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Grounding of Airfield Lighting. Joseph Vigilante, PE Penn State/FAA Hershey Conference 2009. Grounding of Airfield Lighting. “In the Fairytale of electrical design, Grounding is the Ugly Duckling.” Definition Objective Standards Recommended Values Soil Resistivity Calculation Design

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Grounding of airfield lighting

Grounding of Airfield Lighting

Joseph Vigilante, PE

Penn State/FAA Hershey Conference 2009


Grounding of airfield lighting1

Grounding of Airfield Lighting

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

  • Definition

  • Objective

  • Standards

  • Recommended Values

  • Soil Resistivity

  • Calculation

  • Design

  • Summary


Grounding of airfield lighting2

Grounding of Airfield Lighting

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.”


Grounding of airfield lighting3

Grounding of Airfield Lighting

The Objective:

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


Grounding of airfield lighting4

Grounding of Airfield Lighting

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


Grounding of airfield lighting5

Grounding of Airfield Lighting

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…


Grounding of airfield lighting6

Grounding of Airfield Lighting

NEC 250.53 & 58

…. Two or more grounding electrodes that are bonded together shall be considered a single grounding electrode system.


Grounding of airfield lighting7

Grounding of Airfield Lighting

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.


Grounding of airfield lighting8

Grounding of Airfield Lighting

FAA -STD- 19e

Section 4.2.4 Earth Electrode System Requirements

Sub-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.


Grounding of airfield lighting9

Grounding of Airfield Lighting

Recommended Values:

Using a 20% buffer, Design Range is

10-20 ohms


Grounding of airfield lighting10

Grounding of Airfield Lighting

Soil Resistivity Measurement

ρ = (2) (п) (A) (R)

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

A = Distance between Electrodes in cm

R= 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


Grounding of airfield lighting11

Grounding of Airfield Lighting

Soil Resistivity Tabulated

Boring Log

  • 0 – 2ft – Poorly Graded Gravel

  • 2 – 4 ft – Sandy Lean Clay w/Gravel

  • 4 – 13 ft – Silty Sand


Grounding of airfield lighting12

Grounding of Airfield Lighting

Soil Resistivity Tabulated

  • Data table

  • Average values

  • Non-specific to site


Grounding of airfield lighting13

Grounding of Airfield Lighting

Soil Resistivity Tabulated

  • 10 ft. ground rod

  • 0 – 2ft – Poorly

    Graded Gravel

  • 2 – 4 ft – Sandy Lean

    Clay w/Gravel

  • 4 – 13 ft – Silty Sand


Grounding of airfield lighting14

Grounding of Airfield Lighting

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


Grounding of airfield lighting15

Grounding of Airfield Lighting

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


Grounding of airfield lighting16

Grounding of Airfield Lighting

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


Grounding of airfield lighting17

Grounding of Airfield Lighting

Bonding Jumper

  • #6 AWG , Bare Copper

  • NEC Table 8 Conductor Properties

  • 0.491 Ω/kFT


Grounding of airfield lighting18

Grounding of Airfield Lighting

Grounding System

  • 2 Ground Rods At Each Location

  • 500 FT Spacing

  • #6 AWG , Bare Copper


Grounding of airfield lighting19

Grounding of Airfield Lighting

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


Grounding of airfield lighting20

Grounding of Airfield Lighting

Questions?


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