John Chapman, Product Manager for ADB Airfield Solution Control and Power Solutions, explains how to protect your lighting system against the forces of nature.
Satellite data tells us that there are around 3 million lightning flashes per day throughout the world, or about 30 flashes per second on average. Most of those discharges are cloud-to-cloud, but about 30% end up as the cloud-to-ground discharge that we are most familiar with.
There are a number of websites that describe the atmospheric electrical discharge that we know as lightning, but the average bolt carries 30 to 40 kilo amperes (kA) of current, at millions of volts.
Current can exceed 120 kA, and reach temperatures of 20,000˚C (36,000˚F), with a stroke lasting around 30 microseconds.
In airfield applications, we are also very concerned with how far that current will travel in soil once a lightning bolt contacts the earth.
The current will begin to dissipate as it travels from the contact point at a rate that varies depending on the soil conditions, but current can still radiate out around one mile (1.4km) from the strike.
Some parts of the world are more prone to lightning than others. Figure 1 shows a map provided by the NASA MSFC Lightning Image Sensor (LIS) Science Team from its satellite data for North and Central America. It shows that for North America, Florida has by far the highest density of lightning strikes, along with various Caribbean islands.
A similar map for worldwide lightning density is shown in Figure 2, utilising data collected by NASA satellites from April 1995 through to February 2003. This map includes cloud-to-cloud activity, but still gives an impressive representation of lightning activity. It indicates that Central African countries generate the highest density of lightning strikes anywhere in the world.
Airfield Lightning Protection Specifications
Most FAA Advisory Circulars dealing with airfield lighting equipment specify that the equipment must be protected against a lightning surge of anywhere between 3,000 and 20,000 Amps for a duration of 8/20 microseconds.
This rating would indicate a 3-20 kA pulse rising to 90% of its peak amplitude in 8 microseconds, and decaying to 50% of that value within 20 microseconds.
FAA Engineering Brief 67 further states in Paragraph 2.11 that an:
“LED fixture shall be designed to withstand and/or include separate surge protection devices which have been tested against defined waveforms detailed in Table 4, Location Category C1 of ANSI/IEEE C62.41-1991 Recommended Practice on Surge Voltages in Low Power AC Power
Circuits, namely 3,000 Amps, 8/20 microsecond – short circuit pulse and 6,000 Volt, 1.2/50 microsecond – open circuit voltage pulse.”
Category C1 is defined as Low Exposure–Systems in geographical areas known for low lightning activity, with little load or capacitor switching activity. Because LED fixtures are in an exposed area on the airfield, and may be in a high lightning area, we believe that Category C2 is a more appropriate standard. Category C2 is defined as Medium Exposure–Systems in geographical areas known for medium to high lightning activity or with significant switching transients. Category C2 is 5,000 Amps, 8/20 microsecond – short circuit current pulse and 10,000 Volt, 1.2/50 microsecond – open circuit voltage pulse. We have designed and internally tested our LED fixtures to this higher standard.
The latest version of IEEE C62.412002 no longer breaks down Category C into three groups (C1, C2 and C3), but defines Category C as either high exposure risk (high probability of strikes) and low exposure risk. These categories would define that devices be protected for a surge of:
|Level||1.2/50 pulse||8/20 pulse|
Note that these surge specifications are not designed to sustain a direct lightning hit, but are designed to protect against surges through the earth and other conductors, including arcing from other circuits.
Counterpoise Lightning Protection System
FAA AC 150/5340-30 Design and Installation Details for Airport Visual Aids section 12.5 describes the use of a counterpoise or lightning protection system to provide a path of low resistance for the energy of a lightning strike to safely dissipate without causing damage to airfield equipment or injury to personnel. Note that the counterpoise is a separate system, and is not to be confused with an equipment safety ground that provides protection from electric shock hazards. The purpose of a safety ground is to protect personnel from possible contact with an energized light base or mounting stake that may result from a shorted power cable or a primary to secondary short in the isolation transformer.
The counterpoise conductor is a bare #6 AWG solid copper wire connected to ground rods spaced a maximum of 500 feet (152m) apart. When the cable or conduit run is adjacent to pavement, such as along runway or taxiway edges, the counterpoise is installed eight inches (20.3 cm) below grade and located half the distance from the edge of the payment to the cable or conduit run. Note that the counterpoise is not connected to the base or mounting stake of any elevated fixture.
Figure 4 is the section of Figure 116 in AC 150/5340-30 dealing with the counterpoise installations associated with in-pavement lighting. When cables or conduit runs are not adjacent to pavements, the counterpoise is installed 4 inches (10.2 cm) minimum above cable or conduit.
Counterpoise connections are made to the exterior ground lug on in-pavement light fixtures. When non-metallic light bases are used, the counterpoise is not connected to the base and must be routed around it.
