Abstract:
A ground fault interrupt (GFI) circuit detects and interrupts a ground fault, even in the presence of AC power induced currents. The GFI circuit can be used for power delivery between nodes in a network, such as from a central office to remote network equipment. The GFI circuit meets GFI specifications covering such an application. The GFI circuit detects rapid changes and slow rises in ground fault current. Safety features, such as intermittent interruptions of power on power lines in an event of a ground fault, are supported by the GFI circuit to protect field personnel. A digital processor may be used to implement aspects of the GFI circuit to support changes of or various operating environments.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    As the number and variety of services provided by telecommunications service providers has grown, so too has the demand for power necessary to operate the associated equipment. To provide additional power, equipment providers have increased the source voltage of the power supplies used to power the remote units. These increases have resulted in potentially dangerous operating conditions for service persons that install and maintain the equipment. 
         [0002]    Within a telecommunications system, network power is commonly generated in a centralized location and distributed to a number of remote locations, particularly with equipment close to end subscribers. For example, a remote device terminal may contain an optical interface unit that provides power over a transmission link, such as a twisted pair of wires, to a number of remotely located optical network units. In addition to being more economical, it may be more practical than generating power at each remote unit. However, the twisted pair of wires often use the same transmission delivery infrastructure (e.g., telephone poles) that is used to deliver alternating current (AC) power to end subscribers. High power levels in AC power lines and resulting electromagnetic induction may induce an AC power line current on network power lines (e.g., the twisted pair of wires). 
         [0003]    To protect service persons, a number of safety standards have been created. These industry standards define operating conditions and requirements for telecommunication equipment. To obtain certification by a particular standards board, equipment must adhere to the standards required by that particular standards committee. Underwriters Laboratories Inc. (UL) has published the “Standard for Safety for Information Technology Equipment—Safety—Part 21: Remote Power Feeding, UL 60950-21,” the entire teachings of which are incorporated herein by reference, which defines various safety standards. For example, section 6.2.3 of that standard states that a voltage limited remote feeding telecommunications (RFT-V) circuit whose open circuit voltage exceeds 140 volts (V) direct current (DC) shall limit the ground fault current to 10 milliamps. Another standard, Telcordia GR-1089 Issue 3, Table 5-2, the entire teachings of which are incorporated herein by reference, further requires that the 10 milliamps be detected in the presence of 7.1 milliamps root mean square (RMS) of AC power line induced current on a two-pair circuit. 
         [0004]    To protect service personnel, equipment should be able to detect ground fault current conditions that result when a person is accidentally exposed to a hazardous operating condition and shut down the power source should the ground fault current condition continue. Ground fault currents may occur rapidly or they may occur more slowly. Thus, there is a need to be able to detect both rapidly and slowly occurring ground fault currents in the presence of AC power line induced currents. 
       SUMMARY OF THE INVENTION 
       [0005]    A system in accordance with an embodiment of the present invention includes a switch to interrupt current on a power line when a ground fault is detected. The system may further include a fast response unit and a slow response unit. The fast response unit may produce a state change of a fast response output based on the current on a power line to indicate a ground fault at a first time after the ground fault occurs. The slow response unit may produce a state change of a slow response output based on the current on the power line to indicate a ground fault at a second time after the ground fault occurs. The second time may be later than the first time. The system also includes a switch that may be responsive to the state change of the fast and slow response outputs and may interrupt the current on the power line in an event the state change of the fast or slow response output indicates a ground fault. 
