Transcript Lightning - SARC - Surrey Amateur Radio Club

HF OPERATORS
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Notes on Lightning
by
John White
VA7JW
27 Jan 2011_rev 2
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LIGHTNING
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THUNDER
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Generation of Lightning
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Thunderstorms
Cold front - air aloft sinks
Warm air at ground rises
Vertical air flow, up and down
Friction between water droplets
Droplets become charged
Charges separate within cloud
High voltages develop
Within, cloud to cloud
Cloud to Earth
Air breakdown occurs
LIGHTING DISCHARGE
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Some Facts
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Average duration 50 microseconds
Average speed of Lightning stroke 20,000 mph
Average Temperature 30,000 degrees C
Average Length 3 km
Average Energy 300,000,000 joules
Average Power 10,000,000,000,000 watts (10 terawatts)
Average number of strokes per flash, 4
200 thunderstorms in progress world wide any time
 100 flashes per second worldwide any time
 Astraphobia – fear of thunder and lightning
Reference “Lightning and Lightning Protection”, William Hard and Edgar Malone.
Don White Consultants publisher 1979. and other internet sources.
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Forms of Lightning
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 Cloud to Ground – our major concern !
 Cloud discharge to ground
 Within a cloud
 Discharge in a cloud
 Cloud to cloud
 Discharge between clouds
 Heat lightning
 Intracloud, far away
 Thunder not audible
 Sheet Lightning
 Intracloud, diffuse
 Cloud to air
 Bolt-from-the-blue
Don White Consultants
27 Jan 2011_rev 2
NSARC HF Operators
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Annual Thunderstorm
Days in America
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 Numbers are storm days
 Florida is Worst - ( Adam AB4OJ/VA7OJ will vouch for that)
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Don White Consultants
27 Jan 2011_rev 2
NSARC HF Operators
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Annual Thunderstorm
Days in Canada
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IEEE ANSI/IEEE Std 142-1982
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Number of Discharges
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 World wide distribution of Lightning Discharges
 Our part of the world 10 to 30
 Central Africa 5400 !
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Don White Consultants
27 Jan 2011_rev 2
NSARC HF Operators
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Strikes vs Tower Height
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 Lower Mainland @ 5 thunderstorm days per year = low risk
 Until you get hit of course
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Thunder
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 Sound of the explosion along the superheated lightning
channel
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 30,0000 degrees
 Superheated air, gas pressures 10 to 100 atmospheres
 Shockwave is what we hear
 Rumblings are primarily due to the various distances
between observer and tortuous path of the lightning
discharge
 Speed of sound is ~ 1000 ft per second
 count the seconds between the flash and the onset of thunder to
determine your distance to strike; seconds = thousands of feet
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Strike Current Waveform
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 Example for a Typical Strike
 Rise Time ~ 5 seconds
 Crest ~ 25 kA
 Fall time ~ 50 seconds to half of crest value
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Lightning Parameters
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Percentage of Strokes EXCEEDING Value indicated
Parameter
90%
50%
10%
Max Observed
Crest (peak) Current
2 to 8 kA
10 to 25 kA
40 to 60kA
230 kA
Rate of Rise to Crest
2 kA/us
8 kA/us
25 kA/us
50 kA/us
0.3 to 2 us
1 to 4 us
5 to 7 us
10 us
0.1 to 0.6 ms
0.5 to 3 ms
20 to 100 ms
400 ms
Time between Strokes
5 to 10 ms
30 to 40 ms
80 to 130 ms
500 ms
Total Stroke Duration
0.01 to 0.1 s
0.1 to 0.3 s
0.5 to 0.7 s
1.5 s
Time to Crest
Duration of Single Stroke
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Current Distribution
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 Percentage exceeding a given current
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 50 % will exceed 10,000 amps
Don White Consultants
27 Jan 2011_rev 2
NSARC HF Operators
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Strike Current Spectrum
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 Most Energy concentrated DC to 1 kHz.
 Destructive energy
range < 1 kHz
 Not energy > 1MHz
that destroys radio
installations
 It will sound loud
on radio though!
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Primary Protection
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 Cloud to Ground discharges of concern to us
 Need to direct the lightning current to earth as
directly as possible
 Protection of Life and Property
 Fire Protection
 Shock Protection
 Equipment Protection
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Ground
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 Cloud to Ground Strike current seeks earth ground
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 the strike point
 directly to surface or via tree, tower, antenna etc.
