BS : C&G 2382 Outcomes 4

Transcription

1 BS : C&G 2382 Outcomes 4 Outcome 4 Protection for safety The candidate will be able to: 4.1 Identify the differences between basic and fault protection 4.2 State means of protection against electrical shock by a basic protection b fault protection c both basic and fault protection (excluding IT) d additional protection 4.3 Describe how the requirements for shock protection are affected by a value of the external loop Impedance (Ze) b compliance with Zs = Ze + R1 + R2 c compliance with tables 41.1, 41.2, 41.3, 41.4, 41.5 and 41.6 January 08 Legh Richardson 1

2 Protection Against Shock Methods of reducing the likelihood of electric shock 1. Limit the current flow to a person 2. Limit the current flow through a person 3. Limit the duration of shock

3 Protection Against Shock 1. Limit the current flow to a person (Basic Protection) Insulation (416) Class II equipment (412) Barriers and Enclosures (416.2) Non conducting location (418.1) restricted to trained personnel Obstacles and placing out of reach (417) Electrical Separation (413, 418.3) also (411.6) 2. Limit the current flow through a person Basic and Fault protection) SELV, PELV, FELV, RLV (414,411.8) Earth free protective earth bonding zone (418.2) 3. Limit the duration of shock (Fault Protection) Automatic Disconnection of Supply (RCDs, OPDs not shock protection) (411) RCDs in TT systems (411.5)

4 Protection for Safety Part 4 Protection from Electric Shock Basic Protection (Direct Contact) 1. Basic Insulation of Live Parts 2. Barriers or Enclosures Obstacles, Placing out of Reach 417.2, Class II Equipment January 08 Legh Richardson 4

5 Protection for Safety Part Basic Protection where supervised by a skilled or instructed person 1. Non-conducting Location (418.1) (No earth contact at sockets) 2. Earth Free local equipotential bonding (Faradays Cage) 3. Obstacles 4. Placing out of Reach 5. Electrical separation from more than one item of electrical equipment (common neutrals in IT systems)

6 Protection from Electric Shock Part Fault Protection (Indirect Contact) 1. Automatic Disconnection (ADS) 2. SELV, PELV, FELV The use of: 1. RCDs 2. Class II equipment Are additional measures added to ADS and ELV

7 Protection from Electric Shock Part 4 Additional Protection mA RCDs to be used on Socket outlets Where: 1. Socket outlets <20A 2. Mobile equipment < 32A Exceptions permitted to 1. above: 1. Under the supervision of a competent person (Industrial, commercial installation) 2. Identified and labelled for a specific item of equipment (freezers, fire alarm) January 08 Legh Richardson 7

8 Protection from Electric Shock Why RCD protection is provided for ordinary persons (Electricity is the same in the USA as in Europe!) January 08 Legh Richardson 8

9 Protection from Electric Shock January 08 Legh Richardson 9

10 Protection Against Shock January 08 Legh Richardson 10 Certain Conditions apply for Automatic protection (ADS) Under fault conditions the supply must be disconnected from a circuit within the time stated Table Have an Earth Fault Loop Impedance below or equal to Uo/Ia Tables 41.2, and 41.3 Zs U O Ia

11 BS : Protection from Electric Shock January 08 Legh Richardson 11 Fault Protection A low Earth Fault path controlling the duration of the fault Circuit Fuse Load Supply Origin of Installation Fault Earth-Fault Path Suppliers Earth Alternative Earth-Fault Path

12 Protection Against Shock 1. Stop the voltage potentials on exposed and extraneous conductive parts rising beyond the safety voltage - touch voltage in fault conditions AC systems : R 50V I DC Systems : R 120V a I a Where R is the resistance between two simultaneously touchable exposed/extraneous metalwork 1. Use an RCD for additional protection to detect earth fault currents at 50V Zs.I N (Where I N 30mA, t 40mS at 5I N, )

13 January 08 Legh Richardson 13 BS : Protection Fault Protection using SELV or/and local additional supplementary bonding by controlling the current flowing through the body

14 Protection against Shock January 08 Legh Richardson 14

15 Fault and short circuit protection January 08 Legh Richardson 15 Table 41.3 Max Zs for 0.4-5sec disconnection at 230V For the above devices : Type B Type C Type D 46/In 23/In 11.5/In

