Abstract:
A trip unit, such as one comprising a current sensor generating a current signal indicative of current in a power circuit, an analog to digital converter converting said signal into digital form, and a controller monitoring the current signal and sensing whether an over current condition exists and whether a ground fault condition exists. In case of an over-current condition, the controller causes a trip signal to be output resulting in the power circuit being opened. In the case of a ground fault condition, the controller causes an annunciation output signal but does not generate a trip signal in the case of a ground fault condition and therefore does not cause the power circuit to be opened.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to circuit breakers. More specifically, this invention relates to trip units for circuit breakers having switchable protection or annunciation for various fault conditions. 
     Circuit breakers are designed for opening a circuit upon the detection of a fault condition. A trip unit monitors current sensors and outputs a trip signal when a fault condition is detected. Some trip units monitor the current for overload or over-current conditions but have no ground fault sensing capability. Other trip units are available for opening a circuit when an over-current condition or ground fault condition exists. In addition, electronic circuit breakers having sensors to determine when these conditions exist sometimes include annunciation capability, so that an engineer or electrician can determine what fault condition arose to cause the circuit breaker to open the circuit. 
     However, in certain situations it is desirable or even necessary to provide annunciation that a ground fault condition exists, but without opening the circuit. For example, in the case of critical life-saving equipment in hospitals and in the case of fire pumps, it is required by the National Electric Code that ground fault conditions be detected but for annunciation only, not for breaking the circuit. The National Electric Code also permits annunciation-only in the case of industrial processes, where shutting down a manufacturing process due to a ground fault in one sub-system can cause expensive down time. 
     Accordingly, there remains a need in the art for a circuit breaker implementing a ground-fault sensing circuit that annunciates a ground fault condition but does not open the power circuit in the case of a ground-fault condition. 
     BRIEF SUMMARY OF THE INVENTION 
     The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a trip unit for annunciating a ground fault condition including a current sensor generating a current signal indicative of current in a power circuit, an analog to digital converter converting said signal into digital form, and a controller monitoring the current signal and sensing whether an over current condition exists and whether a ground fault condition exists. In the case of an over-current condition, the controller causes a trip signal to be output resulting in the power circuit being opened. In the case of a ground fault condition, the controller causes an annunciation output signal but does not generate a trip signal and therefore does not cause the power circuit to be opened. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the exemplary drawings wherein like elements are numbered alike in the several figures: 
     FIG. 1 is a perspective view of a circuit breaker of the invention; 
     FIGS. 2,  3 ,  4 , and  6  are alternative embodiments to ground fault detection arrangements; 
     FIG. 5 is an alternative embodiment to FIG. 6 employing sensor configurations such as those in FIGS. 2,  3 , and  4 ; 
     FIG. 7 is a block diagram of a trip unit of the invention; 
     FIG. 8 shows a flow chart for operation of a trip unit of an embodiment of the invention; 
     FIG. 9 is a block diagram of implementation of the invention in a control or monitoring network; 
     FIG. 10 shows a block diagram of a remote monitoring computer; and 
     FIG. 11 shows a flow chart for operation of a remote monitoring computer of an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The above-discussed and other drawbacks and deficiencies of the prior art are overcome or by a trip unit having a typical sensing means to monitor the power line for a ground fault condition and annunciating means for annunciating a ground fault condition. 
     FIG. 1 shows a circuit interrupter  10  of the type consisting of a molded plastic cover  11  secured to a molded plastic case  12 . The provision of an accessory cover  13  and accessory doors  14 ,  15  allows field as well as factory-installed electric accessories such as described in U.S. Pat. No. 5,302,786 issued Apr. 12, 1994 to Rosen et al. An externally-accessible operating handle  16  controls the open and closed conditions of the movable contact  20 , and fixed contact  21  located within the case to allow and interrupt current flow through an associate electrical distribution circuit. 
     Automatic circuit protection against overload circuit conditions is provided by means of an electronic trip unit  18  located within the circuit interrupter cover. A rating plug  17  allows the circuit interruption rating to be set by externally accessing the electronic trip unit as described within U.S. Pat. No. 5,204,798 issued Apr. 20, 1993 to Scott. Connection with an external distribution circuit is made by means of the load strap  9  that extends within the modular current transformer  19  for sensing the current therethrough. While current transformer  19  is shown, any means of sensing current may be used, such as Hall-effect sensors or giant magnetic resistors (GMR) such as are disclosed in U.S. Pat. No. 5,933,306 issued Aug. 3, 1999 to Santos et al. 
     Although a molded case circuit breaker is shown, the invention is not limited to molded-case circuit breakers, which are generally limited to current capacities of 1200 Amperes. Indeed, the invention may be implemented in other breaker classes including power breakers up to 4000 Amperes and steel frame breakers up to 5000 Amperes. 
