Fundamentals and Earthing

Transcription

1 Ανάλυση και Εφαρμογή κανονισμών BS 7671 για ηλεκτρολογικές εγκαταστάσεις (17 η έκδοση, 3 η τροποποίηση)» Μονοεπιχειρησιακό πρόγραμμα εγκεκριμένο από την ΑνΑΔ Fundamentals and Earthing Ιούνιος 2017 Εισηγητής Δρ Νικόλας Χριστοφίδης

2 Basis of Electrical Installations 1. Use good workmanship. 2. Use approved materials and equipment. 3. Ensure that the correct type, size and current-carrying capacity of cables are chosen. 4. Ensure that equipment is suitable for the maximum power demanded of it. 5. Make sure that conductors are insulated, and sheathed or protected if necessary, or are placed in a position to prevent danger. 6. Joints and connections should be properly constructed to be mechanically and electrically sound. 7. Always provide overcurrent protection for every circuit in an installation (the protection for the whole installation is usually provided by the Distribution Network Operator [DNO]), and ensure that protective devices are suitably chosen for their location and the duty they have to perform. 8. Where there is a chance of metalwork becoming live owing to a fault, it should be earthed, and the circuit concerned should be protected by an overcurrent device or a residual current device (RCD). 9. Ensure that all necessary bonding of services is carried out 10. Do not place a fuse, a switch or a circuit breaker, unless it is a linked switch or circuit breaker, in an earthed neutral conductor. The linked type must be arranged to break all the line conductors. 2

3 Basis of Electrical Installations 11. All single-pole switches must be wired in the line conductor only 12.A readily accessible and effective means of isolation must be provided, so that all voltage may be cut off from an installation or any of its circuits. 13. All motors must have a readily accessible means of disconnection. 14.Ensure that any item of equipment which may normally need operating or attending by persons is accessible and easily operated. 15.Any equipment required to be installed in a situation exposed to weather or corrosion, or in explosive or volatile environments, should be of the correct type for such adverse conditions. 16.Before adding to or altering an installation, ensure that such work will not impair any part of the existing installation and that the existing is in a safe condition to accommodate the addition. 17.After completion of an installation or an alteration to an installation, the work must be inspected and tested to ensure, as far as reasonably practicable, that the fundamental requirements for safety have been met. 3

4 Who am I and what do i do? LABC-local Authority Building Control 4

5 Examples of Work Notifiable and Not Notifiable 5

6 Examples of Work Notifiable and Not Notifiable 6

7 Examples of Work Notifiable and Not Notifiable 7

8 Examples of Work Notifiable and Not Notifiable 8

9 Distribution Supply Voltage 9

10 Earthing - Definitions Bonding a protective conductor providing equipotential bonding Circuit Protective Conductor (CPC) a protective conductor connecting exposed conductive parts of equipment to the main earthing terminal Direct Contact contact of persons or livestock to live parts Earth the conductive mass of the earth, whose electric potential at any part is conventionally taken as zero 10

11 Earthing - Definitions Earth Fault Loop Impedance the impedance of the phase-to-earth loop path starting and ending at the point of fault Earthing Conductor a protective conductor connecting a main earthing terminal of an installation to an earth electrode or other means of earthing 11

12 Earthing - Definitions Equipotential bonding: electrical connection maintaining various exposed conductive parts and extraneous conducive part at a substantially equal potential Exposed conductive part: a conductive part of equipment which can be touched and which is not a live part but which may become live under fault conditions Extraneous conductive part: a conductive part liable to introduce a potential, generally earth potential and no forming part of the electrical installation Functional earthing: connection to earth necessary for proper functioning of electrical equipment 12

13 Earthing - Definitions Indirect contact: contact of persons or livestock with exposed conductive parts made live by a fault Leakage current: electric current in an unwanted conductive part under normal operating conditions Live part: a conductor or conductive part intended to be energised in normal use, including a neutral conductor but, but by convection not a PEN PEN conductor: a conductor combining the functions of both protective conductor and neutral conductor Phase conductor: a conductor of an AC system for the transmission of electrical energy, other than a neutral conductor 13

