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  5. Information on Fuses and Fuse Holders

More Information on Fuses and Fuse Holders

Electronic equipment is often designed with built in circuit protection. This protection usually comes in one of two forms: a circuit breaker or a fuse. Fuses offer a couple of advantages over circuit breakers. First, they are more cost effective. Second, and perhaps more importantly, they are easier to modify when a product is being designed for multiple markets. For example, a North American product operating at 115VAC might be protected with a 6A fuse. The same product could be shipped into an international market where it will run on 230VAC. To accommodate the 230VAC power supply, the 6A fuse could be exchanged for a 3A fuse. This change can be made quickly and easily, by simply using a Fuse Holder that is compatible with both North American and international fuses and fuse carriers. The designer using a circuit breaker, rather than a fuse in this scenario, could be faced with extensive redesign requirements when shifting from a circuit breaker compatible with North American power requirements to a circuit breaker compatible with international power requirements.

Interpower fuses come in two different sizes. In North America the standard is 1/4 x 1-1/4", or 6.35 x 31.75mm. This size fuse may be found outside of North America, in countries where the power system is based on the North American system, but this is not common. More commonly found in international markets is the 5 x 20mm fuse. The smaller international fuse is also becoming more prevalent in North America. It will be important to keep this information in mind when designing a product for multiple markets.

Fuse and Fuse Holder Design Strategies
It is possible to design a product for most markets and specify the common (larger) North American fuse. This will simplify the design process and eliminate inventory problems such as multiple fuse sizes and Fuse Holders. The downfall is that the global customer will find the North American fuse difficult to source and replace when a failure occurs. Of even greater concern, the North American fuse will not carry any international approvals, as are usually required. These two shortcomings make this strategy unattractive.

An alternative strategy might be to specify the smaller 5 x 20mm fuse. This strategy offers the ease of carrying only one size fuse and Fuse Holder. It also allows the fuse to be positioned in a slightly smaller space inside the equipment, when a design finds space at a premium. Additional benefits include more approvals, as the 5 x 20mm fuses are increasingly approved with UL recognition, CSA certification, or SEMKO and VDE certification for international markets. The smaller fuse is also more likely to be compatible with a variety of other components, such as multi-function modules.

While the fuse size remains constant under this strategy, it is important to remember that separate fuses for North America andå international markets are still required, due to the different approvals. No one fuse carries all of these approvals; the size is the only real constant. Each market has different time-current characteristics for their fuses, based on the approval requirements for the fuses. Since the fuses all have nearly identical appearances, it is quite possible to confuse them. The only real difference is the marking on the fuse caps, which may be difficult to read. It is also worth noting that the 5 x 20mm fuse, while accepted more often in North America, is not as particularly common as used in North American markets.

A third design strategy would be to design a product that accommodates either the international sized, or the larger North American sized fuse. Interpower Corporation offers Fuse Holders that will accept both sizes of fuse. This is accomplished by switching the fuse carrier (sold separately) that fits inside the Fuse Holder. This strategy allows the end design to be user friendly in all markets, as the user will be able to source replacement fuses. It also ensures the component will be compliant regardless of the market in which it will be used, since the necessary approvals will be in place.

Fuse Holder Product Line
International “Touchproof” Fuse Holders are a popular choice. This type of Fuse Holder is available in low-profile, high-profile, and low-profile/snap-in versions. Each of these are available with a variety of terminal styles including quick disconnect, solder/quick disconnect and angled versions of both. Interpower Fuse Holders will also accept both the North American and the international fuse carrier (sold separately) and can be used in nearly every market worldwide. To accommodate designers for worldwide markets, they also carry the necessary approvals, UL recognized/CSA certified for North America and SEMKO and VDE for international markets. These Fuse Holders offer the additional benefit of being touchproof, providing more safety to the end user—the blown fuse can only be accessed from the outside of the equipment.

PC-Board Mount
Like a post-style Fuse Holder, PC-board mount styles are available to use with either North American or international fuse carriers. These Fuse Holders are also “touchproof,” offering the end user protection when changing a blown fuse and are only accessible from the outside of the equipment. Like the international “touchproof” style, they offer the necessary approvals to be used in markets worldwide. The fuse carrier and fuses are sold separately.

Fuse Blocks
Fuse blocks are also PC-board mount components. However, they are not accessible from outside of the equipment, and for this reason, VDE does not test, nor approve them. Changing a blown fuse requires opening the equipment, so care should be taken to ensure only qualified personnel are performing the repair. Fuse blocks offer two important advantages to the designer. First, they are less expensive than other types of Fuse Holders. Second, they do not require a mating fuse carrier: one component is all that is needed. When the equipment will be limited to a North American market, the fuse block is an attractive choice.

