Lightning phenomena and protection systems
3.5 Risk factors and protection
3.5.6.4 Protector devices
Lightning transients can be effectively limited by protection so as to avoid data and software corruption, partial discharges and irreversible damage to hardware. Such protection is arranged so that the transient control level (TCL) limits overvoltages to below the equipment transient design level (ETDL).
The type of protector device will relate to its location in the electronic system.
Locations are defined in the standards as falling into three categories, from a low exposure category A (e.g. plug-in equipment that is expected to experience only low transient levels) to category C which is appropriate for major exposure to transients, such as an incoming mains supply. High vulnerability data, signal or telephone lines are also included in category C because transient attenuation is weak in such networks.
Mains power protectors are tested during manufacture with 1.2/50 impulses of up to 20 kV peak and 8/20 impulse currents of up to 10 kA. Data system protectors are tested up to 5 kV and with 10/700 125 A impulses.
In order to achieve the required TCL, the let-through voltage of the protector must be specified [126]. For mains supplies, the primary series fuse, residual current and miniature circuit breaker boards have secondary shunt varistor protectors together with L and C filters. Filter design, whether high or low pass, is not straightforward for fast transients because of the reduction of inductance by magnetic saturation effects and the effect of series and shunt capacitance.
For electronic devices, two-tier protectors using gas discharge tubes (GDT) followed by metal oxide varistors (MOV) provide transient control. For steep front transients, the let-through voltage of the GDT can be as high as 1 kV before the tube fires and reduces the follow-through arc voltage to about 20 V.
Fortunately, the solid-state technology which has increased the vulnerability of electronic equipment has also offered new protection techniques. The metal oxide varistor with its almost ideal non-ohmic current–voltage characteristic:
I = AVn (3.74)
where n is 25 to 30 has found widespread application after its development in the 1970s. Its high capacitance and limited speed remain a limitation for high band-widths and bit rates, and ageing and condition monitoring are also problems. Recent developments in thyristor technology offer crowbar protection with a fast response as an alternative to the GDT for robust primary protection. These devices can now be manufactured with a drain current of less than 10μA, a response time below 50 ns and a peak current of 750 A with good resealing.
The efficiency of fortress screening, combined with conventional hybrid GDT/diode and GDT/MOV protection of electronic components even against full triggered lightning currents of up to 52 kA was proven in a test programme of triggered lightning in Japan at a 930 m altitude site [127].
3.5.7 Strikes to aircraft and space vehicles
Lightning strikes to airborne structures are essentially triggered events, and are thus common, especially since altitude effects are also present. On average, civilian airliners receive one stroke per annum [128], 90 per cent of which arise from positive
leaders triggered from the aircraft. The standards for the protection of aircraft against lightning strikes define zone 1 as the initial lightning attachment region. This zone is usually taken as being restricted to aircraft extremities where electric field enhance-ment will tend to favour attachenhance-ments to such sites, which are defined as the extremity plus a region 0.5 m aft or inboard of it. Severe lightning strikes with currents and action integrals greater than l00 kA and 0.25× 106A2s, respectively, should be avoided outside zone 1 regions. Reported flight experience, however, indicates that occasionally very severe strikes do occur outside this zone. A known hazard to aircraft arises from the nose to tail sweeping of the lightning attachment points due to the aircraft motion. This has led to new schemes for determining the initial attachment zone such as the rolling sphere method and the swept leader method. The definition of aircraft attachment zones has been improved by the work of the FULMAN pro-gram [129]. Methods for determining the initial attachment zone are electrical field analysis or test methods such as arc attachment to scale models.
Aircraft flight safety is sometimes discussed in terms of the probability of a catas-trophic incident per flying hour. As an example, to illustrate aircraft zoning, it is essential [130] to assume a vanishingly small probability of a hazardous lightning strike, for example less than one in 109flying hours, assuming:
• one strike every 1000 flying hours
• that the most severely damaging lightning strikes are associated with cloud to ground strikes and there is only one strike to ground involved in every ten aircraft strikes
• only one in ten of ground strikes has severe parameters exceeding the protection levels appropriate to a swept stroke zone (zone 2).
These assumptions give a lightning strike to aircraft exceeding zone 2 protection requirements every 105flying hours. Hence, in order to achieve the required prob-ability of a potentially hazardous strike to a zone 2 region, zone 1 has to contain 99.99 per cent of all cloud to ground strikes.
To model the electric field of a real aircraft, the boundary element method derives a solution to the Laplace equation over the surface. The method has the advantage that only the surface of the aircraft has to be meshed. The far-field boundary must also be included as a second two-dimensional surface, but this outer mesh can be remote and simple.
Radomes to protect aircraft radar systems from the elements do not prevent occasional but costly damage from lightning strikes, even after being tested to industry-agreed standards [130]. Radomes usually form the nose cones of aircraft, a zone 1 location which makes them susceptible to damaging strikes. To conduct these lightning currents and prevent damage to the insulating surface of the radome, while maintaining effective transparency to radar signals, thin metal diverter strips consisting of a large number of gaps in series may be fitted.
The new lightweight composite materials which are now being used for radomes, and the introduction of a new generation of airborne weather radar with forward-looking wind shear detection, require increased radar transparency from the radome.
Since 1993 all aircraft carrying more than 30 passengers are required to have
windshear detection capabilities. Other non-conductive aircraft parts include a new generation of satellite communication and antenna fairings installed on the exterior of aircraft.
Strikes that prejudice air worthiness are fortunately rare, but a Boeing 707 was lost to a lightning strike in the USA in 1963. A survey of USAAF aircraft loss and damage between 1977 and 1981 revealed a financial cost of M$10. Boulay [52]
has described inflight data obtained from strikes, both triggered and intercepted, to suitably instrumented aircraft. These strikes confirmed that at an altitude above 3 km these usually involved cloud discharges. This suggests that both low altitude military sorties and helicopter flights are most at risk from high current cloud-to-ground lightning. Uhlig et al. [131] express the need for an agreed test waveform for aeronautical equipment, since the mechanism of the flow of return stroke current in mid channel is unclear.
Lightning research directly associated with the NASA space shuttle activity, both at the launch pad and in space using the optical transient detector (section 3.2.2), followed serious lightning incidents in Apollo 12 and the total destruction of an Atlas-Centaur rocket in 1987. Thayer and colleagues [132] analysed the considerable distortion of the ambient electric field by a vertical space vehicle at launch, and the consequent risk of triggered lightning. An interesting example of the use of the generic Rizk model to the design of lightning protection for a satellite launch pad is given by Joseph and Kumar [133].
3.6 Note
1 Figure from British Standards reproduced with the permission of BSI under licence number 2003SK/0157. British Standards can be obtained from BSI Customer Ser-vices, 389 Chiswick High Road, London W4 4AL (Tel +44(0)20 8996 9001).
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