4.1 Air Terminals. Since Franklin's
day lightning rods have been installed upon ordinary structures as sacrificial
attachment points, intending to conduct direct flashes to earth. In
1876 JC Maxwell suggested that Franklin rods on buildings attracted
a greater number of flashes than their absence. Such rods should not
be located on explosives storage structures. This integral air terminal
design does not provide protection for electronics, explosives,
or people inside modern structures. Inductive and capacitive coupling
from lightning-energized conductors can result in significant voltages
and currents on interior power and signal conductors.
Overhead shield wires and mast systems
located above or next to the structure are suggested alternatives in
many circumstances. These are termed indirect air terminal designs.
Such methods presume to collect lightning above or away from the sensitive
structure, thus avoiding or reducing flashover attachment of unwanted
currents and voltages to the facility and equipments.
Investigation into applicability
of dielectric shielding may provide additional protection where upward
leader suppression may manipulate breakdown voltages (Schnetzer et al,
Sandia Laboratories, 1997). Unconventional air terminal designs which
claim the elimination or redirecting of lightning (charge dissipators)
or lightning preferential capture (early streamer emitters) deserve
a very skeptical reception (NASA/Navy Tall Tower Study; 1975, R.H. Golde
"Lightning" 1977; FAA Airport Study 1989; T. Horvath "Computation
of Lightning Protection" 1991; D. MacKerras et al, IEE Proc-Sci
Meas. Technol, V. 144, No. 1 1997; National Lightning Safety Institute
"Royal Thai Air Force Study" 1997; A. Mousa "IEEE Trans.
Power Delivery, V. 13, No. 4 1998; International Conference on Lightning
Protection - Technical Committee personal correspondence 2000). Merits
of radioactive air terminals have been investigated and dismissed by
reputable scientists (R.H. Golde op cit and C.B. Moore personal correspondence,
4.2 Downconductors. Downconductor
pathways should be installed outside of the structure. Rigid strap is
preferred to flexible cable due to inductance advantages. Conductors
should not be painted, since this will increase impedance. Gradual bends
always should be employed to avoid flashover problems. Building structural
steel also may be used in place of downconductors where practical as
a beneficial subsystem emulating the Faraday Cage concept.
4.3 Bonding assures that unrelated
conductive objects are at the same electrical potential. Without Bonding,
lightning protection will not work. All metallic conductors entering
structures (ex. AC power lines, gas and water pipes, data and signal
lines, HVAC ducting, conduits and piping, railroad tracks, overhead
bridge cranes, roll up doors, personnel metal door frames, hand railings,
etc.) should be electrically referenced to the same ground. Connector
bonding should be exothermal and not mechanical wherever possible, especially
in below-grade locations. Mechanical bonds are subject to corrosion
and physical damage. HVAC vents that penetrate one structure from another
should not be ignored as they may become troublesome electrical pathways.
Frequent inspection and resistance measuring (maximum 1 milliohm) of
connectors to assure continuity is recommended.
4.4 Grounding. The grounding
system must address low earth impedance as well as low resistance. A
spectral study of lightning's typical impulse reveals both a high and
a low frequency content. The grounding system appears to the lightning
impulse as a transmission line where wave propagation theory applies.
A considerable part of lightning’s current responds horizontally when
striking the ground: it is estimated that less than 15% of it penetrates
the earth. As a result, low resistance values (25 ohms per NEC) are
less important that volumetric efficiencies.
Equipotential grounding is achieved
when all equipment within the structure(s) are referenced to a master
bus bar which in turn is bonded to the external grounding system. Earth
loops and consequential differential rise times must be avoided. The
grounding system should be designed to reduce AC impedance and DC resistance.
The use of counterpoise or "crow's foot" radial techniques can lower
impedance as they allow lightning energy to diverge as each buried conductor
shares voltage gradients. Ground rings connected around structures are
useful. Proper use of concrete footing and foundations (Ufer grounds)
increase volume. Where high resistance soils or poor moisture content
or absence of salts or freezing temperatures are present, treatment
of soils with carbon, Coke Breeze, concrete, natural salts or other
low resistance additives may be useful. These should be deployed on
a case-by-case basis where lowering grounding impedances are difficult
an/or expensive by traditional means.
4.5 Corrosion and cathodic
reactance issues should be considered during the site analysis phase.
Where incompatible materials are joined, suitable bi-metallic connectors
should be adopted. Joining of aluminum down conductors together with
copper ground wires is a typical situation.
4.6 Transients and Surges.
Ordinary fuses and circuit breakers are not capable of dealing with
lightning-induced transients. Surge protection devices (SPD aka transient
limiters) may shunt current, block energy from traveling down the wire,
filter certain frequencies, clamp voltage levels, or perform a combination
of these tasks. Voltage clamping devices capable of handling extremely
high amperages of the surge, as well as reducing the extremely fast
rising edge (dv/dt and di/dt) of the transient are recommended.
Protecting the AC power main panel;
protecting all relevant secondary distribution panels; and protecting
all valuable plug-in devices such as process control instrumentation,
computers, printers, fire alarms, data recording & SCADA equipment,
etc. is suggested. Protecting incoming and outgoing data and signal
lines (modem, LAN, etc.) is essential. All electrical devices which
serve the primary asset such as well heads, remote security alarms,
CCTV cameras, high mast lighting, etc. should be included.
Transient limiters should be installed
with short lead lengths to their respective panels. Under fast rise
time conditions, cable inductance becomes important and high transient
voltages can be developed across long leads. SPDs with replacable internal
modules are suggested.