         [0006]    The fast response unit and the slow response unit may detect ground fault currents in the presence of large loop DC currents and common mode AC induced currents. Thus the system may detect both rapidly and slowly occurring ground fault currents in the presence of AC power line induced current. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0008]      FIG. 1  is a block diagram of a communications network including a system in which an embodiment of a ground fault interrupt (GFI) unit of the present invention may be deployed; 
           [0009]      FIGS. 2A and 2B  are detailed block diagrams of an optical interface unit of  FIG. 1  with a ground fault interrupt unit in accordance with embodiments of the present invention; 
           [0010]      FIG. 3  is a functional block diagram of a circuit in the ground fault interrupt unit of  FIG. 1  in accordance with one embodiment of the present invention; 
           [0011]      FIG. 4  is a more detailed diagram of the ground fault interrupt circuit of  FIG. 3  in accordance with one embodiment of the present invention; 
           [0012]      FIG. 5  is a functional diagram of the switch in connection with the ground fault interrupt circuit of  FIGS. 3 and 4  in accordance with one embodiment of the present invention; 
           [0013]      FIG. 6  is a schematic diagram of the sensor of  FIG. 4  in accordance with one embodiment of the present invention; 
           [0014]      FIG. 7A  is a symmetrical impulse response plot of a fast response unit in accordance with one embodiment of the present invention; 
           [0015]      FIG. 7B  is a step response plot of a fast response unit in accordance with one embodiment of the present invention; 
           [0016]      FIG. 7C  is a gain response plot of a fast response unit in accordance with one embodiment of the present invention; 
           [0017]      FIG. 8A  is a plot of a ground fault current that may be detected in accordance with one embodiment of the present invention; 
           [0018]      FIG. 8B  is an output response plot of a fast response unit and a slow response unit in accordance with embodiments of the present invention; 
           [0019]      FIG. 9  is a flow diagram illustrating a method for interrupting the current on a power line due to a ground fault interrupt in accordance with one embodiment of the present invention; and 
           [0020]      FIG. 10  is a flow diagram illustrating a method for interrupting the current on a power line due to a ground fault interrupt and attempting to restore power in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    A description of example embodiments of the invention follows. 
         [0022]      FIG. 1  is a network diagram of an exemplary portion of a telecommunications network  100  in which an embodiment of the present invention may be deployed. A Central Office (CO)  105  communicates with a Remote Device Terminal (RDT)  115  via communications link  110 . Within the RDT  115  is an Optical Interface Unit (OIU)  120 . Within the OIU  120  is a Ground Fault Interrupt (GFI) unit  125 . The OIU  120  in turn, communicates with Optical Network Units (ONU)  140  via fiber connections  135  and power lines  130  (e.g., twisted pair wires). The ONUs  140 , in turn, communicate via copper links  145  (e.g., twisted pair wires) with devices (not shown) in end user premises  150  (e.g., homes or workplaces). 
         [0023]    With the recent development of and increased demand for telecommunications services, such as Internet Protocol television (IPTV), higher bandwidth signals between the OIU  120  and ONUs  140  have caused an increase in power usage by the ONUs  140 . To support an increased power draw, the OIU  120 , which supplies power for the ONUs  140  rather than a remote power source such as a power line, drives −190V DC rather than, for example, the −140V DC of earlier systems. The voltage increase results in safety and ground fault interrupt (GFI) issues, such as AC power line induced signals, that are not issues at the lower voltage. Embodiments of the present invention address the difficulty of detecting ground fault currents in the presence of AC induced signals. Moreover, embodiments of the present invention address fast and slow rising ground fault currents and support safety issues by providing, for example, a mechanism to allow a service person to release the power line  130  should the person&#39;s handling of the power line be the source of the ground fault. Other features are described in reference to  FIGS. 2A-10 . 
         [0024]      FIGS. 2A and 2B  are block diagrams of an exemplary portion of a power supply section  200  of an OIU, such as the OIU  120  of  FIG. 1 , in accordance with embodiments of the present invention. Referring to  FIG. 2A , two power conversion units are provided: a network power conversion unit  205  and a local power conversion unit  215 . Both power conversion units  205 ,  215  are connected to a −48V power supply via a −48 V supply line  235  and a  48  V return (RTN) line  240 . The network power conversion unit  205  also communicates with a complex programmable logic device (CPLD)  210  via a variety of control and status signals, such as GFI trip  245 , GFI reset (RST)  250 , and power fail  255 . 
