Evaluating lightning susceptibility.
On a macroscopic level, when an object or person is struck by lightning, it probably has been struck multiple times in that single event because of the way the lightning waveform is generated.
How Lightning Is Formed
A thundercloud behaves like an electrostatic charge generator. Charge separation occurs when ice particles circulating within the cloud and bumping into one another take on or lose electrons. During collisions, small particles give up electrons to larger particles regardless of whether they are ice, dust, flour, or whatever. Because the smaller ice particles are lighter, they are blown higher in the cloud so the top of the cloud becomes increasingly more positive and the bottom more negative.
Since like charges repel each other, a positive area like a charge shadow exists under the cloud on the Earth's surface and in those objects mounted or standing on it. This charge shadow follows along under the cloud as it moves and may result in corona being launched from sharp objects located in the shadow region.
Electrostatic potential between the cloud and ground continues to increase until the dielectric breakdown potential at approximately 1 MV/m of the air occurs. This may be the result of continued charge accumulation or a localized reduction in the separation distance between the cloud and ground from some high object such as a radio tower, church steeple, hill, or tree located within or along the edge of the charge shadow region.
Partial charge neutralization occurs via high conduction current through a low-impedance path formed by a filament of ionized air molecules stretching from the ground to the cloud. At this point, the lightning event changes abruptly from developing a high electrostatic potential voltage to creating a high current magnetic field, just like any other ESD event.
Even though the electrostatic potential between the cloud and the ground that initiated breakdown is shorted and no longer can sustain the original breakdown voltage, the circulating air currents continue to charge the cloud. This raises the cloud-to-ground potential difference, causing breakdown to reoccur multiple times and creating a crude sawtooth waveform. This charge-discharge cycle continues until sufficient charge has been neutralized that breakdown during this strike event can no longer occur.
Several lightning tests such as RTCA DO-160 and MIL-STD-464A attempt to emulate lightning's sawtooth discharge behavior. Although the maximum strike intensity can reach 400 kA, a 200-kA strike generally is used as the design maximum, and 98% of the occurrences actually are between 3 kA and 140kA. The average peak current for the first strike is approximately 20 kA with the ongoing strikes at about half that value. The typical interval between strikes is approximately 50 ms.
The Destructive Effects of Lightning
Lightning can destroy most anything in multiple ways in just a few milliseconds, as described in Table 1. The worst destructive effects are thermal, mechanical, and direct current injection. These are called the direct effects and generally handled at the facility level.
The minimum facility requirements for lightning protection are those specified in NFPA 70: National Electric Code and NFPA 780: Standard for Installation of Lightning Protection Systems. These documents provide facilities design and installation methods that reduce lightning surges to a more manageable level.
If a system's electronics were to encounter a 200-kV x 200-kA transient accompanied by a 50,000[degrees] F air blast traveling faster than the speed of sound, there would be damage. Lightning surges of this magnitude have been known to punch holes through 3/8 inch thick cold-rolled welded steel boxes, and they routinely turn sand into glass. So a collection of little surface-mount devices on a PCB doesn't stand a chance.
Lightning's indirect effects are not as severe as the direct effects and can be minimized by design and installation.
EMC engineers advocate the suppression of culprit RF energy at its source. With ESD, steps such as raising the humidity can be taken to reduce the amplitude. With lightning, that's not possible so the design approach is to keep as much of the energy out of the equipment as possible and then reduce the amplitude of the remaining energy to a level that the equipment can withstand.
Protecting the Equipment
Just like any other EMC coupling problem, there are two ways that lightning transient energy may enter the equipment: radiated and conducted. The worst indirect threats are the conducted transients. The highest of these generally are the result of lightning striking exposed power, control, or signal cables. Since the lightning-conducted current is returning to earth ground by all possible paths, any device or component in the way and unable to carry the lightning current will be destroyed.
As a result, it is absolutely essential that lightning currents be kept out of the equipment. This is done with a combination of earth grounding, bonding, shielding, and filtering. These hardening techniques also will reduce the currents induced onto cables and directly into circuit boards by electric-and magnetic-field coupling. Since the lightning currents predominantly enter systems on exposed power, control, and signal leads, the hardening discussion begins there.
Power, Control, and Signal Cables
All power and non-RF control and signal cables should be shielded twisted pairs with the shield terminated to an earth-referenced ground at both ends. This usually is the chassis ground.
With some signal and control cables, this may not be the best EMC practice. In these cases, the design may require double-shielded cables with the innershield grounded only at one end. As an alternative, fiber-optic cables could be used to eliminate lightning electromagnetic effects. Cable shields should terminate in a conductive backshell with a 360-degree circumferential bond.
