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WEEK TWO and three 2391 IMPORTANT REMINDER Your exam fees must be paid by Friday 15th April 2005 for the June exam. If this deadline is missed City and Guilds levy an additional fee! It may be prudent to remind ourselves that our inspection and testing procedures are required to confirm compliance with BS7671 2001 which, as we have seen, will help demonstrate compliance the statutory obligations of the duty holders of the installation concerned under the EAW Regulations 1989. Part One of BS7671 2001, entitled Scope, Object and Fundamental Principles, requires that protection is required for three distinct elements. State them. The particular hazards fro the use of an electrical installation are also identified in Part One of BS7671 2001. State them. We have begun by looking at the methods of protection against electric shock. These were identified in the Week One workbook. It is, however, important to ensure that these other aspects of protection are fully appraised in our inspection. We continue now to look at one very important method of protection against indirect shock. EBADOS is by far the most common method employed to provide such protection. It should, however, be emphasised that this system will need support from the recognised methods of protection against direct contact in order to have an effective shock protection regime. 5 Firstly, some clarification may be required in the terminology. State the meaning of indirect contact as it refers to electric shock. State three examples of exposed conductive parts in a typical installation 1 2 3 State three examples of extraneous conductive parts that are required to be connected to the main earth terminal. 1 2 3 State the meaning of the acronym EBADOS. Outline the basic principle of EBADOS. 6 In the space below, sketch the drawing on the board, which shows the fault voltages that might exist in the event of a fault to earth in a typical circuit in a typical installation. The importance of appropriate main equipotential bonding is emphasised in BS7671 2001 for both TN and TT systems. The effect of such bonding then is to minimise fault voltages between exposed conductive parts and extraneous conductive parts in the event of an earth fault. These fault voltages will only be in existence for the time it takes to operate the protective device. The Regulations dictate the required maximum disconnection times. Three common maximum disconnection times are identified for TN and TT systems operating at 230volts to earth. The maximum time depends on whether the circuit supplies fixed equipment, socket outlets or portable equipment and where that equipment is to be used. Complete the table below. CIRCUIT Ring final circuit in a dwelling DISCONNECTION TIME REQUIRED Shower circuit in a dwelling Lights in a dwelling 230v supply to a lighting circuit on a construction site 230v sockets where livestock might be kept in a farm. Three phase 400v motor in a joiners workshop. It is obvious that the disconnection time is dictated by the fault current which in turn is dictated by the impedance of the circuit. The voltage responsible for the fault current is not the nominal voltage to earth, 7 Uo, at the terminals of the installation but the open circuit voltage to earth, Uoc, at the distribution transformer. Uoc is taken to be 240v for a nominal supply voltage of 230v. State this relationship in terms of the maximum tolerable earth fault loop impedance. The operating time of a protective device can be found by reference to the Time Current Characteristics in Appendix 3 of BS7671 2001. From these curves we can determine the maximum value of earth fault loop impedance. Lets look at the previous table this time with a stipulated protective device. Complete the table below. DEVICE In CIRCUIT DISCONNECTION CURRENT TIME REQUIRED REQUIRED Ia MAXIMUM EARTH FAULT LOOP IMPEDANCE Zs=Uoc/Ia BS1361 30A BSEN60898B 40A BS1361 5A BS88 10A Ring final circuit in a dwellingcircuit Shower in a dwelling Lights in a dwelling 230v supply to a lighting circuit on a construction site BS3036 230v sockets 20A where livestock might be kept in a farm. BSEN60898C Three phase 40A 400v motor in a joiners workshop. Check your results against the maximum values of earth loop impedance found in Tables 41B1, 41B2 and Table 41D in BS7671 2001. You should find a direct correlation (providing you have used 240v as Uoc). The value of Zs is obviously important and there are a number of ways of determining it. The general formula is; Zs = Ze + (R1+R2) 8 We could find the value by measurement, calculation and to some extent by enquiry. The latter is used to establish the maximum value of Ze from the DNO in situations where perhaps a design is being considered and the supply is not yet available. Thus we can design or indeed test circuits using the R1 and R2 values 9