Abstract:
A method for reducing the occurrence of false ground fault detections in a central office terminal is provided. The method includes generating a no-fault signal when no ground current is detected, delaying generation of a fault signal when ground current is detected at least for the duration of an expected pulse in AC induced signal, and when the ground current persists for a sufficient period, generating a signal indicating a fault condition.

Description:
BACKGROUND  
       [0001]     In many telecommunications applications, repeaters and other electronic devices are housed in remote units scattered throughout a geographical region in the vicinity of a central office. In one example, a remote unit communicates with the central office and also receives power from the central office through the same cable or other communication medium. This cable is also referred to as a “span cable,” “plant,” or “cable plant.” An example of a span cable includes a set of twisted-pair conductors over which telecommunications data is transferred between the central office and the remote units, and over which DC power is supplied by the central office to the remote unit.  
         [0002]     The remote unit typically utilizes the power received from the central office over the span cable to power one or more electronic devices within the remote unit. The power delivered via a span cable is often susceptible to disturbances (such as faults, voltage spikes and surges) caused by environmental factors such as lighting and nearby electrostatic discharges. Left unmitigated, such power disturbances can interrupt telecommunications operations and permanently damage equipment.  
         [0003]     Many electrical protection and personnel safety systems have been developed to detect these disturbances. One such system is generically referred to as ground fault detection system. With ground fault detection, the system looks for excessive current flowing to ground. When such current is detected, the ground fault detection system takes appropriate action such as shutting down the power supply that transmits power over the span cable.  
         [0004]     AC power lines are often located within the vicinity of the span cable or plant of the telecommunications network. The signals on the AC power lines can adversely affect signals on the span cable through a phenomenon known as “AC induction.” With AC induction, an AC signal from the power lines or other source of AC power is induced onto the copper plant. When the electronic devices of the network are separated by a large distance, the plant is more susceptible to AC induction.  
         [0005]     AC voltages typically are induced longitudinally upon span cables which cause currents to flow through the longitudinal noise filter circuits to ground at both the Central Office Terminal (COT) and Remote Terminal (RT) equipment. The earth ground maintained between the COT and RT installation completes the circuit, allowing the induced voltage to maintain current flow in the communication systems grounding path. The longitudinal noise filter circuits present a relatively high impedance to ground at the AC power line frequencies to avoid large currents from flowing in the filters ground path, as would be the case in a direct contact of an AC power line with the span cable (known as a power cross event). The ground fault detection circuit is designed to monitor the level of DC current flowing in the grounding system as the result of leakage currents to ground along the cable span and equipment. AC induction currents are imposed on the DC leakage currents and can look like a ground fault to the ground fault detection circuit during the half of the AC cycle which is additive to the DC current. Thus, the AC induced signal could trip the ground fault detection circuit causing the power supply to be inadvertently turned off. This could be compensated for with a large filter, e.g., a large capacitor, in the ground fault detection circuit to filter out the AC signal. However, the filter would have to be prohibitively large and expensive due to the large voltages involved. Further, if a large capacitor is incorporated into the ground fault detection circuit, any alternating longitudinal voltage on the span would be exposed to a low (longitudinal) impedance to ground. If the power lines came into direct contact with the cable plant of the telecommunications network, the power lines would be shorted to ground through the network device. Software filters have also been used to attempt to address this phenomenon. However, the effectiveness of software filters tend to roll off at higher frequencies. It has been discovered that some of the most relevant frequencies for AC immunity are harmonics that fall outside the effective range of traditional software filters.  
         [0006]     Therefore, there is a need in the art for enhanced AC immunity in ground fault detection.  
       SUMMARY  
       [0007]     Embodiments of the present invention provide improvements in ground fault detection in a central office terminal. More specifically, in one embodiment, a method for reducing the occurrence of false ground fault detections in a central office terminal is provided. The method includes generating a no-fault signal when no ground current is detected, delaying generation of a fault signal when ground current is detected at least for the duration of an expected pulse in AC induced signal, and when the ground current persists for a sufficient period, generating a signal indicating a fault condition. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a block diagram of a telecommunications system with enhanced AC immunity for ground fault detection according to one embodiment of the present invention.  
         [0009]      FIG. 2  is a block diagram of one embodiment of an AC immunity circuit according to one embodiment of the present invention.  
         [0010]      FIGS. 3A and 3B  are timing diagrams illustrating one embodiment of a process for providing AC immunity to a ground fault detection circuit.  
         [0011]      FIGS. 4A and 4B  are timing diagrams illustrating one embodiment of a process for detecting a ground fault with a ground fault detector with increased AC immunity. 