RF circuits should have the outer braid of the coaxial cable grounded at both ends and, whenever possible, at multiple points along the length of the shield. The outer braid of a coaxial cable is not a shield; it is the RF signal return. If the transients are being coupled into the signal through its return, then it may be necessary to shield the coax. This is done by using triaxial cable.
When the power, control, and signal leads reach their destination, any lightning currents on leads going to sensitive circuits must be diverted to the earth ground reference, bandwidth limited, or attenuated. This is accomplished using bonding, grounding, and filtering.
Table 1. Destructive Effects of Lightning Thermal Voltage Potential Gradient Light, heat, melting, Earth voltagepotential differences vaporization, are welding of mayreach hundreds of kilovolts parts, fire, explosions close in to the Even though Lightning heats the air to potential gradient decreases with 50,000[degree]F in ~1 [mu]s distance from the strike point, Power line pole transformers it still may affect sensitive disintegrate. Thermal is a equipment located 100 to 200 major problem around feet away.Potential on conductors combustibles, explosive gases, resulting from V = L di/dt may and pyrotechnic devices. reach thousands of kilovolts Mechanical Electromagnetic Interference Strong electromagnetic Broad bandwidth event (20 kHz to attraction/repulsion forces 2 MHz) with high-level can be exerted on conductors, radiated EF and HF that may high-level acoustic shockwave cause malfunctioning disruption, (thunder is but a mild distant failure, and creation of latent example), vaporization defects in electronic equipment. pressure. May affect equipment 10 km or more from the strike. Direct Current Injection Personnel Hazards Dielectric High touchvoltages which may breakdown/arc-over/arcthrough cause electrocution, electrical from overvoltage resulting in shock, or involuntary muscle thermal and mechanical jerk injuries. Use 30-30 effects, shorts, or opens. rule for safety. Seek Unusally direct current shelter if flash-bang injection into electronics is 30 s or less.Stay results in irreparable there until flash-bang is 30 catastrophic damage. min.
The filter circuits may control the frequency, amplitude, or both depending on the circuit bandwidth and generally use a two-or three-stage design to handle the large surges. The stage facing the incoming lightning transient might be a gas spark gap or carbon block, followed by a metal oxide varistor (MOV) that in turn is followed by a silicon avalanche diode.
In a multistage transient suppressor, each stage is separated using resistors or inductors that, in conjunction with the component capacitance, form a frequency filter. Multiple stages are required because no one device is capable of protecting solid-state components.
The gas gap isn't fast enough, and the silicon diode can't handle the surge amplitude. The frequency filtering provides a time delay between the stages so the silicon diode doesn't die trying to shunt the entire surge. For example, a 2,500-V surge might be reduced to 650 V by the gas gap, to 27 V by the MOV, then to 12 V by the silicon diode, which protects the circuit from being damaged.
For all practical purposes, the lightning surge transient appears as a zero-impedance current source. Regardless of whether the transient is 200, 2,000, or 20,000 A, it will develop a voltage (V = IR) across any resistance in the path. Voltages can be so high between grounded and ungrounded equipment that are-over will occur. MIL-STD-464A bonding requirements can be used as a guide for the design:
* 15 m[ohm] or less from cable shields to the equipment enclosure including the cumulative effect of all connector and accessory interfaces.
* 10 m[ohm] or less from the equipment enclosure to the system structure including the cumulative effect of all faying surface interfaces.
* 2.5 m[ohm] or less across individual faying interfaces within the equipment, such as between subassemblies or sections. Even with a 2.5-m[ohm] bond impedance, a 200-kA transient will produce 500 V.
As the bonding or grounding impedance increases, voltage potentials reach levels where side flashes can occur. If the equipment items are not part of the same system, are-over can be prevented by spacing them apart. To minimize the voltage potential between individual equipment items, they should all be grounded to the same system.
Large-scale systems with a distributed architecture spanning several widely separated locations must meet the requirements of NFPA-780. The big problem with widely separated equipment grounded at their respective locations is the IR drops that result from currents through the lossy earth.
Nearby lightning strikes also generate high radiated electric and magnetic fields. Shielding is the most effective way to protect the equipment. The primary shield design issues will be controlling the size of any apertures and handling the pickup by the system's cables.
The most cost-effective approach uses any inherent shielding provided by interconnecting and bonding structural members to create an equipoten-tial surface. To do this, all electrical and mechanical components such as connections, enclosures, and electrical metallic tubing (EMT) should be bonded together and to the ground system. All faying surfaces must be conductive and free from nonconductive contaminants such as paints, oxides, and corrosion byproducts.
