Protecting Remote Monitoring Sites from Lightning Damage
Protecting monitoring sites from lightning damage is an important consideration for monitoring equipment and stations that are deployed in areas that are prone to lightning strikes. Even lightning that does not directly strike a site may cause damage by traveling through the ground and into buried sensors or cable. Proper planning for such events can help reduce the risk of lost data or expensive equipment replacement and site downtime.
Basic Introduction to Lightning
Lightning is a discharge of atmospheric electricity which can take place within one cloud, between two or more clouds, or from a cloud to the ground. There are many potential causes of lightning, but one of the most frequent is the interaction between ice particles that can be found within clouds. As these particles collide (air and water molecules) they break apart to form negatively and positively charged ions.
In general, charged particles such as the ones developing in thunderstorm clouds will want to lose their positive charge and become neutrally charged. Lightning potential is the desire of the charged particles to become neutral and a “strike” occurs when a massive surge of negatively charged particles find a path of least resistance to the positively charged particles to satisfy the desire to have charge neutrality. In other words, a lightning strike is a large voltage surge propagating along the path of least resistance in order to achieve terrestrial/atmospheric charge equilibrium.
Protecting Sites and Sensors against Lightning Strikes
While no system can ever be made resistant to all lightning strikes, taking care to install proper lightning protection equipment can provide a much greater level of protection than would be provided if the equipment was installed without protection, increasing the chance of equipment survival during a storm and minimizing the amount of data that can be lost to equipment that is destroyed. The general strategy for lightning protection is to redirect the path of least resistance of the surge around electronics to ground. For example, if a series of sensors and a data logger is the path of least resistance to ground and a lightning strike occurs, the equipment will be subjected to a harsh electric surge. Below are some tips on lightning protection solutions.
1. Determine if equipment is being installed in a lightning-prone area.
Not all areas are as prone to lightning strikes as others. Many government agencies and companies collect data about global lightning strikes and compile charts that can be used to determine a level of risk for a given area.
Many sensors, such as the Stevens Hydra Probe, have built-in lighting protection. In the case of the Hydra Probe, it has been tested by an independent laboratory to withstand repeated exposure to 2,000 volt strikes. Such built-in protection is considered quite adequate for areas with a low number of lightning strikes. For use in areas with a high risk of lightning strike, additional protection against surges may be necessary.
Figure 1: Average number of lightning strikes per square kilometer per year. Image courtesy NASA.
Using the map as a starting point, it can be determined whether the installation site will be in a location that is more likely to be at risk for lightning strikes.
More detailed maps and information, such as the graphic below for the United States, are also available online and can provide more detailed data regarding lightning strike potential.
Figure 2: Average density of lightning strikes in the United States, showing a heavy concentration in the Southeastern area of the US. Image courtesy Vaisala.
2. Install surge suppression equipment to reduce risk of damage from lightning strikes
When equipment is being installed in an area prone to lightning strikes additional precautions need to be taken in order to prevent damage to equipment that and disruptions in data collection.
The two most common ways of protecting against lightning strikes are:
Lightning dissipation and diversion systems are used to provide a lightning strike the most direct route to ground, in this case, away from sensitive instruments. As lightning passes through the air, it is naturally seeking the path of least resistance to ground. Unfortunately for un-protected systems, this path is often through a communication antenna or other wires that connect to a ground plane. The sudden spike in voltage can easily damage sensors, data loggers, or other system components.
By installing a lightning dissipation system the chances of lightning striking system components and causing damage can be reduced by allowing the increased voltage to flow into the ground and dissipate more harmlessly.
Figure 3: Example of a lightning dissipation system used to protect a common soil sensor installation.
Figure 3 shows an example of a simple lightning dissipation system. The “Lightning Dual Dissipater” atop the installation acts as a draw for potential direct lightning strikes. The current is then shunted down the copper cable and then directed into the ground by a buried copper grounding rod, located some distance away from other buried sensors.
Another useful item for preventing lightning damage is the gas discharge tube. These are small devices that are either built into a product, or more commonly connected in-line on communication or data lines.
A gas discharge tube has a special gas that is held in a chamber between two electrodes. When a current of sufficiently high voltage is passed through, this causes the gas to ionize and conduct the voltage. Gas discharge tubes will allow lower voltage current to continue to flow to the system, while helping to block damaging high voltage spikes by diverting this excess energy to the ground line and away from sensitive system components.
Gas discharge tubes are commonly sold as self-contained units that can be connected to sensitive areas of a system, such as between an antenna and communication cable. A lightning strike on the antenna that would normally pass excessive voltage down the cable and into the system can be regulated by installing a gas discharge tube onto the cable.
Gas discharge tubes have a finite life, and will eventually be rendered ineffective over the course of several surges. They should be replaced periodically during routine site maintenance to ensure that they will work properly during the next lightning storm.
Placement of sensors and equipment in relation to builds, trees, antennas, and other nearby objects can also influence the likelihood of a destructive lightning strike.
Using these techniques, it's possible to mitigate the potential of lightning strikes to harm an installation. While nothing can guarantee 100% success at stopping lightning damage, following basic precautions and best practices can help minimize site downtime, lost data, and hardware replacements costs.
Contact Stevens today at (503) 445-8000 or firstname.lastname@example.org to discuss how to properly protect new or existing installations from the lightning strikes.