     FIGS. 2-4 and  6  show four methods of sensing a ground fault condition. These sensing methods are more fully described in a 1991 General Electric publication entitled, “Ground-Fault Protection for Solidly Grounded Low-Voltage Systems,” pp. 4-7. 
     The ground fault current can be monitored either as it flows out to the fault or on its return to the neutral point of the source transformer or generator. When monitoring the outgoing fault current, the currents in all power conductors are monitored either individually, such as in FIGS. 2,  3 , and  6 , or collectively, as shown in FIG.  4 . When monitoring the return fault current, only the ground fault return conductor is monitored (not shown). 
     FIG. 2 shows a broken delta ground fault sensing configuration. Current transformers CT 1 , CT 2 , and CT 3  are connected in series on phase conductors A, B, and C. Voltage V Δ1  provides a signal indicative of a ground fault current. During normal operation, the vectorial addition of voltages from three or four sensors is essentially zero. When a downstream ground fault occurs, the outgoing ground fault current causes a voltage to appear. If the current magnitude is sufficient to produce a voltage equal to, or greater than a predetermined threshold for a predetermined minimum time delay, then the circuit is considered to be in a ground fault condition. 
     FIG. 3 shows a residual ground fault sensing configuration. In this case, current transformers CT 1 , CT 2  and CT 3  are connected in parallel on phase conductors A, B, and C. Here, V Δ2  provides a signal indicative of a ground fault current. The operation is based on the concept that the phase currents in a balanced three-phase system add vectorially to zero. If current transformers correctly transform phase currents to secondary currents, these secondary quantities will also add up to zero. As a consequence, a residually connected ground fault relay will sense zero current during normal, balanced, three-phase operation. For three-phase, three-wire systems, only three current transformers are required as shown in FIG.  3 . Three-phase, four-wire systems require four current transformers to “blind” output voltage V Δ2  to any unbalanced line-to-neutral loading current. The fourth current transformer makes it possible to set the output device a sensitive pick-up level regardless of the anticipated unbalanced load current magnitude. If the anticipated worst-case unbalanced line-to-neutral load current is lower than the pick-up setting of the output device, the neutral current transformer in a four-wire system can be omitted. 
     FIG. 4 shows another ground fault sensing technique known as “Ground Sensor Protection” which is provided by a combination of a window or donut type current transformer CT 4 , which surrounds all power conductors A, B, C, and N, and provides voltage out V Δ3 . In a balanced three-phase system or an unbalanced three-phase, four wire system, the magnetic flux produced by each of the phase and neutral currents has a mutual canceling effect as observed by current transformer CT 4 A ground fault current, however, will return through a circuit external to the current transformer, e.g., through conductor G, and therefore not produce a canceling magnetic flux to that produced by the ground current flowing in the phase conductor. Thus the current transformer produces a current output to the relay only for ground fault currents but no significant current output to the relay for normal phase currents. Phase currents in excess of a predetermined threshold for a minimum period of time will be regarded as an indication of a ground fault condition. The current magnitude is dependent on the current transformer configuration and turns ratio, special distribution of the power conductors within the window or donut current transformer, and sensitivity of the relay. 
     For the above sensing methods, an output voltage V Δ1 , V Δ2 , and V Δ3  is indicative of a ground fault condition. If the voltage exceeds a predetermined threshold for a predetermined period of time, then the circuit is considered to be in a ground fault condition. The output voltages V Δ1 , V Δ2 , and V Δ3  maybe used in a variety of ways. They may connect to a relay  80  matched to the current transformer(s) for closing an annunciator circuit  59  as shown in FIG.  5 . Alternatively, the voltage may be applied, after any necessary conditioning, to the inputs in an A/D converter connected in a trip unit as shown in FIG. 6 which is described in more detail below. 
     A circuit breaker of a first embodiment of the invention will include both a ground-fault sensing and annunciating circuit and a separate over-current trip circuit as shown in FIG.  5 . The ground fault sensing circuit in FIG. 5 includes one or more current transformers  133  as previously discussed in connection with FIGS. 2,  3 ,  4 , or  6 . The output of current transformers  133  is provided to a relay, processor, or similar device which activates an annunciation circuit  58  when a ground fault condition is sensed. The over-current trip circuit includes current sensors  233  which generate at least one signal indicative of the current in power lines  30 . When the current exceeds a predetermined minimum threshold for a predetermined minimum period of time, e.g., when it exceeds the time-current limits defined in associated time-current limit parameters such as are known in the art, relay  82  or similar device activates trip module and separates movable contacts from stationary contacts  21  thereby shutting off current in power lines  30 . 
