Abstract:
The electrical ground fault protection circuit ( 10 ) includes power and ground LINE connections ( 12 ) that are connectable to power ( 18   a   , 18   b   , 18   c ) and ground lines of an electrical distribution system. They also include power and ground LOAD connections ( 14 ) that are connectable to a load ( 29 ). Power and ground paths extend from the power and ground LINE connections to the power and ground LOAD connections and include an interrupter ( 72 ) having a connect position in which it allows current flow from the LINE connections to the LOAD connections and a disconnect position in which it interrupts such current flow. A ground line monitor ( 64 ) detects the presence or absence of a fault condition in the ground line ( 20 ). In response to the presence of a fault condition, the circuit switches the interrupter from its connect position to its disconnect position. The power path monitor ( 66 ) detects the presence or absence of a fault condition in the power path ( 18   a   , 18   b   , 18   c ). In response to the presence of a fault condition in the power path, the circuit switches the interrupter from its connect to its disconnect position. The circuit ( 10 ) includes a ground path and plural power paths extending from the power and ground LINE connections to the power and ground LOAD connections ( 18   a   , 18   b   , 18   c ). A voltage monitor (VM 12 , VM 21 , VM 3 ) is interconnected between each power path and the ground path ( 20 ). The monitors detect the presence or absence of a voltage drop in the power path. In response to the presence of a voltage drop of a predetermined amount, the circuit switches the interrupter from its connect position to its disconnect position.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 09/335,259, filed Jun. 17, 1999, and entitled “Electrical Ground Fault Protection Circuit.” Application Ser. No. 09/335,259 claims priority based on provisional application Serial No. 60/089,864, filed Jun. 19, 1998, and entitled “Ground Fault Interrupter.” 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to electrical equipment that in use is subject to fault conditions that can cause harm to users of the equipment. More particularly, it relates to the provision of an electrical ground fault protection circuit that monitors the electrical equipment and its installation and in response to the detection of a ground fault will disconnect the equipment from its power supply.  
         BACKGROUND OF THE INVENTION  
         [0003]    There are electrical ground fault protection circuits available that detect and provide protection against some electrical fault conditions, such as leakage of current to ground. These circuits are termed ground fault interrupters (GFIs). These protective circuits detect leakage of current to ground by comparing the input current to the output current. This comparison, however, fails to detect all harmful conditions that may occur. For example, if a primary leg of the power source is shorted across the primary ground, standard GFIs will not detect this condition. This is because the input and the output current could remain the same. In addition, known GFI&#39;s do not detect an open ground or an elevated voltage on the primary ground or equipment housing.  
           [0004]    What is needed is an electrical ground fault protection system that continuously tests for numerous conditions to determine whether one or more conditions exist that could cause harm to a user. The continual testing for potentially harmful conditions would provide desirable safeguards to the user. In addition, the system should alert a user to some harmful condition or conditions before operation is commenced. This would provide an additional safeguard to the user. Herein, the term “user” refers to and includes any and all persons in the vicinity of the equipment and/or potential ground fault condition.  
           [0005]    The present invention is directed to the provision of an electrical fault protection system that tests for several conditions to determine whether any individual condition or simultaneous conditions exist that would provide a harmful condition or conditions to the user of the equipment.  
           [0006]    An object of the present invention is to detect harmful conditions including 1) current leakage of one of the primary legs; 2) current through the primary ground; 3) voltage leak from a primary leg to primary ground or case ground; 4) open primary ground; 6) lack of ground to work area continuity; and 7) elevated voltage on the work area. By continually testing for these potentially harmful conditions, this invention provides desirable safeguards to the user. In addition, this system alerts a user to some harmful conditions before operation is commenced. This provides an additional safeguard to the user.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    The electrical ground fault protection circuit of the present invention is basically characterized by power and ground LINE connections that are connected to power and ground lines of an electrical distribution system and power and ground LOAD connections that are connectable to a load. Power and ground paths extend from the power and ground LINE connections to the power and ground LOAD connections and include an interrupter having a connect position in which it allows current flow from the LINE connections to the LOAD connections and a disconnect position in which it interrupts such current flow.  
           [0008]    According to an aspect of the invention, a ground line monitor is provided for detecting the presence or absence of a fault condition in the ground line. In response to the presence of a such a condition, the circuit switches the interrupter from its connect position to its disconnect position. Also, the circuit includes a power path monitor for detecting a fault condition in the power path. In response to the presence of such a fault condition, the circuit switches the interrupter from its connect to its disconnect position.  
