Abstract:
The invention comprises a set of methods to detect the resistance of a connection to safety ground and to detect the presence of voltage hazardous to people in systems where an unknown amount of current intermittently flows in the safety ground circuit. Samples of voltage between two points in the safety ground circuit are taken repeatedly. When the voltage caused by the unknown currents in the safety ground circuit is below a preset threshold, a known current is applied and another voltage sample is taken to detect the electrical resistance of the safety ground circuit. If an excessive resistance or voltage is detected, one or more outputs are activated or deactivated to indicate the problem and remove power from the system. The invention requires no current sensor for its operation.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority of U.S. Provisional Patent Application No. 61/126,928 filed on May 9, 2008 and titled “Method for Measuring Low Resistance Values in the Presence of Ground Loop Currents and Electrical Noise”, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     In art, this invention is believed to be in class 340, subclass 649. 
     Industrial machinery often has internal wiring and components which operate with high voltages that are dangerous to people. Protection is accomplished by covering the components where high voltage exists with electrical insulation and then enclosing the high voltage components with protective metal covers or cases. All of the exterior metal parts of the machinery are then connected directly to safety ground. If any of the insulation around the high voltage components deteriorates or is broken, and a current path from the high voltage to the surrounding metal exists, the connection to safety ground is capable of carrying enough current to open the fuses or circuit breaker supplying power to the system. This prevents high voltage from appearing on any exterior metal part of the machine, where a person could touch it. 
     To assure protection against electrical shock, it is necessary to constantly check the connection between safety ground and the machine containing high voltage. The usual way is with a low-ohm resistance measurement. 
     However, certain types of industrial machinery operate in a way which causes unknown amounts of current to intermittently flow in the safety ground circuit. This current in the safety ground circuit happens during normal operation and it is not practical to eliminate it. Although these unknown currents are almost always low enough to not create any safety issues, they do interfere with the measurement of the resistance between the machine and safety ground. 
     The invention is comprised of a set of methods together with apparatus to detect the resistance of a safety ground circuit and to assure that no voltage harmful to humans exists on the safety ground circuit or the devices which are connected to the safety ground circuit. If the resistance of the safety ground circuit rises above a predetermined value or a harmful voltage is found anywhere on the safety ground circuit or an attached device, outputs are provided to indicate an unsafe condition and to cause power to be removed from the devices which contain high voltage and are connected to the safety ground circuit. 
     The invention is distinguished from the prior art by the ability to detect the resistance of the safety ground circuit even if intermittent and unknown currents are flowing in the safety ground circuit. The invention is further distinguished from the prior art by the measurement of voltage potential to detect hazardous voltage all of the time, even when the intermittent and unknown currents make resistance detection impossible. The invention uses only simple switched DC for resistance detection and does not require any sort of external sensors to measure things like current or temperature. 
     The resistance of safety ground is normally in the range of 0.1 to 10 ohms. This is because the resistance path from any point of the grounded system back to safety ground must be capable of handling enough current to quickly cause the circuit protection device (fuse or circuit breaker) feeding power to whatever is enclosed by the grounded cabinet, housing, cover or case to open if a short-circuit between incoming power and the enclosure occurs. Otherwise, the incoming power voltage would appear for significant time in places that could be touched by people. In many industrial devices, the incoming power feed is at a voltage of approximately 500 volts and is fused at hundreds of amperes. Safety ground resistances no more than a few ohms are required. 
     Within a building, safety ground is ultimately earth ground. Earth ground is achieved by driving one or more metal grounding rods 1 meter or more into the ground to establish electrical contact with the underlying soil. In a building with a steel frame, where the steel supporting members contact the ground as part of the foundation structure, the grounding rods are connected to the steel frame of the building to improve the electrical connection to the underlying soil. 
     Inside the building, where machinery is installed, there is usually a separate conductor, made of copper or aluminum which is used only for safety ground connection. This safety ground conductor is usually routed to one or more grounding rods and is also connected to the steel frame (where present) of the building at many points throughout the building. The safety ground conductor carries no powerline current and is only tied to the neutral point of the incoming power at a single point. The purpose of the safety ground is to provide a place to connect the metal protective barriers (such as cabinets and housings) which surround high voltage components to earth ground. 
