The precursor to a lightning event is called a “step leader.” It consists of a low current, (200 amp) barely visible filament, extending from the charge centre of the cloud towards its mirror image potential (cloud to cloud or cloud to earth). We will discuss a cloud to the earth path.
The stepped leader will traverse a statistically averaged 50 meters in 1 microsecond, then pause for 49 microseconds until the next jump. There can be branching out from the original step leader to many 50 meter jumps. Since the step leader is a conductive ionized path, the furthest out end(s) of the step leader is at the same potential as the cloud charge centre. As the branches reach out toward earth, a strong voltage difference (gradient) is established between the step leader’s end and any object on earth (representing the charge centre’s opposite potential). As this gradient becomes more intense, the air above any structure on earth begins to ionize, or – breakdown. If the structure is pointed (like a lightning rod) it breaks down air sooner do to the point discharge effect (corona). As the breakdown continues, the structure sends up an “upward going streamer” (an opposite polarity ionized path) towards the approaching step leader. When they meet, a conductive path from cloud to earth is established for the main series of strokes to follow. The step leader timing above is from observation of effect. Different charge accumulations within the cloud will change step length and timing somewhat. This effect is also called a “stepped leader”.
The sequence then is: The energy from the cloud charge centre is dumped into the established conductive channel until electron migration within the cloud cannot keep up with discharge. Then all the existing step leader branches that have not been “completed” discharge back through their respective paths towards the completed path to earth. This is seen as a large earth stroke and many smaller branches discharging back to the main stroke. The smaller branches seem to be reaching for earth but have already been suspended incomplete for some time. The energy of their back discharge illumines near the main stroke, and the illumination continues outward until the entire branch is discharged. There can be several additional strokes as the electrons in the cloud centre migrate towards the discharge path. The first stoke is usually twice (or more) the current of the following additional strokes. This is because the first stroke is also discharging the incomplete step leaders (and other unknown factors).
There are several theories regarding multiple strikes. Two are presented below:
– The inability of the cloud to continuously discharge at a high current rate due to limited electron mobility inside the cloud. Electron flow towards the charge centre cannot keep up with the demand for the necessary potential to continue the strike.
– Since any conductor generates a magnetic field around it, the lightning conductive path is no exception. As the current rise occurs within the channel, an increasingly intense magnetic field is forming around the conductive gaseous plasma. This increasingly intense magnetic field could begin to constrict the cross-section of the plasma. If the plasma is constricted, its inductance is increased. As the inductance is increased, the ability to sustain high current flow is reduced until the cloud charge centre can no longer provide the potential to overcome the inductance. The initial stroke is over. The standing magnetic field collapses and dumps its energy back into the path. Once the standing magnetic field has collapsed, the channel is ready for another burst of current. This “cycle” continues until the cloud is discharged to a potential where it can no longer generate the potential required.
Lightning is a “constant current” source. It will develop whatever voltage is necessary (within cloud charge centre limitations) to overcome the resistance of the struck object. Series path inductance or cloud electron migration will sooner or later stop it.
Even though the strike is dc, the typical current waveshape of the strike includes a very fast rise time to peak current flow. It is this characteristic that creates havoc with electronics.
To prevent damage to the building structure and roof-mounted equipment, an air termination network needs to be provided to capture the lightning strike at preferred points.
Down conductors, capable of carrying the lightning current, should be installed to conduct the lightning strike to the ground without the possibility of flashover to extraneous metalwork and electrical systems.
Provision of low impedance earthing is of major importance to the efficiency of the lightning protection system. It is essential that low impedance earth is provided to facilitate the dissipation of the lightning energy into the mass of earth as quickly as possible.
A structure may have several service earth installed that may comprise telephone, electricity supply, communications or special purpose earth. All these may be in addition to the lightning protection earth. When potential differences occur between any of the multiple earths, equipment damage is possible. By providing equipotential bonding to all earths, this problem can be eliminated.
If lightning strikes the power lines some distance from the structure, a surge will travel in both directions, including into the structure containing sensitive electronic equipment. This equipment can be protected by the installation of shunt connected surge diverters or inline surge reduction filters.
Telephone, communications and signal cables entering a structure (overhead or underground) are subject to induced surges or direct strikes that can also damage the electronic equipment to which they are connected. The protection of such equipment by installing suitable protection devices should be considered.
Today, Computers and computer-based equipment are used extensively in virtually every sector of industry and commerce – as well as hospitals, government facilities, laboratories and banking.
Whether a company operates a local area network, automated factory equipment, a sophisticated building security system, or even a small telephone exchange, it depends totally on that system to operate efficiently. However, many organisations ignore the single biggest threat to their systems – voltage surges caused by lightning and electrical switching events. Main power supplies and data communications lines are highly susceptible to surge voltages and the result is usually a total systems failure.
Apart from the enormous cost involved in replacing damaged equipment, consider other expenses that are difficult to quantify: severe disruption to your operations – data loss, downtime, loss of fax and phone, inconvenience to your staff and your customers. But the situation CAN be avoided!
Lightning activity near to a building can cause voltage surges (transient overvoltages). These brief but significant increases in voltage on mains power supplies or data communication lines can be conducted into the electronic circuitry of computers and other sensitive electronic equipment. This results in · Data loss · Loss of fax and phone · System disruption · Physical damage to equipment (either immediate or later through component degradation) · Costly downtime Electrical switching events can cause the same problems although chiefly affecting the mains power supply.
