For the circuit designer, few tasks can be quite as complex as choosing an appropriate fuse for an application. Simply deciphering the information in the various manufacturers' datasheets and comparing them with the application's requirements can be extremely time-consuming.
Fuses have been a staple of circuit protection for more than a century. Littelfuse offers a brief history on its website:
The first fuses were simple, open-wire devices, followed in the 1890's 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. The renewable type fuses and automotive fuses appeared in 1914, and in 1927 Littelfuse started making very low amperage fuses for the budding electronics industry.
The fuses used in electrical/electronic circuits today are current-sensitive devices designed to serve as the weak link in the circuit. They protect discrete components or complete circuits by melting reliably under current overload conditions.
There are 11 critical factors involved in fuse selection:
- Normal operating current: A fuse's current rating is typically derated 25% for operation at 25ºC to avoid nuisance blowing. A fuse with a current rating of 10 A is not usually recommended for operation at more than 7.5 A in a 25ºC ambient environment.
- Application voltage (AC or DC): The fuse's voltage rating must be equal to or greater than the available circuit voltage.
- Ambient temperature: The higher the ambient temperature, the hotter the fuse will operate, and the shorter it will last. Conversely, operating at a lower temperature will prolong fuse life. A fuse also runs hotter as the normal operating current approaches or exceeds its rating.
- Overload current condition: This is 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. Keep in mind that the curves are based on average data.
- Maximum fault current: The fuses's interrupting rating must meet or exceed the circuit's maximum fault current.
- Pulses (surge currents, inrush currents, startup 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 fuse's life. Initial or startup pulses are normal for some applications in which fuses incorporate a thermal delay design to survive normal startup pulses and still provide protection against prolonged overloads. The startup pulse should be defined and compared to the fuse's time-current curve and I2 t rating.
- Physical size limitations: Refer to the manufacturer's data sheet for information on a fuse's length, diameter, and height.
- Agency approvals required: Refer to the manufacturer's data sheet for information on a specific device's agency approvals, such as UL, CSA, VDE, METI, or MITI. Military requirements need special consideration.
- Fuse features: Refer to the manufacturer's data sheet for information on mounting type/form factor, ease of removal, axial leads, visual indication, etc.
- Fuseholder features and rerating: Refer to the manufacturer's data sheet for information on clips, mounting block, panel mount, PC board mount, RFI shielded, etc.
- Pre-production application testing and verification: Verify the selection by requesting samples for testing in the actual circuit. Before evaluating the samples, make sure the fuse is 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 the fuse will operate properly in the circuit.
To learn more about choosing fuses appropriate to the application, download a free Fuseology selection guide from Littelfuse Inc.