         [0025]    The local power conversion unit  215  may be a low voltage power supply, for example, providing +3.3 volts via a 3.3V line  265  and a low voltage return (LV RTN) line  260 . The network power conversion unit  205  may also generate a high voltage signal, for example, −190V via a −190V line  225  and 190 RTN line  230  for transmission along a backplane  268 . In the example embodiment, the CPLD  210  is isolated from sections of the network power conversion unit  205  and local power conversion unit  215  via an isolation barrier  220 . The −190V signals  225  and  230  and associated circuitry (not shown) may also be isolated from the −48 volts signals  235  and  240  via the isolation barrier  220 . 
         [0026]      FIG. 2B  depicts, in further detail, the network power supply section  200  of the OIU  125  of  FIG. 1 . A local power conversion unit  270 , i.e., +3.3V conversion unit  270 , and a network power conversion unit  275 , i.e., −190V conversion unit  275 , are connected to the −48V source  243  via lines  235  and  240 . The output of the +3.3V conversion unit  270  is connected to the PWB distribution unit  295  via a connection  265 . In this embodiment, the local power conversion unit  270  is a low power DC supply that typically provides two voltages: +3.3V for the OIU optics and logic sections (not shown), and +12V bias voltage for the network power conversion unit  275 . 
         [0027]    Since the local power conversion unit  270  is a low power supply (typically less than 5 watts), a flyback converter (not shown) may be used to minimize parts count. A converter switch (not shown) may drive the primary of a coupled inductor (not shown) with three secondary windings that provide the source for three voltage rails. Voltage rails track closely in a flyback converter, so the voltage feedback is simplified by regulating a +12V DC bias voltage that is referenced to the −48V input voltage. Current mode control may be used to improve stability and inherent voltage feed-forward. To minimize power loss, a high voltage startup regulator allows the control circuit to begin operation on the −48V source  243 , but switched to the +12V DC bias source (not shown) once the flyback converter is operating. 
         [0028]    Because a precision +3.3 V DC (e.g., ±5%) is required to power the logic and optics sections, a post regulator (not shown) may be used on the 4V DC output of the flyback converter. The post regulator circuit provides an accurate +3.3 V DC with minimal effect on the overall circuit efficiency. The +3.3 V DC and +12 V DC voltage outputs of the local power conversion unit  270  are fed to the PWB distribution unit  295  via connection  265 . 
         [0029]    The network power conversion unit  275  may be a high power, high voltage DC power supply with outputs  225 ,  230  that are current limited and ground fault-protected by a GFI circuit  280 . The network power conversion unit  275  may be a DC isolated forward DC to DC using a forward converter topology. The secondary winding (not shown) may have a center tap winding (not shown) to allow two 95V outputs to be summed to 190V. A common core coupled inductor (not shown) may be used on the outputs  225 ,  230 . A split secondary approach causes lower voltage stresses on all high voltage components and minimizes losses on secondary rectifiers as well as lower cost, lower voltage filter capacitors. The network power conversion unit  275  may provide 100 watts: 50 watts for the ONU  140  and  50  watts that may be “dropped” across the power lines  130  that provide power to the ONU  140  of  FIG. 1 . 
         [0030]    At a power level of 100 watts, the added complexity of the forward topology over that of a flyback design may be warranted because forward converters are inherently more stable. Forward converters are isolated BUCK converters and, therefore, supply power on both ON and OFF cycles of the primary switch. Flyback converters transfer power to the output only when the primary switch is turned OFF. The output of a flyback should be sustained by the output capacitor (not shown) while the primary switch is ON. This is useful because the ONU may have the added requirement of a ground fault interrupter (GFI), which means that a load may swing from 100 watts (full load) to 0 watts (no load) and back quickly again. The design may be implemented as a single switch, active clamp/reset, forward converter with current mode control. An active clamp/reset circuit maximizes power density and efficiency. The voltage feedback may be provided over a high bandwidth opto-coupler circuit, which may be driven by a precision voltage feedback operational amplifier and secondary side current limit circuit. 
