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17th edition first amendment Section 534

17th edition first amendment Section 534. Selection and erection of surge protection devices (SPDs). Samad Khan MEng (Hons) CEng MIET, MIEEE Furse, Thomas & Betts. Section 534. Covers selection and erection of surge protection devices (SPDs):

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17th edition first amendment Section 534

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  1. 17th edition first amendment Section 534 Selection and erection of surge protection devices (SPDs) Samad Khan MEng (Hons) CEng MIET, MIEEE Furse, Thomas & Betts

  2. Section 534 • Covers selection and erection of surge protection devices (SPDs): • For electrical installations to limit transient overvoltages of atmospheric origin transmitted via the supply distribution system and against switching overvoltages • For the protection against transient overvoltages caused by direct lightning strokes or lightning strokes in the vicinity of buildings External 35% of all transients originate outside the facility • Internal • 65% of all transients originate inside the facility

  3. What are Transient Overvoltages (surges)? Normal mains power supply Large Up to 6,000 volts (Almost 20 times mains supply) Transient overvoltage Fast Typically 50 microseconds duration (20,000 transients per second)

  4. Causes of transient overvoltages External - atmospheric over-voltage (lightning) Both direct stokes to structure and indirect strokes near structure (up to 1km away)

  5. Causes of transient overvoltages Internal - from switching surges (inductive loads) Motors – lifts, air con Transformers Welding equipment Internal transients as a result of fault initiations

  6. Frequency of transients – statistical dataAnalysis of Power Line Disturbances 88.50% Oscillatory Decaying Transients 49.0% Impulse Transients 39.5% Under/Over Voltages 11.0% Reference: G. W. Allen and D. Segall, IBM Systems Development Div. “Monitoring of Computer Installations for Power Line Disturbances”. Voltage Outages 0.50%

  7. The problems transients cause • Disruption – loss of data, RCD’s tripping • Degradation – reduced equipment lifespan (~95% cases) • Components weakened, until unexplained equipment failure • Will insurance cover this? • Damage – immediate loss of equipment (~5% cases)

  8. The problems that transients cause 6000V Damage Degradation Downtime Damage Size of transient overvoltage Typical hidden costs of system downtime Lost business Delays to customers Lost productivity Staff Overtime Disruption Damage 0V No effect

  9. Growing problem Rate of surge damage • Increased use of electronics, less able to withstand surges • Protection of electronics now normative part of BS EN 62305 • - previously only an informative part (Annex C) of BS 6651 1950 2000

  10. Failure Analysis – industrial equipment Causes of premature failure Fire Water 3% 5% Lack of maintenance 25% Theft and vandalism 22% Other 28% 15% Storm 2% Overvoltage (lightning discharge and switching operations) Damage to electronics: Analysis of approx. 7,750 damages NOTE – transients can also cause damage via metallic data, signal & telecommunications lines Ref: Contingency planning insurance data

  11. Surge Protective Device SPD - basic principle Divert surge currents and limit over-voltages, survive and repeatedly protect personnel, buildings and equipment

  12. BS7671 Section 534 • Covers selection and installation of all Types of SPD Type 3 Overvoltage SPD Terminal equipment level Type 2 Overvoltage SPD Sub Distribution Board Type 1 Equipotenial bonding or lightning current SPD Main Switch Board Tested to BS EN 61643

  13. Typical Location of Type 1, 2 and 3 SPDs Type 1 handles high energy surges – required as part of lightning protection to prevent fire/shock Type 2 & 3 SPDs limit overvoltages respectively - required for equipment protection Combined Type SPDs exist (e.g. Type 1+2, 2+3)

  14. L1 L2 L3 N Protective conductor Differential modes Common modes Section 534 key areas • Connection of SPD’s at or near origin of installation • Transients can exist between any two pairs of conductors • Mode of protection – protection between two conductors • Common mode – protection between live and protective conductors • Or between live and main earthing terminal if this distance is shorter • Differential mode – protection between live conductors • Protection of electronic equipment and against switching transients

