Abstract:
A device for monitoring a neutral grounding resistor (NGR), including first and second NGRs electrically connected in parallel, a rectifier circuit electrically connected in series with the second NGR and a voltage source and a logic resistor electrically connected in series with the second NGR. A logic circuit measures current passing through the logic resistor and determines the resistance of the first NGR based on the measured current and the resistance of the second NGR. As such, a failed-open or failed-short condition of the first NGR may be identified based at least in part on the determined resistance of the first NGR.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present disclosure generally relate to an apparatus for connecting a neutral conductor of an electric circuit to ground through a neutral grounding component, and for monitoring the neutral grounding component. More particularly, the present disclosure relates to an improved apparatus that is operative to detect open states and shorted states in the neutral grounding component. 
         [0003]    2. Discussion of Related Art 
         [0004]    A wide variety of electric circuits comprise neutral conductors, which carry current in normal operation but are connected to ground. Connection to ground is desirable in order to prevent the accumulation of dangerous transient voltages between neutral and ground that can be hazardous to operators of equipment housing the circuitry. However, connecting neutral conductors to ground without intervening resistance may allow excessive current flow between neutral and ground, which can cause damage to circuit components. Therefore, some system connect the neutral conductor to ground across a neutral grounding resistor (NGR), the resistance of which is selected in order to limit ground current to safe levels in the event of a ground fault. 
         [0005]    An NGR may fail in at least two ways. The NGR may enter a failed-open condition, in which the connection between neutral and ground is broken. Alternatively, the NGR may enter a failed-short condition, in which the NGR provides no resistance, and thus the neutral conductor may be shorted to ground. As noted above, it may be undesirable to have no connection between neutral and ground, and may also be undesirable to connect neutral to ground without an intervening resistance. As such, it may be desirable to monitor an NGR, in order to detect the occurrence of failed-open conditions and failed-short conditions. However, typical systems are not capable of reliably detecting failed short conditions in NGRs. Accordingly, there is a need for a neutral grounding solution that can be monitored to detect both failed-open and failed-short conditions. 
       SUMMARY OF THE INVENTION 
       [0006]    Accordingly, there is a need for an NGR monitoring device that may identify both failed-open and failed-short conditions. Exemplary embodiments of the present disclosure are directed towards apparatuses for monitoring an NGR. For example, some embodiments describe an apparatus comprising a first neutral grounding resistor (NGR) and a second NGR electrically connected to a neutral conductor, a rectifier circuit electrically connected to the second NGR, a voltage source electrically connected to a logic resistor and the second NGR, and a logic circuit, the logic circuit operatively enabled to measure the current passing through the logic resistor. The logic circuit may determine the resistance of the first NGR based at least in part on the measured current and the resistance of the second NGR. Additionally, the logic circuit may identify a failed-short and/or a failed-open condition with the first NGR based at least in part on the measured current and the determined resistance of the first NGR. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates a conventional apparatus to monitor an NGR. 
           [0008]      FIG. 2  illustrates an apparatus to monitor an NGR based on at least some embodiments of the present disclosure. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0009]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout. 
         [0010]      FIG. 1  illustrates a conventional apparatus  100  for monitoring an NGR. As depicted, the apparatus  100  includes an NGR  102  and circuitry for monitoring the NGR  102 . A neutral conductor  110  is connected to ground  120  across NGR  102 . A voltage source  130  is connected in series with a sensing resistor  104 , NGR  102  and a logic resistor  106 . Under normal operation, current supplied by voltage source  130  passes through sensing resistor  104 , NGR  102 , and logic resistor  106 . Logic circuit  108  measures the current passing through logic resistor  106 , and computes the resistance of NGR  102  based on this measurement and upon the known resistances of sensing resistor  104  and logic resistor  106 . A failed-open condition may correspond to NGR  102  becoming the substantial equivalent of an open switch. As such, NGR  102  would pass substantially no current, and thus current also would not substantially pass through logic resistor  106 . In such a scenario, logic circuit  108  may compute an extremely large or infinite resistance value of NGR  102  based on this substantially zero current, and may detect the failed-open condition on this basis. 
