WARNING:
Electro-magnetic discharge (lightning) damage is not covered under warranty. The recommendations in this guide, when followed correctly, give the user the best protection from the harmful effects of EMD. However 100% protection is neither implied nor possible.
The need for power surge protection
Structures, equipment and people must be protected against power surges (typically caused by lightning) by conducting the surge current to ground via a separate preferential solid path. The actual degree of protection required depends on local conditions and applicable local regulations. Cambium recommends that PMP 450 installation is contracted to a professional installer.
Standards
Full details of lightning protection methods and requirements can be found in the international standards IEC 61024-1 and IEC 61312-1, the U.S. National Electric Code ANSI/NFPA No. 70-1984 or section 54 of the Canadian Electric Code.
Lightning protection zones
The ‘rolling sphere method’ (Figure 19) is used to determine where it is safe to mount equipment. An imaginary sphere, typically 50 meters in radius, is rolled over the structure. Where the sphere rests against the ground and a strike termination device (such as a finial or ground bar), all the space under the sphere is considered to be in the zone of protection (Zone B). Similarly, where the sphere rests on two finials, the space under the sphere is considered to be in the zone of protection.
Assess locations on masts, towers and buildings to determine if the location is in Zone A or Zone B:
- Zone A: In this zone a direct lightning strike is possible. Do not mount equipment in this zone.
- Zone B: In this zone, direct EMD (lightning) effects are still possible, but mounting in this zone significantly reduces the possibility of a direct strike. Mount equipment in this zone.
WARNING:
Never mount equipment in Zone A. Mounting in Zone A may put equipment, structures and life at risk.
General protection requirements
To adequately protect a PMP 450 installation for both ground bonding and transient voltage, surge suppression are required.
Basic requirements
The following basic protection requirements must be implemented:
- The equipment must be in ‘Zone B’
- The AP must be grounded to the supporting structure.
- A surge suppression unit (600SS) must be installed close to the SM.
- The distance between the SM and 600SS must be kept to a minimum.
- The drop cable length between the SM and 600SS must be less than 600 mm.
- A surge suppression unit (600SS) must be installed within 600 mm (24 in) of the point at which the power cable enters the building or equipment room.
- The drop cable must be grounded at the building entry point.
- The drop cable must not be laid alongside a lightning air terminal.
- All grounding cables must be a minimum size of 10 mm2 csa (8AWG), preferably 16 mm2 csa (6AWG), or 25 mm2 csa (4AWG).
Grounding cable requirements
When routing, fastening and connecting grounding cables, the following requirements must be implemented:
- Grounding conductors must be run as short, straight, and smoothly as possible, with the fewest possible number of bends and curves.
- Grounding cables must not be installed with drip loops.
- All bends must have a minimum radius of 203 mm (8 in) and a minimum angle of 90° (Figure 20 ). A diagonal run is preferable to a bend, even though it does not follow the contour or run parallel to the supporting structure.
- All bends, curves and connections must be routed towards the grounding electrode system, ground rod, or ground bar.
- Grounding conductors must be securely fastened.
- Braided grounding conductors must not be used.
- Approved bonding techniques must be used for the connection of dissimilar metals.
Protection requirements for a mast or tower installation
If the AP or SM is to be mounted on a metal tower or mast, then in addition to the general protection requirements (above), the following requirements must be observed:
- The equipment must be lower than the top of the tower or its lightning air terminal.
- The metal tower or mast must be correctly grounded.
- A grounding kit must be installed at the first point of contact (top), between the drop cable and the tower.
- A grounding kit must be installed at the bottom of the tower, near the vertical to horizontal transition point. This grounding kit must be bonded to the tower or tower ground bus bar (TGB), if installed.
Schematic examples of mast or tower installations are shown in Figure 21.
Protection requirements for a wall installation
If the SM is to be mounted on the wall of a building, then in addition to the general protection requirements (above), the following requirements must be observed:
- The equipment must be lower than the top of the building or its lightning air terminal.
- The building must be correctly grounded.
Schematic examples of wall installations are shown in Figure 22.
Protection requirements on a high rise building
If the AP is to be mounted on a high rise building, it is likely that cable entry is at roof level (Figure 23 ) and the equipment room is several floors below (Figure 24). The following additional requirements must be observed:
- The AP must be below the lightning terminals and finials.
- A grounding conductor must be installed around the roof perimeter, to form the main roof perimeter lightning protection ring.
- Air terminals are typically installed along the length of the main roof perimeter lightning protection ring typically every 6.1m (20ft).
- The main roof perimeter lightning protection ring must contain at least two down conductors connected to the grounding electrode
Protection inside a high rise building
The following protection requirements must be observed inside multi-story or high rise buildings (Figure 24):
- The drop cable shield must be bonded to the building grounding system at the entry point to the building.
- The drop cable shield must be bonded to the building grounding system at the entry point to the equipment area.