Bonding conductors should not have sharp bend radiuses. Sharp bends create localized areas of high inductance and are potential arcing points. Bend radiuses should be 8 inches or more.
Testing the Design
Testing is the only way to determine that the design will handle lightning surges. There are a number of specifications that define lightning tests. Some are simple single transient tests; others are very comprehensive and attempt to simulate actual lightning strikes.
The military and aerospace industries use RTCA DO-160, which forms the basis of SAE ARP-5412, MIL-STD-464A, and MIL-STD-1757. Some organizations use MIL-STD-461C test method CS06. It's a simple transient test that wasn't originally designed as a lightning test but many organizations use it that way. It's often called the poor man's lightning test. This test method is back in MIL-STD-461F as CS106. The commercial world uses ANSI/IEEE C62.41 and IEC 61000-4-5.
There's an expression that states lightning goes wherever it wants. Even for test planning, lightning is very uncooperative. Most of the time, the surges are negative. But there is an occasional positive strike, and these are the worst. The waveforms are complex, and their characteristics are quite random.
Within the assortment of real waveforms occurring during a typical lightning strike, the fastest rise time (~1 [mu]s) and the maximum amplitude (~200 kA) do not coincide. Combining the two would result in a 200-kA/[mu]s di/dt, which is not very likely to happen. As a result, a di/dt of 140 kA/[mu]s typically is used.
The RTCA DO-160 test uses five different waveforms to represent lightning. In the simplest case, the most popular waveform used to simulate a lightning surge is the ANSI/IEEE C62.41 combination wave. This is a double exponential 1.2 x 50 [mu]s open-circuit voltage/23 A 8 x 20 [mu]s short circuit current ([I.sub.sc]) waveform that also is called out in IEC 61000-4-5.
Lightning currents enter the equipment principally through the power, control, and signal cables on their way to earth ground so the lightning test emulates the same thing. The DO-160 and MIL-STD-464 test setups are virtually identical to that used for MIL-STD-461 EMC testing. Cable lengths and power line breakout are slightly different, and a 5-[mu]H line impedance stabilization network (LISN) is used in conjunction with 10-[mu]F capacitors. Otherwise, the appearance is the same.
The lightning generator and the EUT are set up in a shielded enclosure on a ground plane that is used to represent the earth return. The EUT subsystems are arranged to allow the lightning surge currents to follow the same general path they would travel in their installed configuration. EUT bonding/grounding should match the actual installation.
When the setup is complete, the pulse generator output is charged and injected into the EUT. Because lightning can be either polarity, both positive and negative pulses are used. Sufficient modes of EUT operation are performed to assure that all circuits have been evaluated. This may require creating specialized software.
Since the conducted transients also are being used to represent the transient being coupled from the lightning's radiated magnetic fields, care should be taken to prevent radiation from the test cables into the test sample. This usually is done through cable, box, and overall enclosure shielding.
The lightning surge generator must be able to produce the required open-circuit voltage and short-circuit current with a waveform and sufficient energy to meet the requirements. The specification defines the waveform, which may be a single stroke or a combination of single stroke, multiple stroke, and multiple bursts.
When applied to the test sample, the generator outputs a signal that is determined by the EUT's impedance at the injection point. Low impedances, such as a cable shield, will have high currents; high impedances will have high voltages.
The limit is met whenever either the required peak current or voltage reaches the specification limit. The specification limit is determined by the operational location of the EUT. For example, if the EUT is an antenna located on the exterior skin of an aircraft, then it will have to withstand higher amplitude surges than a controller located deep within the fuselage.
Unless you are absolutely sure the unit will meet the limit, always start with a lower amplitude transient and gradually increase the level. This allows the susceptibility threshold amplitude to be determined before the EUT is destroyed.
Whether simple or complex, most of the lightning tests are very similar. What varies is the purpose of the test and how the test signals are coupled onto the cables.
There are three primary ways to do this:
* Capacitively couple the signals as done in IEC 61000-4-5.
* Use transformer coupling as done in DO-160 and MIL-STD-464. Although not a lightning test, this is the same method used for MIL-STD-461D/E/F CS114, CS 115, and CS 116.
* Isolate everything from the ground plane. Inject the current into the chassis furthest from the ground point and let the lightning current find its way home.
Direct capacitive coupling tends to be more severe than transformer coupling so the levels often are adjusted to account for this. SAE ARP-5416 does a great job in explaining the various procedures as applied to lightning.
Figure 1 shows the three signal injection methods. Figure 1a illustrates both the differential-mode (DM) and the common-mode (CM) capacitive coupling onto cables. It typically uses a stub cable or sometimes a break-out-box (BOB) to allow injection at each connector pin to assess lightning damage.
