Neutrals cable sheaths

The extension of this technique to the distribution board or feeder pillar where four or five core cables are concerned is relatively simple. Traditionally this termination was made using the iron box filled with bitumen. The cable box was replaced by a preformed plastics glove which is pulled down over the prepared cores so that the lower end overlaps the cable sheath and is then shrunk into position. The individual paper-insulated cores are protected by shrinkable tubing and the terminating lug is sweated or compressed on to the end of the conductor. In a conventional termination the cable box itself forms the direct connection between the cable sheath and the neutral earth bar. 

Ground- Refers to electricity s habit of seeking the shortest route to earth. Neutral wires carry it there in all circuits. An additional grounding wire or the sheathing of the metal-clad cable or conduit— protects against shock if the neutral leg is interrupted. 

New materials and manufacturing techniques have been used to minimize the problem of drag and friction that occurs with heavy umbilicals. Tufflite, a surface diving umbilical, combines the strength of its rope-like cabled lay-up with kink resistance and neutral buoyancy that allows improved maneuverability and lower drag. Tufflite is based on textile braid reinforced polyurethane (PU) hoses for gas and air supply and has a color-coded outer sheath of abrasion-resistant PU. 

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. 

This type of cable comprises conducting cores insulated with paper or plastics insulation or dielectric. Around the outside is a lead or nowadays more commonly aluminium sheath with finally an outer armour. This is the type of cable which provides the normal house or office service. The house service version has one phase and one neutral core, but in the distribution box the incoming cable will contain four or five cores. 

The first weather-protected application concerned house service installations comprising one phase and one neutral core. The termination for this consists of a Y-shaped heat-shrinkable moulding. The house service cable is prepared by stripping off the lead sheath and separating the phase and neutral cores with the insulation intact. These are then threaded up and out of the arms of the Y piece. The Y piece is then shrunk so that it overlaps the lead sheath and the upper arms are covered with tubing. An ordinary propane gas torch is used as a heat source and the total shrinking time is less than 5 min. A suitable earth clip is fitted to the lead sheath to provide consumer earthing facilities. 

As shown in Fig. 8, the high voltage power cable consists of three main aluminum cores, each of which has a diameter of 300 mm, and a neutral core of 185 mm diameter. The three main cores and the neutral core consist of 61 and 37 insulated stranded aluminum rods, respectively. The four cores are warped with polyethylene tap>e that is supported by a layer of insulation. Finally, the cable is armored with steel and wires for protection and is sheathed by an additional layer of insulated. The flow chart shown in Fig. 9, describes the flow of the cable across the different stations. 

Electrode boilers are covered by subsection 554-03. As some earth leakage is inevitable, earthing is important, so the boiler shell has to be connected to the proteetive eonductor and to the metallie armour and sheath, if any, of the incoming supply cable and also to the supply neutral. Earthing the neutral will affeet the operation of an RCD as the earth leakage current will be shared by the neutral and protective conduetors. Regulation 554-03-04 refers to a three-phase electrode boiler supplied direetly from an HV supply. In this case there is no neutral connection, but an RCD is a requirement. The neutral does not have to be connected to the boiler shell for boilers not piped to the water supply see section 554-03-07. 

The burn accidents invariably result from manual excavation when, for example, an operator using a pneumatic drill penetrates a concealed, buried cable with the tool. A short circuit will occur when the bit comes into contact with one or more of the live conductors and the metallic armouring and/or sheath of SNE cables or the CNE conductor where a PME distribution system is in use. The initial phase/earth or phase/neutral fault usually develops and involves the other phases, causing arcing which may emerge as a flame arc and blast from the hole made by the tool, injuring the operator. 

In the past, there were fewer accidents because buried cables were better protected. Most of the supply authorities used paper-insulated, lead-sheathed, single-wire or steel-tape armoured and served cables and very often protected them with cable tiles. For economic reasons cable tiles are seldom used now and many of the mains laid in recent years are unarmoured CNE cables with plastic insulation and sheathing, such as PVC and cross-linked polyethylene (XLPE). The outer concentric conductor, which is the earthed neutral, is copper or aluminium tape and consequently more vulnerable to damage than a steel-wire-armoured cable. Every year more new mains cables of this type are laid so the resultant hazard to excavators is growing and more accidents can be expected to occur unless the construction industry becomes more effective at taking precautions against striking buried cables. 

Despite its many drawbacks the lead sheath has found widest application in the course of the decades up to the present time to protect the cable core against the entry of liquids and vapors and - frequently in connection with a suitable reinforcing - against mechanical stresses. In some special cases the sheath served as grounding wire or neutral conductor, too. 

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