           [0009]    According to a further aspect of the invention, the circuit includes a plural power paths and a ground path extending from the power and ground LINE connections to the power and ground LOAD connections. A voltage sensor is interconnected between each power path and the ground path. Each voltage sensor detects a voltage drop in the power path. In response to the presence of a voltage drop of a predetermined amount, the circuit switches the interrupter from its connect position to its disconnect position.  
           [0010]    In some embodiments, the circuit is connectable to an electrical distribution system that includes three primary legs and a primary ground. The circuit includes three power paths, one for each primary leg, each connected to a separate one of the primary legs, and a ground path connected to the primary ground.  
           [0011]    In a preferred embodiment, the circuit includes a transformer connected to receive power from the power paths and to supply power to the voltage sensors.  
           [0012]    According to another aspect of the invention, the circuit includes a ground continuity monitor for detecting the ground continuity of the circuit. The circuit may also include an elevated voltage monitor for detecting an elevated voltage at the load that is above a predetermined voltage. In response to the presence of such an elevated voltage, the circuit will switch the interrupter from its connect position to its disconnect position.  
           [0013]    A further object of the invention is to provide an electric welding installation that includes an electrical fault protection circuit of the type described.  
           [0014]    Other objects, advantages and features of the invention will become apparent from the description of the best mode set forth below, from the drawings, from the claims and from the principles that are embodied in the specific structures that are illustrated and described. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0015]    Like reference numerals and letters are used to designated like parts throughout the several figures of the drawing, and:  
         [0016]    [0016]FIG. 1 is a pictorial diagram of an installation of electrical welding equipment in a building, showing the installation connected to power and ground lines of an electrical distribution system and further showing in it an electrical fault protection circuit that exemplifies the present invention, for providing protection to persons that are using or are in the vicinity of the installation;  
         [0017]    [0017]FIG. 2 is a view like FIG. 1 from which the electrical fault protection circuit has been omitted, such view showing some harmful conditions that could occur in the welding machine installation;  
         [0018]    [0018]FIG. 3 is a block diagram of a first embodiment of the invention;  
         [0019]    [0019]FIG. 4 is a block diagram of a second embodiment of the invention;  
         [0020]    [0020]FIG. 5 is a schematic diagram showing the first embodiment in greater detail;  
         [0021]    [0021]FIG. 6 is an enlarged scale view of the lower left corner portion of FIG. 5;  
         [0022]    [0022]FIG. 7 shows a portion of the circuit shown by FIG. 5 but with an alternative embodiment of the arrangement of voltage sensors between the primary legs of the power supply and the primary ground line;  
         [0023]    [0023]FIG. 8 is a table that identifies most of the components that are in FIG. 5;  
         [0024]    [0024]FIG. 9 is a schematic diagram of a modified arrangement of the circuit shown by FIG. 5, such view showing a preferred way of positioning the components of the circuit on a supporting circuit board;  
         [0025]    [0025]FIG. 10 is a schematic diagram identifying components in the circuit of FIG. 9;  
         [0026]    [0026]FIG. 11 is a front view of a control panel;  
         [0027]    [0027]FIG. 12 is a component table like FIG. 8, but identifying components shown by FIGS. 9 and 10;  
         [0028]    [0028]FIG. 13 is a diagram of a fully rectified sine wave that is associated with a microprocessor that is adapted to trip the circuit in the event the peak level is sensed twice within a half cycle;  
         [0029]    [0029]FIG. 14 is a diagram of an unrectified sine wave that is associated with a microprocessor that samples the amplitude of the unrectified sine wave in intervals, e.g. every {fraction (1/10)} cycle, and trips the circuit if the area under the actual curve equals or exceeds the area under a preset sine wave;  
         [0030]    [0030]FIG. 15 is a diagram of a sine wave having an amplitude twice the trip level that is associated with a microprocessor that is set to trip the circuit in less than a half cycle if interval sums exceed the trip level before a full half cycle; and  
         [0031]    [0031]FIG. 16 is a diagram of a sine wave having an amplitude less than the trip level on which higher frequency waves are superimposed, that is associated with a microprocessor that is adapted to trip the circuit if the sum of the total readings exceeds the sum of the sine wave. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    Referring now to the drawings, in which like reference numerals and letters identify like parts throughout the several views, FIG. 1 shows a pictorial view of a typical installation having a need for the present invention. A welding machine is shown as the load in this installation. However, the present invention has application with other installations having other loads, in either a commercial or residential setting.  