Fuse Clips
Fuse clips are an alternative to fuse blocks, with many of the same features. Interpower fuse clips come in two sizes to accommodate both North American and international fuses. They are PC-board mounted and accessible only from within the equipment. Both sizes have “stops” on them, to help position the fuse when it is being inserted. No approvals are available on fuse clips, except as part of the finished product. Because they are inexpensive, they are another attractive alternative to post style and PC-board mount Fuse Holders. If fuse clips are used, the designer must remember that each end is a separate pIECe. Two clips will need to be ordered for each fuse used

Fuse Product Line
A fuse is a built-in weak link in an electric circuit. Its main function is to provide protection to both the equipment and the equipment operator in the event of a circuit overload. When the overload occurs, a filament within the fuse snaps, breaking the circuit. A variety of factors dictate that a variety of fuses are available to handle the circuit protection. Does the equipment have a large in-rush at start-up? How long can the equipment handle an overload? Is the overload large enough to physically destroy the fuse? Fortunately, there are fuses available to handle nearly any scenerio.As was discussed previously, there are two different sizes of fuses: the North American 1/4 x 11/4" and the international 5 x 20mm. Within these sizes, Interpower offers fuses with different breaking characteristics. For North American fuses, the breaking characteristics are defined as “fast-acting” and “time-delay.” Interpower offers North American 1/4 x 1 1/4"  with ceramic/sand filled construction, to protect the fuse from being physically destroyed during the overload.

International fuses have similar designations. They are available in “quick-acting/low-breaking” capacity, “time-lag/low breaking” capacity and “quick-acting/high breaking” capacity. As the names imply, these fuses have both fast and slow-breaking times. The high-breaking capacity fuses are also ceramic/sand filled. (For more information on North American or international fuses and their comparative time-current characteristics. It is important to note that while the terminology is similar for North American and international fuses, the actual characteristics are usually quite different, due to the differences in the standards to which the fuses are built. Please refer to UL 248.1 and UL 248-14 for the U.S. and/or CSA 22.2, no. 248.1 and no. 248.14 for more information.

Fast-Acting: The fuse will blow quickly when a circuit overloads. Offers protection for equipment that cannot withstand current overloads, even briefly.

Time-Delay: The fuse is designed to withstand some current overload for a limited amount of time. Used in equipment that can sustain a brief overload situation.

Quick-Acting: Similar to fast-acting, this fuse blows very quickly when overloaded.

Time-Lag: Similar to time-delay, the fuse allows the overload to exist briefly before blowing.

Low-Breaking Capacity: A minimal ability of the fuse to remain physically intact when overloaded. A low-breaking capacity fuse is readily subject to being physically destroyed if the current surge is great enough.

High-Breaking Capacity: A high level of ability of the fuse to remain physically intact even when the overload is great. The fuse is usually constructed with a ceramic, rather than glass body and filled with silica sand to absorb a large overload and protect the fuse itself.

Fuse Accessibility is Limited Under International Standards Users of Fuse Holders designed to comply only with UL/CSA requirements are accustomed to a fuse carrier with a knurled cap that makes it easy to access and change a fuse without tools. Various international equipment standards, however, limit the degree of user accessibility to a Fuse Holder in order to minimize the possibility of electrical shock to a non-technical user. The limited access Fuse Holder is designed to require the use of a tool (usually a screwdriver). It also incorporates additional insulation and insulating barriers that eliminate the presence of live conducting surfaces during fuse change operations. The limited access (“touchproof”) Fuse Holder possesses four important characteristics as shown below

British Plug and Ring Wiring System

The United Kingdom is the only country that requires a fused power plug. The British use a ring wiring system in their houses and buildings, which provides a secondary protection device at the plug, to minimize safety hazards. Circuits leave the local branch protection device, travel out to the loads (outlets or lamps, for example), and then return to the circuit protection device. In the Continental European and North American systems, all wiring is done on a linear basis—a circuit leaves the circuit protection device and goes straight to the outlets or lighting circuits, that represent the load, as illustrated below.

In the British ring wiring system, a fault condition at an outlet, for example, will be sourced with current from both directions. This minimizes the amount of heat generated in the conductor, as the fault condition occurs, but before the circuit protection device can clear the fault. By minimizing the heat generated, the degradation of insulation (which accompanies overheating due to repeated fault conditions) is also diminished, improving the long-term safety of the insulation system.

Until the circuit protection device clears the fault, however, the fault condition is sourced from two different directions in the supply system. There is therefore a much greater potential fault current condition. The British feel that a secondary protection device at the plug minimizes the safety hazard this condition creates; hence, the power plug fuse.

The fuse installed in all British plugs conforms to BS 1362 ("General purpose fuse links for domestic and similar purposes…"). The size of this fuse is 6.3 x 25.4mm. It has a breaking capacity rating of 6000A and is constructed with a sand-filled ceramic tube. The time-current characteristic on this family of fuses is not necessarily consistent with any other international standard, but it appears to have most of the characteristics of a fast acting fuse. The British Standards Institution standard for fuses in general is BS 4265.