- - - - - - - - - - - NSARC HF Operators127 Jan 2011_rev 2 HF OPERATORS Notes on Lightning by John White VA7JW

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NSARC HF Operators 127 Jan 2011_rev 2

HF OPERATORS

Notes on Lightning

by

John White

VA7JW

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LIGHTNING

THUNDER

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Generation of Lightning

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

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

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

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Annual Thunderstorm Days in America

Numbers are storm days Florida is Worst - ( Adam AB4OJ/VA7OJ will vouch for that)


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Annual Thunderstorm Days in Canada

We are lucky, only ~ 5 days per year

IEEE ANSI/IEEE Std 142-1982

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Number of Discharges

World wide distribution of Lightning Discharges Our part of the world 10 to 30 Central Africa 5400 !


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Strikes vs Tower Height

Lower Mainland @ 5 thunderstorm days per year = low risk Until you get hit of course

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Thunder

Sound of the explosion along the superheated lightning channel 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

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

ParameterPercentage of Strokes EXCEEDING Value indicated

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

Time to Crest 0.3 to 2 us 1 to 4 us 5 to 7 us 10 us

Duration of Single Stroke 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

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Current Distribution

Percentage exceeding a given current

50 % will exceed 10,000 amps


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Strike Current Spectrum

Most Energy concentrated DC to 1 kHz.

Destructive energyrange < 1 kHz

Not energy > 1MHzthat destroys radio installations

It will sound loud on radio though!

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Primary Protection

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

Cloud to Ground Strike current seeks earth ground 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

Strike current = 20,000 A for 10 usec

Voltage along feedline = 2000 V bye bye coax

Voltage across ground rod = 200 V 4 MW for 10 usec

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 >

0.1 ohms

0.01 ohms

10 -100 ohms Earth

Rod

Feed line& Tower

ANTENNA

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Station Grounds

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

Multiple unconnected Grounds > Problem

Lightning currents flowing in each ground system not equal

Dangerous voltages will develop between equipments due to differentground system impedances

Extreme shock hazard.

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Safer Ground System

Multiple, Connected Grounds much Safer

Connecting all grounds together createsan EQUIPOTENTIAL environment

Voltage drop between ground systemsideally ZERO if wire has zero resistance

Ground rise will be same everywhereand 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

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


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Bonding

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

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.

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

(how could the inductance of straight wires be of any consequence?)

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Inductance

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

Relationship between Voltage and Current for an inductance

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

1 foot of #6 AWG copper Inductance = 0.26 H per foot Resistance = 0.0004 ohm per foot 2 S rise time

Resistive Voltage drop / foot at 20 kA = 8 volts / foot

Inductive voltage drop / foot at 10 kA/s = 2600 volts / foot

The impedance to ground is clearly limited by L

K7MEMCalculator

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Voltage Flashover

A 50 foot vertical run of coax from feed point to ground could develop 130 kV (ignoring Ground rise)

Very difficult to make all ground and bonding systems

run in a straight line

90o corners and bends in cable runs INCREASE inductance

Higher yet voltages are developed

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

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

Grounded plate at base of tower

Coax protected with arrestors

Copper strap tying off to

the system ground

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Secondary Protection

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

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


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Arrestors

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


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Arrestor Requirements

Designed for TRANSIENT performance, the strike.

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


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Gas Tubes

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


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Gas Tubes

Available with various firing and clamping voltages and current ratings

Operating voltage up to 250 VDC Transient strike voltage 500 VDC Clamp voltage 100 V High current conduction


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Varistors

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


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Surge Rated Zener Diodes

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

General Semi

Vz = 24V

Vpwr = +15V

Vz = 24V

Vz = 24V

Vsig = +/1 15V

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System Approach

Combination MOV - Gas Tube protector for Lines

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Comparison’s

Comparison of common arrestors

Use Gas Tubes and then MOV’s closer to threat

Use Diode clamps closer to protected equipment

TYPE SURGE CURRENT NUMBER of SURGES RESPONSE TIMEGAS TUBE > 20 kA > 20 @ 20 kA 5 uS

MOV to 70 kA 1000 @ 100A 1 nSDIODE 100 A infinite @ 50 A 1 uS

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Coax Surge Suppressors

Placement in series with Coax Typically gas tube Place on grounded Service Entrance Plate

RF Parts $55

DX Engineering$55

MFJ $35

Alpha Delta$50

R & L Electronics$45

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Cable Suppressors

For use on rotors or other control lines Internal arrestor devices not known Place on Grounded Entrance Plate

Array Solutions$46

DX Engineering$133

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NSARC Antenna Protection

Copper Plate

Connected to Building

ground System

(big bare copper wire)

In Roof Top Equipment

Room

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NSARC Rotor Protection

Copper Plate

Connected to Building

ground System

(green wire)

In Roof Top Equipment

Room

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Home System

RADIO

Service Entrance

Pan

el

Line Cord

Branch Circuit

PlugSocket

COAX

COAX

DIPOLE

TOWER

ENTRANCE PLATESurge Supressors

1. TOWER TO BE GROUNDED

2. TOWER COAX TO BE GROUNDED

3. ENTRANCE PLATE TO BE GROUNDED

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

8. GROUND SYSTEMS TO BE TIED TOGETHER

ORANGE WIRING OWNER INSTALLED

GREEN WIRING EQUIPMENT INSTALLED

SYSTEM GROUNDING

GROUND RODS

GROUND RODS

SERVICE ENTRANCE GROUND SYSTEM (Hydro)

Eqpt

Eqpt

Documents

- - - - - - - - - - - NSARC HF Operators127 Jan 2011_rev 2 HF OPERATORS Notes on Lightning by John White VA7JW