November 2010 Electrical Apparatus

November 2010 Electrical Apparatus

This is a summary of the Electrical Apparatus November 2010 featured technical article,  by Richard L. Nailen, P.E.  

Although fuses were among the first electrical system components, safety concerns and new technologies have fostered development of new types of fuses as well as some different approaches to fuse application.

For example, fast-acting current-limiting (CL) fuses were for many years designed to interrupt within a few milliseconds a fault current that could otherwise reach 200,000 amperes. Increasingly robust electrical grids have led to newer CL designs extending that capability to 300,000 amperes.

Rather than simply protecting downstream components from overcurrent damage, CL fuses are now intended to control the energy that can feed a hazardous arc flash incident.

Fuses in general, and the dual element or time-delay type in particular, are subject to unintended operation through fatigue, brought on by thermal cycling and electromagnetic stress repetition from motor starting or low-level faults. As inverter drives proliferate, harmonics are a source of high-frequency alternating stress. Fuse link design modifications have been made to minimize fatigue failure.

Various “fuse indicators” allow personnel to check fuse condition from a safe distance, without the use of test instruments. Safe inspection and replacement of fuses is also promoted by the use of infrared thermography to find overheated connections or surroundings that can distort normal fuse operating characteristics. Thermography readily detects loose or corroded contact between fuse and fuseholder, which can be mitigated by “fuse clip clamps” of several types.

Protection of semiconductor circuits, involving not only harmonics but highly sensitive downstream electronics, has spurred development of many specialty fuses. That’s also true for photovoltaic (PV) generation systems. Whereas many standard a-c fuses with 600 volt ratings can be applied in d-c circuits, PV fuses are rated for up to 1,500 volts d-c, offering high-speed interruption of fault currents as low as 130% of the continuous rating. They must also withstand frequent cycling as sunlight varies.

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