Fuse Parameters

From an electrical standpoint, fuses are categorized by three major parameters: current rat- ing, voltage rating, and most important, “let-through” current, or I²t rating.

Current Rating

It is common knowledge that a fuse has a current rating and that this must exceed the maximum 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 (I²t 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 I²Rt for a particular fuse. As the fuse resistance is a constant, this is proportional to I²t, normally referred to as the I²t 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 character- istic 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 I²t rating categorizes fuses into the more familiar “slow-blow”, normal, and “fast-blow” types. Figure 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 I²t 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 I²t 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 I²t rating can range from 5 A²s for a 10-A fast fuse to 3000 A²s for a 10-A slow fuse.

Fuse Parameters
FIG. 1 Typical fuse I²t ratings and pre-arcing fuse clearance times for fast, normal, and slow fuse links

Related Products:

No responses yet

Leave a Reply