The intent of the counterpoise system is to intercept lightning strikes and dissipate lightning current in the ground without arcing to the airfield lighting system.
There is continued debate within the industry on grounding of the counterpoise to the light fixtures, but data to support arguments either way is lacking at this time. See the Advisory Circular for specific recommendations on implementing a counterpoise lightning protection system.
Protection from Lightning
There are a number of electrical components that can be used to help provide electrical surge or lightning protection, such as avalanche diodes or gas discharge tubes, but the primary protection device in use is the
Metal Oxide Varistor (MOV). The MOV represents the best compromise of response time to multiple surges, if applied properly. The MOV contains zinc oxide particles placed between two metal plates, with one plate usually attached to earth ground. The varistor has a sharp breakdown characteristic that enables it to provide surge suppression. When presented with a voltage transient, the impedance of the MOV changes from a near open circuit to a highly conductive level, clamping the transient voltage to a safe level. The destructive force of the transient pulse is absorbed by the varistor, protecting the circuit from further damage. MOVs are rated by the voltage at which they become conductive (clamping voltage) and the amount of current they are able to handle.
Use of MOVs in Airfield Lighting
MOVs are the surge/lightning protection device of choice for airfield lighting applications. They guard incoming power to vault and field equipment, control wiring, series circuit outputs, and the electronics of any lighting equipment powered by a series circuit.
The key to any lightning protection of airfield equipment lies with proper earth grounding. Good grounding practices include total connectivity to assure a common potential for all equipment. It is critical that the manufacturer grounding recommendations be implemented, both to help dissipate lightning surges and to allow the protection devices a proper grounding path to operate against.
MOVs have a large, but limited, capacity to absorb energy, and are subject to failure. The most common failure modes include: electrical puncture, thermal cracking, and thermal runaway, all usually the result from non-uniform heating.
Initial failure modes for an MOV include: short circuit, open circuit, or high resistance. A short circuit failure is caused by a large fault current fusing the zinc oxide material, usually resulting in a visible puncture outside the device.
The MOV can also fail as an open circuit between the two plates. The good news with this failure is that it does not affect the operation of the electrical circuit. The bad news is that due to the nature of the failure, there is no indication that the MOV is no longer able to provide any surge protection. This is the most common failure mode, but the most difficult to detect.
The varistor can also begin to fail acting more as a resistor and the device will then begin to overheat, causing it to fail. Larger MOVs can actually become a fire hazard at this point.
Degradation of MOVs
MOVs experience degradation due to one or more surge impulses outside of their rated specifications: voltage, current, and length of the surge. This produces excess heat stress on the device, affecting the zinc oxide composition. This stress may not be enough to cause the MOV to fail, but will result in degraded operation with the clamping voltage rising higher and higher until it will no longer conduct current in an overload.
Identifying MOV Failure
An MOV that has failed as a short circuit can usually be identified easily by placing an ohmmeter across the device, reading a very low resistance. As this will normally place a direct short to ground, the failure will be associated with unexplained fuse failures on the device.
An open MOV failure can usually be tested only with specialised equipment designed to verify the clamping voltage at which the zinc oxide material becomes conductive. For this reason, we recommend that larger MOVs, such as those found in CCRs, be replaced on a routine schedule, or after a nearby direct lightning strike.
A visual inspection of all MOVs should be made on an annual basis, looking for blackened material, punctures, or signs of excess heat generated.
Series Circuit Field Lightning Arrestor
To help reduce the susceptibility of airfield series circuits to lightning surges ADB has developed the Field Lightning Arrestor, which is designed to be inserted into the airfield series circuit at 2,000 ft (600m) intervals. When properly grounded, the Field Lightning Arrestor will provide protection of 25,000 Amps (8/20 µs waveform). It is designed to operate on both 6.6 and 20 Amp circuits with any size regulator.
This article is designed to give a basic understanding of lightning protection of airfield lighting equipment. I have listed some additional references below that may provide more information.
1: National Weather Service Lightning Safety
web site (www.lightningsafety.noaa.gov)
2: Global Hydrology and Climate Centre
3: National Lightning Safety Institute
4: FAA STD-019e Lightning and Surge Protection,
Grounding, Bonding and Shielding
5: IEEE C62.41 – IEEE Practice on Surge Voltages
in Low-Voltage AC Power Circuits
6: FAA AC 150/5340-30 Design and Installation
Details For Airport Visual Aids. (www.faa.gov)
7: FAA Engineering Brief 67 – Light Sources Other
Than Incandescent and Xenon (www.faa.gov)
8: Rakov and Uman, Engineering Analysis of
Airfield Lighting System Lightning Protection
(Department of Electrical and Computer
Engineering, University of Florida)