In all instances the use high quality,
high speed, self-diagnosing SPD components is suggested. Transient limiting
devices may use spark gap, diverters, metal oxide varistors, gas tube
arrestors, silicon avalanche diodes, or other technologies. Hybrid devices,
using a combination of these techniques, are preferred. SPDs conforming
to the European CE mark are tested to a 10 X 350 us waveform, while
those tested to IEEE and UL standards only meet a 8 X 20 us waveform.
It is suggested that user SPD requirements and specifications conform
to the CE mark, as well as ISO 9000-9001 series quality control standards.
Uninterupted Power Supplies (UPSs)
provide battery backup in cases of power quality anomalies…brownouts,
capacitor bank switching, outages, lightning, etc. UPSs are employed
as back-up or temporary power supplies. They should not be used in place
of dedicated SPD devices. Correct Category A installation configuration
is: AC wall outlet to SPD to UPS to equipment.
4.7 Detection. Lightning detectors,
available at differing costs and technologies, are useful to provide
early warning. Users should beware of over-confidence in detection equipment.
It is not perfect and it does not always acquire all lightning data.
Detectors cannot "predict" lightning. An interesting application
is their use to disconnect from AC line power and to engage standby
power, before the arrival of lightning. A notification system of radios,
sirens, loudspeakers or other means should be coupled with the detector.
See the NLSI WWW site for a more detailed treatment of detectors.
4.8 Testing & Maintenance.
Modern diagnostic testing is available to "verify" the performance
of lightning conducting devices as well as to indicate the general route
of lightning through structures. With such techniques, lightning paths
can be forecast reliably. Sensors which register lightning current attachments
can be fastened to downconductors. Regular physical inspections and
testing should be a part of an established preventive maintenance program.
Failure to maintain any lightning protection system may render it ineffective.
- PERSONNEL SAFETY ISSUES
Lightning safety should
be practiced by all people during thunderstorms. Measuring lightning's
distance is useful. Using the "Flash/Bang" (F/B) technique, for every
five seconds - from the time of seeing the lightning flash to hearing
the associated thunder - lightning is one mile away. A F/B of 10 = 2
miles; a F/B of 20 = 4 miles, etc. The distance from Strike A to Strike
B to Strike C can be as much as 5-8 miles. The National Lightning Safety
Institute recommends the 30/30 Rule: suspend activities at a F/B of
30 (6 miles), or when first hearing thunder. Outdoor activities should
not be resumed until 30 minutes has expired from the last observable
thunder or lightning. This is a conservative approach: perhaps it is
not practical in all circumstances.
If one is suddenly exposed
to nearby lightning, adopting the so-called Lightning Safety Position
(LSP) is suggested. LSP means staying away from other people, taking
off all metal objects, crouching with feet together, head bowed, and
placing hands on ears to reduce acoustic shock from nearby thunder.
When lightning threatens, standard safety measures should include: avoid
water and all metal objects; get off the high ground including rooftops;
avoid solitary trees; stay off the telephone. A fully enclosed metal
vehicle – van, car or truck – is a safe place because of the (partial)
Faraday Cage effect. A large permanent building can be considered a
safe place. In all situations, people should avoid becoming a part of
the electrical circuit. Benjamin Franklin’s advice was to lie in a silk
hammock, supported by two wooden posts, located inside a house.
Every organization should
consider adopting and promulgating a Lightning Safety Plan specific
to their operations.
6. CODES AND STANDARDS
In the USA there is no
single lightning safety code or standard providing comprehensive assistance.
The most commonly referenced USA commercial lightning protection installation
standard is incomplete, out-dated, and largely pre-empted by commercial
interests. US Government lightning protection documents should be consulted.
The Federal Aviation Administration FAA-STD-019c is valuable. Other
recommended federal codes include military documents MIL HDBK 419A,
NAV OPSEA 5, KSC STD E0012, MIL STD 188-124B, MIL STD 1542B, MIL B 5087B,
UFC 3-570-01 and AFI 32-1065. The British Code BS 6551 is helpful. The
new German lightning protection standard for nuclear power plants KTA
2206 places special emphasis on the coupling of overvoltages at instrument
and control cables. The European International Electrotechnical Commission
IEC 61024 series for lightning protection is the single best reference
document for the lightning protection engineer. Adopted by many countries,
IEC 1024 is a science-based document applicable to many design situations.
Ignored in most Codes is the very essential electromagnetic compatibility
(EMC) subject, especially important for explosives safety and facilities
containing electronics, VSDs, PLCs, and monitoring equipment.
Lightning has its own agenda
and may cause damage despite application of best efforts. Any comprehensive
approach for protection should be site-specific to attain maximum efficiencies.
In order to mitigate the hazard, systematic attention to details of
grounding, bonding, shielding, air terminals, surge protection devices,
detection & notification, personnel education, maintenance, and
employment of risk management principles is recommended.
Conference on Lightning Protection (ICLP) Proceedings, Rhodes Greece
- ICLP Proceedings,
Birmingham UK Sept. 1998
- ICLP Proceedings,
Florence Italy Sept.1996
- IEEE Transactions
on Electromagnetic Compatibility, Nov. 1998
- National Research
Council, Transportation Research Board, NCHRP Report 317, June 1989
Electrotechnical Commission (IEC), International Standard for Lightning
Protection. See: http://www.iec.ch
- Gardner RL,
Lightning Electromagnetics, Hemisphere Publishing, NY NY 1991
Note: Permission to copy
and to re-print this paper is freely given. Please credit NLSI as the
original author. The National Lightning Safety Institute is a non-profit,
non-product independent organization providing objective information
about lightning safety issues.