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Acceptable Lightning Ground?
" wrote in
oups.com: I would like to propose a grounding arrangement for my [hypothetical] antenna and get some feedback on it. I have access to the solid-copper You haven't described here the antenna and its support structure, and they are relevant. One of the strategies for hardening a site against lightning is to divert as much of the strike current to ground rather than having it flow in the conductors within the facility. The value / necessity of this measure will depend on the antenna, its support structure, nearby structures that might protect your antenna to some extent, and the risk of lighting in your locality. cold-water pipe that enters my home through the basement wall close to the basement floor. This pipe is used [in addition to cold water] for the service entrance [circuit breaker box] ground. I was thinking of putting an antenna outside on a pole and running the coax into the basement. Then I would strip back several inches of the outer jacket of the coax [axposing the braided shield] and connect the coax braid to the cold water pipe using several hose clamps. This should ground the coax directly to the service ground - the single point ground for the house. I would add an arrester near the ground point. I would then run the coax upstairs [about 10 feet] to the radio, where it would [via the PL-259] connect to the transceiver chassis. The radio chassis will be electrically bonded to peripheral equipment chassis'. Lightning conductors need to be substantial enough to withstand the strike scenario without physical failure (eg melting). A common scenario is 20kA for 0.1s, but it does depend on the situation. Is your coax braid and your termination / bonding method sufficient to survive strike current? Equipotential bonding of the lightning protection ground and the AC service ground systems is important. Low resistance and more importantly low inductance conductors that will withstand the current are required. Depending on your coax shield as a bonding conductor from your equipment to the ground point as you describe sounds unwise for several reasons, but most notably because it uses coax connectors (and probably PL259s at that) for the connection, they are not reliable enough, and they would not be a permanent connection). If your equipment runs on low voltage DC, do not assume that the power supply provides a connection between the AC ground and the -ve DC lead, some powersupplies have floating output. In any event, bonding of the external metal of all equipment in your station to a single point ground is advisable to reduce the risk of substantial potential differences between equipments, and the risk that poses to life in the event of lightning or even an electrical fault. If addressing these issues seems extreme, it is probably a good indicator that many ham station earth systems are quite inadequate, offset by the low probability of an adverse event and not usually talked about when poor implementation exacerbated the situation. Like one of the other respondents, I recommend the Polyphaser site, it is a usefull source that canvasses many of the issues. Additionally, your wiring codes or standards may provide guidance or set requirements, they do here. Having done all that, and with better knowledge, review the risk, cost and measures. You may not want to resource a lightning hardened solution for 24x7 connection of the stations to antennas. Owen |
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