 Current flows outwards from strike point through earth
 Earth ground is not a good conductor
 Thousands of amperes flow through ohms of resistance
 Thousands of volts per foot exist outwards from strike point
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A Simple Calculation
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 Strike current = 20,000 A
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ANTENNA
 for 10 usec
 Voltage along feedline = 2000 V
 bye bye coax
0.1 ohms
 Voltage across ground rod = 200 V
 4 MW for 10 usec
Feed line
& Tower
0.01 ohms
Rod
10 -100 ohms
Earth
 Voltage at top of ground rod = 200,000 V
 Side flashing may occur
 This is called GROUND RISE
 This 200 kV will diminish exponentially with
distance from the ground point
 Voltage gradient immediate vicinity is dangerous
 See cow >
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Station Grounds
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 Multiple grounds exist out of necessity
 Electrical - AC Power “green wire” power safety
 Lightning - Towers, feed lines
 Signal – chassis, shields, coax,
 Antenna RF – ground planes, counterpoises
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Unsafe Ground System
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 Multiple unconnected Grounds > Problem
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 Lightning currents flowing in each
ground system not equal
 Dangerous voltages will develop
between equipments due to different
ground system impedances
 Extreme shock hazard.
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Safer Ground System
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 Multiple, Connected Grounds much Safer
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 Connecting all grounds together creates
an EQUIPOTENTIAL environment
 Voltage drop between ground systems
ideally ZERO if wire has zero resistance
 Ground rise will be same everywhere
and differential voltages will be minimal
 Multiple ground points leads to lowering resistance to
ground thus lowering of Ground Rise overall
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Wire Sizing
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 What Gauge wire is needed to carry a strike current
 Wire Melt, called FUSING as in blowing a fuse, is the issue
 #6 is typical code
 For 50 sec, fusing
current ~ 800 kA
Don White Consultants
27 Jan 2011_rev 2
NSARC HF Operators
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Bonding
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 Objective is to create an EQUIPOTENTIAL AREA
 Bonding means an electrical connection between equipments
 mechanically connected hardware is not bonding.
 Independent, random unconnected ground systems where
conductivity is not assured is unacceptable
 All grounds and equipments must be electrically connected
 voltage differences are small and shock hazard is suppressed
 lower impedances are achieved
 large currents are distributed over many paths lowering voltages
 “All grounds … . must be bonded together in order to protect life
and property (ARRL 2010 Handbook pg 28.7)
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Grounding Impedance
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 Grounding is not just a simple Resistance problem
 The rate of rise of current, kA / microsecond, is same as a High
Frequency Signal and must be treated the same way.
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LOW IMPEDANCE to Ground is the requirement
 DC resistance can be achieved with large diameter copper
 INDUCTANCE of the ground system is the limiting factor
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(how could the inductance of straight wires be of any consequence?)
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Inductance
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 Conductors carrying the rapidly increasing strike current
generate a rapidly changing magnetic field.
 A changing magnetic field produces a back EMF that
opposes the applied voltage thus constraining the rate at
which the current can rise.
 This is Inductance
 Current cannot rise instantly in the presence of inductance
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Inductive Voltage
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 Relationship between Voltage and Current for an inductance
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V is the voltage developed across and inductor
L is the inductance value
i is the current
t is time
di/dt is the rate of change of current with time,
i.e amps per sec
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Wire Inductance
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1 foot of #6 AWG copper
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K7MEMCalculator
 Inductance = 0.26 H per foot
 Resistance = 0.0004 ohm per foot
 2 S rise time
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Resistive Voltage drop / foot at 20 kA = 8 volts / foot
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Inductive voltage drop / foot at 10 kA/s = 2600 volts / foot
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The impedance to ground is clearly limited by L
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Voltage Flashover
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A 50 foot vertical run of coax from feed point to ground could
develop 130 kV (ignoring Ground rise)
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Very difficult to make all ground and bonding systems
run in a straight line
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90o corners and bends in cable runs INCREASE inductance
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Higher yet voltages are developed
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High voltage will flash over from cable to cable or equipments or
other structures – whichever forms the lowest impedance to earth!
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Magnetic Field
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 Mechanical forces develop between conduction paths due to
their magnetic fields
 2 Conductors carrying 20,000 amps
 Side x side, 1 cm separation
 Force between conductors ~ 500 lbs / foot
 Cable bundles burst, wires break, cables straps rupture,
brackets break, cables deform etc.
ARRL Handbook 2010, sec 28.1.8
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Tower Grounding
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Grounded plate at base of tower
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Coax protected with arrestors
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Copper strap tying off to
the system ground
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Secondary Protection
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 Primary Protection
 Diversion of high currents and voltages to ground
 Secondary Protection
 Limiting dangerous Voltages to non destructive values
 Divert excessive Currents to non destructive values
 Lightning Arrestor Devices
 Placed on cables and equipments
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Cone of Protection
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A Rule of Thumb (old theory)
You are protected from a strike if a tall structure is close by.
