A Methodical Approach to Fuse Selection

October 21, 2015 // By EDN
Bharat Shenoy, Director of Technical Marketing, Electronics Business Unit, Littelfuse
Electrical fuses were originally developed to help protect telegraph stations from lightning strikes. These first fuses were simple, open-wire devices, followed in the 1890s by Edison’s enclosure of thin wire in a lamp base to make the first plug fuse. By 1904, Underwriters Laboratories had established size and rating specifications to meet safety standards. Renewable type fuses and automotive fuses appeared in 1914. In the 1920s, manufacturers began producing very low amperage fuses for the burgeoning electronics industry. Today, the fuses used in electrical/electronic circuits are current sensitive devices designed to serve as the intentional weak link in the circuit. Their function is to provide protection of discrete components, or of complete circuits, by reliably melting under current overload conditions, much like those first fuses used at telegraph stations.

Electrical fuses were originally developed to help protect telegraph stations from lightning strikes. These first fuses were simple, open-wire devices, followed in the 1890s by Edison’s enclosure of thin wire in a lamp base to make the first plug fuse. By 1904, Underwriters Laboratories had established size and rating specifications to meet safety standards. Renewable type fuses and automotive fuses appeared in 1914. In the 1920s, manufacturers began producing very low amperage fuses for the burgeoning electronics industry. Today, the fuses used in electrical/electronic circuits are current sensitive devices designed to serve as the intentional weak link in the circuit. Their function is to provide protection of discrete components, or of complete circuits, by reliably melting under current overload conditions, much like those first fuses used at telegraph stations.

As technology continues to progress, so does the need for more complex circuit protection. Selecting the proper fuse for an application can be a daunting task for a circuit designer. Searching the data sheets of multiple manufacturers to extract the relevant information and then comparing that data with the requirements of the application at hand can be an extremely tedious process. But, because of the critical importance of a properly designed circuit, it is a process that must be met with a well thought out, step by step approach.

Circuit designers can take a methodical approach to the fuse selection process if they consider these eleven critical factors:

 

1.     Normal operating current: The current rating of a fuse is typically derated 25% for operation at 25ºC to avoid nuisance blowing. For example, a fuse with a current rating of 10A is not usually recommended for operation at more than 7.5A in a 25ºC ambient environment.

2.     Application voltage (AC or DC): The voltage rating of the fuse must be equal to, or greater than, the available circuit voltage.

3.     Ambient temperature: The higher the ambient temperature, the hotter the fuse will operate, and the shorter its life. Conversely, operating at a lower temperature will prolong fuse life. A fuse also runs hotter as the normal operating current approaches or exceeds the rating of the selected fuse.

4.     Overload current condition: The current level for which protection is required. Fault conditions may be specified, either in terms of current or, in terms of both current and maximum time the fault can be tolerated before damage occurs. Time-current curves should be consulted to try to match the fuse characteristic to the circuit needs, while keeping in mind that the curves are based on average data.

5.     Maximum fault current: The Interrupting Rating of a fuse must meet or exceed the Maximum Fault Current of the circuit.

6.     Pulses (surge currents, inrush currents, start-up currents, and circuit transients): Electrical pulse conditions can vary considerably from one application to another. Different fuse constructions may not react the same to a given pulse condition. Electrical pulses produce thermal cycling and possible mechanical fatigue that could affect the life of the fuse. Initial or start-up pulses are normal for some applications and