17th Edition Part 4 3

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  • This slide marks the start of Section 434 relating to fault protection. This is worth pointing out saying that the previous slides related to overload and now fault current protection was going to be discussed. Discuss the important of determining prospective fault current in relation to protection of electrical equipment and personnel etc. Discuss the high values that can occur. Calculate typical current that could flow on an earth fault at a socket close to an industrial intake position (Earth fault loop 0.23 Ω gives 1000 A at 230 V). Discuss the difference between line to earth, line to neutral and line to line fault currents and the fact it is important to determine the highest value. Discuss the different methods listed in BS 7671 for determining prospective fault current. Explain that enquiry normally only provides a standard maximum figure and not one relating specifically to the installation in question
  • Explain that in this diagram that where a system is TN-C-S the fault current at the origin for line to neutral and line to earth will be the same. With TN-S system they could well be different. Also point out that prospective fault current measurement should be taken with the main protective bonding in place which can have the effect of lowering the impedance but raising the fault level.
  • The values of line to line fault cannot be measured with some instruments. Where a prospective fault current measurement has been taken with a single-phase instrument the value for a three-phase system should be taken as double the single-phase value (unless more accurate methods can be proven)
  • Like overload protection, and except where not required, a device providing protection against fault current is to be installed at the point where a reduction occurs in the cross-sectional area or other change causes a reduction in the current-carrying capacity of conductors
  • Explain that the intention of this calculation is to ensure conductors are not overheated when a fault occurs. Refer to Table 43.1 and discuss the different conductor insulation temperatures and their k values t is the duration of the fault, in seconds, s is the conductor cross-sectional area in mm 2 I is the effective fault current, in amperes, expressed for a.c. as the rms value, due account being taken of the current limiting effect of the circuit impedances k is a factor, taking account of conductor material, insulation material and the initial and final conductor temperatures See Table 43.1 for values of k for common materials Provide sample example (from Learning Guide or make up example)
  • 17th Edition Part 4 3

    1. 1. Requirements for Electrical Installations IEE Wiring Regulations 17 th Edition Part 4 (continued)
    2. 2. IEE Wiring Regulations 17 th Edition - Part 4 <ul><li>As previously stated: </li></ul><ul><li>It consists of only 4 chapters, </li></ul><ul><li>41 – Protection against Electric Shock . </li></ul><ul><li>42 – Protection against Thermal Effects . </li></ul><ul><li>Tonight / Today we are going to look at: - </li></ul><ul><li>43 – Protection against Overcurrent. </li></ul><ul><li>Still to come in this session: - </li></ul><ul><li>44 – Protection against voltage disturbances and Electromagnetic Disturbances. </li></ul>
    3. 3. Requirements for Electrical Installations <ul><li>IEE Wiring Regulations </li></ul><ul><li>17 th Edition </li></ul><ul><li>Part 4 </li></ul><ul><li>Chapter 43 </li></ul>
    4. 4. Chapter 43 Protection against Overcurrent <ul><li>There are 7 sections in this chapter </li></ul><ul><li>430 – Introduction </li></ul><ul><li>431 – Protection according to the nature of the supply and the distribution system </li></ul><ul><li>432 – Nature of protective devices </li></ul><ul><li>433 – Protection against overload current </li></ul><ul><li>434 – Protection against fault current </li></ul><ul><li>435 – Co-ordination of overload and fault current protection </li></ul><ul><li>436 – Limitation of overcurrent by the characteristics of the supply </li></ul>
    5. 5. Section 430 Introduction <ul><li>430.1 – Scope </li></ul><ul><li>This chapter provides the requirements for the protection of live conductors from the effects of overcurrent </li></ul><ul><li>It describes:- </li></ul><ul><li>How live conductors are protected by one or more devices for automatic disconnection of the supply in the event of:- </li></ul>
    6. 6. Section 430 Introduction <ul><li>An overload current. (section 433) </li></ul><ul><li>A fault current. (section 434) </li></ul><ul><li>Exceptions where the overcurrent is limited as in section 436 </li></ul><ul><li>Or - by conditions in section 433.3 </li></ul><ul><li>Omission of devices as described in 434.3 </li></ul><ul><li>Co-ordination of overload and overcurrent protection (section 435) </li></ul>