     In an industrial environment, there are many things which could cause the safety ground to become disconnected. Ground cables attached to moving machinery may break due to metal fatigue in the constantly-flexing cable. Moving machinery or fork lift trucks might snag on cables and break them. Ground cables may be disconnected during repair or maintenance. In places where corrosive vapors and/or moisture are present, corrosion at joining points may occur. 
     The goal of this invention is a simple, reliable means to detect a broken safety ground connection where an apparatus connected to safety ground may cause an intermittent and unknown current, sufficient to disrupt a resistance measurement, to flow in the safety ground circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows, schematically, an example safety ground circuit in a system which includes a resistance welding control, an integral transformer resistance spotwelding gun and a fixture to hold the part which is to be welded. 
         FIG. 2  illustrates the apparatus used to perform the methods of the invention. 
         FIG. 3  is a flowchart which shows the operation of the apparatus in  FIG. 2 , when it is performing the methods of the invention to detect hazards in the safety ground circuit in the system illustrated in  FIG. 1 . 
         FIG. 4  shows a simplification of the flowchart in  FIG. 3 , where no specific timing is used, but the functions remain the same. 
     
    
    
     All of the blocks in the flowcharts of  FIGS. 3 and 4  are numbered. Where a block is identical in both figures, it carries the same number. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In order to illustrate the features of this invention, a resistance spotwelding gun is used. However, the inventive concept is useful with any type of equipment that may cause intermittent and unknown currents to flow in a safety ground circuit. 
     The resistance spotwelding process is often used to weld two or more pieces of relatively thin sheet steel together. In the spotwelding process, two copper electrodes, about the size and shape of a man&#39;s thumbs, are placed end to end on opposing sides of the sheet steel. These electrodes are normally hollow and are cooled by circulating water. The two electrodes are driven toward each other with a force of about 500 pounds, thus clamping the sheet steel between them. A large heating current, about 10,000 amperes at a low voltage (less than 10 volts) is passed between the electrodes, through the sheet steel for typically ¼ second. The steel immediately between the welding electrodes is heated quickly to the temperature which causes it to soften. The force applied by the welding electrodes forges the softened steel into a homogeneous mass to form the actual weld. As soon as the current stops, the copper electrodes, being water cooled and excellent conductors of heat, rapidly cool the steel and cause it to solidify. A spotweld to join two pieces of sheet steel 1 mm each in thickness is normally done in 1 second or less. For more details on the resistance welding process, please see U.S. Pat. No. 4,447,700, particularly FIG. 1, incorporated herein by reference. 
     The spotwelding process is often done with an integral transformer welding gun. For a detailed description of the appearance and construction of an integral transformer resistance spotwelding gun, please see U.S. Pat. No. 6,455,800, especially FIG. 1 of that patent, incorporated herein by reference. 
     The spotwelding gun is normally moved to different positions around the part, to make the welds where needed. The spotwelding gun is moved by a person, a robot or other automated means. Because the spotwelding gun is moved around, any cables connected to it are flexible and are not enclosed for protection in a rigid steel conduit pipe. However, because of the welding transformer which is built-in to the welding gun, where the primary of that transformer is operated at voltages between 400 and 800 volts, grounding of the integral transformer welding gun is very important for safety. It becomes very important to have a way to assure the integrity of the grounding cable to protect a human who might be touching or operating of the welding gun. 
       FIG. 1  illustrates schematically an integral transformer resistance spotwelding gun making a weld on a part which is held in a fixture. The three major components of  FIG. 1  are the welding control  2 , the welding gun  14  and the fixture  12 . 
     The safety ground point, item  34  in  FIG. 1 , is the connection point for safety ground for the entire system shown in  FIG. 1 . The safety ground point is a terminal of some sort where direct connection can be made to the safety ground conductor for the building. In a typical installation in a building with a steel frame, the safety ground point  34  is a bolt or stud which is rigidly attached, by welding or otherwise, to a structural member of the building, such as a steel supporting column. The safety ground conductor for the building (not shown in  FIG. 1 ) is brought to point  34  along with conductors  10  and  32 . All three conductors are securely connected together with each other and with the steel frame of the building, by bolting or otherwise, at ground point  34 . 