Distance out (radius) = height.
Defines a cone
Theory - Safe inside
from a “hit”
 Your Tower / Antenna
probably IS the
Air Terminal !
 A big tree might help
but don’t depend on it
27 Jan 2011_rev 2
Don White Consultants
NSARC HF Operators
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Arrestors
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 Coax’s, Rotor Cables, any wires, to outdoor antennas are
prime conduits for destructive energy to enter house / shack.
 Arrestors are placed across cables to ground
 Zero current flow to ground under normal conditions
 Does not shunt your signal to ground
 Elevated voltages to ground will cause conduction to ground
to divert harmful current and limit excessive voltages
Don White Consultants
27 Jan 2011_rev 2
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Arrestor Requirements
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 Designed for TRANSIENT performance, the strike.
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 NOT for continuous application of high voltage or current
 Excessive power dissipation will cause failure
 Industry Standard test waveform is 8 x 20 s
 Rises to peak in 8 s and falls to 50% in 20 s
 Arrestors pass currents / clamp voltages for the 8 x 20 s
test without self destructing
Don White Consultants
27 Jan 2011_rev 2
NSARC HF Operators
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Gas Tubes
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Gas filled ceramic or glass cylinder
Metal ends for circuit connection
Often in a fuse-like holder, replaceable
Fire on transient, divert current, clamp voltage to safe level
Don White Consultants
27 Jan 2011_rev 2
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Gas Tubes
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 Available with various firing and clamping voltages and
current ratings
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Operating voltage up to 250 VDC
Transient strike voltage 500 VDC
Clamp voltage 100 V
High current conduction
Don White Consultants
27 Jan 2011_rev 2
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Varistors
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Commonly called MOV - Metal Oxide Varistor
A resistor that changes value when voltage is applied
Resistance decreases with increasing voltage
Clamps excessive voltage
Conducts high surge currents to ground
Don White Consultants
27 Jan 2011_rev 2
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Surge Rated Zener Diodes
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Low Operating Voltage Applications
High surge current rating 100A / 10 s
Clamps voltage to rated Zener Voltage
Used singly or back to back
Power supply rails, AC signal lines
Vsig = +/1 15V
Vpwr = +15V
Vz = 24V
Vz = 24V
Vz = 24V
General Semi
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System Approach
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 Combination MOV - Gas Tube protector for Lines
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Comparison’s
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 Comparison of common arrestors
TYPE
GAS TUBE
MOV
DIODE
SURGE CURRENT
> 20 kA
to 70 kA
100 A
NUMBER of SURGES
> 20 @ 20 kA
1000 @ 100A
infinite @ 50 A
RESPONSE TIME
5 uS
1 nS
1 uS
 Use Gas Tubes and then MOV’s closer to threat
 Use Diode clamps closer to protected equipment
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Coax Surge Suppressors
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 Placement in series with Coax
 Typically gas tube
 Place on grounded Service Entrance Plate
Alpha Delta
$50
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DX Engineering
$55
R&L
Electronics
$45
RF Parts $55
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MFJ $35
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Cable Suppressors
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 For use on rotors or other control lines
 Internal arrestor devices not known
 Place on Grounded Entrance Plate
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Array Solutions
$46
DX Engineering
$133
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NSARC Antenna Protection
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 Copper Plate
 Connected to Building
ground System
(big bare copper wire)
 In Roof Top Equipment
Room
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NSARC Rotor Protection
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 Copper Plate
 Connected to Building
ground System
(green wire)
 In Roof Top Equipment
Room
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Home System
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SYSTEM GROUNDING
1. TOWER TO BE GROUNDED
2. TOWER COAX TO BE GROUNDED
ORANGE WIRING OWNER INSTALLED
3. ENTRANCE PLATE TO BE GROUNDED
GREEN WIRING EQUIPMENT INSTALLED
4. RADIO TO BE GROUNDED TO ENTRANCE PLATE
5. RADIO IS GROUNDED TO ELECTRICAL SYSTEM BY LINE CORD CODE
6. ELECTRICAL SYSTEM IS GROUNDED AT SERVICE ENTRANCE BY CODE
7. SERVICE ENTRANCE IS GROUNDED BY CODE
DIPOLE
8. GROUND SYSTEMS TO BE TIED TOGETHER
TOWER
Panel
Service
Entrance
Eqpt
RADIO
Branch
Circuit
Plug
Socket
Line
Cord
ENTRANCE PLATE
Surge Supressors
COAX
COAX
Eqpt
GROUND
RODS
GROUND
RODS
SERVICE ENTRANCE
GROUND SYSTEM
(Hydro)
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