Grounding of Airfield Lighting Joseph Vigilante, PE Penn State/FAA Hershey Conference 2009.

Grounding of Airfield Lighting

Joseph Vigilante, PEPenn State/FAA Hershey Conference

2009


“In the Fairytale of electrical design, Grounding is the Ugly Duckling.”

•Definition•Objective•Standards•Recommended Values•Soil Resistivity•Calculation•Design•Summary


NEC 250.2 – Effective Ground-Fault Current Path“An intentionally constructed, low impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the OCPD or GFD on high-impedance grounded systems.”


The Objective:

In ground system design, the objective is to achieve the lowest ground resistance value possible that makes sense economically and physically.


Standards:•National Electrical Code (NEC)•FAA AC 150/5340-30D•FAA-STD-19e – Lightning and Surge protection, Grounding, Bonding and Shielding Requirements for Facilities And Electronic Equipment


National Electrical Code (NEC) 250.56

A 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…


NEC 250.53 & 58…. Two or more grounding electrodes

that are bonded together shall be considered a single grounding electrode system.


FAA AC 150/5340-30D

12.6. 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.


FAA -STD- 19eSection 4.2.4 Earth Electrode

System RequirementsSub-section 4.2.4.3 Design

….The design goal for the resistance to earth of the EES shall be as low as practicable and not over 10 ohms.


Recommended Values:

Using a 20% buffer, Design Range is

10-20 ohms


Soil Resistivity Measurement

ρ = (2) (п) (A) (R)

ρ = Average Soil Resistance to Depth A in ohm-cm

A = Distance between Electrodes in cmR= Measured Resistance Value in ohms

Using a 3 m (10’) Ground Rod, Assume R measured value of 100 Ω

ρ = 2 x 3.1416 x 3 m x 100 Ω = 1885 Ω·m


Soil Resistivity Tabulated

Boring Log• 0 – 2ft – Poorly Graded

Gravel• 2 – 4 ft – Sandy Lean Clay

w/Gravel• 4 – 13 ft – Silty Sand



• Data table• Average values• Non-specific to site

Type of Soil SoilResistivity - Ω·cm

Poorly graded gravel, gravel/sand mix

100k – 250k

Well graded gravel 60k – 100k

Silty Sands 10k – 50k

Silty Soils 8k – 30k

Clayey Sands 5k – 20k

Clayey Fine Sands 3k – 8k

Rock 10M



• 10 ft. ground rod• 0 – 2ft – Poorly

Graded Gravel• 2 – 4 ft – Sandy Lean Clay w/Gravel• 4 – 13 ft – Silty Sand

Type of Soil SoilResistivity - Ω·cm

Poorly graded gravel, gravel/sand mix

100k – 250k

Well graded gravel 60k – 100k

Silty Sands 10k – 50k

Silty Soils 8k – 30k

Clayey Sands 5k – 20k

Clayey Fine Sands 3k – 8k

Rock 10M


Resistance 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


Range 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


Ground 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


Bonding Jumper

• #6 AWG , Bare Copper

• NEC Table 8 Conductor Properties

• 0.491 Ω/kFT


Grounding System

• 2 Ground Rods At Each Location

• 500 FT Spacing

• #6 AWG , Bare Copper


Summary

• 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


Questions?

Grounding of Airfield Lighting Joseph Vigilante, PE Penn State/FAA Hershey Conference 2009.

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