of the surrounding filler and fuse housing.
In higher-voltage circuits, an arc will be struck after the fuse element has melted and a further amount of energy will be passed to the output circuit while this arc is maintained.
The magnitude of this additional energy is dependent on the applied voltage, the characteristic of the circuit, and the design of the fuse element. Consequently, this parameter is not a function of the fuse alone and will vary with the application.
The I^2 t rating categorizes fuses into the more familiar "slow-blow", normal, and "fast-blow" types. Figure 1.5.1 shows the shape of a typical pre-arcing current/time let through characteristic for each of the three types. The curve roughly follows an I^2 t law for periods of less than 10 ms. The addition of various moderators within the fuse package can greatly modify the shape of this clearance characteristic. It should be noted that the I^2 t energy (and hence the energy let-through to the protected equipment) can be as much as two decades greater in a slow-blow fuse of the same DC current rating! For example, the I^2 t rating can range from 5 A^2 s for a 10-A fast fuse to 3000 A^2 s for a 10-A slow fuse.
FIG. .5.1 Typical fuse I^2 t ratings and pre-arcing fuse clearance
times for fast, normal, and slow fuse links.
The total let-through energy of the fuse (pre-arcing plus arcing) also varies enormously.
Further, it depends on the fusible link material, construction of the fusible element, applied voltage, type of fault, and other circuit-linked parameters.
3 TYPES OF FUSES
Time-Delay Fuse (Slow-Blow)
A time-delay fuse will have a relatively massive fuse element, usually of low-melting-point alloy. As a result, these fuses can provide large currents for relatively long periods without rupture. They are widely used for circuits with large inrush currents, such as motors, solenoids, and transformers.
Standard-Blow Fuse
These fuses are low-cost and generally of more conventional construction, using copper elements, often in clear glass enclosures. They can handle short-term high-current transients, and because of their low cost, they are widely used. Very often the size is selected for short-circuit protection only.
Very Fast Acting Fuses (HRC, or High Rupture Capacity, Semiconductor Fuses)
These fuses are intended for the protection of semiconductor devices. As such, they are required to give the minimum let-through energy during an overload condition. Fuse elements will have little mass and will often be surrounded by some form of filler. The purpose of the filler is to conduct heat away from the fuse element during long-term current stress to provide good long-term reliability, and to quickly quench the arc when the fuse element melts under fault conditions. For short-term high-current transients, the thermal conductivity of the filler is relatively poor. This allows the fuse element to reach melting temperature rapidly, with the minimum energy input. Such fuses will clear very rapidly under transient current loads.
Other important fuse properties, sometimes neglected, are the long-term reliability and power loss. Low-cost fast-clearance fuses often rely on a single strand of extremely thin wire. This wire is fragile and is often sensitive to mechanical stress and vibration; in any event, such fuse elements will deteriorate over the long term, even at currents below the rated value. A typical operating life of 1000 h is often quoted for this type of fuse at its rated current.
The more expensive quartz sand-filled fuses will provide much longer life, since the heat generated by the thin element is conducted away under normal conditions. Also, the mechanical degradation of the fuse element under vibration is not so rapid, as the filling gives mechanical support.
Slow-blow fuses, on the other hand, are generally much more robust and have longer working lives at their rated currents. However, these fuses, with their high "let-through" energies, will not give very effective protection to sensitive semiconductor circuits.
This brief description covers only a very few of the ingenious methods that are used in modern fuse technology to obtain special characteristics. It serves to illustrate the number of different properties that fuses can exhibit, and perhaps will draw a little more attention to the importance of correct fuse selection and replacement.
4 SELECTING FUSES
Off-Line Switchmode Supplies
The initial fuse selection for off-line switching supplies should be made as follows: For the line input fuse, study the turn-on characteristics of the supply and the action of the inrush-limiting circuitry at maximum and minimum input voltages and full current limited load. Choose a standard- or slow-blow fuse that provides sufficient current margin to give reliable operation and satisfy the inrush requirements. Its continuous current rating should be low enough to provide good protection in the event of a genuine failure.
However, for long fuse life, the current rating should not be too close to the maximum rms equipment input current measured at minimum input voltage and maximum load (perhaps 150% of I rms maximum). Note: Use measured or calculated rms currents, and allow for the power factor (approximately 0.6 for capacitor input filters) when calculating rms currents.
The voltage rating of the fuse must exceed at least the peak supply voltage. This rating is important, as excessive arcing will take place if the voltage rating is too low. Arcing can let through considerable amounts of energy, and may result in explosive rupture of the fuse, with a risk of fire in the equipment.
5 SCR CROWBAR FUSES
If SCR-type overvoltage protection is provided, it is often supplemented by a series fuse.
This fuse should have an I^2 t rating considerably less (perhaps 60% less) than the SCR I^2 t rating, to ensure that the fuse will clear before SCR failure. Of course, a fast-blow fuse should be selected in this case. The user should understand that fuses degrade with age, and there should be a periodic replacement policy. The failure of a fuse in older equipment is not necessarily an indication that the equipment has developed a fault (other than a tired fuse).
6 TRANSFORMER INPUT FUSES
The selection of fuses for 60-Hz transformer input supplies, such as linear regulator sup plies, is not as straightforward as may have been expected.
Very often inrush limiting is not provided in linear power supply applications, and inrush currents can be large. Further, if grain-oriented C cores or similar cores are used, there is a possibility of partial core saturation during the first half cycle as a result of magnetic memory of the previous operation. These effects must be considered when selecting fuses. Slow-blow fuses may be necessary.
It can be seen from the preceding discussion that the selection of fuse rating and type for optimum protection and long life is a task to be carried out with some care. For continued optimum protection, the user must ensure that fuses are always replaced by others of the same type and rating.
7 QUIZ
1. Quote the three major selection criteria for supply or output fuses.
2. Why is the voltage rating of a fuse so important?
3. Under what conditions may the fuse voltage rating exceed the supply voltage?
4. Why is the I^2 t rating of a fuse an important selection criterion?
5. Why is it important to replace a fuse with another of the same type and rating?
Also see: Our other Switching Power Supply Guide