Arc Flash Arc Flash Mitigation Mitigation Remote Racking and - PowerPoint PPT Presentation

Arc Flash Arc Flash Mitigation Mitigation Remote Racking and Switching for Arc Flash danger mitigation in distribution class switchgear.

Distance Distance is is Safety Safety  We will discuss through examples of actual occurrences and possible scenarios the inherent dangers and best possible procedures for using Remote Racking/Switching and to mitigate damage during an arc flash or limit the occurrence.  If procedures require the operation or racking of switchgear while energized these techniques and safety tools will limit exposure to arc flash and other dangers.

Arc Arc Flash Flash Basics Basics  An arc flash is measured is calories per cm 2 . This value is referred to as the Incident Energy (E I )  Current industry standards require workers to wear PPE with a rating (ATPV) > than the E I  This is a 480V arc flash set up in a cubic box to simulate an MCC bucket or breaker cell by PSE&G  E I for this test was 51 cal/cm 2

Arc Arc Flash Flash Haz Hazard ard  There is no possible way to completely avoid arc flash hazards  The preferred method for any electrical work is to de-energize the equipment you will be working on, in order to do this the equipment must be switched off, this action is considered an “Arc flash hazard”

Arc Arc Flash Flash Haz Hazard ard  2009 NFPA 70E Definition: “Arc Flash Hazard” – A dangerous condition associated with the possible release of energy caused by an electric arc – FPN #1 – An arc flash hazard may exist when energized electrical conductors or circuit parts are exposed or when they are within equipment in a guarded or enclosed condition, provided a person is interacting with the equipment in such a manner that could cause an electric arc….. – FPN #2 – See table 130.7 (C)(9) for examples of activities that could pose an arc flash hazard

2009 2009 NFPA NFPA 70E 70E Tables Tables Task 2004 70E HRC 2009 70E HRC Insertion or removal (Racking) of CB’s from 3 4 cubicles, doors open (600V class switchgear) Insertion or removal (Racking) of CB’s from 2 4 cubicles, doors closed (600V class switchgear) Insertion or removal (Racking) of CB’s from 4 4 cubicles, doors open (Metal clad switchgear 1kV-38kV) Insertion or removal (Racking) of CB’s from 2 4 cubicles, doors closed (Metal clad switchgear 1kV-38kV)

NFPA NFPA 70E 70E Tables Tables  Many companies decide to use the tables for PPE selection to save money and avoid doing an analysis.  The tables can only be used if the available fault current and clearing times are known for the equipment to be worked on.  The tables assume a maximum amp-cycle value. If these limits are not met an arc flash analysis is required.  Example: 600V rated switchgear – Note 4: Maximum of 35kA available short circuit current available, maximum of up to 30 cycle clearing time

Table Table Limitat Limitations ions  Typical MV/LV substation  Fuses on MV side will not react quickly to secondary fault  Due to coordination issues the Main breaker usually does not employ INST trip  ST delays can be 0.3 seconds or longer  Arc Flash reduction switches or “Maintenance switches” can be effective

Flash Flash Haz Hazard Analysis ard Analysis  Excel program using Arc Flash Protection (bolted fault) Boundary NFPA 70E Equations NFPA eq. MVA bf = bolted fault MVA at point involved 2 MVA V = volts 480 volts  Example: % Z = percent impedance based on transformer MVA 5 % Z t = time of arc exposure in seconds, (cycles / 60 cycles) 2 seconds I SC = {{[MVA Base x 10 6 ] / [1.732 x V]} x 100 / % Z} B- 2-1 – 2000kVA 480V 48,114 Amps I SC = Short Circuit Current ANSWER transformer P = 1.732 x V x I sc 10 -6 x 0.707 2 B- 2-2 19.994 P = Maximum Power (in MW) in a 3-phase arc ANSWER MW D c = [ 2.65 x MVA bf x t ] 1/2 – 2 second clearing B-2-3.2 14.56 D c = Curable Burn Distance Ft ANSWER time 174.72 D c = Curable Burn Distance ANSWER inches Arc Flash Protection (transformer) Boundary NFPA eq. MVA = rating of transformer 2 MVA V = volts t = time of arc exposure in seconds, (cycles / 60 cycles) % Z = 2 % Z seconds I D C =14.56 ft D c = [ 53 x MVA x t ] 1/2 B-2-3.3 14.56 D c = Curable Burn Distance Ft ANSWER D c = Curable Burn Distance 174.72 ANSWER inches

