Abstract:
A grounding assembly for electronic equipment provides a discharge path for the energy arising from lightning strikes. A conductive strap connects the ground lug of the equipment to a ground plate buried in the earth. The conductive strap has a length, width and thickness, with the width being greater than the thickness. This reduces the inductance of the conductor from the controller to the ground plate, thereby enhancing the ability of the grounding assembly to dissipate energy from lightning strikes.

Description:
BACKGROUND 
     The present application relates to grounding assemblies for electronic equipment and particularly relates to grounding assemblies for use with outdoor electronic equipment that is subject to being damaged by lightning. An example of such electronic equipment is the electronic controllers used on irrigation systems, such as those used on golf courses and the like. Irrigation systems include numerous sprinkler heads located throughout a property which are turned on and off by a plurality of solenoid valves located at or near the valves. The solenoid valves have control boards which are typically connected to a central control computer and a power source by wires buried under ground. Lightning striking the ground far from a particular control board can induce voltage spikes in the wires leading to the control board that can destroy the board. Lightning arrestors are typically incorporated in such equipment to prevent this but for such arrestors to protect the equipment adequately they must have an effective connection to ground. 
     It is the responsibility of the installer to connect all electronic irrigation equipment to earth ground in accordance with Article 250 of the National Electrical Code (NEC.) Grounding, bonding, and shielding components will, at a minimum, include the following items. Earth grounding must be done with grounding electrodes that are UL listed or manufactured to meet the minimum requirements of Article 250.52 of the 2008 edition of the NEC. At the very minimum, the grounding circuit will include a copper clad steel ground rod, a solid copper ground plate installed under ground and in contact with a suitable amount of an earth contact material, such as the carbon backfill products sold under the trademarks PowerSet or PowerFill by Loresco International of Hattiesburg, Miss. This is the minimum requirement for supplementary grounding of any electronic equipment. 
       FIGS. 1 and 2  illustrate the components required for supplementary grounding. The electronic equipment, such as an irrigation controller, is shown at  10 . A ground rod  12  has a minimum diameter of ⅝ inches and a minimum length of 10 feet. The ground rod  12  is driven into the ground in a vertical position or an oblique angle not to exceed 45 degrees at a location 10 feet from the electronic equipment  10 , the ground plate  14 , or the wire  16  connecting the ground plate  14  to the equipment  10 , as shown in  FIG. 1 . A 6 AWG solid bare copper wire  18  (about 12 feet long) is connected at  19  to the ground rod  12  by the installer using an exothermic welded connection, such as that provided by the Cadweld® GR1161G “One-Shot” welding kit available from Erico International Corporation of Solon, Ohio. The wire  18  shall be connected to the electronic equipment&#39;s ground lug. 
     The copper ground plate  14  must meet the minimum requirements of Article 250.52(A)(7) of the 2008 NEC. It is made of a copper alloy intended for grounding applications and has minimum dimensions of 4 in.×96 in.×0.0625 in. A 25-foot continuous length (no splices allowed unless using exothermic welding process) of 6 AWG solid, round, bare copper wire  16  is attached to the plate by the manufacturer using an approved welding process. This wire  16  is also connected to the electronic equipment&#39;s ground lug. In the past the round wire  16  has sometimes been replaced with a braided copper strap for connecting the electronic equipment to the ground plate  14 . But braided copper straps have complicated geometry that contributes to higher inductance characteristics. The ground plate  14  is to be installed to a minimum depth of 30 inches, or below the frost line if the frost line is lower than 30 inches, at a location 8 feet from the electronic equipment  10  and underground wire  18 . A suitable amount of earth contact material  20  must be spread so that it surrounds the copper grounding plate  14  evenly along its length within a 6 inch wide trench. Salts, fertilizers, bentonite clay, cement, coke, carbon, and other chemicals are not to be used to improve soil conductivity because these materials are corrosive and will cause the copper electrodes to erode and become less effective with time. 
     The grounding circuit components are to be installed in straight lines, to the extent possible, with no sharp turns. To prevent the electrode-discharged energy from re-entering the underground wires, all electrodes are installed away from such wires. The spacing between any two electrodes is as shown in  FIGS. 1 and 2 , so that they don&#39;t compete for the same soil. 
