Arc Flash Mitigation & Selective Co-ordination

Transcription

1 Arc Flash Mitigation & Selective Co-ordination Michael Hodder March 2015

2 Overview This seminar will provide information on: Mitigating arc flash hazards Selective Coordination What can we do to make these two co-exist? 2 2

3 Mitigation - Options CSA Z462 provides information on safety related design requirements in Annex O. This annex encourages the choice of design options that eliminate hazards or reduce risk to increase effectiveness of safety-related work practices by: 1. Reducing the likelihood of exposure 2. Reducing the magnitude or severity of exposure 3. Enabling achievement of an electrically safe work condition 3 3

4 Hierarchy of Risk Control Methods Hierarchy of risk control methods from CSA Z Eliminate the hazard 2. Substitute: materials, processes, or equipment 3. Engineering controls 4. Systems that increase awareness of potential hazards 5. Administrative controls: training and procedures, instructions, and scheduling 6. PPE: including measures to ensure its appropriate selection, use, and maintenance. 4 4

5 Mitigation - Safety by Design Safety by design is the most effective approach in minimizing electrical hazards while improving system reliability. The power system designer must make decisions that impact cost, safety and serviceability of the installed system that has an expected life of up to 50 years. 5 5

6 Mitigation - Safety by Design Safety by Design example (FlashGard TM MCC)s 6 6

7 Equipment Labeling CEC Rule CEC rule applies to equipment installed or modified after the date the rule became effective (2006 in most jurisdictions). The owner of the electrical distribution equipment is ultimately responsible, either directly or through the installing contractor, for ensuring that the warning labels are installed when required. Below is an example of a label that satisfies the requirements. 7 7

8 Electrical Hazard Arc Flash The Arc flash hazard is defined by CSA Z462 as: A dangerous condition associated with the possible release of energy caused by an electric arc. The heat reaching the worker s skin (incident energy) is the only one which is calculated and reported on warning labels. There are other hazards associated with the arc such as blast pressure, shrapnel, toxic smoke and bright light. 8 8

9 Solutions to Reduce Arc Flash Hazards Incident energy is dependent upon: Distance of the worker to the arc Solution Move further away (task dependent) Power of the arc at the arc location Solution Reduce fault current (may or may not work) Time duration of the arc exposure Solution Faster fault clearing time 9 9

10 Mitigation Distance is your friend! Mitigation of the arc flash burn hazard using an increase in worker distance during a specific task Arc flash burn hazard is inversely related to the distance from the arc to the worker s skin. Increasing worker distance can be achieved by: Remote switching Shunt trip on disconnect switches Remote circuit breaker racking Remote plugging of buckets on MCCs 10 10

11 Solutions to Reduce Arc Flash Hazards Increase Worker Distance (remote racking) 11 11

12 Mitigation Reduce Fault Current Mitigation of the arc flash burn hazard using a reduction in fault current Arc flash hazard is directly related to fault current. However, reducing fault current may increase fault clearing time which could increase incident energy. Current limiting circuit breakers or current limiting fuses in new design or retrofit. Designing smaller kva sized unit substations and lower amp rated motor control centres. Downside is extra cost when compared to larger unit subs and MCCs

13 Mitigation Reduce Fault Current Mitigation of the arc flash burn hazard using reduction in fault current, continued: High resistance grounding (HRG) to limit phase to ground faults (5 to 10 A). Phase to ground faults are more likely to occur when compared to three phase faults. Appropriate arc flash PPE is still required to be worn as the three phase hazard still exists

14 Mitigation Reduce Clearing Time Mitigation of the arc flash burn hazard using a reduction in fault clearing time Arc flash hazard is directly related to fault clearing time. Reduction in time can be achieved by: Current limiting circuit breakers or current limiting fuses in new design or retrofit. Fast response requires a level of arcing current through the protective device to trigger current limiting action. This is called the current limiting threshold

15 Arcing Fault Clearing Time Fault clearing time is determined from Time Current Curves Increasing time Increasing current 15 15

16 Mitigation Reduce Clearing Time Mitigation of the arc flash burn hazard using a reduction in time, continued: Energy-reducing maintenance switching with a local status indicator. This allows a worker to set a circuit breaker trip unit to operate faster for a potential arc fault in a downstream work location. The local status indicator reminds workers to return to the normal setting after the potentially hazardous work is complete