16 BS Update

17 Protection against Shock Prove that the following tables provides instantaneous disconnection for a BS EN Type B 32A MCB using tables 41.3, App 3, conforms to: Zs U O Ia Type B Type C Type D 48/In 24/In 12/In Type B Type C Type D 46/In 23/In 11.5/In 16 th Edition 17 th edition s = 1.5Ω = 1.5Ω = 5.In 48 32A 5 = 160A Zs = In = 1.44Ω = 1.44Ω multiplier for inst dist. Min.Amps.for inst.dist. (Appendix3) Max Zs (table41.2 Most manufacturer s data show 3-5 * In for instantaneous disconnection

18 MCB instantaneous tripping currents January 08 Legh Richardson 18 BS EN60898 and BS EN Instantaneous tripping currents Type Amperes 1 * > 2.7 I N * 4.0 I N 2* > 4.0 I N * 7.0 I N 3* > 7.0 I N * 10.0 I N 4* > 10 I N * 50.0 I N B > 3 I N 5.0 I N C > 5 I N 10.0 I N D > 10 I N 20.0 I N

19 Basic and Fault Protection Additional Protection for ADS in accordance with 415 Two methods are used: Protection by Residual Current Devices max value = 30mA, At 5 I N < 40ms Not recognized as a sole means of protection Supplementary Bonding Supplementary Bonding to localised and generalized zones To include local cross-bonding to all extraneous and exposed conductive parts with structural elements and metallic service pipes Where doubt exists to the effectiveness of supplementary bonding then it should be tested against R 50 / Ia for AC systems and R 120V / Ia for DC systems January 08 Legh Richardson 19

20 BS Update The use of Supplementary bonding ( ) Where doubt exists regarding the effectiveness of extraneous conductive parts having the same potential then: 50V 120V AC systems : R DC Systems : R I Where protection is given by a RCD then: a I a = 1.67kΩ Where protection is given by a 20A type B MCB then: R = 230 I a = = 100A or Appendix3 Fig3.4 = 100A 50 R'2' = = 0. 5Ω As long as the impedance of the circuit (exposed conductive part) is < 0.5Ω then the touch voltage on any simultaneously touchable metalwork for the time the fault is in operation will not rise above 50V 50 3

21 Protection Against Shock January 08 Legh Richardson 21 Protection provided by a Residual Current Device TN systems. Following condition applies: Regulation : Zs = Earth fault loop impedance in Ohms; Ian = rated residual operating current in Amps Protection in TT systems. The use of over current protective devices are not excluded although it is preferred to use a RCD with a disconnection time of not greater than 1 sec Regulation Zs I n 50V Ra Ia 50V RA = sum of all the resistances of earth electrode and protective conductors connected to the exposed conductive parts; Ia =the current causing automatic operation of the protective device with 1 sec

22 Protection against Shock Allowances from table TN systems disconnection time < 5sec for a distribution circuits TT systems disconnection time < 1sec for a distribution circuits Not covered by (>32A circuits) where disconnection times cannot be met then supplementary bonding must be applied January 08 Legh Richardson 22

23 January 08 Legh Richardson 23 BS : Protection Fault protection using an RCD to BS EN 61008/9 by controlling the duration of current flowing through the body You do not need an earth with RCDs but you do need an imbalance of current between the Line and Neutral

24 Basic and Fault Protection January 08 Legh Richardson TT earthing systems Where disconnection times cannot be met, Zs is too high, then the Use of RCD protection is used for Earth Fault currents

25 Basic and Fault protection in TT systems Where RCDs are used in series, each RCD can be earthed via an earth electrode The method of achieving the disconnection times as stated in and table 41.1 can be achieved by the use of: 1. RCD 2. OPD The former being preferred

26 Basic and Fault protection January 08 Legh Richardson TT earthing systems

27 Protection against shock and Fire January 08 Legh Richardson 27 The TT earthing system in conjunction with RCD fault protection

28 Protection against shock and Fire The use of and terminology of IT systems are more prevalent in the 17 th edition ( ) What is an IT system? High impedance to earth or no connection January 08 Legh Richardson 28

29 Basic and fault protection in IT systems IT systems Insulation Monitoring Devices IMDs Supply Source Main Switch. Gear Limiting Impedance Dist. Board RCBOs IMD Supply Electrode Single fault condition must behave like a TN system and disconnect within the times stated in 41.1 Final Circuits IMD IMD IMD

30 Basic and fault protection in IT systems IT system of protection V1 R1 Resistance of the Phase conductors and Transformer windings Point of first fault V3 R3 Resistance Fault path V2 R2 Resistance of Earth. Voltage across body in contact with exposed conductive part V poc (V2) = V S (R2 / R1 + R2 + R3) Where R3 is fixed by the distributor around 50,000 Ohms R2 is the resistance of the earth path across a human body (1.0 kohms) and R1 is the resistance of the phase conductor (1.0 Ohm) = 4.5V The Max voltage across body to the source = ,000 (Voltage drop sits outside EQBZ) January 08 Legh Richardson 30

