Single Point Ground

I mentioned earlier that the primary purpose of the protector is relatively simple – short circuit when threatened.  By shorting all of the wires associated with an interface no current can flow between the wires of the interface.  Extending this premise further, by mounting all of the protectors in common, no current will flow between the I/O interfaces.  Hence, no lightning surge current will flow through a protected piece of electronic equipment. 

To make this possible in the radio room it is necessary to establish what is known as a ‘Single Point Ground.’  This is the one and only point in the radio room where a ground connection is present.  We need to be a little careful with the term ground.  During a strike event a ground can be anything that is capable of being an energy sink.  By this definition anything, absolutely anything that is not at the same electrical potential can be a sink.  This applies to the electrical domain as well as the time domain – remember that electrons flow at about a nanosecond a foot.

The creation of a single point ground will be different for every installation.  It can be as simple as a couple of protectors bolted together, to a through the wall entrance panel, or as complex as a copper-covered wall upon which the protectors are mounted.  Whatever form your single point ground takes it must be the only ground point for all of the equipment within the circle of the box-level schematic diagram.

To create a common vision for us, I’ll use the PolyPhaser CU-SPGP, a high-density fiberboard-backed copper panel suitable for small to medium radio rooms.  This item comes with a 1 ½ inch wide copper strap to connect the panel to the external ground system and a second 1 ½ inch copper strap to connect to all your operating table equipment.  The panel is intended to be mounted on a wall near the radio equipment.  For convenience of reference, I’ll use is abbreviation – SPGP, for our Single Point Ground Panel.

Now that we have a mounting surface that will become our single point ground, a lot of consideration must be given to the physical placement of the protectors on the SPGP.  Remember, that a protector is required for each I/O line that leaves the circle of the box-level schematic.  As you examine a PolyPhaser protector, most are labeled with respect to which connector faces the surge (the outside world) and which connector faces your equipment.  This is important to observe, since the protectors are not symmetric in their design.  Therefore, they cannot be reversed and then expected to function properly. 

A significant factor in the layout of the protectors on the SPGP is maintaining a physical separation between the incoming unprotected cables (antenna feed lines, incoming ac power, rotor lines, etc.) and the protected side of the same cables.  As a result of going through a PolyPhaser inline protector, there will be a “spark-gap level” voltage difference in time between the input and output sides of the protector.  You must take this into consideration when planning the layout of the SPGP.

A general guideline is to draw an imaginary diagonal line near the center of the panel.  Designate the area above the line as protected and the area below the line as unprotected (or vise versa).  Make sure you consider how the panel will be mounted; how the (unprotected) cables will enter the unprotected area and how the (protected) cables will leave the panel.  One of the nice things about a two-dimensional drawing is that the affects of gravity do not show.  In the drawing to the right, the cables leaving the panel above the dotted line must be anchored.  If they are not, real world gravity will cause them to eventually bend down and come close, maybe even touch, and the unprotected cables.  If this happens, during the strike event there is the potential for a spark-gap breach of the entire protection plan between the cables.

Neatness counts - cables (transmission lines, power (ac and dc), speaker, microphone, computer, control) should be cut to length and routed neatly and cleanly between boxes using the most direct practical route.  The coiling of excess cable length on the protected side should be avoided since it could act as an air-wound transformer coupling magnetic energy from a nearby lightning strike back into the protected equipment.

The chassis ground for each element of radio equipment must also be connected to the SPGP.  The SPGP is our reference point during the strike event and it is important that all elements of the radio station be at the same potential at the same time (nanoseconds).  For small to medium size stations, where all for the equipment fits on a desk/table top, a single interconnect copper bus or strap to the SPGP is usually a sufficient connection.