     An alternative approach is to provide an integral ground fault sensing configuration as shown in FIG.  6 . In this case, the ground fault sensing is integral to the static trip programmer unit. Here, current sensors, such as current transformers CT S1 , CT S2 , and CTS S3 , are connected to an electronic trip unit  18 . Ground fault sensing is similar to the residual ground fault sensing configuration previously described. The current transformers are defined as current sensors since these transformers are designed for use only with low burden tip unit  18 . Other types of current sensors, such as Hall-effect sensors and giant magnetic resistors may used in place of the current transformer type current sensors shown. For a three-phase, three-wire system, three current sensors, mounted within the circuit breaker, are required. For a three-phase four-wire system (not shown), a fourth current sensor monitoring the neutral may be mounted externally from the circuit breaker, provided the neutral conductor is radial and not grounded after passing through the current sensor. If the worst-case unbalanced line-to-neutral is lower than the pick-up setting of the static relay, the neutral current transformer in a four-wire system can be omitted. 
     The operation of the trip unit  18  is best seen by now referring to FIGS. 7 and 8. Trip unit  18  includes inputs for voltage and current information from sensors  33 ,  31  via signal lines  34 ,  32 , respectively. Sensors  33 ,  31  detect the voltage and current in power lines  30  between a source and load  35 . Sensor inputs are directed to analog-digital converter  52  where the analog output from sensors  33 ,  31 , are converted into digital information which is then provided to micro controller  54  via data path  53 . Micro-controller  54  is connected via data bus  60  to random access memory RAM  69 , read-only memory ROM  67 , non-volatile memory  65 , display  57 , communications port  55 , and output  63 . Non-volatile memory may include, for example, EEPROM (electrically erasable programmable read-only memory), EPROM (erasable programmable read-only memory), flash memory, or other non-volatile memory. 
     Output  63  generates a trip signal which is transmitted along line  61  to an external trip module  40  which actuates movable contacts  20  to separate from fixed contacts  21  thereby opening the circuit. Trip unit  18  is also capable of outputting information via display  57  which may be LEDs, LCD, or other display means. 
     Read-only memory  67  or non-volatile memory  65  includes a software program containing instructions readable by controller  54 . Non-volatile memory  65  may also include control parameters such as time-current curve information necessary for over-current protection. These control parameters may be updated as needed depending on the application. 
     In operation, the microprocessor receives information from voltage sensors  31  and current sensors  33 . This information is monitored by the processor to determine if a fault condition exists. Referring to FIG. 8, controller  54  begins at step  110  labeled “START”, and then proceeds to step  111  where current characteristics are compared with the time-current limits defined by the control parameters. If the current exceeds time-current limits defined by the control parameters, then an over-current condition is determined at step  111  and controller  54  proceeds to step  114  wherein controller  54  instructs output  63  to generate a trip signal. Output  63  generates a trip signal in line  61  in response to a signal from controller  54 . Trip module  40  actuates movable contacts  20  in response to a trip signal from trip unit  18 , causing movable contacts  20  to separate from stationary contacts  21 . Control then proceeds to step  115  wherein, in addition to tripping contacts  20 , controller  54  is programmed to annunciate the condition which precipitated the trip by changing display  57  and/or providing an output with such an indication via communications port  55 . Once a trip signal is output at step  114  and the fault condition is annunciated at step  115 , the control stops at step  117 . 
     If there is no over-current condition, controller  54  proceeds to step  112  to determine if a ground fault condition is detected. A ground fault condition is detected as described above with reference to FIG.  6 . In the case of a ground fault condition, controller  54  proceeds to step  113  wherein an annunciation signal is produced via display  57  and/or communications port  55 . Controller then proceeds to step  116  which is repeated until an over-current condition occurs. In other words, after annunciating a ground fault, controller  54  continues to monitor for an over-current condition. If an over current condition exists at step  116 , then control proceeds to step  114  and continues as discussed above. Note that in a ground-fault condition, controller  54  is not programmed to produce a trip signal and therefore the contacts  20  are not separated from fixed contacts  21  as would be the case in other circumstances. 
     Any fault condition sensed may be annunciated by illuminating a representative LED on display  57  and/or displaying a message on display  57  that the particular fault condition exists if display  57  has alphanumeric capability. An audible alarm may also be provided for directing attention to the display. As with other fault conditions, the annunciation may take the form of outputting a signal via communications port  55  to a remote monitoring state. 
     FIG. 9 shows a remote monitor/controller  70  connected via network  80  to the communications ports of a plurality of trip units  18 . Any number of trip units may be monitored, as indicated by dashed line  78  extending from network  80 . Each trip unit  18  includes a local annunciator  58  which may include an audible and/or visible alarm and/or a display. 
