Design Strategies with Fuses

Posted on 9/1/20 3:50 PM

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When designing a new electrical or electronic equipment product, an important aspect of your design will be how to implement circuit protection. How do you ensure the safety of both the equipment and the equipment operator in the event of a circuit overload?

“The primary reason for circuit protection is to protect the end user from harm,” said Dan Ford, Technical Support Specialist at Interpower. “In the event a fault condition occurs, circuit protection can save expensive equipment housed with highly sensitive microprocessors prone to overcurrent faults.” Simply defined, a fault condition occurs when equipment draws too much current into the system. This can cause equipment failure and shock hazards to the end user.

Basic Fuse & Circuit Breaker Differences

Typically, circuitry in equipment is protected by one of two devices: fuses or circuit breakers. Circuit breakers provide automatic switches that prohibit the flow of electricity when rapidly overloaded. Fuses area safety devices protecting circuitry from excessive voltage. A circuit breaker is an automatic switch that prohibits the flow of electric current when rapidly overloaded or unusually stressed. Circuit breakers only protect against low-level faults and are not meant to clear faults with high-voltage levels. A circuit breaker is not a surge suppressor for catastrophic events, such as lightning strikes or high-voltage line shorts.

A fuse is a replaceable circuit protection device that provides protection to both the equipment and the equipment operator in the event of a circuit overload. A typical fuse is made with a thin metal strip or filament sheathed in a glass or ceramic enclosure. The metal strip is attached on each end to a separate terminals on the outside of the fuse. These external terminals mate with matching terminals attached to the protected circuit.

Any current, or flow of electricity, that moves through the circuit flows through the metal inside the fuse. The metal strip will melt when the current exceeds the amount that the fuse is designed to withstand, opening the circuit path, and disconnecting the equipment from the power source. When a traditional fuse disconnects (or “blows”) it must be removed and replaced with a new fuse.

If choosing to include a fuse as the circuit protection in the design, there are various aspects to consider. Among them are fuse size, equipment market, and a variety of design strategies.

Fuse Size and the Equipment Market

Predominately, there are two standard fuse sizes which are used worldwide: the internationally accepted 5 x 20 mm fuse, and North America’s ¼˝ x 1¼˝ fuse. However, you will find both of these fuses used on multiple continents.

“Where your equipment will be marketed plays an important role in which of these fuses should be selected,” Ford said. “If marketing mainly in North America, the designer may choose the common ¼˝ x 1¼˝ fuse since your customer base may already use this fuse. Products made strictly for markets in Europe, Asia, Australia, etc., would likely utilize the 5 x 20 mm fuse as this size is readily available.”

If the equipment will be marketed both in and outside North American markets, then the 5 x 20 mm fuse is recommended. However, if marketing globally, it may be in your best interest to carry a combination of the ¼˝ x 1¼˝ and 5 x 20 mm fuse and their fuse holders. Though less common, the 5 x 20 mm fuse is used in North America but is not nearly as prevalent as the ¼˝ x 1¼˝. A strategy of using both common fuse sizes when marketing globally will help keep component costs down as well as overhead costs.