  15. Section 534 key areas Selection of SPD’s • Protection level (sometimes known as let-through voltage of SPD) • It is voltage clamping level (per mode) of the SPD (for a given transient test) • Key parameter, to be sufficiently lower than equipment withstand levels • Lower than impulse immunity levels for critical equipment (61643)

  16. Section 534 key areas • Maximum continuous operating voltage of SPD shall be sufficiently greater than system voltage • At least 10% higher than nomimal a.c. rms voltage to earth • Surge current • SPD has to be sufficient to survive in the environment where it is located • Type 1 SPDs (higher surge current handling) for service entrance (lightning - equipotential bonding) • Type 2 and 3 SPDs provide overvoltage protection (lower surge current capability than Type 1 SPDs) • Provide protection against switching transients Selection of SPD’s

  17. Section 534 key areas Selection of SPD’s • Co-ordination of SPD types • SPDs on same installation need to operate together effectively • Ensure Type 1 handles high energy surges and Type 2 & 3 SPDs limit overvoltages respectively • Poor co-ordination could result in damage to SPDs and equipment • Refer to SPD manufacturers guidance • Co-ordination is dependent on technology used within SPD

  18. Section 534 key areas Selection of SPD’s • Fault protection integrity • Fault protection to remain effective even in case of SPD failure • SPDs in practice have dedicated OCPD in-line • End of life conditions of SPDs • OCPD provides protection against SPD short circuits • SPDs should have internal thermal safety disconnection e.g. for safe disconnection from abnormal supply conditions

  19. Section 534 key areas Selection of SPD’s • SPD status indication • SPDs installed in shunt/parallel – if they fail, user unaware, equipment unprotected • SPDs to indicate if they provide limited or no protection • Indication could be visual, audible and/or remote • Remote connection to BMS or external panel light

  20. Section 534 key areas Selection of SPD’s • SPD in conjunction with RCDs • SPDs installed before or upstream of RCD – no tripping problem from overvoltages • SPDs installed load side of RCD – nuisance tripping possible • Time delay or Type S RCDs should be used • RCDs to have immunity to surge currents of 3kA (8/20 surge current waveform)

  21. Section 534 key areas Installation of SPD’s • Correct installation critical for effective protection • Shunt (parallel) installed protector has additive inductive voltage drop from connecting leads • Additive voltage also seen by equipment • Connecting leads therefore need to be kept as short as possible

  22. SPD installation • 534 recommends connecting leads (a + b) should not preferably exceed 0.5m • Under no circumstances should leads exceed 1m • In practice, connecting leads should be kept as short as possible (0.25cm if practicable) • Minimum size of connecting leads copper (or equivalent): • 16mm2 for Type 1 SPDs • 4mm2 for Type 2, 3 SPDs • Or equivalent size to line conductors if smaller OCPD overcurrent protective device SPD surge protective device E/I equipment or installation to be protected against overvoltages

  23. SPD installation Key : 1 distribution board 2 main switch 3 main earthing terminals 4 neutral terminals 5 enclosure for SPD 6 first OCPD 7 alternative first OCPD

  24. SPD installation - examples Unit connected via dedicated OCPD and mounted such to facilitate short connecting leads which improves performance. Housed within suitable enclosure allowing visible status indication Unit connected via dedicated OCPD and DIN rail mounted such to facilitate short connecting leads which improves performance.

  25. Summary - Section 534 • Section 534 covers selection and installation of all SPD types • Type 1 for protection against direct lightning strikes • Type 2 and 3 for equipment overvoltage protection • Section 534 in-line with latest IEC/BS EN 62305 standards • BS EN 62305 “Protection against lightning” – personnel, structures, systems • BS EN 62305 (2006) eventually replaced BS 6651 “Protection of structures against lightning” in 2008. • Correct selection and installation of SPDs critical for effective protection

  26. Questions & Answers

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