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.
You might love reading a beautiful essay on the phenomenon of lightning.
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.
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?
- Not having a down conductor connected to the rod. This is the predicament of many houses that have old TV antennas.
- Not using a down conductor that does not have enough current carrying capacity. (Buildings that are taller than 75 feet are classed as Class II and require down conductors of larger cross sectional area.)
- Not bonding the building and its various metallic protrusions to the down conductor. Metallic roof structures like antennas should be bonded to the down conductor.
- Not connecting the earthing of the building’s wiring with the earthing of the down conductor to equalize the potential of the two conductors. This connection must be done properly and not by simply interconnecting the two conductors.
- Improper lay of down conductor, including bends with radius less than 4 feet and a portion of the down conductor going up while it is coming down to connect with the earthing rod.
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. This bonding is an equipotential bonding and must be done using an equipotential connector.
Damage to equipment can arise from two reasons. (1) Transient surges from the power line and (2) induced voltage from a lightning strike on or near the building. Unplugging prevents (1). To prevent (2), the equipment casing must be connected to the house wiring ground and the wiring ground bonded to the lightning ground.
CAUTION: A building with LPS has greater need for surge protection, for when the LPS catches lightning, charges flow mainly through the LPS down conductor and partly through the wiring. This happens because although the charge from lightning is supposed to cause DC and flows to earthing through the LPS down conductor, the rapidly varying quantity of charges presents the DC as AC and electric potential is induced in the wiring and surge of charges flow in it. To quickly transfer this charge to the wiring ground, an LPS is needed.
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.