The Twelve Cardinal Rules of Fuseology

Protecting your circuits means ensuring the safety of your design.

Whether you’re on the road to getting your electronics engineering degree or are a seasoned electronics circuit design professional, there are few tasks that have the potential to be as complex as choosing an appropriate fuse for your application. As designs become smaller, faster and more complex, so do circuit protection decisions. It's not enough to be able to review the various manufacturer’s datasheets and compare them with your application’s requirements: even if you’re savvy enough to decipher the fact from fiction, it can still be an extremely time-consuming exercise.
Fuses have been a staple of circuit protection for more than a century. The 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.

Fuse selection options are varied and can feel overwhelming.

Understanding fuseology helps you more easily and systematically determine which fuse options are right for your application.
There are 12 cardinal rules of fuseology and proper fuse selection:
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 utilize 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.

Circuit protection prevents surges from destroying systems.

7. Physical size limitations: Refer to the manufacturer’s data sheet for information on a fuse’s length, diameter, and height.
8. 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.
9. Fuse features: Refer to the manufacturer’s data sheet for information on mounting type/form factor, ease of removal, axial leads, visual indication, etc.
10. Fuseholder features and rerating: Refer to the manufacturer’s data sheet for information on clips, mounting block, panel mount, PC board mount, R.F.I. shielded, etc.
11. Application testing and verification prior to production: Verify the selection by requesting samples for testing in the actual circuit. 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.
12. Circuit protection should be a forethought, not an afterthought in your electronics circuits design process. I placed this one at the end for a reason. All too often, we get caught up in what you could call the ‘thrill of the design process’ and working with the latest technologies for our design. When that happens, it's all too easy to forget the fundamentals, like circuit protection. This became readily apparent to me during my visit with EE students at Stanford University. It’s bothersome that something as important as circuit protection doesn’t receive more educational attention at our institutions of higher learning. However, I’m not alone in that discovery. Chad Marak, a colleague of mine, also had the opportunity to speak with student engineers at his alma mater, Texas A&M. After his presentation, one of the students said: “Boy, I wish I’d have known all of this before the (robotics) competition!” As it turns out, his team’s entry in the competition was sidelined when, not just one, but three circuit boards were rendered non-functional. Later, they realized the logical culprit was electrostatic discharge (ESD).

While it’s important to observe these 12 cardinal rules of fuseology, you’ll also learn that circuit protection goes beyond the importance and value of fuseology as we discussed here. You also need to protect your designs from ESD and other transients -- right down to the board level (we’ll be covering that topic in a future article).

To learn more about choosing the right fuse for your application, download the Fuseology Selection Guide from Littelfuse, Inc. Do you need further help with your selection and just aren’t sure where to start? Try the free fuse selection tool.