     Conductor  32  in  FIG. 1  goes from the safety ground point  34  to ground connection point  30  located inside the weld control cabinet  2 . Ground connection point  30  is securely connected, via welding or other means, to the metal of the welding control cabinet. 
     In most cases there are several redundant paths, in addition to conductor  32  in  FIG. 1 , to connect the metal cabinet of welding control  2  to safety ground. In most factories, multiple welding controls are grouped together and mounted to the same frame, bracket, or automation machine, which is usually made of steel. Each welding control is connected to safety ground point  34 , each with a separate conductor. Within a group of welding controls, even if one conductor  32  on one of the welding controls becomes disconnected, connection to ground is still maintained through the mounting bolts which hold the welding controls to their common mounting bracket. In addition, it is common practice to weld or bolt the mounting brackets for welding controls to a steel building support column (almost all industrial buildings have steel frames), so this forms another redundant connection to safety ground. 
     Conductor  10  in  FIG. 1  goes from the safety ground point  34  to connection point  36  located in or on Fixture  12  which is holding the part to be welded  38 . Connection point  36  is securely connected, via welding or other means, to the metal frame of fixture  12 . Because the part being welded  38  is rigidly clamped into fixture  12 , by means which do not include any electrical insulation, it is in good electrical contact with the frame of fixture  12  and thus with safety ground. 
     As shown in  FIG. 1 , there is a multi-conductor cable which runs between weld control  2  and welding gun  14 . The two dotted-line ovals, located near welding control  2  and welding gun  14  encircle the conductors which are included in this cable. There are three main conductors in the cable connecting the integral transformer welding gun  14  to the welding control  2 . These main conductors are sized to carry several hundred amperes. Two of these main conductors, identified as item  4  in  FIG. 2 , carry the high voltage power to the primary of the welding transformer. The voltage on these power conductors is normally in the 400 to 800 volt range. The third main conductor, identified as item  6  in  FIG. 1  is the safety ground connection between the safety ground point in the welding control and the welding gun. At the welding gun end of the cable, the safety ground is connected to grounding point  18 , which is usually a threaded hole in the metal case of the welding transformer. The welding transformer itself is securely bolted to the frame of the welding gun. 
     In addition to the three main conductors, the cable between the welding control and the welding gun contains at least one pilot wire, identified as item  8  in  FIG. 1 . The pilot wire is a relatively small insulated wire, 14 gauge or smaller. This pilot wire is used by apparatus  24  to detect the resistance of the safety ground circuit between welding gun  14  and the safety ground connection  30 , located inside the welding control cabinet. In an assembled integral transformer welding gun, depicted schematically in  FIG. 1 , the circuit created by the secondary of the welding transformer, the arms  22  and  28  of the welding gun, the welding electrodes and the part which is clamped between them, is referred to as the secondary circuit of the welding gun. The current necessary to make the weld flows through the secondary circuit of the welding gun. 
     On the welding gun itself, a connection must be made directly between the secondary circuit of the welding gun and ground point  18  on the case of the welding transformer. The conductor for this connection is identified as  20  in  FIG. 1 . The secondary circuit of the welding gun must be connected to safety ground because the most common failure of a welding transformer is insulation failure between the primary and secondary windings of the transformer. Since the primary of the welding transformer operates at 400 to 800 volts and the secondary of the transformer is connected directly to the arms of the welding gun which are made of large, exposed, uninsulated copper parts which can easily be touched by a person, a good connection to safety ground is essential to provide protection from electrical shock. Conductor  20  in  FIG. 1  is of similar size to the main ground conductor  6  in the cable feeding power to the welding gun. Depending on the mechanical details of the transformer and welding gun, conductor  20  is attached by bolting, welding or other means between either the transformer secondary itself or one arm of the welding gun and ground point  18  on the transformer case. 
