fields and waves i lecture - rensselaer polytechnic institute · 2008. 3. 26. · lightning. 26...
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Fields and Waves I
GroundingZero Reference and Safety
K. A. ConnorElectrical, Computer, and Systems Engineering Department
Rensselaer Polytechnic Institute, Troy, NY
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26 March 2008 Fields and Waves I 2
Grounding
ExamplesLightningCurrent FlowResistance
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26 March 2008 Fields and Waves I 3
Grounding of Electrical Systems
Electrical Systems Require a Solid Ground
The Ground Connection Has a Finite Resistance
As Current Flows Away From the Ground Rod, the Voltage Drops to Zero
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26 March 2008 Fields and Waves I 4
Examples
Grounding Can Occur at Several Locations
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26 March 2008 Fields and Waves I 5
Grounds are Most Often Provided by Driving a Conducting Rod Into the Ground
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26 March 2008 Fields and Waves I 6
Lightning
Continent Country / State strikes / km² / yr
Africa Dem Rep of Congo 158South
America Columbia 110
Asia Northern Pakistan 87
North America USA / Florida 59
Europe Northern Italy 28
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26 March 2008 Fields and Waves I 7
Lightning
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26 March 2008 Fields and Waves I 8
Current Flow From Ground Rod
The current flows into the soilThe contact with the soil has a finite resistanceDriving a large current (from lightning) will produce a very large voltage
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26 March 2008 Fields and Waves I 9
Calculating Resistance
Recall the resistance of a cylinder of length L and area A
What is the resistance of a hemisphere?
RLA
LA
= =ρ
σ
NEC guidelines require the ground resistance to be less than 25 Ohms.
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26 March 2008 Fields and Waves I 10
Stoneridge Engineeringhttp://205.243.100.155/frames/home.html
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26 March 2008 Fields and Waves I 11
References
Wikipedia• Ground• Resistance
Ground Rods & Grounding• From Hubbell• From USDA• From IEEE• From Fluke
IEEE Standard 142-2007