31 Basic and Fault protection Methods of protection IT systems High impedance to earth means that the majority of the volt drop under fault conditions is outside of the equipotential bonding zone of the installation (Earth to phase monitoring device < 50kΩ causes an alarm to sound IEC 60364) suitable protective devices Insulation Monitoring Devices (IMD) 2. Residual Current Monitoring Devices (RCM s) 3. Insulation Fault Location Systems 4. Overcurrent Protective Devices 5. Residual Current Devices (RCD s) January 08 Legh Richardson 31

32 PELV and IT systems IT systems and earth monitoring equipment The earth conductor is not separated with PELV but monitored

33 Basic and Fault Protection FELV Systems Designed where SELV and PELV are not fulfilled (414) Basic Protection Insulation 2. Barriers and/or enclosures Fault Protection Exposed conductive parts shall be connected to the primary earthing system to provide Automatic disconnection in the event of a fault Methods of reducing voltage for a FELV system do not include 1. Autotransformers 2. Semiconductor devices 3. Potentiometers January 08 Legh Richardson 33

34 Basic and Fault Protection Reduced Low Voltage Systems (RLV) (Voltage range < 110V ac 55V to earth single phase, 63.5V ac to earth three phase) Basic Protection Insulation Barriers and Enclosures Fault Protection Automatic Disconnection < 5secs by OPD or RCD For RCDs: 50V I N x Zs ( I N x Zs 50V) See Table 41.6 January 08 Legh Richardson 34

35 Basic and Fault Protection January 08 Legh Richardson 35 Zs requirements for reduced voltage systems

36 Protection against shock Examples: 1/ What is the maximum Zs for a BS EN OPD protecting a 230V ac 20A Type B A3 radial final circuit? 2/ What is the maximum Zs for a 110V AC final circuit protected by a 32A Type C BS EN OPD? 3/ What is the maximum Zs for a BS 88 type 2 100A HRC fuse protecting a 400V sub-main cable to a distribution board?

37 Protection Against Shock Requirements for Basic and Fault protection for Electrical Equipment Where the protective measure of double or reinforced insulation is used for the complete installation or part of the installation, electrical equipment shall comply with the following: 1. equipment type tested and marked to the relevant standard equipment with basic insulation only shall have supplementary insulation applied in the process of being erected equipment having no insulation around live parts shall have reinforced insulation applied in the process of being erected relates directly to enclosures which is applicable to (ii) and (iii) above

38 Protection Against Shock Symbols used for indicating protection by insulation and Insulating enclosures must be rated at IP2X or IPXXB And shall only be removed by the use of a tool or a key

39 Protection Against Shock Separated Extra Low Voltage (SELV) 50V ac phase to earth and 120V DC Positive and negative terminals Supplied to Double Insulated Appliances and Equipment Supplied from: Double wound isolating transformer 2. Generator 3. Battery 4. Certain types of electronic SELV equivalent Equipment The Primary Earthing functions separated from the secondary earthing No secondary earth path back to source Nominal ELV voltage 50V ac and 120V DC January 08 Legh Richardson 39

40 Protection Against Shock Conditions for SELV systems (i) insulated to the highest voltage present (ii) enclosed in a separate sheath or conduit additional to the basic insulation (iii) conductors of higher voltages to be separated by earthed metallic screen or sheath (iv) where installed in a multi-core cage or grouped together with higher voltage conductors the SELV conductors are insulated to the highest voltage present (v) Separation via earthed metallic screen (vi) Physical separation SELV/PELV circuits >25V ac or 60V DC or equipment immersed underwater require additional protection by Insulation Barriers or/and enclosures January 08 Legh Richardson 40

41 January 08 Legh Richardson 41 BS C&G 2382 Outcomes 4 continued 4.4 Describe means of protection against fire, burns and harmful thermal effects and identify precautions where particular risks of danger of fire exists 4.5 Identify the difference between overcurrent and fault current 4.6 Identify the differences between overload currents, earth fault currents, short circuit currents and shock currents 4.7 Describe methods of overcurrent protection and the need for co-ordination with conductors and equipment. 4.8 State the requirements for protection against a voltage disturbances I. overvoltage II. undervoltage b electromagnetic disturbances.

