Conductors protection

Cable sheath. Additional overhead line conductor Protective multiple earthing fP.M.E.) Buried Strip/rod/plate Earth-leakage circuit breaker, Voltage-operated Earth-leakage circuit breaker, Current-operated... 

The illustration shows the outlines of a building with a lightning conductor protecting the chimney which constitutes the highest point. In the attic is an electrical point or a water pipe which in turn is connected directly or indirectly to ground. Bear in mind that an electrical supply is still almost at ground poten-... 

Phase conductor Protective conductor Copper Aluminium... 

The majority of fixed apparatus is of Class I construction, as defined in BS 2754 1999 Construction of electrical equipment for protection against electric shock. This means that it usually has a metal case enclosing the live parts and that the five parts are insulated from each other and from the carcass by basic insulation only. The case, which is earthed by connection to the protective conductor, protects the internal wiring and components from damage and prevents direct contact. 

Conformal coatings are protective coatings appHed to circuit board assembHes. They protect the interconnect conductors, solder joints, components, and the board itself they reduce permeabiHty to moisture, hostile chemical vapors, and solvents in the coating. Use of conformal coatings eliminates dendritic growth between conductors, conductor bridging from moisture condensation, and reduction in insulation resistance by water absorption. 

Only Ee O and y-Ee202 are considered to be protective films. Both are adherent and good electronic conductors. Alpha-Ee202, which forms in water and steam containing oxygen, is not adherent, is less protective, and is an insulator. EeO, which does not form at temperatures below 570°C, is nonprotective. 

The contact ends of printed circuit boards are copper. Alloys of nickel and iron are used as substrates in hermetic connectors in which glass (qv) is the dielectric material. Terminals are fabricated from brass or copper from nickel, for high temperature appHcations from aluminum, when aluminum conductors are used and from steel when high strength is required. Because steel has poor corrosion resistance, it is always plated using a protective metal, such as tin (see Tin and tin alloys). Other substrates can be unplated when high contact normal forces, usually more than 5 N, are available to mechanically dismpt insulating oxide films on the surfaces and thereby assure metaUic contact (see Corrosion and corrosion control). 

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. 

Where wires or conductors may pass through a metal sheet, a rubber grommet, bushing or other mechanical protection should be provided to prevent the wires from insulation damage. 

For instance, when lightning of, say, a nominal discharge current of 10 kA strikes a 400 kV (r.m.s.) overhead line, having a surge impedance of 350 Q, then two parallel waves will be produced each of amplitude 10 x 350/2 or 1750 kV which may be more than the impulse withstand level of the system and cause a flashover between the conductors and the ground, besides damaging the line insulators and the terminal equipment (Table 13.2). It is therefore imperative that the system is protected against such eventualities. 

A. To calculate the main ground conductor size, assume that the system is protected through HRC fuses. Then, based on the previous assumptions. 

The fault level, whieh is a function of the size of the feeding transformer, is generally considered to last for only one second, as discussed in Section 13.4.1 (5), unless the system requirements are more stringent. This duration of one second on fault may cause such a temperature rise (not the electrodynatnic forces), that unless adequate care is taken in selecting the size of the current-carrying conductors, they may melt or soften to a vulnerable level before the fault is intenupted by the protective devices. 

Anodes similar to cables are used which consist of a copper conductor covered with conducting plastic. This creates an electrolytically active anode surface and at the same time protects the copper conductor from anodic dissolution. 

Anodes are connected to the object to be protected or to the transformer-rectifier by insulated conductors that are resistant to mineral oil (e.g., Teflon-coated cable) with a cross-section of 2.5 mm of Cu. The transformer-rectifier must meet the demands according to Ref. 6 and have the capability for monitoring and controlling its operation. The life of the anodes is in every case designed to be at least 15 years. 

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