     As shown on welding gun  14  in  FIG. 1 , pilot wire  8  is connected to a point on the secondary circuit separate from the point where conductor  20  is connected. This is done to assure that conductor  20  and pilot wire  8  can&#39;t get disconnected from the welding gun secondary circuit but remain connected to each other. 
     In  FIG. 1 , the part being welded  38  usually doesn&#39;t move during the welding process. The two or more individual pieces of stamped sheet metal which are welded together to make a finished part are tightly clamped and held in the correct positions in relation to each other by fixture  12 , usually made of steel. Fixture  12  is connected to safety ground via conductor  10 . 
     The problems which this invention addresses can be understood by looking at the current flow in the secondary circuit of welding gun  14  in  FIG. 1 . The normal flow of current to make a weld at point  16  is shown with arrows  40  and  42 . The current flow is through the upper arm  22  of welding gun  14 , through the weld at point  16  and then back to the welding gun through its lower arm  28 . 
     From the standpoint of welding current in the secondary circuit of the welding gun, there are two possible paths to complete the circuit. As shown in  FIG. 1 , one path is directly through the area to be welded  16 , as illustrated by arrows  40  and  42 . The other path is through the part to the fixture, illustrated by arrow  44 , then through the grounding conductors  10 ,  32  and  6 , illustrated by arrows  46  and  48 , then back to the secondary of the secondary circuit of the welding gun as illustrated by arrow  50 . 
     In operation to make a spotweld, the current  40  in  FIG. 1  divides along the two paths according to the relative resistances of the two circuits. For example, the electrical resistance of a resistance spotweld in steel is approximately 0.0002 ohms. The resistance of the part being welded  38  plus the ground conductors in the current path indicated by arrows  44 ,  46 ,  48  and  50  is approximately 1 ohm. Thus if current  40  is 10,000 amperes, current  42  (current through the lower arm of the welding gun) will be:
 
1 ohm/(1 ohm+0.0002 ohm)=9,998 amperes
 
and current  44  (current in the ground circuit) will be:
 
0.0002 ohm/(1 ohm+0.0002 ohm)=2 amperes
 
     Current  44  in  FIG. 1 , flowing in the ground circuit, is sometimes called ground loop current. 
     However, the resistance of the material being welded  38  in area  16  of  FIG. 1  is not a constant value. It normally changes as a weld is being made. In normal welding conditions, the resistance of the weld area  16  varies by a factor of up to three above and below the “nominal” value of 0.0002 ohms as the welding current heats the part and the force applied by the welding electrodes forges the metal to create the weld. In addition, abnormal welding conditions, such as (but not limited to) misshaped parts, rust, dirt or a misplaced paper label on the part can make the resistance in the weld area  16  any value up to infinity. Because the resistance of the weld area  16  is unpredictable, the way current  40  divides into currents  42  and  44  in  FIG. 1  will also be unpredictable. 
     The current flowing in the safety ground circuit is also affected by the magnetic field surrounding the 10,000 amperes or more of welding current in the weld area. Depending on how ground cables  6  and  10  of  FIG. 1  are positioned in relation to this strong magnetic field, the magnetic field can induce currents of several amperes in the ground cable. 
     The current flowing in the safety ground circuit is also affected by other welders, especially arc welders, which may be welding on the same part at the same time as the spotwelding operation. If the arc welding machine is not properly grounded to the part (usually through the fixture holding it), some of arc welding current may flow back through the secondary of the spotwelding gun and then through safety ground cable  6  in  FIG. 1 , thus causing an unknown current in the ground conductor. 
     Also, arc welding creates high-frequency currents. Because of the unavoidable inductance of the grounding cables (because of their length), this high-frequency current causes voltage drops across the grounding cables, which causes high-frequency noise to be injected into the safety ground circuit. 
     All of the above-mentioned factors plus others could cause an unknown current to flow through the safety ground cable at any time, even when not welding. Trying to detect the resistance of a cable while it is carrying an unknown current is impossible. The object of the invention is one or more methods to overcome this difficulty and provide increased safety. 
