When a cloud filled with negative charges of electrical energy passes over earth it repels the negative charges below and attracts the positive charges. These positive charges still bound to the atoms in the earth move on the ground with the cloud. Along with the air between the cloud and the earth the configuration is now that of a large capacitor. The material between the two sides of a capacitor is an insulator – in our case here, it is air. But there is no perfect insulator. Every insulator has a resistance value that can be measured under steady DC flow conditions. It is the ratio V/C, where V is the voltage applied and C is the leakage current or current observed after an elapse of time. The higher the insulation resistance the longer a capacitor can store the charges. When the voltage across our capacitor becomes larger and the insulation resistance of the air decreases, such as when a reduction occurs in the gap, the capacitor breaks down and a large current flows – the lightning strike. This is the cloud-to-earth lightning. The gap reduction occurs when the cloud passes over the high point of a hill, building or a tree. Cloud-to-cloud lightning, which is more common, occurs when the voltage difference between the top and bottom of the cloud exceeds the breakdown voltage of the capacitor formed by those two parts of the cloud. The question remains as to how a cloud acquires the negative charges? A cloud is a mixture of water droplets and aggregates of ice crystals. Now, in a cloud, there are updrafts of warm air and downdrafts of cold air. This movement of air past water droplets and ice crystals produces electric charges – the relative movement of molecules generates charges as in the case of a plastic comb acquiring electrons from the dry hair that is combed. Elsewhere on the web one can learn how the movement of air produces the charges and also why the bottom of the cloud becomes negatively charged while the top of the cloud becomes positively charged. It is an amazing phenomenon that in addition to the charge polarization that places the negative charges on the bottom of the cloud, a separate smaller aggregation of positive charges is produced elsewhere on the bottom of the cloud. This separate smaller aggregation arcs over to the negative portion which then arcs over to the top of the cloud or the ground.
A lightning protection system (LPS) is a metal rod, on the highest point of a structure, connected to earth with a high capacity conductor – called down conductor – with low impedance. When a charged thundercloud comes near enough to the rod, the induced positive charges moving on the ground with the cloud go up as an ‘early streamer’ from the lightning rod. At around 30 meters above the rod this streamer meets the streamer from the cloud – think how paper bits rise toward a charged comb – and neutralize. But there is more negative charge in the cloud and they continue to flow through the rod and the attached down conductor to meet the positive charges remaining on the ground below – this flow is known as a return stroke. The charges flow through the down conductor since it has the least impedance among competing paths. The key phrase is ‘low impedance.’ Achieving this requires technical expertise, which is why LPS must be installed by those who specialize in it and have experience and equipment.
There are three types. They are Franklin rods, ESE (Early Streamer Emission) air terminals, and CTS (Charge Transfer) air terminals.
In the presence of a charged cloud above, all three systems produce a stream of charges upward. This stream of charges is called a corona emission current. In the Franklin rod and the ESE, the emission current is immediate upon the presence of a charged cloud above. ESE is said to produce a stronger emission current because it is aided by the charge generated by the electrical system at the tip of the ESE. In both Franklin rod and ESE, the corona emission current has a higher chance of meeting the charge stream (called ‘leader’) from the cloud and thus produce a path through the air terminal to the ground; this prevents the leader from attaching elsewhere. CTS works differently. It has a crown comprising a cluster of sharp needles which continually dissipate charges. So there is a delay in the formation of the upward discharge. This causes a delay in attaching to a leader coming down from the clouds and the leaders fail to attach to the CTS, thus preventing the lightning. This delay produced is generally accepted as an index of the protective effectiveness of a CTS. But the leader coming down from the cloud may attach elsewhere. REFERENCES
An ESE for even a small building is five to ten times more costly than a Franklin rod. Also, an ESE needs a battery in the electrical system at the tip of the terminal. This battery needs to be changed periodically. This is not always practical for small installations as the tip of the ESE terminal (which is several meters high) needs to be accessed without ladder to change the battery. Where the tip can be accessed without ladder and where a large area needs to be protected, ESE may be suitable and the higher cost can be justified. An example is the Burj Khalifa building in Dubai. To protect a large area, a CTS at the highest point and Franklin rods at additional locations in the area is suitable. The reason is that CTS is said to prevent a lightning strike by delaying the attachment of the corona emission current from the the CTS to a downward leader from the cloud above. The downward leader from the cloud would attach elsewhere in the area; Franklin rods would receive those strikes. We conclude that Franklin rod is an effective and a least expensive solution when only a few of them suffice to protect a structure. For a larger area, an ESE placed at the highest point, with access to change the battery, would be a solution where otherwise several Franklin rods would be required. For large area, CTS also appears to work but additional Franklin rods would be required. CTS is cheaper than ESE but costlier than Franklin rods. Roughly their price is midway of Franklin rods and ESE. We conclude that for small buildings or small areas or where there is one high point in relation to the rest of the area, one or a few Franklin rods suffice. This is the practice in the USA and China. In China, large numbers of Franklin rods cover even large areas to be protected.