         [0033]    In the installation shown in FIG. 1, an electrical ground fault protection system  10  may include a power and ground LINE connection or primary in terminal  12 , a ground sense terminal  13 , and a power and ground LOAD connection or primary out terminal  14 . The LINE connection  12  receives electrical power from primary legs  18   a ,  18   b ,  18   c  and receives primary ground  20 . Before the primary ground  20  enters the system  10 , the primary ground  20  is attached to a building structure  22 , thus creating a building ground  24 .  
         [0034]    The LOAD connection  14  delivers electrical power and provides a primary ground to a load, such as a welding machine  26  as shown. The welding machine  26  has two terminals  25 ,  27 . One end of a electrode lead  28  attaches from the first terminal  25  and the other end attaches to an electrode  29 . The electrode lead  28  delivers the necessary current, either direct current or alternating current, to a worktable  32 .  
         [0035]    A work lead  30  attaches from the second terminal  27  to the worktable  32 . In certain situations, such as the embodiment shown, a worktable ground  34  is attached from the worktable  32  to the building structure  22 . In these situations, a ground sense lead  16  from the system  10  should be connected to the worktable  32 . In this configuration, when no fault conditions are present, a normal secondary current flows through the electrode lead  28  and the work lead  30  between the welding machine  26  and the worktable  32 .  
         [0036]    Referring now to FIG. 2, a pictorial view showing possible configurations resulting in harmful conditions is depicted with the present invention removed. As would be expected, not all of these conditions would occur at the same time and have only been illustrated in this manner for ease of explanation. As will hereinafter be explained in more detail, the present invention may detect harmful conditions resulting from a single occurrence or simultaneous occurrences of two or more of these individual harmful conditions.  
         [0037]    One of such harmful conditions results from an improper grounding hook-up  38  by attaching the work lead  30  to the welding machine  26  rather than the worktable  32 . In this situation, the path available for the secondary current is from the work table  32  through the work table ground  34 , the building structure  22 , and then to primary ground  20 . This path is undesirable because the current is very large and may bond the primary ground  20  with one of the primary legs  18   a ,  18   b ,  18   c , thus creating another harmful condition, depicted as a short  42 . Or, it may melt a portion of the primary ground  20  wire causing an open primary ground, shown at  44 . The short  42  may be from primary ground  20  to any one of the primary legs  18   a ,  18   b ,  18   c.    
         [0038]    Another harmful condition occurs as a result of a simultaneous break  44  occurring in the ground line  20  and a short  42  occurring between the primary ground line  20  and one of the primary legs  18   a ,  18   b ,  18   c . If this happens, there is no convenient path for the high current and the welding machine housing  26  maintains an elevated voltage condition  48 , not shown. A welder or other person in the vicinity may become the path for the high current, resulting in severe injury, most likely death.  
         [0039]    Another harmful condition occurs when the worktable ground  34  is open, shown at  46 . This open condition  46  may result from a missing or improperly connected worktable ground  34 .  
         [0040]    Referring now to FIG. 3, therein is shown a block diagram depicting a circuit  50  of the present invention. For ease of explanation, the circuit  50  may be separated by line  79  into a first circuit part  76  and a second circuit part or contactor circuit  78 . An electrical power supply source  52  with a plurality of primary legs  18   a ,  18   b ,  18   c  serves as input to the electrical ground fault protection system or circuit  10 , shown within the dotted box. The FIG. 3 block diagram shows a power supply source  52  as a three-phase system. However, as will become apparent later in the description, the system  10  will operate properly with a single-phase power supply source.  
         [0041]    The first circuit part  76  includes a transformer  56 , voltage sensing devices  54   a    54   b    54   c , and a primary leg indicator light (not shown). This light is shown in and is designated  92  in FIG. 5. Each voltage sensing devices  54   a ,  54   b ,  54   c  receive input from the primary ground  20  and primary leg  18   a ,  18   b ,  18   c , respectively. Sensors  54   a ,  54   b ,  54   c  detect voltage drop conditions on the primary legs  18   a ,  18   b ,  18   c . With reference to primary ground  20 , they also detect an open primary ground  44  condition. The voltage sensing devices  54   a ,  54   b ,  54   c  are adjustably set for a desired threshold voltage. A transformer  56  receives an input from primary legs  18   a ,  18   b ,  18   c  and supplies power to the voltage sensing devices  54   a ,  54   b ,  54   c  and the contactor circuit  78 .  
