a presentation to the qcwa chapter 70 about lightning and grounding
TRANSCRIPT
A presentation to the QCWA Chapter 70 about Lightning and
Grounding
Many of the following slides courtesy of
Todd Sirola C.O.O.SAE Inc.
Todd provided a presentation to the CCBE last fall on the following topics
Threats To EquipmentGrounding FundamentalsElectrical Protection SystemsCase Studies
How do we get lightning?
• We need convection, cumulo-nimbus clouds, the ones with the anvil shape, the result of a collision of warm and cold air masses
• Ice pellets and grauple• Super cooled water droplets above the
freezing level• Earth has a positive charge, the bottom of the
cloud negative charge• Air is a great insulator so the charges build up
Capacitor Analogy
And then, BOOM!
• At some point the charge is large enough to overcome the insulator
• The leaders build out slowly at relatively low current in both directions
• Once they join the current flows. Upwards of 400,000 amps peak
• Power levels in excess of 1 Gigawatt may be encountered
• Systems need to be engineered with this in mind
• lightning can, and often does, strike the same spot more than once--even the same person. U.S. park ranger Roy Sullivan reportedly was struck seven times between 1942 and 1977.
• Take especially swift action if your hair stands on end, as that means charged particles are starting to use your body as a pathway.
Just remember
Lightning energy and power system ground faults will find a path to earth.
The key is to design an electrical protection
system to ensure it doesn’t damage equipment.
09/03/93 - 29.6 kA strike - Sandia National Labs
Evidence in Nature
Electrical representation of a tree
Roots
Branches
Trunk
Types of Lightning
• Cloud to Cloud (CC)• Cloud to Air (CA)• In Cloud• Cloud to Ground (CG)• Peak or Positive Giant• Blue Streak • Red Sprite
A plug for Todd, he can provide
Design, Supply and Install Professional Engineering SupportGrounding System AuditsSystem Resistance (R-Value) TestingSoil Resistivity TestingForensic AnalysisEducational and Training Seminars
What are the threatsLightning
• Direct• Induced
• AC mains• Telecom twisted pair
Electric power systems• Switching operations• Power system ground faults
A typical Ham installation
Definition of Grounding
An engineered , low impedance path to earth.
Definition of Soil Resistivity
A measurement of the electrical resistance of a unit volume of soil. The commonly used unit of measure is the ohm-m.
Factors Influencing Soil Resistivity
Soil Type (chemical makeup)• natural elements (clays, quartz)• foreign elements (salts, fertilizer)
Moisture Content
Temperature
Soil Type
Soil Type Resistivity (ohm-m)
Clays 10-150Sandy Clays 150-600Pure Sand 600-5000Gravel 5000-30,000
Shale/Slate 400-1,000Limestone 1,000-5,000Sandstone 5,000-50,000Granite 1,000-80,000
Moisture Content
Temperature
Temperature Resistivity (ohm-m)
20 0 C 72 10 0 C 99 0 0 C 130 0 0 C (ice) 300 -5 0 C 790 -15 0 C 3,300
Ground Resistance Formula
R = X fR = ground resistance = soil resistivityf = a function determined by
the shape and size of the
electrode
Electrical Protection Systems
Outside Ground Electrodes• Low R value, Low Impedance, High
capacitance, • High energy dissipation
Inside/Equipment Grounding• Single point
Surge Protection Devices (SPD’s)• AC system, Incoming telecom, Transmission
lines
Structural Lightning Protection• Lightning rods, Down conductors
Proactive Lightning Detection
What makes a good outside grounding system?
Low Impedance• Low Resistance• Low Inductance• High Capacitance
High Energy Dissipation
Proper Orientation
Corrosion Resistance
Theft Resistant
Low Impedance Grounds
Z = V / I or
Z = [ R2 + (2ƒL - 2ƒC-1)2 ] 1/2
Lower ResistanceLower Inductance
Increase Capacitance
Low Impedance Grounds
• Increase electrode surface area• Use a capacitive enhancement
product• Increase conductor size• Minimize bends• Maximize bending radius• Eliminate 90º bends• Decrease # of connections
The Trouble with “T” Connections
Lightning travels in straight lines.90 degree connections offer much higher impedance than a straight horizontal conductor.