Flash Flash Prot Protection b ection boundary oundary  The flash protection boundary determines at what distance from exposed live parts flash protection must be worn  Distance is usually based on not exceeding 1.2 cal/cm 2 of heat energy on a persons skin  1.2 cal/cm 2 will cause a 2 nd degree burn of exposed skin in 0.1 second

Flash Flash Haz Hazard Analysis ard Analysis  A 48 kA fault Equation for Arc in Open Air with a 2 second NFPA eq. D A = distance from arc electrodes, inches (for distances 18 in. and greater) 36 inches clearing time at t A = arc duration, seconds (cycles / 60 cycles) 2 seconds 48 F = bolted fault short circuit current, in kA (for the range of 16 to 50kA) kA 36” E MA = 5271 x D A-1.9593 x t A x (0.0016 x F 2 - 0.0076 x F + 0.8938) B - 5.1 39.673 cal/cm 2 E MA = maximum open arc incident energy ANSWER  E I will be: Equation for Arc in a Cubic Box 39 Cal/cm 2 in – open air NFPA eq. D B = distance from arc electrodes, inches (for distances 18 in. and greater) 36 inches t B = arc duration, seconds (cycles / 60 cycles) 2 seconds – 114 Cal/cm 2 48 F = bolted fault short circuit current, in kA (for the range of 16 to 50kA) kA in a cubic box E MB = 1038.7 x D B-1.4738 x t A x (.0093 x F 2 - .3453 x F + 5.9675) B - 5.2 114.3091 cal/cm 2 E MB = maximum 20 in. cubic box incident energy ANSWER

PPE PPE Limitat Limitations ions  No way to re-test or verify the ATPV rating  Expensive to purchase and maintain  Little protection from arc blast pressures, not recommended for >40cal/cm 2 exposure  Cumbersome, vision and mobility is limited

Mitigat Mitigation ion  Arc resistant switchgear is available that redirects the arc away from the operator using a system of channels and flaps  Existing switchgear may also be modified to be arc resistant  Must meet the requirements of IEEE C37.20.7

Mitigat Mitigation ion  2 basic concepts for arc flash mitigation: – Reduce the total amp-cycles of the arcing fault – Increase the distance from the arc to the worker  Limiting fault current seems to be a simple solution  Keep in mind that reducing the fault current may increase the clearing time of the OCPD and may actually increase the hazard  Current limiting fuses are only effective if the arcing current is in the current limiting range

Validit Validity y of Ar of Arc c Flash Flash Analysis Analysis  The results of the arc flash analysis, or the HRC from the tables assume the OCPD will clear the fault within the manufactures published TCC  A failed OCPD, or even a slow one, will result in higher Incident Energies than the workers PPE is rated for  New 2009 70E Article 205.3 – General Maintenance Requirements – OCPD’s shall be maintained IAW the manufactures instructions or industry standards  FPN: Refer to NFPA 70B or ANSI/NETA MTS for guidance on maintenance frequency, methods, and tests

Validit Validity y of Ar of Arc c Flash Flash Analysis Analysis  Over 30% of low and medium voltage power circuit breakers tested that have been in service for more than 24 months in industrial applications will not perform to specification when “as found” trip tests are performed.  After exercise and operation, cleaning and proper lubrication this is reduced to less than 12 %.  This data comes from a cross reference of Group CBS company results compiled over 2000 breakers.

Validity Validit y of Ar of Arc c Flash Flash Analysis Analysis  Assume a worker Equation for Arc in Open Air is racking out a 600V feeder NFPA eq. D A = distance from arc electrodes, inches (for distances 18 in. and greater) 24 inches t A = arc duration, seconds (cycles / 60 cycles) 0.5 seconds breaker for the 36 F = bolted fault short circuit current, in kA (for the range of 16 to 50kA) kA E MA = 5271 x D A-1.9593 x t A x (0.0016 x F 2 - 0.0076 x F + 0.8938) B - 5.1 purpose of 14.028 cal/cm 2 E MA = maximum open arc incident energy ANSWER LOTO. Equation for Arc in a Cubic Box  Arc Flash label NFPA eq. D B = distance from arc electrodes, inches (for distances 18 in. and greater) 24 inches requires the t B = arc duration, seconds (cycles / 60 cycles) 0.5 seconds 36 F = bolted fault short circuit current, in kA (for the range of 16 to 50kA) kA worker to wear 40 E MB = 1038.7 x D B-1.4738 x t A x (.0093 x F 2 - .3453 x F + 5.9675) B - 5.2 26.8335 cal/cm 2 E MB = maximum 20 in. cubic box incident energy ANSWER cal/cm 2 flash suit