How to Choose a Tube Fuse

Glass and ceramic tube fuses conveniently protect appliances and consumer electronics. Many of these appliances lack space for larger, costlier circuit protection devices and fuses become the preferred method of protection due to their accuracy, small size, and reliability.

These fuses are widely used throughout the world beyond North America, so they are commonly identified in mm instead of inches.

Glass and Ceramic tube fuses designed for electronic/electrical applications are rated at 125 V or better. Four sizes of user-replaceable fuses are found: 3.6 x 10 mm, 5 x 15 mm, 5 x 20 mm, 6.3 x 32 mm.

There are a few side notes to the above:

• 5 x 15 mm easily replaces the 2AG size fuse.
• 6.3 x 32 mm fuses are equivalent to 3AG (1/4” x 1 1/4”).

When talking about fuse response time, it is good to know about overcurrents. There are generally two types of overcurrents – short circuits and overloads. Short circuits consist of very high currents that can cause harm to both people and equipment. Overloads typically occur when too many devices are added to a circuit and the normal rating of the circuit is exceeded.

There are times when temporary overloads cannot be avoided. For example, when a circuit is first energized or when a motor starts up. These overload conditions generally last between 1 and 5 seconds, with the overload subsiding once the motor has reached its operating speed or the circuit has been fully energized.

These temporary overloads are sometimes called transient currents and are normally 2 to 6 times the normal current of the circuit. They are completely normal but can cause problems if the overcurrent protection device isn’t properly selected.

In come fuse response times. In circuits where transient currents exist, a time-delay fuse should be used (sometimes a time delay fuse is also called a time lag or slow-acting fuse. A time delay fuse allows the overload to exist for a few extra seconds before opening and thus avoiding a nuisance opening from the transient current condition.

The terms “time delay” and “fast-acting” are for overloads and/or transient currents only. Both time delay and fast-acting fuses are very quick acting when it comes to opening short circuits.

If fast acting fuses aren’t fast enough, there are also super-fast acting fuses. These super-fast acting fuses are primarily used for protecting sensitive equipment, such as rectifiers, SCRs, and transistors. Because these fuses are so high performing, they can cost as much as ten times normal fast-acting fuses.

Tube fuses are available in glass or ceramic. The physical properties of the ceramic material make the ceramic fuses stronger than their glass counterparts. Ceramic fuses incorporate sand filling to improve their performance during short circuit failures. 

The sand serves to quench the arcing, allowing the fuse to have a higher interrupting capacity. This means they can interrupt higher levels of fault currents without sustaining damage.

When choosing a fuse, glass is a good option for applications where the probability of a high current episode isn’t likely to occur (compare the Interrupt Rating to the potential current the fuse may be subjected to). They also allow for visual checks of a damaged filament. Ceramic has a higher breaking capacity offering a greater level of thermal and arc protection better suited for applications where high current episodes may occur.

In the case of a component shortage, you can usually substitute a ceramic fuse in place of a glass fuse, but you should not substitute a glass fuse for a ceramic fuse. Using a glass cartridge fuse as a substitute could result in the fuse envelope rupturing during a fault.