Protective conductor size

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

When high voltage motors are being considered, it is usually found that the minimum conductor size of the cable is determined by the let-through fault withstand capability rather than the full-load or starting current. Cable manufacturers provide graphical data for fault withstand capabilities of their cables, which are based on practical tests. These aspects are also associated with the protection system used for the motor, e.g. a contactor-fuse combination, a circuit breaker, the protective relay characteristics (thermal, inverse time with or without instantaneous or earth fault elements). 

A house extension has a total load of 6 kW installed some 18m away from the mains consumer unit for lighting. A PVC insulated and sheathed twin and earth cable will provide a sub-main to this load and be clipped to the side of the ceiling joists over much of its length in a roof space which is anticipated to reach 35°C in the summer and where insulation is installed up to the top of the joists. Calculate the minimum cable size if the circuit is to be protected by a type B MCB to BS EN 60898. Assume a TN-S supply, that is, a supply having a separate neutral and protective conductor throughout. 

Double socket outlets must have two terminals for protective conductors. One terminal to be used for each protective conductor, of a minimum size of 1.5 mm ... 

Section 543 deals with the types, sizes and preservation of electrical continuity of protective conductors. Regulation 543-01-01 requires a mechanically unprotected conductor to have a cross sectional area of not less than 4mm. ... 

Regulation 543-01-03 allows for the cross sectional area of the protective conductor to be calculated using the formula S= (I t)Vk. S is the cross sectional area in mm T is the fault current t is the operating time in seconds of the disconnecting device for a current off amps k is a factor that takes account of resistivity, temperature coefficient and heat capacity of the conductor material, and the appropriate initial and final temperatures. Alternatively, the size can readily be selected from Table 54G. There is a series of tables, 54B to 54F, which can be used to select the value of k for different types of conductor. 

To maintain the integrity of the supply, Regulation 561-01-03 advocates that automatic disconnection of the supply should not occur when the first earth fault occurs, but metalwork must be adequately earthed and bonded to minimise the shock hazard see also Regulation 566-01-01. Regulation 563-01-03 permits the omission of overload but not short circuit protection where a reduction in conductor size occurs see Regulation 473-01-01 and 552-01-02. Where there is more than one circuit, however, overload protection is likely to be required to comply with Regulation 563-01-04. 

Post-World War II installations will usually have a consumer unit, which is a small distribution board with a double pole isolating switch and a phase busbar to feed the fuses or MCBs controlling the final circuits. There will be terminal blocks for the neutrals and protective conductors. The earth terminal block may be utilised as the consumer s earthing terminal. Some of the older models may have wood frames and/or may be backless. As BS 7671 requires connections to be made in non-flammable enclosures, the wooden framed type do not comply and the backless ones are acceptable only if mounted on non-flammable material. If rewirable fuses to BS 3036 are used, check that the correct size of fuse wire has been used. For cartridge fuses, check that blown fuses have not been repaired with a bit of fuse wire spanning the contacts or the cartridge replaced by a nail, hairpin or the like. There should be only one final circuit connected to each fuse carrier or MCB. 

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. 

Conductors of sufficient size so as to prevent overheating, and adequately insulated. All circuits to be protected by fuses or automatic circuit breakers, except for specified battery connections... 

The type and size of ground rod used is determined by how many sections are to be connected and how hard or rocky the soil is. Copper-clad % -in. x 10-ft rods are probably the most popular. Copper cladding is designed to prevent rust. The copper is not primarily to provide better conductivity. Although the copper certainly provides a better conductor interface to earth, the steel that it covers is also an excellent conductor when compared with ground conductivity. The thickness of the cladding is important only insofar as rust protection is concerned. 

From a ] actical point of view, gas phase polymerization can lead to the syndiesis of defect-fiee, uniform diin polymeric films of controlled morphology and tailored coiiqiositions with excellent elecdical and optical properties for many technological applications such as protective coatings and electrical insulators. For example, the polymeric species could be deposited fixim die gas phase in a size-selected manner on metal or s conductor surfaces. Furdiomore, gas jdiase potymerization can be easily coupled with the vapor... 

Overloading of conductors is not a frequent fire causation because the exeess current protection is usually matched to the eonductor size and will operate before the conductors are seriously overheated. 

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