         [0042]    In another embodiment, shown in FIG. 4, a disconnect switch  58  controls the operation of the contactor circuit  78 . Typically, this disconnect switch  58  is easily accessible to a user and is manually controllable by the user in emergency type situations. However, in certain situations, the disconnect switch  58  is undesirable or unnecessary, such as in residential use. The disconnect switch  58 , indirectly through an auxiliary switch  59 , controls whether the second circuit part  78  may become operational and capable of supplying power to a load  74  when the user selects a start button. In this embodiment, fuses F 1 , F 2 , F 3  are positioned between the disconnect switch  58  and the load  74 . These fuses are well known in the art.  
         [0043]    Referring to FIGS. 3 and 4, the contactor circuit  78  includes a primary current leakage monitor  66 , a ground current monitor  64 , a continuity monitor  68 , an elevated voltage monitor  70 , an interrupter  72 , a power indicator light  90  (FIG. 5), and a fault indicator light  89  (FIG. 5). The ground current and primary current leakage are separately monitored. The ground current monitor  64  detects current in the primary ground  20 . The primary current leakage monitors  66  detects current leakage of any one of the plurality of primary legs  18   a    18   b    18   c . The primary current leakage monitor  66  and ground current monitor  64  may detect the err condition using a voltage sensing device or a current sensing device. A continuity monitor  68  detects the ground continuity of the system  10 , such as the open worktable ground  46  condition. An elevated voltage monitor  70  detects an elevated voltage  48  on the work area. If any of these devices or monitors  54   a ,  54   b ,  54   c ,  64 ,  66 ,  68 ,  70  detect an err condition, the interrupter  72  disconnects the power source  52  from the load  74 . As indicated by broken lines in FIGS. 3 and 4, the interrupter may be extended to include the ground path. That is, when the interrupter disconnects the power source from the load, it also opens the ground parts.  
         [0044]    Now referring to FIG. 5, an embodiment of the circuit  50  is shown in greater detail in this schematic diagram. In the first circuit part  76 , the transformer  56  is a three-phase transformer wired as an open delta. The primary winding  80  receives the primary legs  18   a ,  18   b ,  18   c  at each of three nodes  81   a    81   b    81   c . The secondary winding  82  having three nodes  83   a ,  83   b ,  83   c , thus has three legs  84 ,  86 ,  88  ( 88  not shown). The secondary winding  82  is tapped across one leg  88  which extends from node  83   a  to node  83   c . A fuse F 7  is between primary ground  20  and the node  83   c . Because the same voltage is available on any of the legs, if one of the primary legs  18   a ,  18   b ,  18   c  is lost at the source, the voltage across the secondary winding  82  is maintained. Configured in this manner, the transformer  56  does not have to be retapped to operate with a single-phase source. For example, with a single-phase source, even though only primary legs  18   a  and  18   b  are active, the voltage across the secondary winding  82  maintains the desired voltage to operate the circuit components.  
         [0045]    In the embodiment including the disconnect switch  58 , by designing the transformer  56  and the voltage sensing devices  54   a ,  54   b ,  54   c  to be positioned before the disconnect switch  58 , the system  10  can detect some harmful shock hazard type of conditions before the system  10  is allowed to deliver power to the load  74 . Therefore, this system  10  may provide additional safeguards to the user. In a further embodiment, once a certain shock hazard type of condition is detected, the primary ground  20  may be disconnected within the system  10 .  
         [0046]    The voltage sensing devices, shown generally at  54   a ,  54   b ,  54   c , include a voltage sensing circuit  55   a ,  55   b ,  55   c , a shock hazard enabling circuit  91   a ,  91   b ,  91   c , and a contactor enabling circuit  93   a ,  93   b ,  93   c . In the embodiment shown, the shock hazard enabling circuits  91   a ,  91   b ,  91   c  are parallel relays RL 1 , RL 2 , Rl 3  in series with a shock hazard indicator light  92 .  