         [0031]    A GFI circuit  280  is provided to protect craftsmen from electrical shock. The GFI circuit has an added benefit of extinguishing protection components, such as gas tubes that create fault currents to ground. The GFI circuit  280  may use a microcontroller  282  to monitor large loop DC current and common mode ground fault current of the network power line and interrupt power if a ground fault current is detected. A ground fault current event may be indicated by a GFI trip signal via a connection  245  between the GFI circuit  280  and CPLD  210 . Status and control signals  285  may be communicated to and from the CPLD  210  via another connection  290 . The network power conversion unit  275  may also communicate a power fail signal  255  to the CPLD  210 . 
         [0032]      FIG. 3  is a block diagram of an example GFI circuit, such as the GFI circuit  280  of  FIG. 2B . Power lines between an OIU and an ONU (see  FIG. 1 ) may be provided as twisted pair wires, sometimes referred to as a tip lead  375  and a ring lead  380 . One side of the tip lead  375  may be connected to a resistor  360 , and the other side of the resistor  360  may be connected via a connection  375  to the ONU (not shown) and an over-voltage protection device, such as a sidactor  365  for lightening strike protection. In the example embodiment of  FIG. 3 , a switch  345  is placed in-line with the −190V lead  355 . The switch  345  is responsive to a signal  335 , which may be analog or digital depending on implementation, to open and close the switch  345 . The switch may be implemented in a variety of ways known to one skilled in the art, for example, a MOSFET. In this example embodiment, the output side of the switch  345  may also be connected to a resistor  360 , and the other side of the resistor  360  is connected to the ONU and the protection device  365 , which may be connected to ground  370 . 
         [0033]    The ground fault interrupter  305  contains a fast response unit  315  and a slow response unit  320  that may be part of, for example, a microcontroller  310  or other electrical/electronic device(s) suitable and configured to support GFI in a manner disclosed herein. The ground fault interrupter  305  may detect a ground fault condition based on input signals RGND  325  and GFI Current  330  (discussed below in reference to  FIG. 6  in more detail). Upon detection of a ground fault condition, the ground fault detector  310  may communicate a signal  335  via connection  340  to open the switch  345 . As described in further detail below, the ground fault detector  310  may open the switch  345  temporarily, intermittently, or until the ground fault detector  310  is manually reset. 
         [0034]    Power lines tip  375  and ring  380  are often run in parallel with AC power lines  390  as they make their way from the RDT  115  to the ONU  140 . Referring to  FIG. 3 , AC power lines  390  may create an AC induction field  385  that results in a current being induced on the tip  375  and ring  380  leads. Typically, induced current appears as a periodic common mode current that may produce a false indication of a ground fault condition. An exemplary embodiment of the present invention described herein provides a system and corresponding method that detects ground fault currents in the presence of the induced currents produced by the AC power line induction  385  and reduces false ground fault events. 
         [0035]      FIG. 4  is a detailed block diagram of an example ground fault interrupter  402 , such as the ground fault interrupter  305  shown in  FIG. 3 . In the example ground fault interrupter  402 , a sensor  490  coupled to sense resistors (discussed below in reference to  FIGS. 5 and 6 ) and is configured to produce an analog signal  492  representative of ground fault currents i gfc    482  in a telecommunications network power system  400 . The analog signals  492  are communicated to a ground fault detection unit  405 . The analog signals  492  may then be digitized into a digital signal  412  using a sampler  410  sampling the output of the sensor  490  at a particular sampling rate  415  using sampling methods known to those skilled in the art. Alternatively, other analog signal processing methods employing, for example, discrete components may also be used. 