         [0011]    Conversely, a failed-short condition may correspond to NGR  102  becoming the substantial equivalent of a closed switch. As such, the resistance of sensing resistor  104  and logic resistor  106  substantially determines the current passing through sensing resistor  104 , NGR  102 , and logic resistor  106 . However, the resolution of the detection capabilities of logic circuit  108  may be too coarse to detect current differentials below a certain threshold, and the current differential caused by the failed-short condition may not exceed this threshold. For example, logic circuit  108  may only be able to detect current differentials of 2.5% or greater. As such, in some instances (e.g., when a failed-short condition does not cause the current to increase above 2.5% of normal) the logic circuit  108  may not be able to differentiate between normal operation and a failed-short condition. 
         [0012]    As a particularly illustrative example,  FIG. 1  depicts the resistance of NGR  102  as 7.9Ω, which is relatively small compared to those of sensing resistor  104  (i.e., 20 kΩ) and logic resistor  106  (i.e., 1 kΩ), and thus the current that passes through logic resistor  106  during a failed-short condition is only slightly higher than it would be under normal operation. Since the resistance of sensing resistor  104  is 20 kΩ, a 2.5% change in current would correspond to a change in resistance of at least 500Ω. Under such circumstances, since NGR  102  has a resistance of only 7.9Ω, logic circuit  106  may be unable to differentiate between normal operation and a failed-short condition, and thus may not detect a failed-short condition of NGR  102 . 
         [0013]      FIG. 2  illustrates an apparatus  200  to monitor an NGR based on at least some embodiments of the present disclosure. The apparatus  200  comprises a neutral grounding component  210  and circuitry for monitoring the neutral grounding component  210 . Neutral grounding component  210  comprises parallel NGRs  212  and  214 . A neutral conductor  220  is connected to ground  230  across NGR  214 . A rectifier circuit  240  comprising rectifiers  242  and  244  (e.g., diodes, or the like) is coupled to NGR  212  and to ground  230 . A monitoring circuit  250  comprising logic circuit  252 , a logic resistor  254 , and a voltage source  256  coupled to rectifier circuit  240  and to ground  230 . Rectifier circuit  240  is operative to clamp the voltage during a ground fault, such that current is shared between NGR  212  and NGR  214 . The voltage clamping provided by rectifier circuit  240  may also provide protection for the components of monitoring circuit  250  from damage caused by ground faults. 
         [0014]    Under normal operation, current supplied by voltage source  256  (or power supply circuit) passes through logic resistor  254 , NGR  212  and NGR  214 . Logic circuit  252  measures the current passing through logic resistor  254 , and computes the resistance of NGR  214  based on this measurement and upon the known resistances of NGR  212  and logic resistor  254 . A failed-open condition may correspond to NGR  214  becoming the substantial equivalent of an open switch. As such, NGR  214  would pass substantially no current, and thus current also would not substantially pass through logic resistor  254 . Logic circuit  252  may compute an extremely large or infinite resistance value of NGR  214  based on this substantially zero current, and detect the failed-open condition on this basis. A failed-short condition may correspond to NGR  214  becoming the substantial equivalent of a closed switch. As such, the resistance of NGR  212  and logic resistor  254  substantially determines the current passing through logic resistor  254 , NGR  212  and NGR  214 . In some examples, the resistance of NGR  212  may be equal to that of NGR  214 , and greater than that of logic resistor  254 , and thus the current that passes through logic resistor  254  during a failed-short condition may be substantially higher than it would be under normal operation. Since the current passing through logic resistor  254  may be substantially higher, the current differential may be above the threshold of logic circuit  252 . 
         [0015]    For example, in one embodiment, the resistances of NGR  212  and NGR  214  may each be 960Ω, the resistance of logic resistor  254  may be 1Ω, and a 2.5% current differential threshold of logic circuit  252  may correspond to a change in resistance of 2.5% of 960Ω, or 24Ω. Under such circumstances, since NGR  214  has a resistance of 960Ω, logic circuit  252  may be able to differentiate between normal operation and failed-short conditions, as well as between normal operation and failed-open conditions. As such, logic circuit  252  may be able to detect both failed-open and failed short conditions. 
         [0016]    While the present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.