دایره المعارف تاسیسات برق (اطلاعات عمومی برق)
|1||3 pointed brass finial mounted on rod & flat parapit mounting bracket|
|3||Push in clips for mounting 8mm aluminium conductor|
|6||Buried earth conductor|
|7||Crows foot earth installation consisting of 3 earth rods|
|9||Lattice copper earth mat|
|10||Solid earth plate|
|11||Universal clamp used for multiple earth connections|
|12||Brass earth rod clamp|
PLEASE NOTE: Aluminium conductors and down conductors to be installed above ground level only.
All installations will be subjected to and conform to the following regulations:
|Push in clips with nail-in anchor||ALC-008|
|Adhesive base with push-in clip||ADB-008|
|8mm Aluminium down conductor sold in 100m rolls||RDC-608-080|
|Line taps for straight "T" and "X" connections
8mm dia. conductor
|Line taps for straight "T" and "X" connections
10mm dia. conductor
|M16 flat parapit finial mounting bracket||FMB-M16|
|M16 swivel-mounting finial mounting
can be set to mount finials at different angles
|3-pointed brass finial nut type
to be screwed on M16 copper rod
|Finial mounting - complete set||MCS-M16|
|Earth Boss test connector - M8 screws||EBTC-M8|
• All test clamps to be installed
|Lid marked “Electrical Earth Connections”||PLSEARINBOX|
• Size: 400 x 200 x 150mm
|Static retractable reel||SER-000|
• Cable 16mm² ±16mm
The role of the building protection system is to protect it against direct lightning strokes.
The system consists of:
When the lightning current flows in a conductor, if potential differences appear between it and the frames connected to earth that are located in the vicinity, the latter can cause destructive flashovers.
Three types of building protection are used:
The lightning rod is a metallic capture tip placed at the top of the
building. It is earthed by one or more conductors (often copper strips)
(see Fig. J12).
Fig. J12: Lightning rod (simple rod or with triggering system)
These wires are stretched above the structure to be protected. They are used to protect special structures: rocket launching areas, military applications and protection of high-voltage overhead lines (see Fig. J13).
Fig. J13: Taut wires
This protection involves placing numerous down conductors/tapes symmetrically all around the building. (see Fig. J14).
This type of lightning protection system is used for highly exposed buildings housing very sensitive installations such as computer rooms.
Fig. J14: Meshed cage (Faraday cage)
As a consequence, the building protection system does not protect the electrical installation: it is therefore compulsory to provide for an electrical installation protection system.
50% of the lightning current discharged by the building protection system rises back into the earthing networks of the electrical installation (see Fig. J15): the potential rise of the frames very frequently exceeds the insulation withstand capability of the conductors in the various networks (LV, telecommunications, video cable, etc.). Moreover, the flow of current through the down-conductors generates induced overvoltages in the electrical installation.
Fig. J15: Direct lightning back current
by Ron Kurtus (revised 14 April 2011)
Lightning is a huge static electric spark created during a thunderstorm. Lightning strikes can be damaging to buildings and equipment, as well as dangerous to people.
Buildings often use a lightning protection system consisting of a lightning rod (also called a lightning conductor) and metal cables to divert and conduct the electrical charges safely into the ground. Another form of lightning protection system creates a short circuit to prevent damage to equipment. The electrically conducting metal skin of commercial aircraft is isolated from the interior of to protect passengers and equipment.
Questions you may have include:
This lesson will answer those questions. Useful tool: Units Conversion
When lightning jumps toward the earth from a cloud during a thunderstorm, it will seek out the highest structure that has collected electrical charges opposite of the static electric charges in the cloud. A high tree in a field or a tall building can be likely lightning targets.
In 1752, Benjamin Franklin invented the lightning rod, which was meant to extend above a building and attract the lightning bolt to the rod, where it could be diverted harmlessly to the ground.
A lightning rod (also called lightning conductor) is just part of a lighting diversion system meant to protect building from damage due to a lightning strike. Included in the system is the metal cable or rod that extends down into the ground and electrodes in the earth to safely dissipate the energy away. Often buildings prone to being struck by lightning will have a network of lightning rods and conductors.
Lightning rods on the roof of a barn
The idea is that the electricity will take the path of least resistance and thus bypass the structure of the building as it travels to the ground. But also, the configuration of tip of the lightning rod is such that it is a good receptor for lightning.
When Franklin invented the lightning rod, he felt that a sharp tip would be best for attracting electrical charges and thus lightning. It wasn't until 2000 that scientists at the Langmuir Laboratory for Atmospheric Research in New Mexico proved that a rod with a rounded or spherical end works better.
When lightning strikes an electrical power line or communication line external to a building or when it strikes a building and jumps to one of those lines, it can create a surge of current that will burn out or damage electronic instrument and computers.