14 Earthing - Definitions PME: an earthing arrangement, found in TN-C-S systems, where an installation is earthed via the supply neutral conductor Protective conductor: a conductor used for some measure of protection against electric shock and intended for connecting together any of the following parts: Exposed conductive parts Extraneous conductive parts Main earthing terminal Earth electrode Earthed point of the source 14

15 Earthing - Definitions Residual Current Device: an electromechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under given conditions Simultaneously accessible parts: conductors or conductive parts which can be touched simultaneously by a person or, where applicable, livestock 15

16 Definitions 17 th Basic protection Protection against electric shock under fault-free conditions. Bonding conductor A protective conductor providing equipotential bonding. Circuit protective conductor (cpc) A protective conductor connecting exposed conductive parts of equipment to the main earthing terminal. Earth The conductive mass of earth, whose electric potential at any point is conventionally taken as zero. Earth electrode resistance The resistance of an earth electrode to earth. Earth fault current An overcurrent resulting from a fault of negligible impedance between a line conductor and an exposed conductive part or a protective conductor Earth fault loop impedance The impedance of the phase-to-earth loop path starting and ending at the point of fault. 16

17 Definitions 17 th Earthing conductor A protective conductor connecting a main earthing terminal of an installation to an earth electrode or other means of earthing. Equipotential bonding Electrical connection maintaining various exposed conductive parts and extraneous conductive parts at a substantially equal potential. Exposed conductive part A conductive part of equipment which can be touched and which is not a live part but which may become live under fault conditions. Extraneous conductive part A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation. Fault protection Protection against electric shock under single fault conditions. Functional earth Earthing of a point or points in a system or an installation or in equipment for purposes other than safety, such as for proper functioning of electrical equipment. 17

18 Definitions 17 th Leakage current Electric current in an unwanted conductive part under normal operating conditions. Line conductor A conductor of an AC system for the transmission of electrical energy, other than a neutral conductor Live part A conductor or conductive part intended to be energized in normal use, including a neutral conductor but, by convention, not a PEN conductor PEN conductor A conductor combining the functions of both protective conductor and neutral conductor PME (protective multiple earthing) An earthing arrangement, found in TN-C-S systems, where an installation is earthed via the supply neutral conductor. 18

19 Definitions 17 th Protective conductor A conductor used for some measure of protection against electric shock and intended for connecting together any of the following parts: exposed conductive parts extraneous conductive parts main earthing terminal earth electrode(s) earthed point of the source. Residual current device (RCD) An electromechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under given conditions. Simultaneously accessible parts Conductors or conductive parts which can be touched simultaneously by a person or, where applicable, by livestock. 19

20 Earth Earth is not a good conductor So why connect anything to it? Understand of Potential Difference (PD) 0 volt potential! Earth: the conductive mass of earth, whose electric potential at any point is conventionally taken as zero Test: Measure voltage between Phase and Earth =? Measure voltage between 12V terminal of a battery and earth =? 20

21 illustration 21

22 Earth Person touching a faulty appliance could suffer an electric shock Lethal shock current level is 50mA and above Protection against electric shock is achieved by bonding all metallic parts between them: Ensures that under healthy conditions all metalwork is at 0 volts Under fault conditions, all metalwork rises to same potential Simultaneous contact with 2 such metal parts would not result in a dangerous shock no PD E.E.B. (A.D.S.) 22

23 Earth Earth presents a high resistance to the flow of current unless it is wet Resistance is enough to restrict fault current to a level well below that of the rating of the protective device leaving a faulty circuit uninterrupted methods to overcome discussed later. 23

24 Connecting to Earth Necessary to have as low an earth path resistance as possible The greater the surface contact area with earth that can be achieved, the better. Methods of connecting to earth Rod earth electrode (most popular, show data) Plates( sufficient depth needed, 1-2 m 2 ) Tapes (earthing of large electricity substations) 24

