Fuses for small, low-voltage, usually residential, wiring systems are commonly rated, in North American practice, to interrupt 10,000 amperes. Fuse elements may be supported by steel or nichrome wires, so that no strain is placed on the element, but a spring may be included to increase the speed of parting of the element fragments. The operating time is not a fixed interval, but decreases as the current increases. Again, AC Fuses are divided in to High Voltage (HV) Fuses and Low Voltage (LV) Fuses.High Voltage (HV) AC Fuses are used for voltages above 1000V and Low Voltage (LV) AC Fuses are used for voltages less than 1000V. Some Wylex standard models were made with an RCD instead of the main switch, but (for consumer units supplying the entire installation) this is no longer compliant with the wiring regulations as alarm systems should not be RCD-protected. The Unicode character for the fuse's schematic symbol, found in the Miscellaneous Technical block, is U+23DB (⏛). [16]. This is a North American, tri-national standard (UL 4248-11; CAN/CSA-C22.2 NO. Fuses only sense overcurrent, or to a degree, over-temperature, and cannot usually be used independently with protective relaying to provide more advanced protective functions, for example, ground fault detection. Normal fast-blow fuses are the most general purpose fuses. [1] A variety of wire or foil fusible elements were in use to protect telegraph cables and lighting installations as early as 1864. The speed at which a fuse blows depends on how much current flows through it and the material of which the fuse is made. to the existing carrier. The exact fusing current is not as well controlled as an enclosed fuse, and it is extremely important to use the correct diameter and material when replacing the fuse wire, and for these reasons these fuses are slowly falling from favour. Rated voltage should be higher than the maximum voltage source it would have to disconnect. IEC standards refer to a "fuse" as the assembly of a fusible link and a fuse holder. This is not the case with fuses, which rely on melting processes where no mechanical operation is required for the fuse to operate under fault conditions. in transformer stations, main distribution boards, or in building junction boxes and as meter fuses. Pole-mounted distribution transformers are nearly always protected by a fusible cutout, which can have the fuse element replaced using live-line maintenance tools. Fuses for high-voltage equipment, up to 115,000 volts, are rated by the total apparent power (megavolt-amperes, MVA) of the fault level on the circuit. This page was last edited on 13 November 2020, at 17:15. It may be installed in a circuit-breaker box with a door. Fuse bodies may be made of ceramic, glass, plastic, fiberglass, molded mica laminates, or molded compressed fibre depending on application and voltage class. The time and current operating characteristics of fuses are chosen to provide adequate protection without needless interruption. Medium-voltage fuses rated for a few thousand volts are never used on low voltage circuits, because of their cost and because they cannot properly clear the circuit when operating at very low voltages.[8]. [11], In North America, fuses were used in buildings wired before 1960. For example, fuse holders for North American class RK fuses have a pin that prevents installation of similar-appearing class H fuses, which have a much lower breaking capacity and a solid blade terminal that lacks the slot of the RK type. In this way, only the faulty circuit is interrupted with minimal disturbance to other circuits fed by a common supplying fuse. There are many types of fuses available for a variety of applications. So-called self-resetting fuses use a thermoplastic conductive element known as a polymeric positive temperature coefficient (PPTC) thermistor that impedes the circuit during an overcurrent condition (by increasing device resistance). Ratings were 5, 10, 15, 20, 25, and 30 amperes. A thermal fuse is often found in consumer equipment such as coffee makers, hair dryers or transformers powering small consumer electronics devices. There are two styles of fuse base that can be screwed into these units: one designed for rewirable fusewire carriers and one designed for cartridge fuse carriers. A maximum current that the fuse can continuously conduct without interrupting the circuit. [12], Edison base (left) and Type S fuses (right), An older fuse box of the type used in North America, Some companies manufacture resettable miniature thermal circuit breakers, which screw into a fuse socket. There are two styles of fuse base that can be screwed into these units: one designed for rewirable fusewire carriers and one designed for cartridge fuse carriers. The main category of Fuses are based on the type of circuit they are used in i.e. It is still used in many installations. When the fuse element blows, the indicating pin extends to activate the micro switch or relay, which, in turn, triggers an event. Large power fuses use fusible elements made of silver, copper or tin to provide stable and predictable performance. In North American standards, the fuse is the replaceable portion of the assembly, and a fuse link would be a bare metal element for installation in a fuse. Fuses are widely used for protection of electric motor circuits; for small overloads, the motor protection circuit will open the controlling contactor automatically, and the fuse will only operate for short circuits or extreme overload. The range of applications for D fuse-links "gG" is the protection of cables and power lines. The fuse is arranged in series to carry all the current passing through the protected circuit. 248. A cable limiter is similar to a fuse but is intended only for protection of low voltage power cables. While glass fuses have the advantage of a fuse element visible for inspection purposes, they have a low breaking capacity (interrupting rating), which generally restricts them to applications of 15 A or less at 250 VAC. Other fuses have other indication methods including: Some fuses allow a special purpose micro switch or relay unit to be fixed to the fuse body. The fuse elements may be shaped to increase heating effect. [clarification needed] Once current is applied, resistance and voltage drop of a fuse will constantly grow with the rise of its operating temperature until the fuse finally reaches thermal equilibrium. High rupture capacity fuses may be rated to interrupt current of 120 kA. The design does not allow for fitting of RCDs or RCBOs. Blade type fuses often require the use of a special purpose extractor tool to remove them from the fuse holder. Some manufacturers have tested their fuses for up to 63 kA short-circuit current. This process is called "coordination" or "discrimination" and may require the time-current characteristics of two fuses to be plotted on a common current basis. In both cases, higher rated carriers had wider pins, so a carrier couldn't be changed for a higher rated one without also changing the base. The D fuse-system is the first generation of the screw fuse-system in the german DIN standard. A fuse is an automatic disconnection of supply which is frequently shortened t… 4248.11-07 (R2012); and, NMX-J-009/4248/11-ANCE). Plasma inside the glass tube may continue to conduct current until the current diminishes to the point where the plasma becomes a non-conducting gas. Today there are thousands of different fuse designs which have specific current and voltage ratings, breaking capacity and response times, depending on the application. The standard ampere ratings for fuses (and circuit breakers) in USA/Canada are considered 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes.
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