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Charge Build Up
The scientific community is unsure of exactly what causes these charges to rise, however, following is some of the current thinking associated with the very large static charge that gives rise to lightning.
If the surface of the planet is considered as one plate of a capacitor; the atmosphere is an insulator, the upper air can be treated as the second plate with the clouds and moisture particles being the electrolyte.
A charged field between these plates can build in two ways. The first is through the continual air movement, effectively brushing over the ground; the second is through the movement of ice particles through a cloud. Both involve movement and subsequent particle on particle friction. [A larger version of cloth on cloth friction that is used to generate a demonstrable static charge.]
Continual dry air movement over the ground gives rise to a charge forming over a large area. The occasional equalisation or discharge from an invisible cloud or charge centre is the cause of the occasional summer wild fire. ‘A bolt from the blue.'
As discussed, how thunderclouds become charged is not fully understood, but most thunderclouds are negatively charged at the base and positively charged at the top. The various hypotheses that explain how the polarization occurs may be divided into two categories: those that require ice and those that do not. Most meteorologists believe, however, that ice is a necessary factor, because lightning is not usually observed until ice has formed in the upper layers of thunderclouds. The falling dry ice particles cause friction that creates a charge.
Experiments have shown that when dilute solutions of water are frozen the ice gains a negative charge but the water retains a positive charge. If, after freezing has started, rising air tears small droplets of water away from the frozen particles, the droplets are concentrated in the upper part of the cloud and the larger ice particles fall towards the base, moving the charge to the bottom of the cloud. Equally, experiments have also shown that large, swiftly falling drops of water also become negatively charged, thereby moving further negative charge to the ground.
The polarization of a thundercloud may thus be due to the rates at which large and small raindrops fall. However formed, the negative charge at the base of the cloud induces a positive charge on the earth beneath it as the second plate of a huge capacitor. When the electrical potential between two clouds or between a cloud and the earth reaches a sufficiently high value (average about 10,000 V per cm), the air ionises along a narrow path and a lightning flash results.
One theory suggests that the electrical polarization in a thundercloud may cause precipitation rather than be a consequence of it, and postulates that the electrical potential existing between the ionosphere-the highest layer of the atmosphere-and the Earth initiates the polarization in a thundercloud. According to this theory, the upward flow of warm air through a thundercloud carries with it positively charged particles. These accumulate at the top of the cloud and attract negative charges from the ionosphere. The negative charges are carried to the base of the cloud by powerful downdrafts at the periphery of the cloud, thus preventing oppositely charged particles from neutralizing each other. Perhaps 90 per cent of all strokes from cloud to ground are negative; the remainder are positive flashes. Rarely, strokes move from ground to cloud. However the equalisation occurs between any two points, particularly from mountain peaks and from tall objects such as radio towers; it is because these points represent the shortest path between the two charged plates.
Studies with high-speed cameras have shown that most lightning flashes are multiple events, consisting of as many as 42 main "strokes" of lightning, each of which is preceded by a "leader" stroke. These leaders are areas where charged ions are collecting in the atmosphere. All strokes follow an initial ionized path, which may be branched, along with the current flows. The average interval between successive lightning strokes is 0.02 sec and the average flash lasts 0.25 sec.
At the time the charged plates equalise, there is a rapid movement of ions between the plates. At this instant, the conductive paths are saturated; they can conduct no more energy. The energy is therefore passed through any available path aside from that which is most direct. To find this path, the pressure exerted on the conductors will rise until release is found. This difference of potential is divided across the various conductive paths in an instant, causing insulation stress levels to be exceeded and subsequent failure of devices which may be connected somewhere in related circuits.
The high speed of discharge, charge build up and discharge is such that the whole cycle is a high frequency AC cycle. As a consequence, the capacitive and inductive effects of a discharge earth circuit have a large influence on the eventual path of the current. Because of the high density of instantaneous current, and the broad extent of the charge source, earth circuits must be more extensive for lightning than for other electrical earths. Additionally, the 0.14mH per meter of discharge circuit means that although the earth circuit must be extensive, it becomes ineffective at around 90m in length as the discharge impedance rises to high. At this instant in time, over this distance the peak current has been injected before the leading edge has travelled the length of the conductor; this brings alternative paths into use as the current must go somewhere and the primary circuit becomes saturated.
Through a failure to bond all earths, the alternative discharge circuit route is an extended length. The total voltage is expressed across the insulated circuit elements; when break over potential is exceeded there is a subsequent system component rupture.
Design of an earth circuit that will allow lightning to discharge in an area that does not cause apparatus damage is the first level of protection. Up to certain limits, a tower or mast will provide a path to earth selected by the owner, effectively creating a shadow for equipment under the tower or mast.
Surge protection in circuits that may provide an alternative earth route will mitigate the possibility of damage. Surge protection is the common name given to ‘transient protection'. The use of the term ‘Surge Protection' has come about through the most common application of devices to mitigate lightning surge. A transient is correctly described as an over voltage condition between any two conductors. These transients can vary from just a few volts to many thousands; they can be of a magnitude that is no problem to an installation to one that causes a failure of insulation within a device.
Protection is afforded by a device that can detect a transient and prevent it from affecting connected electrical / electronic equipment.
Operation of Surge Protection
Every electrical device is designed to operate at a specific voltage. At this design voltage it will pass a predetermined amount of current through the circuit and perform the function or work for which it is correctly designed. This voltage impressed on a device is unable to influence other elements of the device, as there is insulation between the correct circuit element and those in which the current is prevented from flowing.
Any breakdown in inter-circuit insulation causes the device to malfunction.
Australian standards define the level of working voltage a device may be subject to before malfunction. This level is twice design operating voltage. Therefore, a 12-volt device may not withstand transients over 24volts; a transient of more than 830 volts may damage a 415-volt rated device. It is apparent that each device must be protected against voltages that may affect that specific device.
A Surge Protection device is engineered to breakdown or ‘Let-Thru' to earth at a specific voltage. By selecting this let-thru' voltage to be above the operating level of the device and below the insulation failure level, we can connect a device input power to earth under transient conditions only. We therefore prevent a malfunction of the device due to surge, lightning or other.
Transients can arise through;
The transient condition can be caused by direct connection or through inductive, resistive or capacitive coupling. For instance a lightning strike to electrical equipment is direct, however a strike to earth through pipe-work is a resistive coupling; as is a discharge between clouds giving rise to an inductive coupling. With any ‘coupling', it is the effect of the movement of electrons that causes a build up in potential difference within the susceptible equipment. If the build up in potential difference is enough to exceed the rated insulation value of a device, then a malfunction will occur.
The final failure in insulation can be minimized through installation of a system designed to prevent a build up in charge between the insulated conductive paths. This can be achieved through a combination of measures; each of which is essential to ensure an effective transient protection system. These measures are;
1. Identify the anticipated causes of potential failure,
2. Select a surge diverter that provides a balance of risk to cost benefit,
3. Build an effective protection shield for lightning impulse conditions,
4. Build an effective earth and surge protection system.
For an installation where there are a combination of earthing systems and high incidence of lightning, there are many difficulties designing and installing effective protection systems. If there is a selective failure of one device and not another, we must ask the question, Why did this particular device fail? We will often find the problem is not related to the installation or absence of the surge diverter but could be related to the earth configuration of the total system.