         [0036]    The digital signals  412  are then communicated to a fast response unit  425  and a slow response unit  430  via a connection path  420 . Examples of a fast response unit  425  and slow response unit  430  may include, for example, software filters, such as a finite impulse response (FIR) filter, among others. Outputs of the fast response unit  425  and slow response unit  430  may be communicated to a reset unit  440  and a reporting unit  445  via a connection path  435 . The reset unit  440  may then communicate an ‘open/close’ signal  442  to open or close a switch  470  via a connection path  465 , causing power on a downstream portion of the ring lead  485  relative to the switch  470  to be interrupted or restored, respectively. 
         [0037]    A reporting unit  445  coupled to the fast and slow response units  425 ,  430  via a connection path  435  may provide a status indicator signal  495  indicative of a ground fault condition to an operator interface  455 . Indicators may, for example, be visual, such as a light produced by a light emitting diode (LED) (not shown) or an indicator on a display screen at the operator interface  455 , or an audible alert, such as a beeping sound, to notify an operator that a ground fault condition has occurred. The status indicator signal may also be a wireless signal or a wired signal  460  that may, for example, be transmitted on a network (not shown) using methods known in the art to a person&#39;s pager or personal communications device, such as a cell phone or to a central monitoring facility. 
         [0038]      FIG. 5  depicts a circuit diagram of an exemplary embodiment of a switch  512 , such as the switch  470  shown in  FIG. 4 . A tip lead  530  of the network power supply  500  is referenced to RGND  555  through a sense resistor  545 . A ring lead  525 ,  535  is nominally at −190V DC with respect to ground. A ground fault current i gfc    532  from the tip lead  530  to earth ground (i.e., RGND  555 ) results in a current through the sense resistor  545  that is communicated in the form of a corresponding voltage Vsense  548  via a connection  540  and RGND  555  to a sensor described below and in  FIG. 6  in more detail. 
         [0039]    Continuing to refer to  FIG. 5 , when a ground fault interrupt detection unit  505  detects a ground fault event, it may generate a signal  510  and send the signal  510  to an opto-coupler  515 . The opto-coupler  515 , in turn, generates a signal  550  (e.g., closes an internal pathway) that is communicated to a switch, for example, the gate of an n-channel MOSFET  520 , to open or close the MOSFET  520 . The n-channel MOSFET  520 , in turn, interrupts the current or restores the current on the ring  535  lead. Other circuit design techniques known in the art may be used to limit current flow in a network power line. 
         [0040]      FIG. 6  is a schematic diagram of a sensor  600 , such as the sensor  490  shown in  FIG. 4 , to sense ground current. An input signal, GFI Current  615 , shown in  FIG. 5  as signal  540 , represents a sense current through the sense resistor  545 . Note that GFI current  615  may be provided in the form of a voltage Vsense  648 . The GFI Current signal  615  is connected to one side of a resistor  625 . The other side of the resistor  625  is connected to level shifting resistors  660  and  665  for bias purposes and to a positive input resistor  635 . 
         [0041]    A reference sense resistor  620  may have one side tied to RGND via a connection  610  and the other side connected to level shifting resistors  675  and  680 , also for bias purposes, a negative input resistor  630 , and a feedback resistor  645 . The other side of the input resistor  635  is connected to the positive input of a differential amplifier  605 , and the other side of input resistor  630  is connected to the negative input of the differential amplifier  605 . 
         [0042]    The output of the differential amplifier  605  is connected to the feedback resistor  645  and output resistor  650 . The other side of level shift resistors  660  and  675  and a supply lead of the differential amplifier  605  are connected to a power supply, such as the +3.3V source generated in the local power conversion shown in  FIG. 2B . The other side of level shift resistors  665  and  680  and another supply lead of the differential amplifier  605  may be connected to DGND  685 . An output capacitor  655  may be connected to the other side of output resistor  650  to create a low-pass filter to improve signal quality for use by subsequent components (not shown) in the signal chain. 