A lightning arrester is a device that is connected between each electrical conductor in a power and a communications system and the Earth or ground. They create a means to short circuit the surge in power, limiting the rise in voltage. Usually, lightning arresters are placed where the power and communication lines enter the building.
Surge protectors are often used between the power and communications outlets and the computer or other electronic device. They add an extra security. In most residential buildings, there is not lightning arrester, so surge protectors are important.
Although most commercial airliners try to avoid flying through a thunderstorm they occasionally have no choice. In such a situation, it would seem that the airplane could be in danger of being struck by lightning and damaged. In fact, individual airliners are struck by lightning an average of once a year.
The way commercial aircraft are protected from lightning damage is by keeping the lighting current in the outer skin of the airplane. This system works so effectively that the last crash due to lightning was over 40 years ago.
Since most commercial aircraft skins are primarily made of aluminum, a very good conductor of electricity, most of the lightning current remains on the exterior skin of the aircraft. The aircraft are designed to make sure that there are no gaps in this conductive path.
Since the aircraft is speeding through the clouds, the excess charges will then be dissipated into a region of cloud that has an opposite charge.
However, when the lightning is traveling along the exterior of the aircraft, transient effects, similar to electrostatic induction, can create power surges in the aircraft wiring that could damage electronics and aircraft computers. By shielding wiring and equipment and adding proper grounding and surge suppression devices. The problems caused by these lightning effects can be averted.
(See Electrostatic Induction for more information.)
Likewise, the fuel system must be protected from even tiny sparks caused the lightning currents in the aircraft exterior that could result in an explosion. The skin around the tanks, fasteners and fuel filler caps must be able to prevent sparks near the fuel.
A common misconception is that the static wicks on the edges of the wings. Since they look like little lightning rods, many people think they are used to attract lightning to them, as a way to protect the aircraft.
Static wicks on Boeing 737 wings
The only natural predator of wireless equipment is lightning. There are two different ways lightning can strike or damage equipment: direct hits or induction hits. Direct hits are when lightning actually hits the tower or antenna. Induction hits are caused when lightning strikes near the tower. Imagine a negatively charged lightning bolt. Since like charges repel each other, that bolt will cause the electrons in the cables to move away from the strike, creating current on the lines. This is much more current the the sensitive radio equipment can handle. Either type of strike will usually destroy unprotected equipment.
|Figure 5.2: A tower with a heavy copper grounding wire.|
Protecting wireless networks from lightning is not an exact science, and there is no guarantee that a lightning strike will not happen, even if every single precaution is taken. Many of the methods used will help prevent both direct and induction strikes. While it is not necessary to use every single lightning protection method, using more methods will help further protect the equipment. The amount of lightning historically observed within a service area will be the biggest guide to how much needs to be done.
Start at the very bottom of the tower. Remember, the bottom of the tower is below the ground. After the tower foundation is laid, but before the hole is backfilled, a ring of heavy braided ground wire should have been installed with the lead extending above ground surfacing near a tower leg. The wire should be American Wire Gauge (AWG) #4 or thicker. In addition, a backup ground or earthing rod should be driven into the ground, and a ground wire run from the rod to the lead from the buried ring.
It is important to note that not all steel conducts electricity the same way. Some types of steel act as better electrical conductors then others, and different surface coatings can also affect how tower steel handles electrical current. Stainless steel is one of the worst conductors, and rust proof coatings like galvanizing or paint lessen the conductivity of the steel. For this reason, a braided ground wire is run from the bottom of the tower all the way to the top. The bottom needs to be properly attached to the leads from both the ring and the backup ground rod. The top of the tower should have a lightning rod attached, and the top of that needs to be pointed. The finer and sharper the point, the more effective the rod will be. The braided ground wire from the bottom needs to be terminated at this grounding rod. It is very important to be sure that the ground wire is connected to the actual metal. Any sort of coating, such as paint, must be removed before the wire is attached. Once the connection is made, the exposed area can be repainted, covering the wire and connectors if necessary to save the tower from rust and other corrosion.
The above solution details the installation of the basic grounding system. It provides protection for the tower itself from direct hits, and installs the base system to which everything else will connect.
The ideal protection for indirect induction lightning strikes are gas tube arrestors at both ends of the cable. These arrestors need to be grounded directly to the ground wire installed on the tower if it is at the high end. The bottom end needs to be grounded to something electrically safe, like a ground plate or a copper pipe that is consistently full of water. It is important to make sure that the outdoor lightning arrestor is weatherproofed. Many arresters for coax cables are weatherproofed, while many arresters for CAT5 cable are not.
In the event that gas arrestors are not being used, and the cabling is coax based, then attaching one end of a wire to the shield of the cable and the other to the ground wire installed on the towers will provide some protection.
This can provide a path for induction currents, and if the charge is
weak enough, it will not affect the conductor wire of the cable. While
this method is by no means as good of protection as using the gas
arrestors, it is better then doing nothing at all.
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