[FIGURE 1 OMITTED]
This same approach is used for control and signal cables but becomes more difficult to apply if the cables are shielded. This is especially true if high-quality, 360-degree circumferentially bonded connectors have been used to terminate the shield.
Figure 1b illustrates coupling the lightning transient onto cables using clamp-on current probes. These are broad-bandwidth, one-turn transformers clamped around the cable. The cable is the one-turn secondary. If the cable is unshielded, the surge transient energy is coupled directly onto all wires in the cable simultaneously. If it is a shielded cable grounded at both ends, the surge current is coupled onto the shield and, in turn, coupled into all the wires as a function of the shield's transfer impedance.
Cable shields generally are grounded at both ends for lightning protection although that may not always be the case. For cables grounded only at one end, the current probe technique still is valid but the current path impedance is extremely high at the low frequencies, which increases the voltage injected along the shield.
As the frequency increases, the cable's capacitive coupling reduces the current path impedance. Figure 1c illustrates a test that simulates a transient voltage gradient across the ground system. This test is performed by isolating the EUT from ground at all but one point and driving current through the EUT ground system.
As indicated in Table 1, lightning produces severe direct and indirect effects. The lightning transient generators used in these tests are not quite as energetic as the real thing, but they occasionally do cause components to disintegrate, sending fragments around the lab, and they do produce lethal voltages. Test engineers and technicians are aware of the hazards, but everyone involved in lightning testing must observe and follow all safety precautions.
Recommended Test Requirements
The indirect effects of lightning can induce transient energy into all systems cables, resulting in substantial circuit damage. Pin-level tests are used for damage assessment to indirect lightning. One of the best-known and the basis of several military lightning specifications is RTCA DO-160. This specification does a great job emulating lightning strikes and can be used for engineering evaluations and qualification.
This standard specifies indirect lightning waveforms, levels, and test methods including setup and calibration procedures and is suitable for lightning testing of any EUT. Although the DO-160 specification defines several test levels, Levels 1 and 3 cover most situations.
Level 1 is intended for equipment and interconnecting wiring installed in a well-protected environment. Testing to Level 1 is accomplished by applying waveform 3, a 1-MHz damped sine wave with 100-V peak open-circuit voltage ([V.sub.OC]) or 4-A peak [I.sub.sc] and waveform 4, a unipolar 6.4 x 69-[mu]s pulse with 50-V [V.sub.oc] and 10-A [I.sub.sc].
Level 3 is intended for equipment and interconnecting wiring in a moderately exposed EM environment. It specifies higher amplitudes for waveforms 3 and 4, 600-V [V.sub.oc]/24-A [I.sub.sc] and 300-V [V.sub.oc]/60-A [I.sub.sc], respectively.
Regardless of which specification is selected to evaluate the lightning susceptibility of the EUT, get a copy and thoroughly study the procedure before zapping the EUT.
For More Information
* Lightning and Lightning Protection, William C. Hart and Edgar W. Malone, EEC Press. 1988.
* The Aerospace Engineers Handbook of Lightning Protection, Bruce C. Gabrielson, Interference Control Technologies, 1988.
* Lightning Protection of Aircraft, Franklin A. Fisher and J. Anderson Plumer, NASA Reference Publication 1008, 1977.
* MIL-STD-1757A: Lightning Qualification Test Techniques for Aerospace Vehicles and Hardware, July 20, 1983.
* SAE ARP-5416: Aircraft Lightning Test Methods, March 2005.
* "Developments in Lightning Test Standards for Aircraft Avionics," Nicholas Wright, Interference Technology, EMC Directory and Design Guide, 2006.
* RTCA/DO-160E: Environmental Conditions and Test Procedures for Airborne Equipment, Section 22: Lightning Induced Transient, Dec. 9, 2004.
* MIL-STD-461F: Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment, Dec. 10, 2007.
* MIL-STD-464A: Electromagnetic Environmental Effects Requirements for Systems, Dec. 19, 2002.
* "3000 dB and Rising," Ron Brewer, EE--Evaluation Engineering, February 2008.
by Ron Brewer, EMC/ESD Consultant
About the Author
Ron Brewer is a senior EMC/RF engineering analyst with Analex at the NASA Kennedy Space Center. The NARTE-certified EMC/ESD engineer has worked full-time in the EMC field for more than 30 years. Mr. Brewer was named Distinguished Lecturer by the IEEE EMC Society and has taught more than 385 EMC technical short-courses in 29 countries and published numerous papers on EMC/ESD and shielding design. He completed undergraduate and graduate work in engineering science and physics at the University of Michigan. e-mail: firstname.lastname@example.org
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