         [0047]    In the embodiment shown, the contactor enabling circuits  93   a ,  93   b ,  93   c  use well known devices that interact with the interrupter  72  component in the contactor circuit  78 . Each contactor enabling circuit  93   a ,  93   b ,  93   c  includes a relay  106 ,  108 ,  110  and coils  114 ,  116 , 118 .  
         [0048]    [0048]FIG. 6 shows an enlarged scale view of a portion of the schematic used for selecting either a single phase or three phase power source. To provide operation for single and three phase power sources, phase selector switch  104  allows a user to select whether the power source  52  is single phase or three phase. The contactor enabling circuit  93   c  for primary leg  18   c  includes a closed relay  112  which may be operably selected by a corresponding position of the phase selector switch  104 . If single phase is selected, the phase selector switch  104  completes the circuit thru the closed relay  112 . Therefore, a third leg contact  124  (not shown), associated with contactor enabling circuit  93   c , remains closed so that the contactor circuit  78  does not open. In addition, in single phase, the phase selector switch  104  will open the circuit through relay RL 3 , thus preventing the shock hazard indicator  92  from illuminating due to no voltage on primary leg  18   c.    
         [0049]    Referring back to FIG. 5, a device suitable for use as the voltage sensing device  54   a ,  54   b ,  54   c  is available as model SM 125 115 500 1-Phase AC/DC Voltage—AC Current Control Relays from Carlo Gavazzi Inc. of Buffalo Grove, Ill. or a Schmitt Trigger such as used in a model VoltAlert™ 1 AC AC line voltage detector from Fluke Corp. of Everett, Wash. If an SM  125  device, or a similar device, is selected, a separate continuity circuit is not needed because the SM  125  provides continuity enabling along with the voltage sensing circuit. However, if a Schmitt Trigger device, or another voltage sensing device, is used, a separate contactor enabling circuit is necessary. Suitable contactor enabling circuits are well known in the art.  
         [0050]    The voltage sensing devices  54   a ,  54   b ,  54   c  have two inputs: one of the primary legs  18   a ,  18   b ,  18   c  and primary ground  20 . Across the inputs to each of the voltage sensing devices  54   a ,  54   b ,  54   c  is a voltage protection device  101 . In the embodiment shown, the voltage protection device  101  includes two stacked varistors  100 ,  102 . These stacked varistors clamp off harmful voltages and passes current thru the varistor so that only the desired voltage is on the inputs to the voltage sensing devices.  
         [0051]    In preferred form, a first varistor  100   a ,  100   b ,  100   c  is rated at a voltage to be limited, a limiting voltage, and handles up to a somewhat higher voltage, a clamping voltage. A second varistor  102   a ,  102   b ,  102   c  is rated with a limiting voltage just below the clamping voltage of the first varistor  100   a ,  100   b ,  100   c  and has a considerably higher clamping voltage. In this configuration, the stacked varistors  100   102  protect the voltage sensing devices  54   a ,  54   b ,  54   c  when one of the primary legs  18   a ,  18   b ,  18   c  shorts to ground resulting in double the voltage across the inputs to the corresponding voltage sensing device. The second varistor  102   a ,  102   b ,  102   c , in essence, protects the corresponding first varistor  100   a ,  100   b ,  100   c  from damage during this condition and thereby, the combination restricts the voltage without resulting damage to the circuit  50 .  
         [0052]    In an alternative embodiment, shown in FIG. 7, the primary leg  18   a ,  18   b ,  18   c  input of the voltage sensing devices  54   a ,  54   b ,  54   c  may have its input half-wave rectified. A well-known suitable device for performing this function is a diode  105 . This embodiment increases the sensitivity especially on unbalanced lines.  
         [0053]    Referring back to FIG. 5, the transformer  56  also provides power to the contactor circuit  78 . As mentioned previously, the contactor circuit  78  includes a primary current leakage monitor  66 , a ground current monitor  64 , a continuity monitor  68 , an elevated voltage monitor  70 , an interrupter  72 , a power indicator light  153 , a system on indicator light  152 , and a fault indicator light  89 . The interrupter  72  includes a first leg contact  120 , a second leg contact  122 , a third leg contact  124 , a primary leakage contact  134 , and a ground current contact  144 .  