         [0043]    The differential voltage across the sense resistors may be amplified and offset so that ±27.5 milliamps of current corresponds to an input range (e.g., +511 to −512) of a sampler, such as the sampler  410  shown in  FIG. 4 . The sense resistor  545  also senses AC inducted currents. Other circuit designs may be envisioned that produce similar results known in the art. 
         [0044]      FIGS. 7A-7C  represent characteristic response plots  700  of a fast response unit  425  shown in  FIG. 4 . In one exemplary embodiment of the present invention, the fast response unit may be realized through the use of two finite impulse response (FIR) filters implemented using digital signal processing (DSP) techniques. The fast response unit  425 , as shown by way of the plots in  FIGS. 7A-7C , is particularly effective at detecting rapidly occurring ground fault currents in the presence of periodic AC noise. For example, the fast response unit may be configured to detect 10 milliamps of ground fault current in approximately 10 milliseconds in the presence of 7.1 milliamps of AC induced current. Larger amounts of ground fault current may be detected in less time. 
         [0045]      FIG. 7A  is a plot  705  that shows an output response of a first FIR filter with 16-coefficients using a hamming window, a sampling rate of 16 times the power line frequency, and a lowpass cutoff frequency of 100 Hertz. The vertical axis  715  represents the magnitude of the impulse response, and the horizontal axis  720  represents the filter coefficient. As can be seen in the plot  705 , the signal trace  710  is symmetrical. Therefore, the value of coefficient  0  is the same as the value of coefficient  15 , the value of coefficient  1  is the same as the value of coefficient  14 , and so on. Because the value of coefficients  0 - 7  are equal to the value of coefficients  15 - 8 , respectively, filter data for equivalent coefficients may be calculated in one operation. Thus, the symmetrical nature of the output response effectively reduces the time required to calculate the 16-point FIR filter data in half. 
         [0046]      FIG. 7B  is a plot  730  that shows a signal  735  representing a step response of the first FIR filter discussed above in reference to  FIG. 7A  in which a 10 milliamp ground fault current is detected within 10 milliseconds. The vertical axis  740  represents the ground fault detection state, where a ‘zero’ level  750  represents a ‘no ground fault’ condition, and a ‘one’ level  755  represents a ‘ground fault’ condition. The horizontal axis  745  represents the sample number of the sampled signal. The time period for each sample may be determined by calculating the inverse of the sampling frequency. For example, if the sample rate is 960 Hertz (i.e., 16*60 Hertz), the time period for each sample is equal to 1/960 Hertz or 1.042 milliseconds per sample. For the plot  730  shown in  FIG. 7B , the step response of the signal  735  indicates a ground fault condition may be detected within approximately 10 samples or approximately 10.4 milliseconds (i.e., 10*1.042 milliseconds). 
         [0047]      FIG. 7C  is a plot  760  that shows the gain response of a second FIR filter designed to reject periodic AC induced signals according to an exemplary embodiment of the present invention. The vertical axis  770  represents the magnitude of the gain response normalized to 1, and the horizontal axis  775  represents the frequency of the filtered signal  765  in Hertz. A point  780  shown on the signal trace  765  represents the gain response at a power line frequency, for example 60 Hertz. Here, the signal is attenuated approximately −1.71 decibels. The other point  785  shown on the signal trace  765  represents the gain response at 3 times the power line frequency, for example, 180 Hertz. Here, the signal is attenuated by approximately −50 decibels. While the plot  760  shows there is some 60 Hertz rejection, the second FIR filter response provides significant rejection at 180 Hertz. In applications where it is desirable to filter periodic AC signals induced upon a network power line, more energy may be present in the third harmonic, allowing the technique of the present invention to more accurately detect a ground fault condition in the presence of periodic AC induced signals such as the (Telcordia) 7.1 milliamp RMS two-pair specification cited above. 