25 Shock Path 25

26 Earth electrode resistance area 26

27 Ground surface voltage 27

28 Housing the electrode in a pit below ground level 28

29 Earth Rod 29

30 Earthing in IEE regulations Chapter 4 Indirect contact contact with metalwork made live by a fault Can be overcome by EEBADS where we bond together and earth: All metalwork associated with electrical apparatus and systems, termed exposed conductive parts e.g conduit, trunking and metal cases of apparatus All metalwork liable to introduce potential, including earth potential, termed extraneous conductive parts e.g. gas, oil and water pipes, structural steelwork, radiators, sinks and baths 30

31 Earthing in IEE regulations Chapter 4 Conductors in such connections are called protective conductors, subdivided into: circuit protective conductors CPC Main equipotential bonding conductors Other equipotential bonding conductors Supplementary bonding conductors 31

32 Illustration of earthing and protective conductor terms 32

33 Earthing Systems T terre (French for earth) N neutral C combined S separate When letters above are grouped they form the earthing system classification 1 st how supply source is earthed 2 nd how metalwork of an installation is earthed 3 rd, 4 th indicate functions of neutral and protective conductors 33

34 34

35 Earthing Systems TN-S A TN-S system has the neutral of the source of energy connected with earth at one point only, at or as near as is reasonably practicable to the source, and the consumer s earthing terminal is typically connected to the metallic sheath or armour of the distributor s service cable into the premises or to a separate protective conductor of, for instance, an overhead supply 35

36 Earthing Systems TN-S 36

37 Earthing Systems TN-S 37

38 Earthing Systems TN-S 38

39 Earthing Systems TN-C-S A TN-C-S system has the supply neutral conductor of a distribution main connected with earth at source and at intervals along its run. This is usually referred to as protective multiple earthing (PME). With this arrangement the distributor s neutral conductor is also used to return earth fault currents arising in the consumer s installation safely to the source. To achieve this, the distributor will provide a consumer s earthing terminal which is linked to the incoming neutral conductor. 39

40 Earthing Systems TN-C-S 40

41 Earthing Systems TN-C-S 41

42 Earthing Systems TN-C-S 42

43 Protective multiple earthing (PME). Such a supply system is described in BS 7671 as TN-C-S The Electricity Safety, Quality and Continuity Regulations 2002 permit the distributor to combine neutral and protective functions in a single conductor provided that, in addition to the neutral to earth connection at the supply transformer, there are one or more other connections with earth. This protective multiple earthing (PME) has been almost universally adopted by supply companies in the UK as an effective and reliable method of providing their customers with an earth connection. Such a supply system is described in BS 7671 as TN-C-S. 43

44 Protective multiple earthing (PME). Such a supply system is described in BS 7671 as TN-C-S However, under certain supply system fault conditions (external to the installation) a potential can develop between the conductive parts connected to the PME earth terminal and the general mass of earth. Supply system There are multiple earthing points on the supply network, and providing bonding within the building complies with BS 7671 it is unlikely that such a potential as described above would in itself constitute a hazard. Additional earth electrode for PME supplies. In the unlikely event of the PEN conductor of the supply becoming open circuit, touch voltages perhaps causing some discomfort may arise on exposed metal in customers installations downstream of the open circuit. 44

45 Protective multiple earthing (PME). Such a supply system is described in BS 7671 as TN-C-S The effect can be mitigated by connection of a suitable earth electrode to the main earth terminal of the customers installation. The value of the resistance toearth necessary to limit the touch voltages to a given value depends on the load and the network parameters: 45

46 Protective multiple earthing (PME). Such a supply system is described in BS 7671 as TN-C-S Where: Vs is the nominal supply (source) voltage Vp is the touch voltage Re is the external supply resistance RL is the load resistance (Vs2/ wattage) RA is the resistance of the additional earth electrode including parallel earth (e.g. water and gas pipes) RB is the resistance to earth of the neutral point of the power supply. 46