     The apparatus used to implement the method of the invention is shown as  24  in  FIG. 1 . In this example, apparatus  24  is located inside welding control  2 , but it could be located in other places as well. Apparatus  24  has two connections to detect the resistance between ground point  30  inside weld control  2  and the secondary circuit of welding gun  14 . Conductor  26  connects between apparatus  24  and ground point  30 . Pilot wire  8  connects between apparatus  24  and the secondary circuit of welding gun  14 . As mentioned previously, pilot wire  8  connects to the secondary circuit of welding gun  14  at a different point than the connection of grounding conductor  20 . In operation, apparatus  24  detects the resistance of the circuit between pilot wire  8  and connection  26 , which includes ground conductors  6  and  20 . When the welding gun is closed on the part  38 , the resistance will also include the circuit through part  38 , weld fixture  12 , conductor  10  and conductor  32 , as indicated by arrows  44 ,  46  and  48 . If a loose or broken connection happens anywhere in this circuit, the detected resistance will rise and the problem will be detected. 
       FIG. 2  illustrates the internal circuitry of apparatus  24 . As shown in  FIG. 2 , the major components of apparatus  24  are a DC power supply  60 , a switch  62 , a resistor  64  and a controller  68 . 
     Referring to  FIG. 2 , the positive output of power supply  60  connects through switch  62  and resistor  64  to pilot wire  8 . The negative output of the power supply  60  in  FIG. 2  connects to conductor  26 , which connects to ground point  30  in  FIG. 1 . 
     In the preferred embodiment, switch  62  in  FIG. 2  is implemented with a bipolar transistor which has a voltage drop of about 0.25 volts. DC power supply  60  has a regulated voltage output of 12.25 VDC at approximately ½ amp and resistor  64  has a value of 20 ohms. The net effect of the circuitry of the preferred embodiment is to inject a known current when switch  62  is closed, with a source voltage of 12 VDC and a 20 ohm series resistor into the safety ground circuit. In the presence of this known current, the safety ground circuit resistance can be detected with a voltage measurement between conductors  8  and  26 . Other power supply voltages, power supply polarities, switch implementations and series resistor values can be used without departing from the inventive concept. 
     The point of switch  62  in  FIG. 2  is that the actions to detect the resistance of the safety ground circuit are not done continuously. Rather, the actions to detect resistance, by turning switch  62  on and measuring the resulting voltage across conductors  8  and  26  are done only after some other conditions are met. In normal operation, switch  62  is closed only for brief intervals. 
     In  FIG. 2 , controller  68  is equipped with an output  70  to control switch  62 , differential analog inputs  72  to read the voltage between conductors  8  and  26 , digital inputs  74  and digital outputs  76 . 
     Controller  68  in  FIG. 2  is a digital device that can be implemented with discrete logic, a stored program microcomputer or other ways. Controller  68  could also be only a portion of a stored program microcomputer, which is also used for other purposes completely unrelated to measurement of safety ground circuit resistance. Since the details of implementation of controller  68  are not an object of this invention, controller  68  will be treated as a “Black Box” in this discussion; only the external behavior, not the inner workings, of controller  68  will be discussed. 
     Switch  62  in  FIG. 2  can be any sort of switch suitable for the purpose, such as an electromechanical relay or a transistor. In the preferred embodiment, output  70  of controller  68  is used to turn a transistor ON and OFF so that the transistor continuously cycles ON for 1/16 th  of a second and OFF for 15/16 th  of a second. Analog inputs  72  are used to take samples of the voltage across conductors  8  and  26  at the rate of about 2000 samples per second. During the time that switch  62  is turned ON to inject a known current into the safety ground circuit, the measured voltage is used to detect the resistance of the circuit between conductors  8  and  26 . At times when switch  62  is OFF, the voltage measured by analog inputs  72  between conductors  8  and  26  is compared to several different upper and lower limits; different actions are taken depending on the voltage found. 
     Digital outputs  76  of controller  68  in  FIG. 2  are used for user interface; to drive indicator lights, and for controlling an external device to remove power from welding gun  14  in  FIG. 1  if excessive resistance or voltage is detected in the safety ground circuit. 