         [0048]    The fast response unit is particularly well adapted to detect rapidly occurring ground fault currents. However, there are occasions where ground fault currents may increase more slowly, increasing over a period of time until the detected current satisfies a ground fault condition criteria. Since the fast response unit is optimized to detect rapidly changing signals (e.g., within 10 milliseconds) and reject periodic AC signals, slowly increasing ground fault currents may not be detected by the fast response unit. The slowly increasing ground fault currents may also be subject to the same periodic AC power induced signals as described above. In this situation, a slow response unit, such as the slow response unit  430  shown in  FIG. 4 , may be employed to detect slowly increasing ground fault currents in the presence of periodic AC induced current. 
         [0049]      FIG. 8A  is a plot  805  produced by a computer simulation that illustrates a ground fault current  820  in the presence of a periodic AC induced signal. The signal  820  may be represented by the equation: 
         [0000]    
       
         
           
             
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         [0000]    where R=7.1 milliamps of AC power induced current, n=sample number, and Fs=sampling frequency. 
         [0050]    The vertical axis  810  represents the magnitude of the ground fault current in milliamps. The horizontal axis  815  represents the sample number. The plot  805  shows a signal  820  that represents 7.1 milliamp RMS of 60 Hertz induced AC noise with a 10 milliamp ground fault current step. A non-ground fault condition is indicated in the plot region  830  representing samples 0-100. At approximately sample number  100 , depicted as a point  840  on the signal trace  820 , the detected current experiences a 10 milliamp ground fault current step. The 10 milliamp step continues in the plot region  835  representing approximately samples  101 - 200 . The magnitude of the AC induced current frequently may produce false ground fault detection in prior art systems resulting in unnecessary network power shutdown. 
         [0051]      FIG. 8B  is a plot  855  produced by computer simulation that illustrates output of the fast response unit  425  and the slow response unit  430  of  FIG. 4  in response to an ground fault signal as described above in  FIG. 8A  (i.e., 10 milliamp ground fault step in presence of a 7.1 milliamp AC induced current) according an exemplary embodiment of the present invention. The vertical axis  860  represents the detected ground fault current in milliamps, and the horizontal axis  865  represents the sample number of the output signals  870 ,  875 . The transition from a non-ground fault condition to a ground fault condition, represented as a 10 milliamp step, occurs at approximately sample number  100  of the signal trace  820 . 
         [0052]    The solid signal trace  870  represents the output of the fast response unit. In one exemplary embodiment described above, a first FIR filter has  16  coefficients and a step response time of 10 milliseconds and a second FIR filter that subtracts the average of the previous four samples spaced 1/60 Hertz apart. Therefore, fast response unit may detect a quickly increasing ground fault current  895  as shown by the region  885  of the signal trace  870  corresponding to 10 milliamps. However, because of the second FIR filter subtracts the average of the previous 4 samples, both the AC induced signal and the detected ground fault current are rejected after 4 unit intervals of 60 Hertz as shown in the region  880 ,  890  of the trace  870  corresponding to 0 milliamps. Thus, the fast response unit may detect quickly increasing ground fault currents, but may not detect slowly increasing ground fault currents because of the filtering subtraction. 
         [0053]    The dotted signal trace  875  corresponds to the output of the slow response unit. The slow response unit is configured to detect slowly increasing ground fault currents and may be implemented using similar sampling and digital signal processing (DSP) techniques described above. The slow response unit detects the average value over 4 unit intervals of line frequency so that periodic AC induced signals are rejected. For example, if the line frequency is 60 Hertz and is sampled 16 times per unit interval, the average of 64 samples may be determined. The result is that the 7.1 milliamp RMS AC power induced current described above in  FIG. 8A  is rejected but the desired signal is passed. Therefore, the slow response unit may detect slowly increasing ground fault current in the presence of periodic AC induced current that may be too slow for the fast response unit to detect. 