47 Earthing Systems TT A TT system has the neutral of the source of energy connected as for TN-S, but no facility is provided by the distributor for the consumer s earthing. With TT, the consumer must provide his or her own connection to earth, i.e. by installing a suitable earth electrode local to the installation. 47

48 Earthing Systems TT 48

49 Earthing Systems TT 49

50 Earthing Systems TT 50

51 Earth fault loop impedance - Z s Circuit protection should operate in the event a direct fault from phase to earth Attention on speed of operation! Depends on magnitude of fault current which in turn depends on the impedance of the earth return path Path consists of 1. The cpc. 2. The consumer s earthing terminal and earth conductor. 3. The return path, either metallic or earth, dependent on the earthing system. 4. The earthed neutral of the supply transformer. 5. The transformer winding. 6. The line conductor from the transformer to the fault. 51

52 Earth fault loop impedance - Z s 52

53 53

54 Earth fault loop impedance Z s Simplified loop path Z s =Z e + R 1 + R 2 I = U oc / Z s 54

55 Determining Z e - external loop impedance 1. Determine it from details of the supply transformer, the main distribution cable and the proposed service cable 2. Measure it from the supply intake position of an adjacent building having service cable of similar size and length to that proposed 3. Use maximum likely values issued by the supply authority as follows (UK) 1. TT system: 21 Ω maximum? 2. TN-S system: 0.8 Ω maximum 3. TN-C-S system: 0.35 Ω maximum 55

56 Calculating resistance values at conductor operating temperature R t = R 20 { 1 + α 20 ( θ - 20 ) } Where, R t = Resistance at conductor operating temp R 20 = Resistance at 20 C α 20 = the 20 temp coefficient of copper, Ω/Ω/ C θ = the conductor operating temperature For 70 C PVC insulated conductor, the multiplier becomes { (70-20)}=1.2 56

57 Example For a 90 C XLPE type cable the multiplier becomes: 57

58 Copper conductor resistances 1-35 mm 2 58

59 Copper conductor resistances mm 2 59

60 Earth fault loop impedance - Z s Value of Z s should be as low as possible in order to allow enough current to flow to operate the protection as quickly as possible. Tables 41B1, B2 and D give maximum values of loop impedance for different sizes and types of protection for sockets and fixed equipment circuits 60

61 Earth fault loop impedance in 17 th edition- Z s Value of Z s should be as low as possible in order to allow enough current to flow to operate the protection as quickly as possible. Tables 41.2, 41.3 and 41.4 give maximum values of loop impedance for different sizes and types of protection for sockets and fixed equipment circuits 61

62 Disconnection times in 16 th edition Socket outlets 0.4 sec or less Circuits feeding fixed equipment 5 sec or less Bathrooms 0.4 sec or less These times do not indicate the duration that a person can be in contact with a fault. They are based on the probable chances of someone being in contact with exposed or extraneous conductive parts at the precise moment that a fault develops 62

63 Disconnection times in 17 th edition Final circuits 0.4 sec or less Distribution circuits 5 sec or less Bathrooms 0.4 sec or less These times do not indicate the duration that a person can be in contact with a fault. They are based on the probable chances of someone being in contact with exposed or extraneous conductive parts at the precise moment that a fault develops 63

64 Disconnection Times for final circuits not exceeding 32A 64

65 Disconnection Times 65

66 Fuses 66

67 Fuses BS3871 domestic cartridge fuse Description: Metal light switch which is not earthed. This poses a risk of shock. Note that the cables are old steel stranded and that their outer insulation is frayed and extremely worn 67

68 Example 1 68

69 Example 2 69

70 Example 2 - figure 70

71 Additional Protection (17 th edition) Requirements for RCD protection 30mA All socket outlets rated at not more than 20 A and for unsupervised general use Mobile equipment rated at not more than 32 A for use outdoors All circuits in a bath/shower room Preferred for all circuits in a TT system All cables installed less than 50mm from the surface of a wall or partition (in the safe zones) if the installation is unsupervised, and also at any depth if the construction of the wall or partition includes metallic parts 71