17 Mitigation Reduce Clearing Time Provide temporary Faster Tripping Time with a Maintenance Switch (Retrofit shown below) Door Mounted Components Breaker Mounted Components DIGITRIP Lockout Switch Arc Flash Reduction trip element 17 17

18 Mitigation Reduce Clearing Time Mitigation of the arc flash burn hazard using reduction in time, other options: Zone-selective interlocking Arc Flash Relay Energy-reducing active arc flash mitigation 18 18

19 Mitigation Redirect the Arc Mitigation of the arc flash burn and blast hazard using containment and redirection Arc resistant switchgear or MCC Permits working in close proximity Enclosure integrity must be maintained for containment to occur Redirection requires ceiling height and plenum Arc resistant standard is IEEE C

20 Solutions to Reduce Arc Flash Hazards Arc Resistant Switchgear Redirects Arc Energy and Particulates 20 20

21 What Does Selective Coordination Mean? In a Selectively Coordinated System Only the Nearest Upstream Overcurrent Protective Device Opens this Isolates the Overload or Faulted Circuit In a Non-Selectively Coordinated System More than Just the Nearest Upstream Overcurrent Protective Device Opens Causing Unnecessary Outages Worst Case The Fault Opens All the Overcurrent Devices in Series Up to the Main Fault Unnecessary Power Loss 21 21

22 A Different Way to Look at it Medical Centre Operating Normally Fault in Operating Room by Medical Imaging Equipment Without Selective Coordination we Could Lose an Entire Panel -- Leaving Multiple Suites Dark Worst Case Condition -- we Could Actually Lose the Main 22 22

23 The Definition of Selective Coordination Selective Coordination is not defined in the Canadian Electrical Code. Two statements describing selective coordination are shown below: When ONLY the overcurrent device protecting the specific circuit that has an overload or fault opens to clear it. This minimizes disruption to the power system and allows it operate as designed to continue to supply loads. The IEEE Buff Book TM states that, Coordination is the selection and/or setting of protective devices in order to isolate only the portion of the system where the abnormality occurs. Coordination is a basic ingredient of a well-designed electrical distribution protection system and is mandatory in certain health care and continuous process industrial systems. [1] [1] IEEE , Section

24 Selective Coordination mandated in the CEC C CEC C rule (5) Description Requires selective coordination between the circuit breaker installed in the normal power supply circuit to a fire pump (upstream of a fire pump controller) and the circuit breaker provided integrally with the fire pump controller. Elevators, dumbwaiters, material lifts, escalators, moving walks, lifts for persons with physical disabilities, and similar equipment requires selective coordination with any upstream overcurrent protective device

25 Selective Coordination mandated in the CEC C CEC C rule (1) (3)(c) Notes Description The overcurrent device for an emergency power supply shall be coordinated with the overcurrent devices of feeders and branch circuits supplying life safety systems and other electrical equipment connected to the emergency power supply in order to provide selective operation of the branch circuit overcurrent device when a fault occurs in that branch circuit. Requires conformance with CSA C (Standard for Emergency Electrical Power Supply for Buildings) Section 46 covers provisions for installation of electrically connected life safety systems mandated by the National Building Code of Canada, and reliability of operation of this equipment in the event of fire or power loss emergency is paramount

26 Selective Coordination referenced in the CEC C CEC C rule (8) Description In ground fault schemes where two or more protective devices in series are used for ground fault coordination, the upstream protective device settings shall be permitted to exceed those specified in Subrule (2) where necessary to obtain the desired coordination. Passenger ropeways and similar equipment requires selective coordination with any upstream overcurrent protective device 26 26

27 CSA Considerations CSA Z32-09 Standard for Electrical safety and essential electrical systems in health care facilities, Essential electrical systems, Clause requires: For aspects of emergency electrical power supply systems not covered by Clause 6 of this Standard, the requirements specified in CSA C Standard for Emergency electrical power supply for buildings shall be followed

28 Selective Coordination changes in the National Electrical Code (NEC) for 2014 The National Electrical Code (NEC) made changes in 2014 to Article 517 to correlate with the recent changes in requirements of NFPA 99. This new section simply requires coordination for times of 0.1 seconds and greater. Separation of trip curves for all times greater than 0.1 seconds

29 Selective Coordination Challenges Interpretation Do all protective devices need to be Selectively Coordinated? What level of Selective Coordination is required? Design How Do I Design a System that can be Selectively Coordinated? Will Arc Flash incident energy increase? 29 29