     
    
     DETAILED DESCRIPTION  
       [0012]     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense.  
         [0013]     Embodiments of the present invention provide enhanced AC immunity in ground fault detection circuits to avoid problems with AC induced signals on telecommunication lines. Some embodiments use an AC immunity circuit that conditions the output of the ground fault detection circuit in a manner that stretches out AC pulses in the ground fault detection signal to reduce the chances of a false ground fault detection.  
         [0014]      FIG. 1  is a block diagram of one embodiment of a telecommunications system  100  with enhanced AC immunity to ground fault detection. The embodiment of system  100  is a four wire digital subscriber line communication system the includes a central officer terminal  102  and a remote terminal  104  coupled together over a communication medium  105  comprising two twisted copper pairs  106  and  108 . In other embodiments, the teachings of the present application with respect to AC immunity are applied to other systems that use copper wires exposed to potential electrical disturbances, e.g., single pair systems.  
         [0015]     In this embodiment, the central office terminal  102  provides power to and communicates data with the remote terminal  104 . The central office terminal  102  includes communication circuits  110  and  112  that communicate data with corresponding communication circuits  114  and  116 , respectively, over twisted copper pairs  106  and  108 . In one embodiment, these communication circuits communicate data using high bit rate digital subscriber line (HDSL), asymmetric digital subscriber line (ADSL), G.SHDSL, or any other appropriate xDSL or other communication protocol.  
         [0016]     Central office terminal  102  also includes power supply  118  that provides power over communication medium  105  to power remote terminal  104 . Power supply  118  includes two outputs PS+ and PS−. The output of power supply  118  is typically a negative voltage on the order of −190VDC. The power supply  118  injects the power signal on the communication medium through transformers  120  and  122  that are coupled to PS+ and PS−, respectively. The power is received in remote terminal  104  at power supply  124 . Power supply  124  is coupled to communication medium  105  through transformers  126  and  128 . Power supply  124  typically reduces the voltage level received from power supply  118  for use by the circuits of remote terminal  104 , e.g., communication circuits  114  and  116 .  
         [0017]     Central office terminal  102  also includes circuitry that is designed to protect the central office terminal  102  from damage due to electrical surges caused by various natural phenomenon, e.g., lightning strikes. The circuitry  113  appears at each interface of the twisted copper pairs  106  and  108  and is connected to ground  131 . Protection circuitry  113  activate as surge voltages rise above the trigger threshold of the protection devices used and conduct away large amounts of surge currents, thus reducing the surge voltages seen by the end terminal equipment  102  and  104 . Further protection and personnel safety circuitry includes a ground fault detector  130 , an AC immunity circuit  132  and a processor  134 . The ground fault detector  130  is coupled to the power supply signals PS+ and PS− and is adapted to determine when a current to ground  131  exceeds a selected threshold. When such a condition is detected, the ground fault detector  130  produces a signal that indicates a ground fault condition has occurred. Unfortunately, the ground fault detector  130  may provide a false indication of a fault condition due to AC induced signals on the communication medium  105 . Thus, the signal from ground fault detector  130  is conditioned to reduce the likelihood of a false indication of a ground fault condition.  
         [0018]     This embodiment uses a combination of circuit elements to reduce the potential for false indications of a ground fault due to AC induced signals on communication lines  105 . Central office terminal  102  includes, for example, a combination of software and hardware filtering along with circuitry that extends AC pulses in the output of ground fault detector  130 . In one embodiment, the hardware filter comprises a capacitor built into the ground fault detector. An example of this type of hardware filter is shown and described below with respect to  FIG. 2 . Further, the software filtering is typically implemented in processor  134 .  
         [0019]     AC immunity circuit  132  implements the pulse extender functionality. For example, AC immunity circuit  132  receives the ground fault signal from ground fault detector  130 . When AC current is present, periodic pulses corresponding to the various harmonics of the AC fundamental frequency occur in the signal from ground fault detector  130 . The AC immunity circuit  132  stretches out the pulses for a period of time sufficient to allow the software filtering of processor  134  to prevent a false indication of a ground fault condition caused by the higher harmonic pulse rates which exceed the software filters sampling capability. In one embodiment, the AC immunity circuit  132  stretches out the portion of the AC signal that indicates no fault condition such that the output of ground fault detector  130  is conditioned to remain in a no fault state for a prolonged period.  
