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 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