A conventional lightning protection system will protect only the building structure, not its electronic contents. Equally, uninterruptible power supplies alone cannot protect a computer system against the large voltage surges caused by the secondary effects of lightning. These can be up to 6000 volts (3000 Amps). Indeed, UPSs themselves need protecting from the effects of voltage surges.
The cost to recover from losses caused by lightning can be very high. In addition to repair of actual damage to electrical or electronic equipment, there are other costs such as loss of data held on computer, delays whilst staff cannot use their computers, make or receive phone calls, send or receive faxes etc.
These losses are not always covered by insurance and the inconvenience, stress and long hours to co-ordinate repairs and catch up, are almost certainly excluded. It is therefore in everyone’s interest to identify cost-effective ways to reduce the incidence and severity of lightning damage.
A lightning strike is an electrical discharge resulting from a build-up of electrical charge in a thunderstorm. This discharge can be from cloud to cloud or cloud to ground. The current that flows from cloud to ground can be very high, sometimes even in the hundreds of thousands of Amps, but the duration is often less than one-tenth of a second.
When lightning strikes the ground or a structure, it causes the voltage at that point to rising dramatically, causing electrical current to flow from that point to points further away from the strike.
A lightning strike also creates electrostatic and electromagnetic fields, which can induce currents in exposed cables, again causing damage to equipment at both ends of the cable.
One of the major suppliers of lightning suppression equipment suggests a five-step process summarized as follows: –
All equipment inside a boundary is treated as a single entity and the protection equipment is connected to a single earth point. A boundary may be an entire building or in the case of a large multi-story building, it may be a single floor.
All electrical services crossing the boundary require protection and for some sensitive equipment, it may also be necessary to provide additional protection within the boundary. This is described in more detail when discussing the protection of power lines and signal lines.
We suggest that for our purposes we consider boundaries to be either individual buildings up to say three stories or individual floors for buildings above three stories.
The protection of building and structures against a direct lightning strike is usually incorporated in the original construction and ensures that all metallic components of the building are electrically tied together and connected to a single earthing point.
Any lightning arresters mounted on the structure at the time of construction should be also adequately electrically tied to the same earth point.
As this is generally at the time of construction, protection of the structure is outside the scope of this report.
As was discussed previously, a lightning ground strike on or close to a structure causes the voltage of that structure to rise dramatically compared to earth. This can cause current to pass down electrical cables to other structures unaffected by the lightning strike, damaging equipment at both ends of the cable. If the structures have their earth electrically connected, any voltage differences between the structures are minimized and this reduces the current flowing down cables between the structures.
As with protection of individual structures, installation of earth rings is usually done at the time of construction. These issues are again outside the terms of this report.
The most common source of damage to electrical and electronic equipment as a result of a lightning strike is the power supply. A surge or spike entering the incoming power lines has the potential to damage every electrical and electronic item connected to those power lines.
Whilst a surge diverter can substantially reduce spikes entering electrical and electronic equipment, considerable damage can still be done to voltage-sensitive equipment and it is, therefore, necessary to also install a surge filter for each item of sensitive equipment. Surge filters can either be installed in power distribution boards or can be free-standing with a flexible lead for plugging into a standard general-purpose power outlet.
A surge filter located in a distribution board can provide filtering and is, therefore, a lot more cost-effective to all devices connected to that power circuit (e.g. several computers).
Protection of power lines does not prevent surges from entering electronic equipment along with signal and data cables. Even with power line protection, damage to computer networks can be extensive if a surge enters data cabling connecting several computers. A surge can also enter a computer through a telephone line if the computer is connected to a modem. It is therefore also necessary to protect against damage to electronic equipment from this source.
Surges can also enter telephone networks via incoming telephone lines and there is a range of protection devices that can be mounted inside telephone main distribution frames.
We will consider a site with two low set buildings, each with its incoming electrical power and switchboard, with computers communicating between buildings via a local area network and with a telephone PABX in one building with telephone extensions in the other.
Using the five-step process, we define the boundaries as being each building. All electrical cabling in and out of the building, therefore, requires protection.
– All metallic components of each building must be electrically bonded together and tied to the earth.
– The earth of each building must also be tied together to reduce any voltage difference between the two buildings in the event of a lightning ground strike close to one of the buildings.
– Earthing of structures and bonding of their earth together would be considered at the time of construction.
– A surge diverter should be installed in the electrical switchboard of each building. Power to each electronic device should be protected by a surge filter either located in the power distribution board providing power to the device or located adjacent to the device as a free-standing component plugged into the nearest general purpose power outlet via a flexible lead.
– Incoming telephone lines connected to the PABX should be protected by signal line protectors mounted in the main distribution frame where the incoming telephone lines are terminated and each extension line to a telephone in the other building should be protected similarly at the PABX end only. Due to the relatively low cost of the telephone handsets themselves, it would not be necessary to protect the telephone handset end of the extension cables.
– Data line protectors should be installed at the termination point of each computer connected to the local area network.
For new installations or installations where there are long cable runs between buildings, fibre optics can be considered. As the optical fibre cable sends signals by modulating light rather than electrical current, the cable is unaffected by lightning.
Even if fibre optics are used, it is still necessary to protect power lines.
Whilst it is not always possible to ensure adequate protection of structures and correct bonding to an earthing system, as this takes place during the construction period, damage from all but the most intense lightning strikes can be eliminated by the adequate protection of power, signal and data cabling. It is therefore within your power to prevent the stress and sleepless nights when trying to return to normality after a lightning strike takes out your computer systems, telephone system, burglar alarm, access control system, fire detection system and facsimile machine.