         [0054]    In the contactor circuit  78 , the primary current leakage monitor, shown generally at  66 , includes a primary current sensor  126 , a primary current transformer  128 , and a primary current protector device  129 . This monitor  66  has an associated primary leakage contact  134  in the interrupter  72 . Two inputs Y 1 , Y 2  on the primary current sensor  126  receives a current level from the primary current transformer  128 . The primary current protector device  129  includes a primary closed relay  130  on the input Y 2  and a primary open relay  132  connected between the two inputs Y 1 , Y 2 . Because an err condition current may be significantly higher than the trip current, this large current through inputs Y 1 , Y 2  would damage the primary current sensor  126 . Therefore, the relays  130 ,  132  protect the sensor  126  and the transformer  128 . In preferred form, the relays will latch. A device suitable for use as the primary current transformer  128  is available from well known manufacturers.  
         [0055]    Similarly, the ground current monitor, shown generally at  64 , includes a ground current sensor  136 , a ground current transformer  138 , and a ground current protector device  139 . This monitor  64  has an associated ground current contact  144  in the interrupter  72 . Two inputs Y 1 , Y 2  on the ground current sensor  136  receives a current level from the ground current transformer  138 . The ground current protector device  139  includes a ground closed relay  140  on the input Y 2  and a ground open relay  142  connected between the two inputs Y 1 , Y 2 . Because an err condition current may be significantly higher than the trip current, this large current through inputs Y 1 , Y 2  would damage the ground current sensor  136 . Therefore, the relays  140 ,  142  protect the sensor  136  and the transformer  138 . In preferred form, the relays will latch. A device suitable for use as the ground current transformer  138  is available from well known manufacturers.  
         [0056]    Both the continuity monitor and the elevated voltage monitor, shown together generally at  68  and  70 , include a trip device having an associated contact  148   150  (FIG. 5). The contacts  148   150  may be part of the interrupter  72 . In the embodiment shown, a device suitable for use as the continuity monitor  68  and the elevated voltage monitor  70  is available as model  840  Ground Line Integrity Monitor from Time Mark Corp. of Tulsa, Okla.  
         [0057]    Input to the monitors  68 ,  70  is the ground sense lead  16  having a combined internal 1M Ohm resistance. The 1M Ohm resistance provides an additional safety feature for the ground sense lead. For instance, if there is an elevated voltage condition, the 1M Ohm resistance will decrease the current flow through a user in contact with the elevated voltage condition  48 . If there is continuity and no elevated voltage, the monitors  68 ,  70  switch to complete the remaining contact circuit  78  which includes the contacts  120 ,  122 ,  124 ,  134 ,  144  arranged in series. Thus, any contact that opens, due to an err condition, will disconnect the power source  52  to the load.  
         [0058]    In another embodiment, in which a work table ground  34  is not available or used, a ground by-pass switch  146  is operably positioned between the primary ground  20  and the ground sense lead  16 . This ground by-pass switch  146 , thus affects the input to the continuity monitor  68  and the elevated voltage monitor  70 . When closed, a resistor R 2  having a suitable resistance, such as 800K, allows continuity detection to be disabled but the elevated voltage detection to be enabled.  
         [0059]    A device suitable for use as the indicator lights is well known in the art.  
         [0060]    The contactor enabling circuit  93  and the shock hazard enabling circuit  71  may include electromechanical devices, e.g. relays, and solid state switching arrangements or any other non-linear response type device.  
         [0061]    The values of the components may be selected so that each of the above described harmful conditions are adequately detected. In one example circuit, components with the following values were used: three phase input 480v Y system with ground tapped; transformer  56  as 480-240/120; varistors  100   a ,  100   b ,  100   c  clamp voltage of 385; varistors  102   a    102   b    102   c  clamp voltage of 550; voltage sensing devices  54   a    54   b    54   c  set at 277V; ground current monitor  64  set to trip between a range of 2-200 mA depending on the need to compensate for nuisance tripping, preferably at &lt;20 mA; primary current leakage monitor set between a range of 2-200 mA depending on the need to compensate for nuisance tripping; elevated voltage monitor set to trip at 15V potential; and R 1  at 1200 Ohms. FIG. 8 is a table showing a component list with corresponding reference numbers.  
         [0062]    [0062]FIGS. 9 and 10 are schematic diagrams of a preferred circuit layout. Some components are shown in both FIG. 9 and FIG. 10. Some are shown only in FIG. 9. Others are shown only in FIG. 10. A key component of this circuit is the logic and timing unit A-6828. This CPU replaces hard circuit components shown in FIG. 5. The CPU is programmed to add a time element in the equation. This is done to prevent tripping of the circuit each time that the trip level is reached, even though for a short duration of time. Tripping will not occur unless a fault condition is sensed over a period of time.  