    7. 7. Section 430 <ul><li>Refer to Part 2 of BS 7671 and review the following definitions: </li></ul><ul><li>Overcurrent , Overload current and Fault current </li></ul><ul><li>Short-circuit current </li></ul><ul><li>Earth fault current </li></ul><ul><li>What is the difference between an overload current and a fault current? </li></ul><ul><li>What is the difference between a short-circuit current and earth fault current? </li></ul>
    8. 8. Section 431 Protection of line conductors <ul><li>Except in two cases (431.1.2), detection of overcurrent is required for all line conductors and should cause disconnection of the conductor in which the overcurrent is detected. </li></ul><ul><li>Disconnection of other line conductors is not necessary, unless the loss of one phase could cause danger or damage (e.g. in a 3-phase motor) </li></ul>
    9. 9. Chapter 432.4 Characteristics of protective devices <ul><li>Overcurrent protective devices to comply with: </li></ul>BS 88 Cartridge fuses BS 1361 Cartridge fuses
    10. 10. Chapter 432.4 Characteristics of protective devices <ul><li>Overcurrent protective devices to comply with: </li></ul>BS EN 60898 Circuit-breakers BS 3036 Semi-enclosed (rewireable) fuses
    11. 11. Chapter 432.4 Characteristics of protective devices <ul><li>Overcurrent protective devices to comply with: </li></ul>RCBOs to BS EN 61009-1 MCCB to BS EN 60947-2
    12. 12. Chapter 433.1 Co-ordination between conductor and overload protective devices <ul><li>Every circuit is to be designed so that a small overload of long duration is unlikely to occur </li></ul><ul><li>When an overload occurs, the protective device should automatically disconnect the circuit (e.g. by blowing a fuse) </li></ul><ul><li>Where no overload protection is provided, the temperature of the circuit conductors may rise excessively, possibly damaging the insulation, joints and terminations of the conductors, and/or their surroundings. </li></ul>
    13. 13. Chapter 433.1.1 Co-ordination between conductor and overload protective device <ul><li>Co-ordination will be met and comply with Regulation 433.1.1 if: </li></ul><ul><li> I b ≤ I n ≤ I z </li></ul><ul><li>Where: </li></ul><ul><ul><li>I b is the design current of circuit (i.e. the load) </li></ul></ul><ul><ul><li>I n is the rated current or current setting of the protective device </li></ul></ul><ul><ul><li>I z is the current-carrying capacity of the conductor (i.e. a cable, busbar or powertrack) </li></ul></ul><ul><ul><li>And… </li></ul></ul>
    14. 14. Chapter 433.1.1 Co-ordination between conductor and overload protective device <ul><li>ii) Where the current ( I 2 ) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current-carrying capacities ( I z ) of any cable of the circuit, represented as: </li></ul><ul><li> I 2 ≤ 1.45 x I z </li></ul><ul><li>Refer to Regulation 433.1.2 to show stated devices comply </li></ul>
    15. 15. Chapter 433 Rewireable fuses and buried cables <ul><li>433.1.3 – Semi enclosed fuses ( BS 3036 ), also comply if its rated current ( I n ) does not exceed 0.725 times the current-carrying capacity ( I z ) of the lowest rated conductor in the circuit protected. </li></ul><ul><li>433.1.4 – For buried cables and those buried in ducts etc, Regulation 433.1.4 requires a factor of 0.9 to be applied to the current-carrying capacity ( I z ) of the lowest rated conductor in the circuit protected : </li></ul><ul><li>I n ≤ 0.9 I z </li></ul>
    16. 16. Chapter 433.1.5 Ring final circuits (special case) <ul><li>TASK:- For a ring final circuit, complete the table: </li></ul>Minimum csa of PVC cables Minimum csa of MI cables Minimum current-carrying capacity of line and neutral conductors I z The load current should not exceed what for long periods? British Standard of accessories to be: Overcurrent device to be rated at:
    17. 17. Chapter 433.1.5 Ring final circuits (special case) <ul><li>TASK:- For a ring final circuit, complete the table: </li></ul>Minimum csa of PVC cables 2.5 mm 2 Minimum csa of MI cables 1.5 mm 2 Minimum current-carrying capacity of line and neutral conductors I z 20 A (devices listed in 433.1.5) The load current should not exceed what for long periods? The current-carrying capacity of the cable I z British Standard of accessories to be: BS 1363 Overcurrent device to be rated at: 30 A or 32 A
    18. 18. Chapter 433.2 Position of devices for protection against overload (i) <ul><li>Except where not required, a device for protection against overload is to be installed at the point where a reduction occurs in the value of current-carrying capacity of conductors: </li></ul>
