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Think of it as a “lightning arrestor.” Its purpose is to keep high voltages and currents, which can be induced into your wiring by a nearby lightning strike, from making it into your structure. According to the National Electrical Code, a ground block should be used on all installations.
Note: grounding your switch or ground block to a place other than the ground rod where your electrical service enters the structure may result in ground loops and/or the danger of shock from stray electrical currents. Contrary to popular belief, not all grounds are at the same potential.
For more info see: http://www.panamax.com/literature/pdf/gndcbl.pdf
In many installations the switch can be used as the ground block. To be used properly in this manner, your switch should be located in a position that satisfies National Electrical Code requirements for the placement of a ground block. For most installations, this will be where the external wiring enters the structure. If this placement is inconvenient, or you wish to mount the switch indoors, use an actual ground block as recommended by the NEC. Some switches may also function better if they are grounded, and you may wish to do this by running a ground wire from the switch to the point where your electrical service enters the building, or to a nearby cold water pipe.
This practice is a source of great controversy among installers. Use of a separate ground block is the preferred method.
The Dish TechPortal article Grounding the System says a switch can NOT take the place of a ground block. The DPP44 with the “UL-Listed” logo may be used as a ground block as announced on the 11/08 Retailer Chat, followed by the DP34 with the “UL-Listed” logo per the 4/09 Retailer Chat.
For more info see http://panamax.com/pdf/grnskew.pdf