42 Protection against Thermal effects Chapter 42 now combines together chapters 42 and 48 from the 16 th edition Scope: I. Heat generated by electrical equipment II. Ignition, combustion or degradation of materials III.Propagated fire and smoke to other fire compartments IV. Protection of safety services being cut off by the failure of electrical equipment January 08 Legh Richardson 42

43 Protection against Thermal effects protection against hot surface temperatures I. Mounted on a surface of low thermal conductance II. Be screened by low thermal materials III.Positioned to allow dissipation of heat protection against arcs and sparks I. Totally enclosed in arc resistant material II. Screened by arc resistant material III.Mounted to allow safe extinguishing of sparks IV. In compliance with its standard January 08 Legh Richardson 43

44 Protection against Thermal effects and Lamps and luminares must be positioned away from combustible structures and materials 1. < 100W = 0.5m 2. > 100W < 300W = 0.8m 3. > 300W < 500W = 1.0m where MIMS, busbar, powertrack, are not used then: 1. TT,TN systems should be protected by <300mA RCD 2. Where overheating and fire are high use a <30mA RCD 3. IT systems use IMD January 08 Legh Richardson 44

45 Protection against Thermal effects January 08 Legh Richardson Protection against burns

46 Problem Currents Examples: 1. Fault 2. Fault current (434) 3. Overload currents (433) 4. Overcurrents (435) 5. Short Circuit Current (434) 6. Earth Fault currents (435) 7. Shock Currents 8. Prospective Fault Current 9. Protective device s Operating current January 08 Legh Richardson 46

47 Overcurrent Protective Devices Types of Protective Devices (In) 432.1, 433.3, 434.3, BS 3036 Rewireable fuse links BS1361 Cartridge Fuse links BS 88 pt 2 & pt6 HRC or HBC Fuses BS 3871 MCBs - Miniature Circuit Breakers (old type) No longer included in BS7671 BS EN New type MCBs BS-EN MCCBs 10kA+ BS EN RCBOs Residual Current Breaker with overload protection January 08 Legh Richardson 47

48 Overcurrent protective devices January 08 Legh Richardson 48 Fusing Factor ii Actual Fu sin g Current ( I2) Fu sin g Factor = Rated Fu sin g Current ( In) Typical Fusing Factors for: BS88 Pt2 & pt BS BS1361/ BS3871 / BS EN Appendix 3 Time current characteristics

49 Coordination of Protective Devices January 08 Legh Richardson 49 Coordination of Protective Devices and Current Carrying Capacities of Conductors for overload and short circuit , ( a ) ( b) I I I B I I N B N 2. I 1 45 I Z Z Overload conditions Note: It is the tabulated current after taking external factors into consideration Compliance with (i), (ii), (iii) HBC BS88pt2.1, pt6 BS1361 Cartridge Fuse I I Circuit Breaker to BS EN and BS EN RCBO to BS EN b N I I t Z

50 Coordination of Protective Devices January 08 Legh Richardson 50 To Comply with the regs using BS In order to satisfy the terms in (ii) then : I N I Z To satisfy the fact that rewirables have a fusing factor up to 2 times Fusing Current = Fusing Factor * Fuse Rating = 2 * In I 2 = I and 2. I N 2 N I I Z and The use of Semi-enclosed or re-wireable fuses to BS3036 is not recommended for untrained persons Z I I I Z Z

51 Coordination of Protective devices January 08 Legh Richardson 51 Energy Let through (434) All electromechanical devices have a maximum breaking capacity Maximum amount of energy that the component will allow through without exploding or disintegrating General Equation E = I 2 t where P = Joules Seconds Note: The Resistance of the conductor can be regarded as negligible (but not zero) for the short time period and very high fault currents E t

52 Coordination of Protective Devices January 08 Legh Richardson 52 Short Circuit Protection Regulation States that: The regulation is satisfied if the time for disconnection is equal to or less than: 2 2 S. k t = 2 I The fault current must be cleared before the time given in the above equation Note: for short circuits between live conductors and for earth fault currents

53 Coordination of Protective Devices January 08 Legh Richardson and states that for the time that the earth fault exists the cpc must be able to dissipate the heat generated without damage to the other cables 2 I. t S = k. S K t = 2 I So as long as : S. K I. t Then the thermal characteristics of the cable are protected from the energy let through of the protective device The size of the cpc usually works out to be much smaller than anticipated, regulation is applicable and then this must be sized up according to table 54.7

54 Applying Circuit Design 2 Operating Characteristics of Protective Devices Energy Let Through (Heat) I (r.m.s.) I (r.m.s.) Energy (I 2 t) let through by fuse t Energy (I 2 t) let through by one cycle of AC current Fuse Link Cut-off t 0 t1 t2 t1 Pre-arcing I 2 t melting area Fuse duration t2 Total I 2 t area January 08 Legh Richardson 54