         [0020]     Processor  134  over-samples the output of AC immunity circuit  132  periodically to determine if there is a ground fault in the communications system. Without AC immunity circuit  132  or a software filtering means, the processor  134  was prone to false detection of ground faults because it could sample the signal from the ground fault detector in a low state (active state) caused by the pulses from the AC induced signal. With some software filtering means, this problem was partially solved, e.g., at lower harmonic frequencies. However, due to the presence of higher harmonics in the AC induced signal, e.g., harmonics above 180 Hz (especially at 540 Hz and 900 Hz), the software filtering could not achieve the sampling rate to eliminate the problem. With the addition of AC immunity circuit  132 , the effective sampling rate of the processor is increased beyond the harmonic frequencies of the AC induced signal, thereby improving the accuracy of the ground fault detection circuit.  
         [0021]      FIG. 2  is a block diagram of one embodiment of a ground fault detector  200  and an AC immunity circuit  202  for use in a central office terminal, e.g., central office terminal  102  of  FIG. 1 . AC immunity circuit  202  conditions the output of ground fault detector  200  so as to reduce the effect of AC induced signals in the telecommunications system.  
         [0022]     Ground fault detector  200  detects ground fault conditions. Ground fault detector  200  includes transistor  204  and resistor  206 . Resistor  204  is coupled to the supply signal PS+. In most embodiments, the PS+ signal is referenced to ground such that all power signals in the system are below ground potential to prevent corrosion as is known in the art. Resistor  206  is also coupled to the emitter of transistor  204 . The collector of transistor  204  is coupled to chassis ground, e.g., earth. The base of transistor  204  is coupled to the PS− signal through resistor  210 . Ground fault conditions cause current to flow in resistor  206  and transistor  204 . This current is used to indicate a ground fault condition when it rises above a selected level.  
         [0023]     Ground fault detector  200  also includes an optocoupler  208  that is coupled to the base of transistor  204  and the PS+ signal. Optocoupler  208  is turned on when current above a selected level flows to earth through resistor  206  and transistor  204 . When this condition is detected, the output of optocoupler  208  transitions to a low output voltage to indicate the fault condition.  
         [0024]     When an AC signal is induced on the communication medium, e.g., communication medium  105  of  FIG. 1 , this signal causes an AC current to flow in resistor  206  and transistor  204 . Thus, on one-half of the AC cycle of the induced signal, optocoupler  208  is turned on and on the other half the cycle optocoupler  208  is turned off. If left unmitigated or filtered, this AC signal can lead to false indications of a ground fault in ground fault detector  200 .  
         [0025]     Ground fault detector  200  includes a hardware filter that is used to at least partially address this problem. The hardware filter, in this embodiment, comprises capacitor  212  coupled across resistor  206  and transistor  204 . Unfortunately, the value of capacitor  212  cannot be made large enough to fully remove the AC components because such a capacitor would provide a dangerous low impedance path to ground and prevent proper electrical protection circuit function. Further, such a capacitor would be prohibitively large and expensive due to the low frequencies involved with AC signals. Capacitor  212  can be made of sufficient size to provide sufficient filtering of only the highest frequency harmonic components (above 1020 Hz) of the AC induced signal. The lower and middle frequency components (60 Hz through 900 Hz) are addressed through pulse extension and software filtering.  
         [0026]     Pulse extension is accomplished in AC immunity circuit  202 . AC immunity circuit  202  is coupled to the output of ground fault detector circuit at node  214 . AC immunity circuit  202  conditions this signal at node  214  and provides an output at node  216 .  
         [0027]     In one embodiment of AC immunity circuit  202  includes a comparator  218  that compares a reference voltage at input  220  with a signal at input  222 . The reference voltage is established by a voltage divider comprising resistors  221  and  223  and power supply  225 . The signal at input  222  is the signal from node  214  with pulses caused by induced AC.  
         [0028]     AC immunity circuit  202  extends the pulses in the signal at node  214  using two signal paths with different time constants. The two signal paths control the charging and discharging of capacitor  224 . The first signal path charges capacitor  224 . The first signal path includes resistor  226  and diode  228 . Resistor  226  is coupled between node  214  and a power supply  225 , e.g., a 3.3 V supply. Diode  228  is coupled between node  214  and node  222 . Capacitor  224  is coupled between node  222  and ground.  
         [0029]     The second signal path controls the discharging of capacitor  224 . The second signal path includes resistor  230  coupled between nodes  214  and  222 . Resistor  230  has a resistance value that is substantially greater than resistor  226 . This difference in resistance values controls the difference in time constants between the two paths. In one embodiment, resistor  226  is 10 KΩ and resistor  230  is 75 KΩ.  
