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Earthing of VSAT dishes
Former Community Member
I am trying to apply the principles of BS 7671 to VSAT installations in Africa. My chief concern is the connection of the VSAT dish to the electrical supply provided by the customer. The standard practice I've adopted is shown in the attached sketch. The metallic parts of the dish are connected with copper conductors and then to an earth spike (or spikes) near the dish foundations. The earth cable is then run (either in a conduit of buried) to the equipment building. At the entry to the building, the earth cable is used by surge arrestors fitted to the RF cables. The earth cable then runs to an earth bar fitted into the equipment cabinet. The mains supply to the equipment is supplied by the customer, the earth of which is also connected to the cabinet's earth bar. The power supplied by the customer may be from a generator 100% of the time, or more like comes from the town's municipal supply, These supplies are often erratic so a back-up generator is often in use, and as a matter of course we always fit an on-line UPS. Is this approach sensible? Thanks for any assistance.
I am no expert but I think it would be better to have your surge arrestor on the earth outside the building rather than inside. Also I am not over keen in the long runs of earth from your surge arrestor to the earth electrode. I would be much happier to see another electrode directly connected to your surge arrestor outside the building. In the UK BS 7671 requires the total length of the connection of a live conductor to a Surge Protection Device and the earth connection ideally should not exceed 0.5m and not to exceed 1m.
If a few milli Ohms of 1m of cable is an impedement to ideal SPD operation, I can't quite see how an electrode with dozens of Ohms to Earth is going to make a significant difference (skipping over that plain resistance is only part of the story of impedance with rapidly rising spikes). In my head SPDs are less about dumping surges to true earth so much as limiting the voltage difference between the live/signal conductors and the installation's protective conductors - pulling the protective system up as much as pulling the other lines down. Shades of the old equipotential zone - which may well be a long way from true earth potential during faults. I think the important bit if the lenth of the 'branch' between the SPD and the connections feeding the equipment/installation being protected - keep those below 0.5m and the other lengths matter much less I think.
I am no expert but I think it would be better to have your surge arrestor on the earth outside the building rather than inside. Also I am not over keen in the long runs of earth from your surge arrestor to the earth electrode. I would be much happier to see another electrode directly connected to your surge arrestor outside the building. In the UK BS 7671 requires the total length of the connection of a live conductor to a Surge Protection Device and the earth connection ideally should not exceed 0.5m and not to exceed 1m.
If a few milli Ohms of 1m of cable is an impedement to ideal SPD operation, I can't quite see how an electrode with dozens of Ohms to Earth is going to make a significant difference (skipping over that plain resistance is only part of the story of impedance with rapidly rising spikes). In my head SPDs are less about dumping surges to true earth so much as limiting the voltage difference between the live/signal conductors and the installation's protective conductors - pulling the protective system up as much as pulling the other lines down. Shades of the old equipotential zone - which may well be a long way from true earth potential during faults. I think the important bit if the lenth of the 'branch' between the SPD and the connections feeding the equipment/installation being protected - keep those below 0.5m and the other lengths matter much less I think.