Weaknesses of Conventional Grounding Systems
• Poor lightning protection• Higher surge impedance• Seasonal fluctuation of R value• Subject to corrosion• Multiple connections
How can I lower Ground Resistance?
Add more rods?
How can I lower Ground Resistance?
Rods must be spaced appropriately ortheir benefit is diminished.
#Rods *Multiply By
2 1.16 4 1.36 8 1.68 16 1.92
24 2.16
*Multiplier if rods are spaced one length apart.
How can I lower Ground Resistance?
Conductive Concrete
Horizontal Electrode Construction
Vertical Electrode Construction
What about inside the shack
Or how to keep Greg on the air
Single Point Grounding
Buildings should be converted to single point grounding. This method eliminates current loops and creates an environment in which it is easier to protect equipment against power surges from whatever source.
Single Point GroundingTypical Radio Site Layout
Single Point GroundingTypical Radio Site Layout
Single Point Grounding
Single Point Grounding
Single Point Grounding
Materials used for inside grounding
• Green insulated grounding conductor • Rated double holed compression lugs with
stainless steel hardware• Copper ground bars with insulated
mounting brackets• NON-Metallic mounting clips• Parallel Compression Connectors
Label all conductors at both ends with permanent identification labels
Surge Protection
Surge Protection is required for all metallic conductors that enter the building:
• Telephone lines• Intranet Line• AC Power Systems• Transmission lines
Surge Protection
Types of Surge Protection
Voltage Limiting Devices
• Gap devices e.g. air gap carbon arrestors and gas tubes
• silicon avalanche diodes, metal oxide varistors.
Current Limiting Devices
• Fuse links
• Circuit breakers
• Heat coilsOther
• Quarter Wave Stubs• Neutralizing transformers• Isolation transformers• Dielectric fiber optics
Case Studies
CKVR Television, Barrie OntarioTower located at the studioComplex on top of the hill overlooking Barrie
Case Study: CKVR - Barrie Broadcast Tower
The protection system included a comprehensive approach to eliminating damage due to lightning and electrical surges.
• Outside grounding electrodes• Inside single point grounding• LSC2000 Lightning Strike Counter• ESID storm monitor linked directly to a stand-by Generator
CKVR - Barrie Broadcast Tower Outside Grounding
A total of 378 m of horizontal electrode was installed at the tower center and each of the guy anchors. The overall ground resistance of the system is 0.9 ohms.
CKVR - Barrie Broadcast Tower Outside Grounding
Guy Anchor GroundingCompound Grounding
CKVR - Barrie Broadcast Tower Inside Grounding
SAE Inc. installed a Master Ground Bar (MGB) in the equipment building.
MGB
CKVR - Barrie Broadcast TowerLightning Strike Counter
Sensor Unit
Counter Unit
CKVR - Barrie Broadcast TowerESID - Generator Installation
The ESID detects storms in the area and automatically switches the site to generator power.
ESIDGenerator
Is there any science to this?
• It’s a lot of money to spend on the off hand chance it might take a lightning hit.
Case Study:CKVR - Barrie Broadcast Tower
On June 11, 2000 a severe storm rolled through Barrie. Lightning knocked out a substation at 6:21 am cutting off power to the surrounding area. Fortunately at 5:08 am the ESID had identified the storm activity and switched the site to generator power. The site remained on generator until 9:40 am when the storm had passed.
CKVR was never off the air.
Case Study:CKVR - Barrie Broadcast Tower
During the same storm the LSC2000 registered 3 direct strikes to the tower. Global Atmospherics data confirmed the times and provided peak currents: 4:55 am 20 kA 5:10 am 35 kA 5:59 am 59 kA
The grounding system absorbed the energy of the strikes and no damage occurred to any of the sensitive broadcasting equipment.