lightning overvoltage protection

8/11/2019 Lightning Overvoltage Protection

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Lightningovervoltage

protection


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Facts and trivia

Potential difference between a thundercloud chargecenter and ground is estimated to be of the order of 10million to 100 million Volts.

15% are cloud-to-ground strokes. Rest is light-show. Some 95% of ground strokes are negative with respect

to ground (northern hemisphere).

Average flash will light a 100 Watt bulb for 3 months. Flash temperature exceeds 30 000 C (50 000 F) The average thickness of a bolt is 1 - 2 inches. 9/10 people struck by lightning survive the event (>75%

with long term trauma). Ray Sullivan got seven hitsbetween 1942 and 1976.


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Current magnitudes

Discharge current (acc. BS6651) :

1% > 200 kA10% > 80 kA50% > 28 kA90% > 8 kA

99% > 3 kA

Recorded max. >360 kA.Up to 42 separate strokes hasbeen recorded with 10 - 90 msintervals in one discharge.


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Direct current pulse properties

Pulse rise time about 1...10 us (10 to 90%).Pulse duration (to half-value) 50 ... 700 us.

Earth potential differences can last severalmilliseconds.

BS5561 recommends following "direct hit"max. values for design purposes (99%confidence)

Current 200 kA and max di/dt 200kA/us.

Most standard down conductors haveinductance of the range of 1.5 uH/m. ==>Voltage difference over the down conductorcan be several Mega volts.


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Induced surges

The "voltage" of a direct hit is virtually unlimited.High voltage AC distribution lines (20 kV) are air-gap-limited

to around 100 kV at pole transformers.Recommended protection capacities in AC line (IEEE C62.41) : Category C, Open distribution lines 10 kV / 10...20 kA Category B; Building entry point 6 kV / 3 kA

Category A; Distribution outlets 6 kV / 1 kAAn indirect strike within 100m of cables or buildings caninduce surges up to 5 kV and 1.25 kA in AC cables.Telephone (and instrumentation) cables have higher seriesimpedance thus limiting the current


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Transient coupling

Lightning transients have three main coupling paths:

Radio-frequency interference (RFI) Typically only a nuisance

Induced voltages via magnetic or capacitive coupling. Shielded signal cables and twisted pairs are pretty

immune. Common mode voltages can cause component damage.

Surge currents caused by localized shifts in earthpotential.

Relatively low voltage but long common mode pulses Can (literally) burn series impedance components in multi-

phase surge protection circuits.


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Standards for lightning protection (USA)

NFPA-780 (37% of technical committee members are lightning rodvendors)

American Petroleum Institute (API) recommended practice 2003

(1998) DOD Explosives Safety Board 6055.9 (1997) MIL-HDBK-419A (1987) Grounding, bonding and shielding for

Electronic Equipment. (800+ pages) Department of Energy bulletin DOE/EH-0530 (1996), events summary

and risk analysis. FAA-STD-019c (1999) lightning protection for facilities IEEE Std 142-1991 recommended practice for grounding of industrial

and commercial power systems

IEEE Std 1100-1999 recommended practice for powering andgrounding of electronic equipment U.S. Air Force Instruction 32-1065 (1998), civil engineering directive

covering grounding systems in detail. U.S. Navy, NAVSEA OP 5, Vol. (ammunition and explosives ashore)


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Standards for lightning protection (EU + Asia)

TC-81 IEC 1024-1 Protection of structures against lightning

IEC 1024-1-1 Guide A, Selection of protection levels forlightning protection system. IEC 1024-1-2 Guide B, Design, construction,

maintenance and installation of lightning protection

system. IEC 1662 Technical report "Assessment of risk of

damage due to lightning." CP 33 : 1996 - Code of Practice on lighting Protection

(Singapore)


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Standards for surge protection

IEEE C62.41-1991 BS6651-1992 Appendix C

IEC 60079-14 (1996-12) IEC 1312-1 Protection against LEMP Part1: General

principles.


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Protection methods

Structures:The traditional mesh of copper wires on roofs and walls and

their associated earth rods protect the bricks and mortar butnot necessarily the electronics.

Electronics:

Surge Protection Devices (SPDs) cannot protect equipmentagainst direct lightning strikes. Their task is to neutralizevoltage surges on cables caused by inductive or resistive(earth potential variations) coupling from nearby lightning

strikes.


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Surge protectors

Air gaps (specially in HV-lines) very high surge currents

not accurate voltageGDT:s (Gas Discharge Tubes) high surge currents slow and shorting

MOV:s (Metal Oxide Varistors) medium to high surges quite fast but "soft"

Transient suppressor diodes(clamping diodes)

small surge currents accurate clamp voltage fast


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Protect your protectors!

Varistors, GDT:s and Transils caneither be destroyed in a flash ordeteriorate during time.

Like good bodyguards theyusually fail "safe" shorting theconnection.

Overcurrent protection

must be designed!GDT:s trigger voltage has atendency to creep lower afternumerous functioning cycles(spikes or whiskers buildup).

Design forinterchangeability andserviceability!


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Recommended design for data lines

Two-stage protection with a resistor or resistor/inductorcombination in between (only inductor is not recommended).