30 Do all protective devices need to be Selectively Coordinated? UTILITY A UTILITY B GEN #1 GE N #2 MAIN A MAIN B MSG -TIE GEN #1 MAIN GEN #2 MAIN MSG-A MSG -B GEN SW GR CHILLER FDR DISTR PNL FDR ATS-EQ NORM ATS-CR NORM ATS-LS NORM AT S-EQ E MER ATS-CR EMER ATS-LS EMER C BL-CHILLER CBL-DIST R PNL CBL-ATS EQ N CBL-ATS CR N CBL-ATS LS N CBL-0005 CBL-0006 CBL-0007 CHILLER Upstream of the normal side of ATS - Use utility available fault current 480V LRGST DISTR PNL 480V LTG P NL FDR C BL- 48 0V LT G P N L 48 0 V LT G P N L LTG BRANCH N E N E N E ATS-EQ ATS-CR ATS-LS CBL-EQ 480V PNL CBL-CR 480V PNL CBL-LS 480V PNL EQ 480V PNL CR 480V PNL LS 480 V PNL EQ XFM R PR I CR XFM R PRI LS XFMR PRI Upstream of the emergency side of the ATS - Use the generator system available fault current Downstream of the ATS - Use higher of utility or generator system CBL-EQ XFMR PRI P EQ XFMR S CBL-CR XFM R PR I P CR XFM R S CBL-LS XFMR PRI P LS XFMR S CBL-EQ 208V PNL CBL-CR 208V PNL CBL-LS 208V PNL EQ 208V PNL MAIN CR 208V PNL M AIN LS 208V PNL MAIN EQ 208V PNL CR 208V PNL LS 208 V PNL EQ 208V BRANCH CR 208V BRANCH LS 208V BRANCH 30 30

31 What level of Selective Coordination is required? Most real world faults are lower level arcing faults or ground faults Time current curves for time greater than 0.1 seconds show coordination for overloads and typical arcing fault levels Time current curves for time less than 0.1 seconds show coordination for all but the highest levels of fault current 31 31

32 What level of Selective Coordination is required? What level of Selective Coordination is required? Coordination shown for time greater than 0.1 s Selectivity not achieved for time less than 0.1 s 0.1 s 32 32

33 How Do I Design a System that can be Selectively Coordinated? Request assistance from your local Eaton rep! Example of a solution for branch selective coordination with high fault current is shown: Bussmann by Eaton CUBEFuse in a Compact Circuit Protector (CCP) that is installed in a Quick Spec Co-ordination Panel 33 33

34 How Do I Design a System that can be Selectively Coordinated? Use Selectivity Guides Fuse upstream of fuse Circuit Breaker upstream of circuit breaker Moulded case circuit breaker upstream of fuses Alternatively, plot Time Current Curves 34 34

35 Summary Where should Selective Coordination be implemented? The Canadian Electrical Code has several requirements for Selective Coordination. Emergency Power Supply Life Safety Systems Elevators / Escalators / Chairlifts / Gondolas When continuity of service is critical. As long as potential increases in cost or arc flash incident energy have been evaluated. Health Care Facilities Critical Response Facilities Data Centres Industrial Facilities 35 35

36 Summary To achieve Selective Coordination, consider: Current that flows: Overload Short Circuit Types of Overcurrent Protective Device: Circuit Breaker Fuse Time goal for Time Current Curve plots: Greater than 0.1 s (typical for Canada) Less than 0.1 s 36 36

37 Summary Will Arc Flash incident energy increase? Arc Flash incident energy is directly proportional to time Short-time delay of an upstream circuit breaker is often used to achieve selective coordination with a downstream overcurrent device. When a fault occurs, delayed tripping of the upstream circuit breaker will enable more arc flash incident energy than would have occurred if that circuit breaker utilized an instantaneous trip

38 Summary The ideal combination would be arc flash mitigation which does not compromise selective coordination. Arc Flash solutions which can accomplish this are: Energy-reducing maintenance switching Increase Worker Distance for specific tasks Arc resistant switchgear or arc resistant MCC Zone-selective interlocking 38 38

39 Summary Carefully consider the requirements for selective coordination and it s effect on arc flash incident energy Perform an arc flash study Consider safety by design techniques such as remote switching Consider products which offer improved safety features such as arc resistant switchgear or arc resistant MCC Use instantaneous tripping where possible Use energy-reducing maintenance switching where possible 39 39

40 40 40