OMEGA FUSIBILI

United Kingdom

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PHYSICAL SIZES There are numerous physical sizes of electronic fuses, including subminiature fuses. The most common cylindrical designs are 5x20mm and 6,3x32mm. Subminiature fuses are often used when board space is limited. For applications of this type there are both through-hole and surface mount device available. Standard package size for surface mount fuses are 1608, 3216 and 6125. These sizes are standard throughout the electronic industry. Through-hole axial and radial leaded products allow fuses to be PCB mounted. Standard 5x15 and 5x20 fuses fitted with leads

can also be mounted in this way. GENERAL INFORMATION Basic Fuse Technology Miniature fuses serv to main purposes: a) To protect components, equipment and people from risk of fire and shock risk caused by overcurrents. b) To isolate subsystems from the main system once a fault has occurred. Overcurrents Overcurrents exist when the normal load for a circuit is exceeded. It can be either an overload or short circuit. An overload is any current flowing within the normal circuit path that is higher than the normal circuit full load current. A short circuit is an overcurrent which greatly exceeds the normal full load current of the circuit. Components and equipment can be damaged by both types of overcurrent. Selecting overcurrent protection During normal load current conditions, the fuse must carry the normal load current of the circuit without nuisance opening. However, when an overcurrent occurs the fuse must interrupt the overcurrent and withstand the voltage across the fuse after arcing. To properly select a fuse the following items must be considered: • Voltage rating (ac or dc voltage) Available short circuit current Characteristics of equipment or components to be protected Ambient conditions Available board space Automatic or manual insertion Ease of field service Full load currents In-rush characteristics Standards requirements Type of mounting Reliability Electronic circuits frequently exhibit surges, caused by capacitors charging, motors being momentarily stalled, or high voltage components sparking over. It is important that designers take account of these temporary conditions during fuse selection. The ability to resist surges is a function of the fuse design relative to the surge pulse, duration, frequency, etc. Voltage ratings The voltage ratings of the fuse must be greater than or equal to the circuit voltage. Because the fuse has such low resistance the voltage rating becomes critical only when the fuse is trying to open. The fuse must be able to open quickly, extinguish the arc after the fuse element has melted and prevent the system open-circuit voltage from re-striking across the open fuse element. Current ratings Each fuse is marked with a nominal current rating. Several factors can affect the ability of the fuse to carry this rated current. First the base material of the clip in which the fuse is mounted may greatly affect the performance of the fuse. Another important factor is the conductor size used to connect the fuse to other circuit components. If the conductor is too small, it will generate heat. That extra heat will be seen by the fuse, causing the fuse to open before it should. It is also important that the fuse be installed with clean and tight connections. If the connections are dirty or loose, they will cause increased resistance, generating extra heat. That heat will lead to a shortened fuse life. Breaking capacity A fuse must be able to open the circuit under a short circuit without endangering its sorroundings. The breaking capacity of a protective device is the maximum available current, at the rated voltage, that the device can safely open without the possibility of rupturing. Fuse resistance In most applications, the voltage drop across the fuse due to its internal and contact resistances is negligible. There are, however, certain critical applications where the fuse resistance must be considered and it is important that the circuit designer understands the fuse characteristics in order to select the proper fuse. GLOSSARY OF TERMS Ampere squared seconds I2t A measure of heat energy supplied to a circuit while the fuse is clearing a fault. It can be expressed as melting I2t, arcing I2t, or the sum of them “clearing I2t”. This is usually used as a measure of short circuit performance Arcing time The amount of time from the instant the fuse link has melted until the overcurrent is interrupted, or cleared. Clearing time The total time between the beginning of the overcurrent and the final opening of the circuit at the rated voltage by an overcurrent protective device. Clearing time is the total of the melting time and the arcing time. Fast acting fuses A fuse which opens on overload and short circuits very quickly. This type of fuse is not designed to withstand temporary overload currents with some electrical loads. Overcurrent A condition which exists on an electrical circuit when the normal load current is exceeded. Overcurrents take on two separate characteristics: overloads and short circuits. Overload Can be classified as an overcurrent which exceeds the normal full load current of a circuit. Resistive load An electrical load which is characterised by not having any significant in-rush current. When a resistive load is energized, the current rises instantly to its steady state value, without first rising to a higher value. RMS Current The RMS (root mean square) value of any periodic current is equal to the value of the direct current which, flowing through a resistance, produces the same heating effect in the resistance as the periodic current does. Short circuit Can be classified as an overcurrent which exceeds the normal full load current of a circuit by a factor many times (tens, hundreds, thousands greater). Time delay fuses A fuse with a built-in time delay that allows temporary and harmless in-rush current to pass without operating, but is so designed to open on sustained overloads and short circuits. Time lag (surge proof) A fuse which can withstand short high current pulses and is particularly suitable for television sets, amplifiers, etc. Voltage rating A maximum open circuit voltage in which a fuse can be used, yet safely interrupt an over-current. Exceeding the voltage rating of a fuse impairs its ability to clear an overload or short circuit safely
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