Earthing metalwork

Earth bonding tests apply a substantial test current, typically about 25 A, down the earth pin of the plug top to an earth probe, which should be connected to any exposed metalwork on the portable appliance being tested. The PAT Tester will then calculate the resistance of the earth bond and either give an actual reading or indicate pass or fail. A satisfactory result for this test would typically be a reading of less than 0.1 n. The earth bond test is, of course, not required for double-insulated portable appliances because there will be no earthed metalwork. 

The combination of synthetic carpets, man-made footwear materials and dry, air-conditioned buildings contributes to the creation of static electrical charges building up on people moving about these buildings. Individuals only become aware of the charge if they touch earthed metalwork, such as a stair banister rail, before the static electricity has been dissipated. The effect is a sensation of momentary shock. 

Method 2, the use of Class 11 insulated equipment is limited to single pieces of equipment such as tools used on construction sites, because it relies upon effective supervision to ensure that no metallic equipment or extraneous earthed metalwork enters the area of the installation. 

If a person is in contact with earthed metalwork or is inadequately insulated from earth then, because the human body and the earth itself are good conductors of electricity, they can form part of a circuit (albeit an abnormal... 

Short circuits have also been caused by uninsulated tools which, when carelessly used, have bridged between live parts or between a live part and earthed metal. Another common cause is the use of inadequately insulated instrument probes. The bare section of the metal probe, if carelessly used, may bridge between a live part and earthed metalwork or between live parts of different polarity, again causing a short circuit with the consequential arcing and burn injuries. 

Many burn injuries are caused by flame arcs emitted with explosive violence when short circuits occur in apparatus where the fault levels are high. The short circuits often take place during live working when conductive parts such as bolts, nuts and washers are accidentally dropped and bridge between a phase conductor and earthed metalwork, or between phases. Another common cause is the use of uninsulated or insufficiently insulated tools which can bridge conductors in the same way. 

If Class II distribution apparatus or accessories are used. Regulation 471-09-02 requires the protective conductor to be available at each wiring point to allow for the replacement of the Class II by Class I equipment. It is preferable for the apparatus and accessories to be of the all-insulated rather than the double-insulated type to avoid the possibility of exposed conductive parts making fortuitous contact with earthed metalwork. 

In confined and conductive locations the potential electric shock hazard is increased and special precautions are needed. Examples of confined, conductive spaces are inside boilers and other metal vessels or inside metal pipes, flues and ducts where the area of body contact to earthed metalwork is likely to be substantial. Even if the interior is dry, the shock risk is enhanced, but if it is damp it is worse. In these circumstances the llOV system is not considered safe and pneumatic, hydraulic or battery powered tools are advocated. For lighting, battery powered cap and hand lamps could be used or the luminaires could be supplied from a safety transformer at not more... 

An electrical earth system comprises the provision on the supply system of an earth connection to facilitate earth fault current flow, the connecting of all exposed metalwork within the installation to a common grounding terminal and the connection of this terminal to earth. For a complete installation the principal factors which need to be considered, are ... 

In order to minimise the risk of shock and fire, any metalwork other than the current-carrying conductor must be connected to earth. The neutral of the electrical supply is earthed at the source of distribution, i.e. the supply transformer, so that, if all appliances are also connected to earth, a return path for the current will be available through earth when a fault occurs (see Fig. 1.9). To be effective, this earth path must he of sufficiently low resistance to pass a relatively high current when a fault occurs. This higher current will in turn operate the safety device in the circuit, Le. the fuse will blow. 

Accidents happen when the body provides a direct cormection between the live conductors -when the body or a tool touches equipment connected to the supply. More often, however, the cormection of the hirman hody is between one live conductor and earth, through the floor or adjacent metalwork (see Fig. 1.10). Metal pipes carrying water, gas or steam, concrete floors, radiators and machine structirres all readily provide a conducting path of this kind. 

The written records of the monk Reinier of Liege from the early thirteenth century describe workers mining black earth in Europe. Blacksmiths used this black earth as fuel for metalworking. Other historical records contain numerons references to coal mining in England, Scotland, and continental Europe throughout the thirteenth century. 

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Protection against electric shock, both basic protection and fault protection , is provided by insulating and placing live parts out of reach in suitable enclosures, earthing and bonding metalwork and providing fuses or circuit-breakers so that the supply is automatically disconnected under fault conditions. 

Where earthed electrical equipment may come into contact with the metalwork of other services, they too must be effectively connected to the main protective earthing terminal of the installation (lET Regulations Chapter 544). 

Circuit protective conductor (CPC) - a protective conductor connecting exposed conductive parts of equipment to the main earthing terminal. Exposed conductive parts - the metalwork of an electrical appliance or the trunking and conduit of an electrical system which can be touched because they are not normally live, but which may become live under fault conditions. 

In Chapter 13 of the lEE Regulations we are told that where the metalwork of electrical equipment may become charged with electricity in such a manner as to cause danger, that metalwork will be connected with earth so as to discharge the electrical energy without danger. The application of protective equipotential bonding is one of the important principles for safety. 

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