         [0054]    The combination of a fast response unit and a slow response unit enables some embodiments of the present invention to detect fast ground fault currents (e.g., within the 10 millisecond requirement of the Telcordia specification discussed above) and slowly increasing ground fault currents while being immune to false detection from periodic AC power induced currents. As a result, the safety of service personnel is increased and the likelihood of unnecessary network power shutdowns is reduced due to increased immunity to false detection of ground fault events. 
         [0055]      FIG. 9  illustrates, in the form of a flow diagram  900 , an exemplary embodiment of a process to interrupt current on a power line in the event a ground fault condition is detected. In a normal operating condition, current is flowing through a telecommunications power line  905 . If a ground fault condition is detected based on a “fast” response criteria at time t m , a state change in the fast response output is produced  910 . Alternatively, or in addition, if a ground fault condition is detected based on a “slow” response criteria at time t m+n , a state change in the slow response output is produced  915 . The process then determines if a ground fault condition has been detected. If a ground fault condition is no longer detected, current is allowed to flow on the power line  905 . However, if the ground fault condition is still detected  920 , current on the power line is interrupted,  925 . A continuous ground fault condition may result in the power being interrupted, and the detection process ends  930 . Manual intervention may be required to restore power, for example, by a service person performing a manual reset or through an initiation of a reset signal sent from a central office. 
         [0056]      FIG. 10  is a flow diagram of an example process  1000  employed by the ground fault interrupter  305  as shown in  FIG. 3  of the present invention. The process  1000  starts  1005  by determining is a ground fault condition has occurred  1010 . If the sensor detects a ground fault interrupt event  1010 , the process  1000  may determine if the ground fault event is on a tip lead. If the ground fault event occurs on the tip lead  475 , the process  1000  may disable power by opening the ring lead via a solid state or mechanical switch, and power is disabled. Power remains disabled, and manual intervention may be required to restore power, for example, by a service person performing a manual reset or through the initiation of a reset signal sent from the central office. If the ground fault event occurs on a ring lead, the process may interrupt power for a predetermined period of time  1020  (e.g., 60 milliseconds) by opening a switch on the power line. After the interruption time period has expired, power may be restored  1025 , and the process  1000  waits a predetermined period of time (e.g., 3 milliseconds) to allow the power to stabilize. 
         [0057]    The process  1000  then determines if the ground fault condition is still present  1030 . If the ground fault condition is no longer present, the process  1000  continues to monitor the power lines for ground fault interrupt events  1010 . If the ground fault condition is still present, the process checks to determine if power has been restored a predetermined number of times  1035  (e.g., 14). If power has been restored more that the predetermined number of time, power is disabled  1040  and may be restored, for example, through use of a manual reset or a manual reset signal transmitted from a central office. If power has been restored less than the predetermined number of times, the process  1000  continues to monitor the power lines for ground fault interrupt events  1010 . 
         [0058]    It should be understood that the process  1000  described in  FIG. 10  is an example embodiment used for illustration purposes only. Other embodiments within the context of interrupting current on a power line may be employed. 
         [0059]    Some or all of the steps in the process  1000  may be implemented in hardware, firmware, or software. If implemented in software, the software may be (i) stored locally with the ground fault interrupter  305  or (ii) stored remotely and downloaded to the ground fault interrupter  305  during, for example, start  1005 . The software may also be updated locally or remotely. To begin operations in a software implementation, the ground fault interrupter  305  loads and executes the software in any manner known in the art. 
         [0060]    It should be apparent to those of ordinary skill in the art that methods involved in the present invention may be embodied in a computer program product that includes a computer usable medium. For example, such a computer usable medium may consist of a read-only memory device, such as a CD-ROM disk or convention ROM devices, or a random access memory, such as a hard drive device or a computer diskette, having a computer readable program code stored thereon. 
         [0061]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.