PD IEC/TR 62655:2013 pdf free

PD IEC/TR 62655:2013 pdf free.Tutorial and application guide for high-voltage fuses.
4 Tutorial section
4.1 A simple introduction to fuses
4.1.1 General
Fuses have been in use since the very beginnings of electrical power distribution. While the true inventor of the fuse is not known, pioneers of electrical distribution soon incorporated them as “weak points” in their circuits to prevent overheating of wiring, due to excessive current, and to prevent damage to fragile lamps from fluctuations in voltage. Fuses rapidly
developed into devices able to sense a current higher than normal and quickly interrupt (break) that current, all in a self-contained easily replaceable unit. Although the variety and complexity of fuses has grown to the point where user’s guides, such as this one, are necessary, fuses still provide the highest degree of protection for the lowest initial cost.
The simplest definition of a “fuse” is that it is a device that carries current through a conductive part called the fuse element that, when the fuse is subjected to an excessive current, melts due to self-heating and initiates the interruption of the current. All conventional fuses interrupt current after some arcing across breaks in the element produced by the melting process. The melting time of a fuse is therefore also termed the “pre-arcing” time. A characteristic of fuses is that, because current interruption is initiated by a melting process, there are almost no “mechanical” aspects involved in their arc initiation. Fuses therefore have a very inverse time-current relationship (higher currents giving shorter pre-arcing times) as illustrated in Figure 1. This enables extremely short pre-arcing times at high currents, virtually without limit. It is this apparently simple phenomenon that is primarily responsible for the universal success fuses have enjoyed for a very long time.
In general, high-voltage fuses (defined as fuses rated above 1 000 V a.c.) are physically larger and generally more complex than low voltage fuses due to their need to operate at much higher voltages. HV fuses may perform one or both of two primary functions. The first function is to respond to moderately excessive currents, typically termed “overload” currents. In this case, the rated current of the fuse (the current it is designed to be capable of carrying indefinitely without deterioration) is exceeded by a relatively modest amount (typically less than 10 times). Such currents can be caused by too much load being connected to a circuit, or by a fault that by-passes only part of the load. It should be noted that not all types of fuses are designed to have the ability to operate successfully if melted by a very low overcurrent as some types are intended only for operation at high currents (see 4.1.2.1). If melted by a low current, such fuses may arc until a series device interrupts， possibly resulting in physical damage to the fuse and its surroundings. However, some fuses of this type can quickly initiate another device to interrupt such current, containing the arcing without damage until the second device interrupts.PD IEC/TR 62655 pdf download.