     When a trip unit  18  detects a fault condition, a message is sent via network  80  to remote monitor or controller  70  where it is received. Remote monitor or controller may be a general purpose computer programmed to respond to the fault signal in a variety of ways depending on the circumstances. Referring to FIG. 10, remote monitor/controller  70  that comprises a general purpose computer includes a processor  72  which is connected via a data bus  75  to non-volatile memory  74 , random access memory  73 , display  71 , and input/output  76 . Processor  72  responds to programming instructions in the usual way stored on non-volatile memory  74 . Random access memory  73  provides a location to store temporary information as is generally known in the art. Input/output  76  is connected to network  80  via signal line  77 . 
     The remote monitor/controller  70  is capable of receiving messages from trip units  18  via its input/output port  76 . When such a message is received, it can display it on display  71 . It can also query each trip unit to determine its status as to ON, OFF, or TRIPPED, as well as current load levels, i.e., the current and voltage on the line. This information can then be displayed on display  71 . Such a display may be in table format, or it may graphically display the trip units, their interconnections in the electrical distribution network, and the status of each. Additionally, it can transmit a message to any selected trip unit to remotely trip that trip unit. 
     Remote monitor/controller  70  may further be programmed with information as to the nature of the load on each trip unit, and whether any critical systems are served thereby. Critical systems are those that the National Electrical Code permits or requires annunciation-only when a ground-fault condition occurs. For example, in a hospital setting, a particular circuit in each room may be set aside for life-preserving equipment such as a respirator or heart monitor. Alternatively, when such equipment is connected to a circuit, that information may be entered into remote controller/monitor  70 . Thus, if critical equipment is connected to a circuit breaker at the time a ground fault message is received from the trip unit, remote monitor/controller  70  merely displays the information, along with a message that a ground fault condition is a fire hazard but this circuit is connected to critical equipment which must be transferred to another circuit before shutting down. 
     When monitor/controller  70  receives a message from a trip unit indicating a fault condition exists, its programmed response will vary depending on the message and circumstances. The remote monitor/controller is programmed for a desired response which may be to simply display the condition on screen  71 . This fault condition may be accompanied by an audible alarm to attract attention to the on-screen message. Another desired response might be to send a trip signal back through network  80  to trip the processor and avoid a potential fire hazard. 
     FIG. 11 shows a flow chart of one embodiment of remote monitor/controller software implementation. Processor  72  begins at step  120  labeled “START” and proceeds to step  121 . When a fault message is received, processor  72  proceeds to step  122  wherein the message is analyzed to determine if it is an indication that a trip unit has detected a ground fault. If the message does not indicate a ground fault, then processor  72  proceeds to step  123  wherein it is directed to annunciate the fault condition, then to step  131  wherein it is directed to stop. 
     If, at step  122  the message is found to contain an indication that a trip unit has detected a ground fault, then processor  72  proceeds to step  124 , where it determines whether the trip unit is on a critical circuit. A trip unit is on a critical circuit if critical equipment, such as a fire pump, life preserving equipment, or industrial processes are connected to it. If not, processor  72  proceeds to step  129  wherein it checks to see that the circuit breaker that sent the message has tripped. If it has tripped, then its status is annunciated at step  130  and the procedure ceases at step  131 . If it has not tripped, then processor  72  proceeds to step  128  wherein a trip signal is transmitted to the circuit breaker. Processor  72  then returns to step  129 . Once the breaker has tripped, processor  72  proceeds to step  130  where the status of the breaker is annunciated and then to step  131  where the procedure ceases. 
     If, at step  124 , it is determined that the breaker that sent the ground fault message to remote monitor/controller  70  was on a critical circuit, then processor  72  proceeds to step  127  wherein a determination is made whether or not it is safe to shut down, i.e., remotely trip the offending circuit breaker. A circuit is safe to shut down when all such critical equipment is turned off or moved to another circuit. Obviously, if the load has been reduced to zero on that circuit, it is safe to shut down the circuit. However, sometimes that may not be possible and an operator will have to manually authorize the shut-down of the ground-faulted circuit. Thus, if it cannot be verified that the shut-down is safe, then processor  72  proceeds to step  126  wherein a warning message is displayed on display  71 . The warning message will not be removed until it can be determined that the offending circuit is safe to shut down at step  127 . Once that happens, processor  72  continues to step  128  which instructs processor  72  to send a trip signal to the trip unit in order to shut down the circuit. Processor thereafter continues to step  129  where the breaker status is verified as tripped. Once verification is complete, processor  72  continues to step  130  wherein it annunciates the status and finally the procedure is complete at step  131 . 
     A safe and effective system providing for the annunciation-only of ground-fault occurrences having now been described, it is to be understood that various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.