Technical requirements concerning components of cable television (antenna systems)
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Annex No. 21 to the directive of the Minister of Communication of September 4, 1997 (fragments).
10. Protection against lightning overvoltages, elimination of potential differences
The aim of these requirements is to protect antenna systems (including satellite installations) against accidental overvoltages (static electricity, lightning discharges). These requirements cannot be applied to buildings and other constructions (they only concern antenna systems).
There are excluded the following cases:
- antennas systems that are situated outside buildings, at least 2m below the top of the roof and less than 1.5m from the side of the building,
- antenna systems inside buildings.
Antennas cannot be mounted on buildings with roofs made of inflammable materials. (e.g. thatch, cane materials etc.).
Antenna cables and grounding conductors shouldn't be lead through areas applied to store inflammable materials, like: hay, straw etc., and through areas where gases can appear or collect.
AM radio antennas should have protectors connected to neutral conductor.
10.1. Antenna system protection
10.1.1. Buildings equipped with lightning protection systems.
If the building is equipped with lightning protection system compliant with Polish Norms (PN), the metal antenna mast should be connected to the system with shortest conductor possible (see paragraph 10.2 below). The screens of all coaxial cables going from the antenna should be connected to the mast via potential-leveling copper conductor of a minimum cross-section area not less than 4mm2 (Cu).
An example of equalizing potentials in antenna system
10.1.2. Buildings not equipped with lightning protecting systems.
If the building is not equipped with lightning protection system, the metal mast and screens of coaxial cables should be grounded according to par. 10.2.
This applies to individual and community antenna installations in areas where probability of lightning discharge is small and local regulations don't impose use of lightning protection systems (but only advise them).
10.2. Grounding of antenna system
10.2.1. Grounding methods
The antenna mast and the screens of coaxial cables should be grounded with shortest conductor possible. Any loops should be avoided. The grounding conductors have to be led directly (vertically) to the grounding terminals, to obtain the shortest route.
10.2.2. Grounding terminals
Grounding terminals should be made in one of the following methods:
- connection to lightning protection system of the building
- connection to grounding system of the building
- connection to at least two horizontal electrodes, each minimum 5m long, or vertical/inclined electrode minimum 2.5m long - placed at least 0.5m below the ground level, not closer than 1m from the wall.
Minimum cross-section area of each electrode is to be 50mm2 (Cu) or 80 mm2 (Fe). It is possible to utilize "natural" elements like e.g. connected steel or reinforced concrete constructions or other underground metal structures situated in building foundations of dimensions not less than specified above.
10.2.3. Grounding conductors
Appropriate grounding conductor is a single solid conductor with cross-section area not less than 16mm2 (Cu - bare or insulated), 25mm2 (Al - insulated), or 50mm2 (Fe - bare). One can use ''natural'' elements e.g. metal installations like continuous water pipes , continuous central heating pipes, but only when:
- it is allowed by local regulations
- there is guarantee of reliable electrical connection between different elements
- cross-section areas of these elements (of the metal part) are at least equal to described above minimum dimensions of standard grounding conductor
- metal framework of the building structure
- connected steelwork (rods/bars) of the building structure
- metal facade elements, metal beams etc.
- their total dimensions are above the minimum dimensions of standard grounding connector
- the minimum size of individual conductors is at least 0.5mm
- have adequate conductivity in vertical direction (they are connected with braze, welded, screwed together, riveted, or the distance between metal parts is less than 1mm and the lap is at least 100 cm2.
It is banned to use as the grounding conductor :
- protective and/or neutral conductor of power network
- screens of coaxial cables .
10.3. Protection against overvoltages
As the result of electromagnetic induction, there may appear overvoltages in installation nodes, subscribers' outlets, inputs of subscribers' receivers. To avoid such surges one can apply level-equalizing connections and surge protectors.
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