GSS
Technical Notes
1 Introduction Miniature fuses are automatic switchgears that protect electrical plants, appliances and modules from inadmissibly high current loads . They have various applications. Miniature fuses are used in consumer electronics for safeguarding power supplies and power output amplifiers. They are often used in industrial applications as primary fuses, where in the event of a fault they separate the defective module from the voltage supply in order to forestall any greater damage. Fuse-links for direct mounting on printed circuit boards are used among other things for the secondary current supply in low power devices. In the event of a fault they serve to protect components and PCB traces that might be destroyed by excessive current loads. 2 Designs The traditional miniature fuse is cylindrical in shape, with a diameter of 5mm and a length of 20mm. There is also a design used internationally which has the dimensions 6.3 x 32mm. Depending on their rating, these fuses, whose characteristics might be very quick acting to long time-lag, can switch off short circuit currents of up to several kilo amperes Sub-miniature fuses (KS) are used for through hole assembly on printed circuit boards. Whether their characteristics are quick acting or time-lag, these fuses with a rated voltage of 250V are used in primary circuits for protecting transformer mains and in secondary circuits for selective protection of the modules. SMD fuses at rated voltages of up to 250V are available for surface mounting on printed circuit boards. Their applications are similar to those of the sub-miniature fuses (KS) described above. As regards their breaking performance, Miniature fuses consist of the fuse-link and the fuse holder. The fuse link contains the fuse-element and must, after the latter has melted, be replaced by a new fuse-link. The fuse-element can continuously carry the rated current under given conditions. When the rated current has been exceeded, the fuse-element melts above a value prescribed by the standards and the electrical circuit is broken. The duration of the melting process is prescribed in the relevant standards. The fuse-link is inserted into a fuse-holder. The latter consists of the fuse base (mount) and the (screw or bayonet) fuse carrier. The fuse base is firmly mounted in the device to be protected and provides the electrical connection. The fuse carrier receives the fuse-link, allowing easy replacement. Open fuse holders and clips are often used besides such ’enclosed’ fuse holders. 3.1 Rated voltage (nominal voltage) U rat The rated voltage of a fuse link is given as the r.m.s. value of a sinusoidal alternating voltage at 50 Hz. All the test conditions are laid down in accordance with it. The voltage U b driving the short circuit current must not be greater than the rated voltage. at mains voltages of up to1000V. they are also designed with the special requirements of the telecom industry in mind. 3 Terminology
U rat U b
Operation at direct voltage is possible if the rated voltage is reduced; we will be glad to advise you on this matter if you provide us with the maximum fault current and the time constants of the fault current circuit. 3.2 Rated Current I n The rated current given is the r.m.s. value. Under prescribed conditions, the fuse-link can operate permanently at the rated current level. Ambient conditions, cyclic currents and special assembly conditions can lead to a de-rating of the rated current. The I b operating current must not be greater than the rated current during normal operation.
I n I b
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