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D jav grounding D jav grounding Presentation Transcript

  • Grounding of Airfie ld Lig hting Joseph Vigilante, PE Penn State/ FAA Hershey Conference 2009
  • Grounding of Airfie ld Lig hting“In the Fairytale of electrical design, Grounding is the Ugly Duckling.”• Definition• Objective• Standards• Recommended Values• Soil Resistivity• Calculation• Design• Summary
  • Grounding of Airfie ld Lig htingNEC 2 5 0 .2 – Effe ctiv e Ground-Fa ultCurre nt Pa th“An intentionally constructed, low impedanceelectrically conductive path designed and intended tocarry current under ground-fault conditions from thepoint of a ground fault on a wiring system to theelectrical supply source and that facilitates theoperation of the OCPD or GFD on high-impedancegrounded systems.”
  • Grounding of Airfie ld Lig htingThe Obje ctiv e :In ground system design, the objective isto achieve the lowest ground resistancevalue possible that makes senseeconomically and physically.
  • Grounding of Airfie ld Lig htingSta nda rds :• National Electrical Code (NEC)• FAA AC 150/ 5340-30D• FAA-STD-19e – Lightning and Surgeprotection, Grounding, Bonding and ShieldingRequirements for Facilities And ElectronicEquipment
  • Grounding of Airfie ld Lig htingNa tiona l Ele ctrica l Code ( NEC) 2 5 0 .5 6A single electrode consisting of a rod…that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode…
  • Grounding of Airfie ld Lig htingNEC 2 5 0 .5 3 & 5 8… Two or more grounding electrodes that are . bonded together shall be considered a single grounding electrode system.
  • Grounding of Airfie ld Lig htingFAA AC 1 5 0 /5 3 4 0 -3 0 D1 2 .6 . SAFETY ( EQUIPMENT) GROUND…A safety ground must be installed at each light fixture… The safety ground must be a . #6 AWG bare jumper connected to the ground lug at the fixture base or stake to a 5/ 8 inch (16 mm) by 8 foot (2.4 m) minimum ground rod installed beside the fixture. The resistance to ground of the stake must be 25 ohms or less per measurement with a ground tester.
  • Grounding of Airfie ld Lig htingFAA -STD- 1 9 eSe ction 4 .2 .4 Ea rth Ele ctrode Sy s te m Re quire me ntsSub-s e ction 4 .2 .4 .3 De s ig n….The design goal for the resistance to earth of the EES shall be as low as practicable and not over 10 ohms.
  • Grounding of Airfie ld Lig htingRe comme nde d Va lue s :Using a 20% buffer, Design Range is 10-20 ohms
  • Grounding of Airfie ld Lig htingSoil Resistivity Measurementρ = ( 2 ) ( п) ( A) ( R)ρ = Average Soil Resistance to Depth A in ohm-cmA = Distance between Electrodes in cmR= Measured Resistance Value in ohmsUsing a 3 m (10’) Ground Rod, Assume R measured value of 100 Ω ρ = 2 x 3 .1 4 1 6 x 3 m x 1 0 0 Ω = 1 8 8 5 Ω·m
  • Grounding of Airfie ld Lig htingSoil Resistivity TabulatedBoring Log• 0 – 2ft – Poorly Graded Gravel• 2 – 4 ft – Sandy Lean Clay w/ Gravel• 4 – 13 ft – Silty Sand
  • Grounding of Airfie ld Lig htingSoil Resistivity Tabulated Type of Soil Soil• Data table Resistivity - Ω·cm• Average values Poorly graded 100k – 250k• Non-specific to site gravel, gravel/sand mix Well graded gravel 60k – 100k Silty Sands 10k – 50k Silty Soils 8k – 30k Clayey Sands 5k – 20k Clayey Fine Sands 3k – 8k Rock 10M
  • Grounding of Airfie ld Lig htingSoil Resistivity Tabulated Type of Soil Soil• 10 ft. ground rod Resistivity - Ω·cm• 0 – 2ft – Poorly Poorly graded 100k – 250k Graded Gravel gravel, gravel/sand mix• 2 – 4 ft – Sandy Lean Well graded gravel 60k – 100k Clay w/ Gravel Silty Sands 10k – 50k• 4 – 13 ft – Silty Sand Silty Soils 8k – 30k Clayey Sands 5k – 20k Clayey Fine Sands 3k – 8k Rock 10M
  • Grounding of Airfie ld Lig htingResistance To Ground• IEEE Green Book Chapter 4• 10 ft (3m) Ground Rod, 5/ 8” Diameter• Rg = ρ / (2 x п x L x ln (4L/ a -1))• Rg = ρ (Ω·cm) / 335 cm
  • Grounding of Airfie ld Lig htingRange of Soil Resistivity• Rg = 10 Ω – 20 Ω• ρ = Rg (Low) x 335 cm - Rg (High) x 335 cm• ρ (Ω·cm) = 3350 Ω·cm - 6700 Ω·cm
  • Grounding of Airfie ld Lig htingGround Resistance Adjusted• ρ = 10,000 Ω·cm• Rg = 10kΩ·cm / 335 = 29.8 Ω• IEEE Chapter 4, Multiple Rods Factor• Using 2 Rods: Rg (adjusted) = (Rg/ 2) x 1.16• Rg (adjusted) = 17.3 Ω
  • Grounding of Airfie ld Lig htingBonding Jumper• #6 AWG , Bare Copper• NEC Table 8 Conductor Properties• 0.491 Ω/kFT
  • Grounding of Airfie ld Lig htingGrounding System• 2 Ground Rods At Each Location• 500 FT Spacing• #6 AWG , Bare Copper
  • Grounding of Airfie ld Lig htingSummary• Obtain Boring Reports• Field Measure Soil Resistivity• Calculate Ground Resistance• Specify Ground Resistance Testing – During Installation and After• Obtain and Review Results• Improve Your Design Process
  • Grounding of Airfie ld Lig hting Questions?