Demystifying arc flash hazard management

Transcription

1 Demystifying arc flash hazard management Presented by Matt Brazier Senior Electrical Engineer Gove Operations November 2013

2 Standard NFPA 70E IEEE 1584 AS/NZS 4836 TERMINOLOGY Arc arc flash hazard analysis arc flash boundary hazard/risk categories arc flash hazard analysis flash-protection boundary arc blast, arc flash Shock shock hazard analysis shock protection boundary risk of electric shock In this presentation: arc flash = arc blast arc flash hazard category = hazard/risk category 2

3 At the basic level System parameters Arcing current Arc intensity Flash-protection boundary Working distance Incident energy Arc flash hazard category PPE 3

4 Not arc resistant. CB operation with door closed. Does A-F hazard management apply? NFPA70E-2012 p31 4

5 The door was closed What arc flash hazard category applies and how is it arrived at? 5

6 KEY POINT There is no recognised method of establishing prospective incident energy where an intervening obstruction such as a door is in the path of the arc. 6

7 Assess as if door not present? From NFPA70E-2012 p36 (Same as 2004 edition) Where did the 2 come from? 7

8 From NFPA70E-2012 p36 Arcing energy and working distance are identical. The only difference is the nature of the task. The arc flash hazard categories are different. Why? 8

9 KEY POINT Incident energy does not translate directly to Arc Flash Hazard Category. 9

10 b) The desire to avoid providing more protection than is needed. Hazards may be introduced by the garments such as heat stress, poor visibility, and limited body movement. From IEEE p4 Ok. But where did the 2 come from, and the 3? 10

11 From NFPA70E-2012 p31 11

12 KEY POINT Arc Flash Hazard Categories do not correspond to incident energy, they correspond to RISK. 12

13 INCIDENT ENERGY OUTCOMES FOR DIFFERENT SCENARIOS 13

14 From NFPA70E-2012 p31 14

15 KEY POINT Developing an arc flash hazard management strategy is not a purely analytical process. It necessarily involves applying a certain amount of professional judgement and expertise. 15

16 ARC FLASH PPE At the low end, when in doubt default to leather gloves and face shield. There is a large difference between that and nothing. Small impost to cover a small risk. Reinforces good habits. From ISSA Guidelines... Most arc flash injuries are to the hands and face. 16

17 Cross-checker Racking an LV ACB (ACB not compatible with remote racking devices) What is the primary arc flash hazard control measure for the cross-checker? 17

18 KEY POINT The primary operational control measure for arc flash hazard management is DISTANCE. ( Operational as opposed to engineering control measures for example.) 18

19 Cross-checker How far away? First degree burn distance? Racking an LV ACB 19

20 NFPA70E Approach Boundaries Shock Hazard Arc Flash Hazard Limited Approach Boundary Restricted Approach Boundary Arc Flash Boundary Prohibited Approach Boundary From NFPA70E-2012 p24 20

21 AS/NZS m exclusion zone for LV for other than competent persons 21

22 DISTANCE There is no defined answer to the question: How far is safe? Arc flash boundary is useful to an extent but answers the wrong question. Suggested approach is to use an exclusion zone of 3m for LV as default, or otherwise roughly double the calculated theoretical arc flash boundary. A default 3m LV exclusion zone rule is much easier to apply than posting precisely calculated but possibly dubious arc flash boundary figures, with a different figure on every switchboard. Post exceptions where required, but round up the figure. 22

23 KEY POINT For bystanders, think in terms of an exclusion zone rather than arc flash boundary. 23

24 NON-PPE ARC FLASH HAZARD CONTROL MEASURES Prohibit racking by other than qualified electricians Remote racking & remote switching where possible Barricading against inadvertent unauthorised approach Barriers to keep exposed conductors out of arm s reach Arc resistant switchgear mandated for new projects IP2X within MCC modules mandated for new projects Safety observer (e.g. AS/NZS4836 Section 6) Standby person (similar to confined space hole watch) 24

25 KEY POINT When developing or revising an arc flash hazard management strategy: THINK BEYOND PPE 25

26 ARC FLASH HAZARD MANAGEMENT Control measures = f( arc flash hazard category, job-specific hazards ) Arc flash hazard category = f( risk ) Risk = f( consequence, likelihood ) Making significant decisions, e.g. when a safety observer is or isn t mandated. The methodology matters. 26

27 Fuse holder Situation Production down One electrician on site Time: 0200 Task ACB has been racked out 1. Unbolt lexan cover 2. Test for dead 3. Troubleshoot Issue Is a second electrician required as a safety observer until conductors proven dead? 27

28 ARC FLASH HAZARD MANAGEMENT CONTROL MEASURES Safety observer policy. Possible options. A. Not required when testing for dead. (But what if there has been an isolation error?) B. Is required. (But could add an extra hours downtime when it is almost certain that the conductors are actually dead. Defend the policy against challenge.) Note: SO duties include warning of impending danger, e.g. if the electrician is making a mistake. An unqualified person is not competent to make such an assessment. Therefore SO must be an electrician. 28

29 TESTING FOR DEAD Somewhat like Schrödinger s cat. Neither actually live nor actually dead. The truth cannot be known without opening the box and doing the test. Conductors must be treated as if live. But does this mean that the same control measures must be applied as when they are live? (Noting that opening many types of equipment would not be permitted while actually live.) Conductors must be treated as live but are most likely dead. The change in likelihood of an incident is several orders of magnitude. Risk analysis schemes generally increment risk steps according to one order of magnitude change in likelihood. 29

30 ARC FLASH HAZARD MANAGEMENT Safety observer when LV testing for dead. A solution. Apply the principles to establish a methodology 1. Mandate a safety observer for higher level base case categories, e.g. 3 & For LV (not MV/HV) when isolated and testing for dead: Apply the principles behind Informational Note No. 1 and reduce the category by one number to account for the reduction in risk. Category SO Required Isolated? N N N Y Y NFPA70E-2012 p31 30

31 Arc Flash Hazard/Risk categories are a similar concept to IEC61508 Safety Integrity Levels Overpressure explosion. Gramercy alumina refinery IEC61508 SIL 1, 2, 3, 4 Arc fault explosion NFPA 70E Article 130 Cat 0, 1, 2, 3, 4 The level of risk determines the category. 31

32 KEY POINTS 1. There is no recognised method of establishing prospective incident energy where an intervening obstruction such as a door is in the path of the arc. 2. Arc Flash Hazard Categories do not correspond to incident energy, they correspond to RISK. 3. Developing an arc flash hazard management strategy is not purely an analytical process. It necessarily involves applying a certain amount of professional judgement and expertise. 4. The primary operational control measure for arc flash hazard management is DISTANCE. 5. For bystanders, think in terms of an Exclusion Zone rather than arc flash boundary. 6. When developing or revising an arc flash hazard management strategy: THINK BEYOND PPE. 32