         [0030]     The operation of the circuit of  FIG. 2  is described with respect to the timing diagrams of  FIGS. 3A, 3B ,  4 A, and  4 B.  FIGS. 3A and 3B  illustrate the conditioning effect of AC immunity circuit  202  on the output of ground fault detector  200  in the presence of an AC induced signal. Further,  FIGS. 4A and 4B  illustrate the manner in which AC immunity circuit  202  processes a signal without an AC induced component.  
         [0031]      FIG. 3A  illustrates a signal at node  214  when an AC signal is induced on the communication medium, e.g., communication medium  105  of  FIG. 1 , by a co-located power line. At time T 1 , the ground fault detector signal  214  is at a high voltage level corresponding to the time where the induced AC current opposes the DC leakage current, resulting in a net current below the selected level of the ground fault detector. At time T 2 , the signal output by the ground fault detection circuit transitions to a low voltage which corresponds to the point where the induced AC current becomes additive to the DC leakage currents and exceeds the selected level of ground fault detector  200 . In the absence of AC induction, a continuous low voltage state of this signal would indicate that a DC ground fault has been detected.  
         [0032]     During this cycle (between T 2  and T 3 ), the voltage at node  214  is grounded and the capacitor  224  is enabled to discharge through resistor  230 , e.g., through the second signal path. Because the time constant of the second signal path is much longer than the time constant of the first signal path, the voltage at node  222  does not change significantly before it is recharged as described below.  
         [0033]     The first signal path of AC immunity circuit  202  maintains the no fault state during the other half of the AC cycle of the signal at node  214 . At time T 3 , the signal shown in  FIG. 3A  returns to a high voltage level. This turns on the diode  228  and allows the capacitor  224  to be charged from voltage source  225  through resistor  226  and diode  228 . Because the resistor  226  is selected with a lower resistance value, the capacitor is quickly charged up to a level sufficient to maintain the indication of no fault condition.  
         [0034]     The AC immunity circuit  202  produces the conditioned ground fault detection signal based on the voltage across capacitor  224  appearing, at node  222  which is the input to comparator  218 . A sample of comparator  218  output at node  216  of AC immunity circuit  202  is shown in  FIG. 3B . As can be seen in  FIG. 3B , the output voltage at node  216  remains constant at a level indicating no fault despite the AC induced signal on the communication lines. The voltage at node  222  is maintained above the reference voltage set at node  220  by the resistor divider. Thus, comparator  218  produces the high voltage output at node  216 . This indicates to the processor that there is no fault despite the AC induced signal.  
         [0035]     When the DC leakage current increases above the selected level, a true DC ground fault conditions occurs. Increased DC leakage current decreases the high times and increases the low times in signal  3 A, allowing the second signal path of AC immunity circuit  202  to discharge capacitor  224 . The resulting voltage on node  222  falls below the selected level on node  220  forcing comparator  218  output  216  to go low, AC immunity circuit  202  produces a slightly delayed signal indicating the ground fault condition as shown in the timing diagrams of  FIGS. 4A and 4B . This delay is caused by the time constant of the second path in AC immunity circuit  202  that compensates for the AC induced signal. In this example, a ground fault occurs at time T 5 . At time T 5 , the signal from the ground fault detector transitions from a high (no fault) condition to a low (fault) condition. At this time, diode  228  is turned off and capacitor  224  is slowly discharged through resistor  230 . When the capacitor voltage drops below the reference voltage at node  220 , the comparator  218  trips and changes the output at node  216  to a low voltage level indicating a ground fault at T 6 .  
         [0036]     It is noted that the described embodiments have used a low voltage level to indicate a fault condition. It is understood that in other embodiments, a fault condition is indicated by a high voltage signal. Further, capacitors  242  and  244  are standard bypass noise capacitors. In one embodiment, resistor  250  is included as a pull-up resistor because comparator  218  is open collector. Further, optional resistor  248  can be included to add hysteresis to comparator  218 , but, it is not necessary to improve noise performance. Further, with the use of AC immunity circuit  202 , the processor, in some embodiments, does not implement a software filter on the output of AC immunity circuit  202 .  
         [0037]     In another embodiment, AC immunity circuit  202  extends any AC pulse that is present in the output of the ground fault detector  200  through the use of a retriggerable monostable timer to decrease the chances of a false positive indication of a ground fault. The pulse is extended at least for the duration of the low voltage cycle of the AC induced signal. In one embodiment, the pulse is stretched past the edge of the next sampling period. This effectively raises the bandwidth of the software filter.