    19. 19. Chapter 433.3.1 Omission of devices for protection against overload <ul><li>Where: </li></ul><ul><ul><li>A conductor is effectively protected against overload by a protective device placed on the supply side of that point </li></ul></ul><ul><ul><li>the characteristics of the load or the supply, is not likely to carry overload current (e.g. resistive load) </li></ul></ul>
    20. 20. Chapter 433.3.1 Omission of devices for protection against overload <ul><li>iii. The Distributor agrees that their overload device(s) provide overload protection between the origin and the main distribution point of the installation </li></ul>
    21. 21. Chapter 433.3.3 Omission of devices for protection against overload for safety reasons <ul><li>To avoid unexpected disconnection of a circuit (causing danger or damage) the following, as examples, do not require overload protection: </li></ul><ul><ul><li>Exciter circuits of rotating machines </li></ul></ul><ul><ul><li>The supply circuit of lifting magnet </li></ul></ul><ul><ul><li>The secondary circuit of a current transformer </li></ul></ul><ul><ul><li>A circuit supplying a fire extinguisher device </li></ul></ul><ul><ul><li>A circuit supplying a safety service </li></ul></ul>
    22. 22. Chapter 434 Protection against fault current <ul><li>The prospective fault current at every relevant point of the installation is to be determined </li></ul><ul><ul><li>In a single-phase circuit, this would be the higher value of either line to earth, or line to neutral; </li></ul></ul><ul><ul><li>In a three-phase circuit, this would be the highest value between all live conductors. </li></ul></ul><ul><li>The highest value measured is usually at the origin </li></ul><ul><li>Prospective fault current can be determined by calculation, measurement or enquiry. </li></ul>
    23. 23. Protection against fault current Line to Earth fault
    24. 24. Protection against fault current Line to line fault
    25. 25. Chapter 434.2 Position of devices for protection against fault current <ul><ul><li>Like overload protection, a device providing protection against fault current is usually installed at the point where a reduction occurs in the cross-sectional area … causing a reduction in the current-carrying capacity of conductors: </li></ul></ul>
    26. 26. Chapter 434.2.1 Omission of devices for protection against fault current <ul><li>Except where not required, no device for fault current protection is required provided the conductor: </li></ul><ul><ul><li>does not exceed 3 m in length, and </li></ul></ul><ul><ul><li>is installed in such a manner as to reduce the risk of fault to a minimum, and </li></ul></ul><ul><ul><li>is installed in a manner to reduce to a minimum the risk of fire or danger to persons. </li></ul></ul>
    27. 27. Chapter 434.3 Omission of devices for protection against fault current <ul><li>Some installations, like the following, do not require protection against fault current: </li></ul><ul><ul><li>A conductor connecting a generator, transformer … </li></ul></ul><ul><ul><li>A circuit where disconnection could cause danger such as those quoted in omission of overload </li></ul></ul>Conditions apply. Refer to indents (v) and (vi) or 434.3
    28. 28. Chapter 434.3 Omission of devices for protection against fault current <ul><ul><li>certain measuring circuits </li></ul></ul><ul><ul><li>The origin of an installation provided the distributor install one or more devices to provide affordable protection … </li></ul></ul>Conditions apply. Refer to indents (v) and (vi) or 434.3
    29. 29. Typical example of where a device for protection against fault current is not required Conductors having a reduction in current-carrying capacity between the busbars and the switchboard
    30. 30. Calculation of maximum permissible fault clearance time <ul><li>t is the duration of the fault, in seconds, </li></ul><ul><li>s is the conductor cross-sectional area in mm 2 </li></ul><ul><li>I is the effective fault current, in amperes, expressed for a.c. as the rms value, due account being taken of the current limiting effect of the circuit impedances </li></ul><ul><li>k is a factor taking account of resistivity, temperature coefficient and heat capacity of the conductor material, and the initial and final conductor temperatures </li></ul>For common materials, the values of k are in Table 43.1
    31. 31. Chapter 43 Protection against Overcurrent <ul><li>Does this make sense? </li></ul><ul><li>GOOD – We will carry on - - - - - - - - </li></ul><ul><li>To section 44 </li></ul>

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