72 Additional Protection (17 th edition) In zones 0, 1 and 2 of swimming pool locations All circuits in a location containing saunas, etc. Socket outlet final circuits not exceeding 32A in agricultural locations Circuits supplying Class II equipment in restrictive conductive locations Each socket outlet in caravan parks and marinas and final circuit for houseboats All socket outlet circuits rated not more than 32 A for show stands, etc. All socket outlet circuits rated not more than 32 A for construction sites (where reduced low voltage, etc. is not used) All socket outlets supplying equipment outside mobile or transportable units All circuits in caravans All circuits in circuses, etc. A circuit supplying Class II heating equipment for floor and ceiling heating systems 72

73 Additional Protection (17 th edition) 100mA Socket outlets of rating exceeding 32A in agricultural locations. 300mA At the origin of a temporary supply to circuses, etc. Where there is a risk of fire due to storage of combustible materials All circuits (except socket outlets) in agricultural locations. 500mA Any circuit supplying one or more socket outlets of rating exceeding 32 A, on a construction site. 73

74 When max values of Z are not satisfied We have seen the importance of the total earth loop impedance Z in the reduction of shock risk. However, in some systems and especially TT, where the maximum values of Z given in Tables 41.2, 41.3 and 41.4 of the regulations may be hard to satisfy, an RCD may be used: its residual rating being determined from : I 50 / Z n s 74

75 Residual Current Devices Value of Z s is very important in the reduction of shock risk In TT systems, the earth forms part of the fault path and values given in tables 41B1,B2 and D may be hard to satisfy. In addition, climatic conditions alter the earth resistance, constituting Z s satisfactory in wet weather but not in dry! IEE regulations recommend protection for socket outlet circuits in TT systems be achieved by a residual operating device (RCD), such that the product of its residual operating current and the loop impedance will not exceed 50V RCBs, RCCBs, and RCDs are all the same thing RCBO is a combined circuit breaker and RCD in one device 75

76 RCD principle of operation Healthy circuit Same current through phase coil, load and neutral coil Magnetic effects of phase and neutral currents cancel out Faulty circuit phase to earth / neutral to earth Currents no longer equal Out of balance current produces residual magnetism in core and links with the turns of the search coil, inducing an emf in it. The emf drives a current in the trip coil causing operation of the device 76

77 Why do we need residual current devices? The standard method of protection is to make sure that an earth fault results in a fault current high enough to operate the protective device quickly so that fatal shock is prevented. However, there are cases where the impedance of the earth-fault loop, or the impedance of the fault itself, are too high to enable enough fault current to flow. In such a case, either: 1. - current will continue to flow to earth, perhaps generating enough heat to start a fire, or 2. - metalwork which is open to touch may be at a high potential relative to earth, resulting in severe shock danger. Either or both of these possibilities can be removed by the installation of a residual current device (RCD). In recent years there has been an enormous increase in the use of initials for residual current devices of all kinds. The following list, which is not exhaustive, may be helpful to readers: RCD residual current device RCCD residual current operated circuit breaker SRCD socket outlet incorporating an RCD PRCD portable RCD, usually an RCD incorporated into a plug RCBO an RCCD which includes overcurrent protection SRCBO a socket outlet incorporating an RCBO 77

78 RCD principle of operation Note: Phase to neutral fault will appear as a load and therefore RCD will not operate 3Φ RCD 78

79 Nuisance Tripping Certain appliances such as cookers, water heaters and freezers tend to have, the nature of their construction and use some leakage currents to earth Could cause operation of an RCD protecting entire installation Overcome by using split load consumer units, where socket outlets are protected by 30mA RCD, leaving all other circuits controlled by a normal mains switch In TT systems, is recommended to use a 100mA RCD for protecting circuits other than socket outlets Today, easy to protect any individual circuit with RCBO, making use of split load boards unnecessary 79

