Switching protection

Cable hand lamp without switch (Protection class II)... 

To protect the woiksheet, choose Protect Worksheet... from the Tools menu, shown in the bottom part of Fig. 6.34. Worksheet protection comes in three levels presented by switches Protection Level in the Protect Worksheet dialog box. The first level, File, the lowest one, protect the worksheet from being saved in some formats, for example as earlier Mathcad versions. Figure 6.35 shows two lists of file formats for saving the current worksheet even without protection level File. In Mathcad 12, this protection level makes it impossible to save the worksheet as Mathcad 11, 2000i, or 2000. 

Electrical. The plant electrical system is sometimes more important than the steam system. The electrical system consists of the utihty company s entry substation, any ia-plant generating equipment, primary distribution feeders, secondary substations and transformers, final distribution cables, and various items of switch-gear, protective relays, motor starters, motors, lighting control panels, and capacitors to adjust power factor. 

Electrical—panels, switches, motors, conduit, wire, fittings, feeders. Weather protection... 

Manual and Magnetic Starters Manual motor starters are operated by hand. The simplest type of manual starter is a snap switch with no overload protection, used only for motors of 1,492 kW (2 hp) and smaller, usually single-phase motors with integral overload protection. 

The starting of an induction motor does not relate to simple switching alone. It also involves its switchgears to control its starting inrush current, starting torque, or both, and its overload and short-circuit protection. 

This is applicable to thyristor (SCR) circuits to protect all the semiconductor devices used in the switching circuit, such as diodes (also power diodes) or IGBTs, in addition to SCRs. The same protection can be applied to all the semiconductor circuits likely to experience high dv/di. 

The role of SCRs is to vary the supply parameters, which require frequent changes in V, i.e. du/df and in /, i.e. di/di in an energized condition. Becau.se of momentary phase-to-phase short-circuit, dv/di occurs during switching OFF and d 7df during switching ON sequences. Both are transient conditions and may damage the semiconductor devices used in the circuit. To protect the devices, the transient conditions can be dealt with as follows ... 

In protected motors failure may be due to accumulation of fly ash at the overhangs (modern installations use only enelosed motors). Fly ash beeomes an extremely good conductor when damp. The failure will generally occur when a motor is switched on after a prolonged shutdown. 

The selection should be such as to provide proper discrimination at the various levels of a multi-distribution network. Our discussions generally take account of the recommendations in lEC 60947-4-1 regarding coordination between the short-circuit protection and the main components such as switching devices [switch, breaker. 

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