     The earth-to-ground resistance of this circuit is to be no more than 10 ohms. If the resistance is more than 10 ohms, additional ground plates and earth contact material are to be installed in the direction of an irrigated area at a distance of 10 feet, 12 feet, 14 feet, etc. It is required that the soil surrounding copper electrodes be kept at a minimum moisture level of 15% at all times by dedicating an irrigation station at each controller location. The irrigated area should include a circle with a 10-foot radius around the ground rod  12  and a rectangle measuring 1-foot×24-feet around the plate  14 . All underground circuit connections are to be made using an exothermic welding process by utilizing products such as the Cadweld® “One-Shot” kits. Solder cannot be used to make these connections. 
     The above grounding circuit is referred-to as supplementary/auxiliary grounding in the NEC. For safety reasons the NEC requires that all supplementary grounds be bonded to each other and to the service entrance ground (power source). This is also the recommended practice of IEEE Standard 1100-1999. Note that this is in addition to the equipment ground, which is commonly referred to as “the green wire.” The black (line or hot), white (neutral), and green wires must always be kept together in a trench, conduit, tray or the like. The bonding conductors are to be 6 AWG solid bare copper unless the system power conductors are larger than 1/0 AWG, in which case they are to be 4 AWG solid bare copper. All splices to the bonding conductors shall be made using an exothermic welding process. 
     SUMMARY 
     The present invention concerns an improved grounding assembly that is used to ground outdoor electronic equipment, such as irrigation controllers in a fixed irrigation system, for the purpose of providing a discharge path for the lightning induced voltage spikes. The grounding assembly comprises a ground plate and an electrically conductive strap. The conductive strap is electrically connected to the ground plate and electrically connectable to the electronic equipment. The conductive strap has a length, width and thickness. The width of the conductive strap is greater than the thickness. In a preferred embodiment the thickness of the conductive strap is about the same as the thickness of the ground plate and the strap has a generally rectangular cross section. Since flat conductors have lower inductance characteristics than round conductors, this reduces the inductance from the electronic equipment to the ground plate. Lightning follows the path of least inductance. Accordingly, the conductive strap of the present invention improves the ability of the grounding assembly to discharge the energy from lightning and thereby protect the electronic equipment. 
     These and other desired benefits of the invention, including combinations of features thereof, will become apparent from the following description. It will be understood, however, that a device could still appropriate the claimed invention without accomplishing each and every one of these desired benefits, including those gleaned from the following description. The appended claims, not these desired benefits, define the subject matter of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view of the layout of a prior art ground assembly. 
         FIG. 2  is a schematic side elevation view of a prior art ground assembly. 
         FIG. 3  is a schematic circuit diagram of the system in which the ground assembly of the present invention may be incorporated. 
         FIG. 4  is a front elevation view of a printed circuit board for an irrigation controller, a ground wire connecting the wire to a ground bar, and a conductive strap and bonding wires. 
         FIG. 5  is a front elevation view of the ground bar. 
         FIG. 6  is a section taken along line  6 - 6  of  FIG. 5 , with the conductive strap also shown in its slot. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is a circuit diagram of the system in which the ground assembly of the present invention may be used. The electronic equipment is shown generally at  10 . In this case it is shown as an irrigation controller but it will be understood that other types of electronic equipment could utilize the ground assembly of the present invention. The controller is mounted on a pad  22  on the ground  24 . The controller receives electric power from power lines which in this case are labeled black for line or hot, white for neutral and green or bare for equipment ground. The power lines are connected to a service entrance indicated schematically at box  26 . A power source from the utility company indicated schematically at  27  connects to a fuse or circuit breaker  28 . A ground bus bar  30  is also provided, with the white and green lines connected thereto. An earth ground  32  is provided by the utility company. 
     The enclosure for the electronic equipment may also house a ground bar shown schematically in  FIG. 3  at  34 . Details of the ground bar will be described below. The ground bar  34  connects to a ground wire  36 , a conductive strap  38 , and bonding conductors  40  and  42 . Bonding conductor  40  joins the utility earth ground  32 , while bonding conductor  42  extends to other controllers. The conductive strap  38  connects the ground bar  34  to the ground plate  14 . The ground wire  36  is fixed to a ground lug  46  on the electronic equipment for connecting to the ground bar  34 . In the illustrated embodiment the ground wire continues from the bar  34  to the ground rod  12 . Alternately, a separate wire could be used for connecting the ground bar to the ground rod. 