         [0063]    Referring to FIG. 9, the imbalance sensing circuit  126  may include standard filtering adopted to antinuate frequency of the monitored power that is above the primary frequency of the monitored power. It may also include a full wave rectifier for providing full wave rectification of the antinuate signal. A low pass filter is common and is known in the art. Using RC circuits, it will antinuate frequencies such as those above 2000 HZ in a 60 HZ primary circuit. Full wave rectification is also well known in the art and it is commonly accomplished by use of a bridge rectifier.  
         [0064]    [0064]FIG. 13 shows a fully rectified sine wave with a peak value of 5, for example. The microprocessor monitors the level reached every half cycle. If the preset peak level is sensed twice in two consecutive cycles, the microprocessor will register a fault and trip the circuit.  
         [0065]    In another embodiment, the ground fault protection circuit uses standard filtering to antinuate frequencies above the primary frequency of the power being monitored. The microprocessor that is used is adapted to measure input levels at less than {fraction (1/10)} th  the input frequency, and to some the peak input levels of each cycle and register a fault if that sum exceeds the trip level for any time equal to one-half of the primary input cycle of a sine wave with a peak value equal to the trip level. FIG. 14 shows a trip sum value of 218. FIG. 15 shows a sine wave of twice the value of a trip level sine wave. This figure shows a condition in which the microprocessor would register a fault and trip the circuit in less than one-half cycle because the interval sums would exceed 218 before a full half cycle.  
         [0066]    [0066]FIG. 16 shows a sine wave that is less than the trip level with a higher frequency superimposed. It represents a situation in which the sum of the superimposed signals is equal to the sine wave due to the summing of the values. FIG. 16 represents a situation in which above peak level signals are received but for short durations. Because the sum of the signals does not exceed the trip level over a period of time, the circuit is not tripped.  
         [0067]    If the circuit were to be tripped each time that the trip level is reached, even though for a short duration of time, there would be nuisance tripping and the fault protection circuit would have little value. The situations represented by FIGS.  13 - 16  enter a time element in the equation. In the situation illustrated by FIG. 13, the peak level must be sensed twice in consecutive half cycles. In the situation represented by FIGS. 14 and 15, the interval sums in less than a half cycle must exceed the interval sums for the half cycle of a sine wave at a preset trip level. The situation illustrated by FIG. 16 requires the interval sum of the frequencies to exceed the interval sum of a sine wave of a preset trip level. At other times, the circuit is not tripped, thus eliminating nuisance tripping.  
         [0068]    The microprocessor CPU sums the peak values over half cycle periods (FIG. 13) and when the sum is equal to or greater than a sign wave of a preset trip level, the processor registers a trip condition. FIG. 14 shows a sign wave with measured levels on intervals less than {fraction (1/10)} th  the primary frequency. This approaches the true RMS value of the signal. The faster the sample rate, the closer to true RMS value is measured. Thus transients and spikes will have little RMS value and be ignored. High level signals would have a higher RMS value and allow the processor to register a trip faster. See FIG. 15. Trip level is exceeded at less than ¼ cycle (sum of 256).  
         [0069]    [0069]FIG. 11 shows one form of control panel. It shows “Start”, “Test” and “Resent/off” buttons and several indicator lights. At the top of the panel there is a “shock hazard” light. This light is normally off. It goes on when there is a shock hazard condition. Below the “shock hazard” light there are six small lights, two associated with GF, two associated with GC and two associated with GI. The top row of lights are green. The bottom row are red. When conditions are normal, the green lights are on. They show that the monitors are in operation. In there is a ground fault (GF), the green light above “GF” goes off and the red light below “GF” goes on. If there is a ground current fault, the green light above “GC” goes off and the red light below “GC” goes on. If there is an unfavorable ground integrity condition, the green light above “GI” goes off and the red light below “GI” goes on. The on light  152  is on when the system is on. The fault light  92  is on when there is a fault condition. The power light  153  is on when there is power to the system. Element  158  is a start button. Element  154  is a reset/off button. Element  153  is a test light. It is on when the circuit is being tested. At the bottom of the panel there are three lights, one above “L1”, one above “L2” and one above “L3.” These lights may be amber in color. When there is a short in the power supply, all three lights are off. When the system is connected to single phase, lights “L1” and “L2” are on and light “L3” is off. When the system is connected to a three-phase power supply, all three lights “L1”, “L2” and “L3” are on.  