     Digital inputs  74  of controller  68  in  FIG. 2  are used for user interface, such as pushbuttons or switches to select operating parameters or to reset error conditions. Digital inputs  74  can also be used to detect when the external device used to disconnect power from welding gun  14  of  FIG. 1  has operated properly. 
     The digital inputs and outputs at  74  and  76  respectively in  FIG. 2 , can be implemented in many different ways. In the preferred embodiment, described herein, these inputs and outputs are discrete and are used to turn on indicator lights, detect the status of switch inputs and directly control external devices to do such things as shut off power to a machine. However they could also be implemented in other ways, perhaps (but not by limitation) with a computer interface, such as an RS-232 serial port. In the situation where controller  68  of  FIG. 2  is implemented by software in a microcomputer which also does other things, inputs  74  and outputs  76  could simply be bits in the microcomputer memory, which are read or set by other software routines. Since the details of implementation of digital inputs  74  and digital outputs  76  are not an object of this invention, no further description is given herein. 
     The operation of apparatus  24 , shown in  FIG. 2  may be understood with the use of the flowchart in  FIG. 3 . All of the blocks of the flowchart are identified by numbers near the upper right corner of each block. The START position of the flowchart represents the power-up or initialization state of the apparatus. In the power-up or initialization state, all of the outputs  76  of  FIG. 2  are OFF, including the ones which allow power to welding gun  14  of  FIG. 1 . 
     In the flowchart of  FIG. 3 , the variable “V 1 ” represents the digital number resulting from a voltage sample taken between conductors  8  and  26 . 
     The voltage between conductors  8  and  26  is applied to the differential analog inputs  72  of controller  68  in  FIG. 3 . The voltage across analog inputs  72  is sampled at discrete times and converted to a digital number, V 1 , by an A/D converter which is built-in to controller  68 . The voltage samples are taken differentially with conductor  8  as the positive input and conductor  26  as the negative input. If the voltage at conductor  8  is positive with respect to conductor  26 , the digital number which represents the sampled voltage, V 1 , is positive. If the voltage at conductor  8  is negative with respect to conductor  26 , the digital number, V 1 , is negative. In  FIG. 3 , flowchart blocks  81  and  88 , this polarity is indicated by the (+) and (−) following the conductor numbers. 
     In the preferred embodiment, things work on a 1-second time cycle, but there is nothing in the inventive concept which requires a particular timing. In the flowchart of  FIG. 3 , a timer is used to keep track of time within each one second block of time. For the first 0.937 seconds ( 15/16ths of the interval) switch  62  of  FIG. 2  is turned OFF and the voltage between conductors  8  and  26  is sampled multiple times. In the preferred embodiment, samples are taken about once every 500 microseconds (2000 times per second), but almost any other sampling rate could be used. The only limit on the voltage sampling rate is on the low end; this device is, after all, a safety device to protect a person from electric shock. If a too-high resistance or a hazardous voltage is present, the sampling rate needs to be fast enough to detect the condition quickly, so reaction can be done quick enough to prevent injury. 
     During the first 15/16 th  of the second, the voltage sample V 1  obtained in block  81  of  FIG. 3  is compared to two different thresholds and two different actions are taken as a result. 
     If the sampled voltage, V 1 , has an amplitude whose absolute value is greater than 24 volts (block  82  in  FIG. 3 ), then the appropriate output(s)  76  are turned-on to indicate excessive voltage and actions are taken to remove power from the welding gun. The voltage of 24 volts is selected because most regulatory agencies consider voltages below 24 volts as not hazardous to humans. However, nothing in the inventive concept limits this voltage to any particular value. 
     In block  83  of  FIG. 3 , if the sampled voltage, V 1 , is above +0.2 volts or below −0.2 volts then switch  62 , shown in  FIG. 2  is turned OFF (block  92 ) and the 1-second timer is set back to zero (block  80 ). This defers the actions to detect if the resistance of the safety ground circuit is above a preset limit until the voltage resulting from an unknown current in the safety ground circuit is within the limits of +/−0.2 volts. 