At the line end first 10 kA capable GDT, then series impedanceand then transil diodes.


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For unprotected mains connection

First stagearrestor with

>20kA currentcapability.Then inductorwith adequatecurrent rating.Second stage(>3 kA rated)at equipmentmains input.


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Lightning conductor construction

Air terminal (rod) on top. Preferably a blunt tip. No isolating coating.

Sturdy copper wire or strip to earth electrode. min. cross section of 16 sq. mm (10... 15 degrees C

temperature rise with a single 30 kA stroke.) Smooth turns and never over 90 degrees angles.

Copper (or copper plated) ground electrode with a lowimpedance to ground. DC resistance preferably < 10 Ohms.

Preferred topologies: Ring (with min. 2 connecting wires for buildings) X and Y

Massive, non corroding connections between sections.


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Lightning rods

"Classical" air terminal designby Mr. Benjamin Franklin

Widely used but the sharp point isactually a drawback.

Over the seven-year study, not oneof the sharp Franklin rods werestruck by lightning, while twelveblunt rods were hit, Moore and hiscolleagues report in the journalGeophysical Research Letters [15May 2000] 1.


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"Sci Fi approach"

`Early Streamer Emitters' (ESEs)Air terminals equipped with a device orformed such a way that it supposedlycreates an upward propagating streamerfaster than a standard air terminal.Manufacturers claim that they have adifferent protective radius (than a

standard rod).


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Early Streamer Emitters

US Standards committee NFPA 780 and the Standards councilmade an independent study:

Early Streamer Emission, Air terminals, Lightning protectionsystems, Literature review and Technical analysis January1996 .

Three hundred and three different works were reviewed, 301rejected ESE as no better than a conventional rod . Two wereauthored by a manufacturer.

==> The US NFPA Standards Council rejectedconsideration of the ESE for a new standard and doesnot recommend their use.

(Actually the study revealed that no standard protection schemeprovides a 100% protection!)


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ESE realism?

Observe the difference between cartoon physics illustrations about thesuggested protective radius and especially the shape of it, compared towidely accepted IEC standards model.


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ESE favors

Different types of ESE: 1. Radioactive

(Radium 226, Alpha emitter, banned in most countries between 1975 -1989, ionizing found to happen max. 2 cm from surface)

2. Sparking (ionizing) 3. Voltage pulsing (a few kilovolts) 4. Special shape to make corona discharge 5. Special shape to prevent corona discharge

Some models emit a cloud of ionized air in high voltage

laboratory environment, where no other early streamers arepresent and no high winds exist (like in thunderstorms)


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Down conductors

Sturdy wire, pipe orthick foil.Massive steel oraluminumconstructions can beused instead.Straight lines, smooth

curves, no sharpangles.Never over 90 degreeturns!

Special attention tocorrosion betweendifferent metals inconnections.(find 5 errors in picture)


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Routes of a down conductor.

Most military standards recommend isolated down conductor,not connected to structures at all. Some require evenseparate ground electrodes.In building standards an electrically conducting structure isoften used as down conductor (it is impractical to even try tomake a smaller impedance with added wiring!)With isolated conductor, maintaining a very low impedancepath to ground is required, otherwise the charge will sidestep.Over 1.5 m sidesteps through walls have been observed.When using a metal mast as a down conductor, normal signal

cabling can survive because the mast makes a better routethan the cable. This requires isolated electronics on bothends of the mast! (WS425, WT50, MAWS !#%&/)


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Ground electrode

Typically copper wire eitherburied to the ground 0.5 - 1 mdeep, or connected to groundrods.Make as low impedance to theground as possible.Ground rods can be of solid

copper, copper tubes, copperclad steel or stainless steel.Typically 20 - 50 mm thick, insome cases filled with mineralsalt (to decrease groundresistance).Recommended length andnumber of electrodes isdependent on soil moisture and

resistivity.

(What is wrong with this picture?)


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Soil properties

Soil resistivity varies a lot, even at the same locationdepending on the moisture and salt content (and ice).Examples: [Ohm-meters]

Topsoil, loam 1 - 50Gravelly clays, sandy clays 25 - 60Fine sandy or silty clays 80 - 300

Gravel, gravel-sand mixtures 100 - 500Granite, sandstone 20 - 2000Gravel, sand, stones, little clay 600 - 4500

Surface limestone 100 - 10 000


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Buried wire electrode example

Specially when bedrock is close,ground is very dry or rods arenot feasible, a three wire Y -buried electrode is a favoredconstruction.Three pieces of min 16 mm2copper wire are buried into 0.5 -1m deep ditches. Recommendedwire segment length is 10 - 30m.Connect all ground wires to thedown conductor with a heavy-duty terminal block at the same

point! Do not daisy-chain theconnections.Treating the ground (MgSO4,CuSO4) can be used to decreasethe ground impedance.


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Web resources (take always with a grain of salt...)

Comprehensive application notes: http://www.mtlsurgetechnologies.com/downloads/tans/in

dex.htm

Reading: (interesting if true, as everything on the Web) http://www.lightningeliminators.com/technica.htm

http://www.straightdope.com/columns/010824.html http://www.lightningsafety.com/nlsi_bus/CodeReview200

2.html

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