Fuses

Fuses&holders
Continuing the topic on motor control circuit design I started a few weeks ago, today’s topic is on fuses. Last week we discussed circuit breakers: these are circuit protection devices that can be reset when a fault such as a short circuit appears. When a fuse’s current capacity is exceeded, it cannot be reset and must be replaced.

From my book:

A fuse or fusible link is an overcurrent protection device that is designed to melt (or “blow”) when excessive current flows through it. It is composed of a metal strip or wire element rated at a specified current plus a small percentage. This is mounted between two electrical terminals and generally surrounded with a nonflammable insulating housing.

Fuses are placed in series with the current flow to a branch or device. If the current flow through the element becomes too high, enough heat is generated to melt the element itself or a solder joint within the fuse. Dual element fuses contain a metal strip that melts instantly for a short circuit as well as a low melting solder joint for longer-term overloads. Time delay or “slow blow” fuses allow short periods of overcurrent conditions and are used for motor circuits, which can have a higher current inrush as the motor starts.

Fuses are made in many different shapes, sizes, and materials, depending on the manufacturer and application. While the terminals and fuse element must be made of a metal or alloy for conductivity, the fuse body may be glass, fiberglass, ceramic, or insulating compressed fibers. Fuse sizes and mounting methods generally fall into several standardized formats. The figure below shows several cartridge-type fuses; note that the larger fuse on the left has an indented area or groove at the bottom end of the fuse. This is known as a rejection fuse, the feature ensures that the fuse can only be placed into its holder one way.

Cartridge Fuses

Cartridge Fuses

Most fuses used in industrial applications are cartridge fuses. These are cylindrical with conducting caps on each end separated by the fusible link covered by the housing. These may be small glass or ceramic fuses for light loads or larger J or R class fuses. Cartridge fuses are also known as ferrule fuses. The caps may have bladed ends for insertion into clips or have a hole in the blade for bolting to a terminal. The most common methods are spring clips or terminal block–style fuse holders.

Fuses for use on printed circuit boards (PCBs) are generally soldered into place. They may have wire leads or solder pads depending on the desired mounting technique.

Fuses vs. Circuit Breakers:

Fuses are less costly than circuit breakers but must be replaced every time an overcurrent event occurs. This is not as convenient as simply resetting a breaker, though, and makes it more difficult to ignore intermittent faults. Fuses react more quickly than circuit breakers, especially the “current-limiting” variety. This helps minimize the damage to downstream equipment.

Well-known fuse manufacturers include Bussman, Littelfuse, Ferraz-Shawmut, along with many smaller manufacturers outside of the USA.

Knowing how things work is an important element of design. Its not enough to simply specify things based on numbers and “habit”; critical thinking about the purpose of a component is an important part of becoming a better designer and engineer!

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3 comments on “Fuses
  1. Tony says:

    I prefer fuses because they’re much easier to change than a circuit breaker — and, when properly sized, the fuses rarely blow.

    For example, our machines are built for easy change over to and from 120V and 220V operation. Some customers require that each major AC component (computer, power supplies, etc) is protected by a protective device (fuse or circuit breaker) with current rating <125% of the component's rated current.

    So a power supply with a 4A (120V) / 2A (240V) rated current must be protected with a current limit <= 5A / 2.5A. Fuses make this easy: the customer just exchanges the fuses when changing voltage. But it'd be a major pain with circuit breakers.

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