         [0070]    In operation, in the embodiment including the disconnect switch  58  with the disconnect switch  58  open, the user selects either a single phase or a three phase on the phase selector switch  104 . Once the primary legs  18   a ,  18   b ,  18   c  and primary ground  20  are connected to the primary in terminal  12  of the electrical ground fault system  10 , the primary leg indicator light  90  is illuminated and the transformer  56  provides power to the voltage sensing devices  54   a ,  54   b ,  54   c . If three phase is correctly selected and there is no open primary ground  44  or voltage leak from a primary leg  42 , relays RL 1 , RL 2 , RL 3  open and the shock hazard indicator light  92  remains off. If single phase is incorrectly selected, the contactor enabling circuit  73   c  would cause the contactor circuit  78  to open at the third leg contact  124  once powered on. A similar result occurs if the power source  52  is single phase and three phase was selected with the phase selector switch  104 .  
         [0071]    Once the disconnect switch  58  is closed and a start button  158  is pressed, a relay RL 6  closes and the system on indicator  152  and system power indicator  153  is illuminated. If there are no fault conditions, the contactor circuit  78  is closed and power is delivered to the load  74 .  
         [0072]    If there is a current leakage of one of the primary legs  18   a ,  18   b ,  18   c , the primary current leakage monitor  66  will detect the error and open the associated primary leakage contact  134 . Similarly, if there is current through the primary ground  20 , the ground current monitor  64  will detect the err and open the associated ground current contact  144 .  
         [0073]    If the worktable ground  34  is open, (condition  34 ), and the ground by-pass switch  146  is either open or not part of the configuration, the continuity monitor  68  will detect the err and open the associated continuity contact  150 . Similarly if there is an elevated voltage on the load  74 , (condition  48 ), the elevated voltage monitor  70  will detect the err and open the associated elevated voltage contact  148 .  
         [0074]    For each of the above errs, once the associated contact is opened, CR 4  drops out and relay RL 4  closes resulting in the illumination of the fault indicator  89 . The power to the load  74  is stopped by power supply contacts  156  and system power indicator  153  is turned off. A reset button  154  is pushed before the contactor circuit  78  may become operational.  
         [0075]    If either a voltage leak from one of the primary legs  18   a ,  18   b ,  18   c  to primary ground  20  (condition  42 ), or the primary ground  20  is open (condition  44 ), the voltage sensing devices  54   a ,  54   b ,  54   c  will detect the condition, thereby opening the associated contacts  120 ,  122 ,  124  and similarly illuminating the fault indicator  92  and removing power to the load  74 . In addition, the corresponding relay RL 1 , RL 2 , RL 3  will close causing the shock hazard indicator light  92  to illuminate. Once the err is removed, the fault indicator  92  turns off, the contacts are closed, and the circuit  50  is operational. The start button  158  must then be pushed to start the system  10 . If a user pushes the start button  158  while in the fault condition, the contactor circuit  78  will be opened and the load will not receive power.  
         [0076]    As one skilled in the art would recognize, in the embodiment with the disconnect switch  58 , the system  10  would operate if the voltage sensing devices  54   a ,  54   b ,  54   c  and transformer  56  were after the disconnect switch  58 . However, in this arrangement, the additional shock hazard indicator  92  would not be available until after the system  10  was switched on. In addition, the indicator lights are a matter of preference for alerting users to the type of condition. Other indicator mechanisms may by preferable given individual situations, such as audible alerts, readable messages.  
         [0077]    In preferred form, the ground sense lead  16  is 25 feet with a well-known industry standard ground clamp. In preferred form a plurality of components of the circuit  100  are designed on a printed circuit board mounted behind a front access door of the ground fault protection system  10 .  
         [0078]    The illustrated embodiments are only examples of the present invention and, therefore, are non-limitive. It is to be understood that many changes in the particular structure, materials and features of the invention may be made without departing from the spirit and scope of the invention. Therefore, it is my intention that my patent rights not be limited by the particular embodiments illustrated and described herein, but rather determined by the following claims, interpreted according to accepted doctrines of claim interpretation, including use of the doctrine of equivalents and reversal of parts.