55 Energy let-through for MEM 10kA MCBs January 08 Legh Richardson 55

56 MCB Breaking Capacities MCB to BS3871 Obsolete from 2001 (blue) Circuit Breakers BS EN BS EN Category of Duty Prospective current (A) ICN (once only) ka ICS (repeatable) ka M M M M M M January 08 Legh Richardson 56

57 Energy let-through video GEC BS 88 Fuse protection

58 Applying Circuit Design 2 Operating Characteristics of Protective Devices Electromagnetic Energy t Electromagnetic stress(i 2 ) let through by one cycle of AC current Fuse Link Cut-off Fuse duration I 2 (peak) Electromagnetic stress proportional to (I 2 ) peak current January 08 Legh Richardson 58

59 Discrimination of OPDs Discrimination of Protective Devices 10 6 A I2 characteristics of HBC fuses Rated Current plotted against Amperes squared 10 5 A Total (I 2 t) 10 4 A Pre-arcing (I 2 t) 10 3 A 30A 40A 50A 60A 80A 100A January 08 Legh Richardson 59

60 Ferraz Shawmut BS88 cartridge fuses I 2 t Characteristics Discrimination achieved when downstream fuse is ½ the size of the upstream OPD Ampere Fuse rating

61 Protection against Fault Currents Discrimination of Protective Devices to A X 300A 100A Y 80A? 40A 30A 30A 50A Z Discrimination of devices must take place to reduce danger and inconvenience If fuse Z blows then fuse Y should be of such size that it can withstand the energy let through without disconnecting January 08 Legh Richardson 61

62 Coordination of OPDs January 08 Legh Richardson 62 The example, shows the superimposed characteristics of a 5 A semi-enclosed fuse and a 10 A miniature circuit breaker which we shall assume are connected in series. If a fault current of 50 A flows, the fuse will operate in 0.56 s whilst the circuit breaker would take 24 s to open. Clearly the fuse will operate first and the devices have discriminated. However, if the fault current is 180 A, the circuit breaker will open in s, well before the fuse would operate, which would take 0.12 s. In this case, there has been no discrimination.

63 Coordination of OPDs January 08 Legh Richardson 63 Graph A shows discrimination from instantaneous disconnection of the 16A MCB from the 32A fuse at 180A Graph B shows poor discrimination only starting at 100A in an overload condition but none for short circuit

64 Protection against overcurrents Positioning of Device and 434 Where a conductors diameter reduces along the line of a cable run a method of protection is required for that part of the cable which has a reduced cross sectional area Examples of reduced cable conductors are: Fused spur on a ring final circuit Installation method changed (overhead to underground) Type of cable has changed (PVC in conduit to MIMS) Ambient change in temperature (Boiler house to outside) Rules for termination between reduction in current carrying capacity and Protective Device Not Exceed 3m in length 2. Be erected to minimise risk of fault current 3. Be erected to minimise fire and danger to persons January 08 Legh Richardson 64

65 Protection against Overcurrent Omission of protective devices for safety reasons Used where unexpected disconnection would cause a dangerous situation 1. The exciter circuit of a rotating machine 2. The supply circuit of a lifting magnet 3. The secondary circuit of a current transformer 4. A circuit supplying a fire extinguishing device 5. A circuit supplying a safety circuit (fire or gas alarm) 6. A circuit supplying medical equipment in IT systems January 08 Legh Richardson 65

66 January 08 Legh Richardson 66 Voltage and Electromagnetic disturbances 441 Overvoltages due to HV and LV faults Scope (See note) 1. HV faults to earth at the substation Loss of supply neutral on LV systems Line to Neutral Short Circuit in LV systems Accidental earthing of a line conductor to earth in IT systems Stress Voltages created by HV currents circulating around exposed conductive parts producing an electromagnetic effect thus producing a secondary fault voltage (stress voltage U1 and U2) rules for designers and installers of substations 1. Quality of system earth 2. Maximum level of earth fault current 3. Resistance of earthing arrangements

67 Voltage and Electromagnetic disturbances January 08 Legh Richardson 67

68 HV fault conditions affecting LV systems January 08 Legh Richardson 68

69 HV fault conditions affecting LV systems January 08 Legh Richardson 69

70 HV fault conditions affecting LV systems January 08 Legh Richardson 70

71 January 08 Legh Richardson 71 Voltage and Electromagnetic disturbances 443 Overvoltage requirements

72 Problem Voltages 445 Undervoltages Suitable precautions shall be taken to provide protection when the voltage dips or is reduced No automatic restarting of rotating machinery (Note: see external classifications appendix 5) January 08 Legh Richardson 72