     Further details are shown in  FIG. 4 . The irrigation controller includes at least two printed circuit boards, one of which is shown at  44 . Board  44  is a lightning protection board that contains lightning arrestors. It will be understood that other possible arrangements of the various circuit boards are possible and that the lightning protection components could be incorporated in boards having multiple functions, such as an output board or a communication system boards. It is connected to other boards (not shown) that contains the circuit elements for interpreting control signals from a central control computer and actuating one or more solenoid valves in accordance with the control signals. The lightning protection board  44  has components designed to protect it and the second board from lightning induced spikes coming in through any wire connected to the controller. The lightning protection board  44  has a ground lug  46  affixed thereto. A connector block  43  receives the underground wires. Sets of fuses  45  and chokes  47  are provided. The chokes have high inductance which prevents spikes associated with the high frequency lightning strikes on the ground from passing to the second board. The high impedance caused by the chokes redirects such spikes to the ground lug  46  and conductive strap  38 . The ground lug  46  has a set screw connector  48  attached to it for receiving the ground wire  36 . The ground wire extends into and through the ground bar  34 . It is held fixed in the ground bar by a set screw  50 . The conductive strap  38  fits through a slot in the ground bar  34 . Two set screws  52  hold the strap  38  in the slot.  FIG. 4  also shows the bonding wires  40 ,  42  attached to the ground bar  34 . A set screw  54  is used on each bonding wire to retain it in the ground bar. 
     The conductive strap  38  has a length L as seen in  FIG. 4 . It also has a width W and a thickness T (see  FIG. 6 ). The width W is greater than the thickness T. Preferably, the thickness T is about the same as the thickness of the ground plate  14 . For reference purposes only and not by way of limitation, the width W may be about ½″ and the thickness T may be about 1/16″. In the preferred embodiment shown the strap has a generally rectangular cross section but it could be otherwise so long as the width is greater than the thickness. This configuration affords a reduction in the inductance of the strap, as compared to a round wire. The lower inductance provides superior dissipation of energy from lightning strikes at high frequencies. 
       FIGS. 5 and 6  illustrate details of the ground bar  34 . It is an elongated bar made of electrically conductive material, such as aluminum, although other materials could be used. A series of internally-threaded set screw openings  56  are formed in the front face  58 . A slot  60  extends through the center of the bar, all the way from the top face  62  to and through the bottom face  64 . Slot  60  is sized to receive the conductive strap  38  as described above. On either side of the slot  60  there is a pair of bores parallel to the slot. These similarly extend all the way through the ground bar  34 . Bores  66 ,  68  are on one side of the slot, while bores  70 ,  72  are on the other side. Bore  66  receives the ground wire  36 . Bores  70  and  72  receive the bonding wires  40 ,  42 , respectively. Bore  68  is an unused spare in the illustrated version. It could be used if it were desired to separate the ground wire  36  into two separate wires. That is, one wire could extend from the lug  46  to bore  66  and a second, separate wire could extend from the bore  68  to the ground rod  12 . 
     As can be seen from the above description, the present invention has several different aspects, which are not limited to the specific structures shown in the attached drawings and which do not necessarily need to be used together. Variations of these concepts or structures may be embodied in other structures without departing from the present invention as set forth in the appended claims. For example, the cross section of the conductive strap could vary from rectangular. It could have an oval cross section in which the major axis is greater than the minor axis. Or it could have a modified oval cross section with curved side edges and flat top and bottom surfaces. This could be made by starting with a round wire and squeezing it to flatten opposite sides of the wire. Preferably, the cross section has at least portions of the top and bottom surfaces that are planar. Most commonly these planar portions will also be parallel to one another, although they could have a non-parallel relationship. Although a relatively flat cross section is shown in the drawings, the cross section of the conductive strap could have any shape that has a lower inductance than a fully circular cross section. 
     In a further alternate construction the conductive strap and ground plate could be integrally formed from a single piece of copper. This would obviate the need to weld the two pieces together.