     The voltage of +/−0.2 volts used in block  83  of  FIG. 3  is selected according to the apparatus of  FIG. 2  and the requirements of the resistance detection of the safety ground circuit. Referring to  FIG. 2 , when power supply  60  supplies 12 volts, switch  62  is closed, resistor  64  is 20 ohms, and the maximum allowable resistance of safety ground is 1 ohm, then the expected maximum voltage reading between conductors  8  and  26  is 0.57 volts. Thus, any unknown current which produces a voltage which is a significant fraction of 0.57 volts across conductors  8  and  26  in  FIGS. 3 and 4  will cause significant errors in the measurement. If a more accurate detection is needed or the maximum allowable resistance is different than 1 ohm, then the voltage thresholds in block  83  of  FIG. 3  may be a different value. The value of the voltage thresholds in block  83  of  FIG. 3  are selected according to the requirements of the application and are not limited by anything in the inventive concept. 
     If nothing causes the timer to reset to zero, then at times greater than 0.937 seconds (block  84 ), switch  62  is turned on (block  87 ), and the voltage V 1  between conductors  8  and  26  is sampled (block  88 ) for the resistance determination. 
     Because the current injected for the resistance reading is positive in polarity, the resulting V 1  sample should also be a positive value. V 1  is checked to be sure it is not a negative value in block  89  of  FIG. 3 . If a negative value is found, it indicates that there is an unknown current in the safety ground circuit that is interfering with a resistance measurement. In response, switch  62  is turned OFF in block  92 , the timer is reset to zero, and the resistance checks are deferred until the unknown current goes away. 
     During each one second time interval, the resistance checks are only done during the final 0.063 second ( 1/16 th  second) of the one second interval. At all other times, voltage measurements are done both to detect hazardous voltages and to detect the presence of voltage resulting from unknown currents in the safety ground circuit sufficient to disrupt the resistance detection. 
     In block  90  of  FIG. 3 , the sampled voltage, V 1 , is compared with the value (+0.57 volts) which corresponds to 1 ohm. Depending on the requirements of the overall safety system, this comparison can either be single or it can be averaged or it can be with a time delay. For example, the comparison could be done with the average value of V 1  obtained over a time interval of several seconds. In another example, there is a time delay associated with the comparison so that V 1  must be greater or less than 0.57 volts for a stated period of time, perhaps for several seconds. Since this invention is not directed to methods of averaging voltage readings or using time delays in comparisons of them, no further discussion is given herein. 
     If the voltage sample, V 1  is found to be greater than 0.57 volts in block  90  of  FIG. 3 , which corresponds to 1 ohm between conductors  8  and  26 , the appropriate outputs are turned ON or OFF in blocks  93  and  94  to indicate the problem and shut power off to the welding gun. As the flowchart indicates, once power is removed from the welding gun, the inventive apparatus is designed to keep power off to the welding gun indefinitely, until the apparatus is reinitialized (thereby returning it to the START position of the flowchart) or is powered-down. 
     For illustrative purposes, the flowchart of  FIG. 3  shows specific time values and voltage values which correspond to the preferred embodiment. However, the invention does not require any specific values and any use of specific values herein should be taken as examples of the invention, not limitations of it. For example, the invention could be practiced without any specific time intervals at all. If the voltage sample V 1  is found to be below a certain threshold as in block  83  of  FIG. 3 , then switch  62  of  FIG. 2  would immediately be turned on and another voltage sample taken to determine the resistance of the safety ground circuit. Such operation is illustrated in the flowchart of  FIG. 4 . Except for the removal of the blocks related to timing, and other blocks made redundant by that, the functions in the flowchart of  FIG. 4  are the same as  FIG. 3 . Since the flowchart blocks in  FIG. 4  are a proper subset of  FIG. 3 , their numbering is the same. 
     While the inventive methods have been illustrated herein by specific examples within a preferred embodiment, it is to be understood that the inventive methods are far broader, with many possible variations, with many possible applications, and are limited only by the appended claims.