Electric shock. This is the passage of current through the body of such magnitude as to have significant harmful effects. 1 mA – 2 mA barely perceptible, no harmful effects 5 mA – 10 mA throw off, painful sensation 10 mA – 15 mA muscular contraction, can not let go 20 mA – 30 mA impaired breathing 50 mA – and above ventricular fibrilation and death note :- 1 mA means 1 thousanth of an ampere (1/1000) There are two ways we can be at risk, direct contact – touching live parts of equipment or systems that are intended to be alive indirect contact – touching conductive parts which are not meant to be alive To avoid shock by direct contact, live parts must be insulated, or if not insulated then live parts must be housed in suitable enclosures and/or behind barriers. Direct contact. Residual current devices (RCD) are used to disconnect the mains supply under fault conditions i.e. when a live wire is connected to earth, this could be when a person touches a live wire and earth at the same time (just standing on a tiled floor provides the earth connection) so if a person touches a live wire and earth a potential difference of 230 volts exists across their body and a current will flow through them, but as soon as the current exceeds 30mA the RCD will trip out preventing them having a leathal shock. To prevent this situation occuring all live parts must be insulated, or if not insulated then housed in suitable enclosures and/or behind barriers. Indirect contact. Sometimes an appliance can become live, if for instance the live wire in an oven becomes detached and touches the metal casing, then the metal casing will become live with 230 volts. Then anyone touching the oven and earth (standing on a tiled floor) would receive a shock, although the RCD would trip out this situation is not desireable as the only way you would know that an appliance is faulty would be when you had a shock off it. To overcome this situation all appliances that have any exposed metalwork that could possibly become live must be earthed, all such appliances come with the facillity to connect to earth, all that is needed is an earth circuit present in the premises that the appliance is to be used in. Then if a live wire does touch the exposed metalwork a current flows through the earth wire and the RCD opperates before anyone can receive an electric shock. The use of a residual current device (RCD) does not prevent direct contact, but it can be a life saver providing that its rating is 30 mA or less and has a tripping time of not more than 40 ms. It should be noted that RCDs are not the panacea for all electrical ills, they can malfunction, and they should not be used as the sole means of protection. Equipotential bonding. In addition to appliances being earthed the electricity regulations require that all extraneous metalwork be earthed where pracically possible, this includes any baths, sinks, taps, pipes etc, certain things such as metal clamps holding a tube to a wall for example it would not be deemed practically possible to earth them all, also metal window frames, because the part you are most likely to touch is the moving part, would be exempt. It should be noted that it is the responsibility of the installer of the metal part to insure that the requirements are met. So what is earth and why and how do we connect to it. Earth in electrics means the general mass of the Earth and it is taken to be at zero potential, when we say that the live conductor is at a potential of 230 volts it means with respect to the Earth. A point to note here is that we do not use absolute values but relative values (which we call potential difference), so the mains supply is 230 volts relative to the earth, there is a potential difference of 230 volts. The Importance of Earthing and Bonding.
International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT. The first letter indicates the connection between earth and the power-supply equipment (generator or transformer) T : direct connection of a point with earth (Latin: terra); I : no point is connected with earth (isolation), except perhaps via a high impedance. The second letter indicates the connection between earth and the electrical device being supplied: T : direct connection of a point with earth N : direct connection to neutral at the origin of installation, which is connected to the earth TN networks In a TN earthing system, one of the points in the generator or transformer is connected with earth, usually the star point in a three-phase system. The body of the electrical device is connected with earth via this earth connection at the transformer. (see dia below) Earthing Systems.
The conductor that connects the exposed metallic parts of the consumer is called protective earth (PE). The conductor that connects to the star point in a three-phase system, or that carries the return current in a single-phase system, is called neutral (N). Three variants of TN systems are distinguished: TN−S : PE and N are separate conductors that are connected together only near the power source.(see below)
TN−C : A combined PEN conductor fulfills the functions of both a PE and an N conductor. The combined PEN conductor typically occurs between the substation and the entry point into the building, whereas within the building separate PE and N conductors are used. In the UK, this system is also known as protective multiple earthing (PME), because of the practice of connecting the combined neutral-and-earth conductor to real earth at many locations, to reduce the risk of broken neutrals.(see below)
TN−C−S : Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines.(see below)
It is possible to have both TN-S and TN-C-S supplies from the same transformer. For example, the sheaths on some underground cables corrode and stop providing good earth connections, and so homes where "bad earths" are found get converted to TN-C-S. TT network. In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator. The big advantage of the TT earthing system is the fact that its clear of high and low frequency noises that come through the neutral wire from various electrical equipment connected to it. This is why TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing. Also, TT does not have the risk of a broken neutral. In pre-RCD era, the TT earthing system was unattractive for general use because of its worse capability of accepting high currents in case of a live-to-PE short circuit (in comparison with TN systems). But as residual current devices mitigate this disadvantage, the TT earthing system becomes attractive for premises where all AC power circuits are RCD-protected.
IT network. In an IT network, the distribution system has no connection to earth at all, or it has only a high impedance connection. In such systems, an insulation monitoring device is used to monitor the impedance. Home page. Electrics Home page. Previous page. Next page.
