Fuse selection factors critical to circuit design

For more than a century, fuses have been essential to circuit protection. The first fuses were uncomplicated, 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. When selecting a fuse, there are 11 critical factors to consider: 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 require the use of fuses that incorporate a thermal delay design to enable them to survive normal start-up pulses and still provide protection against prolonged overloads. The start-up pulse should be defined and then compared to the time-current curve and I2t rating for the fuse. 7. Physical size limitations Reference the data sheet for information on a fuse's length, diameter, and height. 8. Agency approvals required Reference the data sheet for information on a specific device's agency approvals, such as VDE, TÜV, or SEMKO. Military requirements need special consideration. 9. Fuse features Reference to the data sheet for information on mounting type/form factor, ease of removal, axial leads, visual indication, etc. 10. Fuseholder features and rerating Reference the data sheet for information on clips, mounting block, panel mount, PC board mount, R.F.I. shielded, etc. 11. Application testing/verification prior to production Request samples for testing in the actual circuit to verify the selection. Before evaluating the samples, make sure the fuse is properly mounted with good electrical connections, using adequately sized wires or traces. The testing should include life tests under normal conditions and overload tests under fault conditions to ensure that the fuse will operate properly in the circuit. Bharat Shenoy is director of technical marketing for Littelfuse.