LPS neutralizes lightning strikes. However, an improperly installed LPS can cause more lightning damage than not having a LPS. So in what ways can a LPS installation be improper?
If you are outside the house and you see lightning, count the time until you hear its thunder. If the time before thunder is under 30 seconds, get inside the house immediately. And to go out again, wait for 30 minutes after you heard the last thunder.
Yes! ‘Overcurrents’ which are current surges that flow in to the house from lightning on the power lines or on the house or near the house can cause damage to the equipments. So you need a surge protection device (SPD) at the electrical panel where the current enters the building. The SPD responds to surges by lowering its internal impedance so as to divert surge current to limit the voltage to its protective level-the measured limiting voltage. After the occurrence of surges, the SPD recovers to a high-impedance-state line-to-ground and extinguishes current to ground through the device when line voltage returns to normal.
Connect the SPD earthing to the housewiring earthing, not to the LPS earthing.
The saying goes, “When Thunder Roars, Go Indoors!” That is, you are safer inside even without an LPS installed. There are two reasons for this – the so-called Skin Effect and that the building is the taller object when you are inside unlike the case when you are exposed outside away from the house; outside but near the house is also dangerous since the house is again the taller object and a hit on the house may do a side-flash on objects near it outside.
Now, if a LPS is installed on the house, are you safe inside the house? Lightning protection is more than just the connected triple – air-termination system, down conductor and earth termination system. A lightning strike is a dc current that builds up to its maximum in about 8 microseconds. This fast-varing current has a fast-varying magnetic field associated with it. This varying magnetic field induces an electric field on any conductor near the down conductor that is struck. The largest conductor configuration inside the building is the wiring. So the wiring earthing should be connected with the LPS grounding; the wiring and the bottom of the LPS system will then be at the same potential and hence safe. There are also the power surges that enter the building wiring from power supply outside. To supress these surges, surge suppressors must be installed at the entrance of power to the building. And the suppressor must be earthed with shorted bonding connection to the lightning ground rod.
An MCB is not needed if there is a way to ensure an SPD cartridge will not age or stop working otherwise. Ageing occurs not only due to passage of time but also the number of surge events. The SPD cartridge begins to deteriorate with age – as shown by change in color in its indicator window. As the color begins to change, the cartridge needs to be replaced. What happens if the cartridge is not replaced or there is a catastrophic surge event? Although such an event is rare, it is possible. Then the SPD would protect during the event but get sacrificed in the process. The residual current continuing through the SPD could then cause it to catch fire. This is prevented by a Circuit Breaker connected in series with the SPD so that the circuit breaker breaks the circuit by tripping. A question then is if a standard fuse would be sufficient in place of the MCB. A fuse would not suffice since each wire needs to have its own fuse and the fuses are not coordinated or ‘ganged’. So one may trip and another may not; that would still cause current to flow through the faulty SPD. The conclusion is that an MCB that matches the SPD be connected in series with it so that the SPD is between the MCB and earthing.
Some trees suffer severe damage from lightning. Two trees can be hit by lightning and only one suffer severe damage. The reason is conductivity. The tree that has more water content will sustain less damage because the more the water content, the more the conductivity (or less resistance). This is why electric stoves work. When large current passes through, high resistance wires heat up.