80 Nuisance Tripping Socket outlets intended for the connection of portable (φορητές) appliances outside the main equipotential zone require 30mA RCD. E.g. socket outlets in garages, lawn mowers, hedge trimmers 0.4sec disconnection time for any equipment outside main equipotential zone Exemption of RCD Fixed equipment connected via socket outlet (such as fridges) provided that some means of preventing the socket outlet being used for hand-held appliances is ensured 80

81 Supplementary Bonding Most debated topic in IEE regulations 1. Why do I need to bond the hot and cold taps and a metal sink together? Provided that main protective bonding conductors have been correctly installed there is no specific requirement in BS 7671 to do this. 81

82 Supplementary Bonding 2. Do I have to bond radiators in premises to for example, metal clad switches or socket outlets? Supplementary bonding is only necessary when extraneous conductive parts are simultaneously accessible with exposed conductive parts and the disconnection time for the circuit concerned cannot be achieved. In these circumstances the bonding conductor should have a resistance R 50/Ia Ia = is the operating current for the protection 82

83 Supplementary Bonding 3. Do I need to bond metal window frames? In general no. Apart from the fact that most window frames will not introduce a potential from anywhere, the part of the window most likely to be touched is the opening portion to which it would not be practicable to bond. There may be a case for the bonding if the frames were fortuitously touching structural steel work In any case there would need to be another simultaneously accessible part to warrant considering any bonding 83

84 Supplementary Bonding 4. What about bonding in bathrooms? Bathrooms are particularly hazardous areas with regards to shock risk, as body resistance is drastically reduced when wet. Hence supplementary bonding between exposed conductive parts must be carried out in addition to their CPCs. Also of course, taps and metal baths need bonding together and, to other extraneous and exposed conductive parts. 84

85 Supplementary Bonding 5. What size of bonding conductors should I use? Main protective bonding conductors should be not less than half the size of the main earthing conductor, subject to a minimum of 6mm 2 or where PME (TN-C-S) conditions are present, 10.0 mm 2. For example, most new domestic installations now have a 16.0 mm 2 earthing conductor, so all main bonding will be in 10.0 mm 2. Supplementary bonding conductors are subject to a minimum of 2.5 mm 2 if mechanically protected or 4.0 mm 2 if not. However, if these bonding conductors are connected to exposed conductive parts, they must be the same size as the cpc connected to the exposed conductive part, once again subject to the minimum sizes mentioned. It is sometimes difficult to protect a bonding conductor mechanically throughout its length, and especially at terminations, so it is perhaps better to use 4.0 mm 2 as the minimum size 85

86 Supplementary bonding in bathroom 86

87 Supplementary Bonding 5. What size of bonding conductors should I use? Main equipotential bonding conductors should be not less than half the size of the main earthing conductor, subject to a minimum of 6.0mm 2 or, for TNCS 10.0mm 2. For TT next figure Supplementary bonding conductors are subject to a minimum of 2.5mm 2 if mechanically protected or 4.0mm 2 if unprotected However, if these bonding conductors are connected to exposed conductive parts, they must be the same size as the CPC connected to the exposed conductive part, once again subject to the minimum sizes mentioned 87

88 Copper earthing conductor cross-sectional area (csa) for TT supplies 88

89 Earthing conductor and main protective bonding conductor sizes (copper equivalent) for TN-S and TN-C-S supplies 89

90 Supplementary bonding conductors 90

91 Supplementary Bonding 6. Do I have to bond free-standing metal cabinets, screens, workbenches etc? No. these items will not introduce a potential into the equipotential zone from outside, and cannot therefore be regarded as extraneous conductive parts 91

92 Supplementary Bonding 7. What are the bonding requirements for plumbing installations that incorporate plastic pipes? There is an increasing amount of plastic plumbing installations being used in modern houses for both domestic hot and cold water and C.H. systems. If the pipework is plastic but terminates in copper at taps, radiators etc. no bonding in needed 92

93 More examples 93