I have a bunch of receivers in several rooms around the house, most of which are connected to a multiswitch, a nice big 8-output one :-) An earth on any of the cables connected to it has the effect of earthing all receivers whose LNB is plugged in.
If you aren't using a multiswitch (most people don't - it's usually only hardcore hobbyists, professional installers or rich folk with several plasmas around the house who do!) then there's no need to think you need one just to earth your receiver. You can earth an individual receiver using an earthing kit like those from Dream Multmedia, or using a cable of your own making. Just be certain you know what you're doing before messing with mains wiring!
The first benefit of earthing, the most significant for Dreambox enthusiasts, is you eliminate the possibility of zapping your Dream's serial port, or perhaps other ports, when tinkering with it to install new firmware. The zapping may originate from the Dream's switchmode plugpack or from other switchmode-powered equipment it's connected to (eg TV or hifi system). Damage occurs because the pins on data ports of an unearthed receiver connected to a switchmode supply are often found to be floating 200V or more above ground. The impedence is quite high so it's not enough to electrocute but it's enough to give you a tingle and plenty to destroy interface ICs such as those on your Dream or PC serial port. Earthing your receiver prior to insertion/removal of data leads eliminates that potential.
The second benefit of proper dish earthing, as per the specifications Z80 referred to earlier, is that your equipment is less likely to be destroyed if your dish is hit by lightning. There's no guarantee though. Last time my big dish was hit I lost an LNB, a $700 sat receiver and some attached computer gear. Oh, and one of my 8 precious multiswitch ports got destroyed! (The other 7 survived amazingly but that's another story.)
The main drawback to earthing is it can create an earth loop manifesting as hum in your sound system and sometimes as a wavy picture. To avoid such loops you really need to have just a single earth point in your multimedia system, such as your amplifier. Optical digital audio connections are a lifesaver in this regard (it's also the reason the MIDI standard used by computer musicians requires optoisolators).
The bottom line advice I give people is as follows.
- If you're non-technical just leave everything hooked up and you've nothing to worry about; if you need to move plugs around then turn everything off at the wall and nothing can go wrong.
- If you're flashing your dream and have an earthed LNB lead then leave it plugged in at all times and you can't go wrong. If you aren't earthed then, before hooking up or removing your serial cable at either end, make sure you first unplug every other cable from your dreambox. If you take that precaution you'll avoid port zapping.
- If you're worried about lightning then (a) take out home contents insurance, and (b) unplug your expensive gear when bad thunderstorms are forecast.
- If you find yourself with an earth loop one day, whatever you do don't try to resolve it by cutting the earth lead of equipment that's supposed to be earthed. That's a big no-no! Old-timers will tell you about the times they've got rid of hum by doing so, but if you ask they'll probably confess to receiving horrendous shocks over the years too. It's shocks to others you have to worry about.
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