Abstract:
An open neutral sensing circuit includes a first sensing resistance contacting a first line and a neutral. A second sensing resistance contacts a second line and the neutral and has a value greater than the first sensing resistance. A control switch contacts the first line and the neutral. The control switch is configured to switch from a first state to a second state when the voltage drop across the first sensing resistance is equal to the voltage drop across the second sensing resistance.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to circuits used in power sensing applications. More particularly, the present invention relates to circuits used for detecting open neutral configurations in power circuits for recreational vehicles.  
       BACKGROUND OF THE INVENTION  
       [0002]     Recreational vehicles (RVs) use the same type of power as residential housing. Namely, both recreational vehicles and residential housing are supplied by public utility power (240/120 VAC). For example, as shown in  FIG. 1 , a normal utility source at a breaker panel  12  includes four lines: L 1  line  14 , L 2  line  16 , neutral  18  and ground  20 . The voltage difference between the L 1  line  14  and the L 2  line  16  is 240 volts. The difference between each of the L 1  line and L 2  line  14  and  16  and the neutral line  18  is 120 volts. As power flows from L 1  line and L 2  line  14  and  16  to the neutral, the voltage drop will be 120 volts. Such a system is the common configuration for both residential housing and RVs. RVs, however, may also be powered by diesel or gas generators that can be used to power the 240/120 VAC of the RV if utility power is not available. Because of this difference, and because an RV is mobile and must plug-in to utility power when the RV stops, the connections which are hard wired in residential housing require the use of a multi-pronged plug for RVs.  
         [0003]     When a generator is used to supply power, it is necessary to inhibit the flow of power to the utility power grid. In order to minimize the risk of putting power from the generator into the power grid, a switch over box switches from utility power to generator power. A common switch over box  28  is shown in  FIG. 2 . The switch over box  28  includes L 1  line  30 , L 2  line  32 , and two neutral lines  34 . Line contactors C 1   36  and neutral contactors C 2   38  mechanically couple the power sources (utility source and generator source, respectively) to an AC main panel  40 . A transfer delay board  42  delays power to the main panel from the generator source and converts the AC power from the generator source to a DC current. A line coil  44  actuates the line contactors  36 . An auxiliary contactor  46  is also controlled by the line coil  44 . A neutral coil  48  actuates the neutral contactors  38 .  
         [0004]     In one embodiment, the line contactors  36  are 50 Amp rated contactors while the line coil  44  which actuates the line contactors  36  is a 125 VDC coil. The neutral contactors  38  are 50 Amp rated contactors while the neutral coil  46  which actuates the neutral contactors  38  is a 125 VAC coil.  
         [0005]     When power is supplied from the generator source, the transfer delay board  42  delays the opening and closing of the line contactors  36  and the neutral contactors  38 , generally for 15 to 20 seconds so that the generator source may stabilize. Once the delay has passed, the line coil  44  is energized, and opens the line contactors  36  on the lines  30  and  32  from the utility and closes the line contactors  36  on the lines  30  and  32  from the generator. The line coil  44  also closes the auxiliary contactor  46  so that power will flow to the neutral coil  48 . When the neutral coil  48  is energized, the neutral contactors  38  change state and power flows from the generator to the AC main power panel  40 . When the generator is turned off, the coils  44  and  48  are de-energized and power flows from utility to the AC main power panel  40  as the line contactors  36  and neutral contactors  38  which are normally closed are returned to a closed state and the de-energized line contactors  36  and neutral contactors  38  which are normally open are returned to an open state.  
         [0006]     The use of a mechanical connection to the utility power grid creates potential problems over time as the mechanical connections tend to degrade or erode causing an unreliable power source. It is also possible that the power plug may be improperly inserted into the utility supply receptacle. When either problem exists, an open neutral condition may occur. An open neutral condition can allow the full voltage differential between lines L 1  and L 2 , that is 240 VAC, to be applied to all devices within an RV. Many devices within an RV are designed only for 120 VAC sources. When 120 VAC devices are energized with 240 VAC supplies, failure of the devices and possible injury to the RV and the owner may occur, including the ignition of RV materials.  
       SUMMARY OF THE INVENTION  
       [0007]     An open neutral sensing circuit, comprising a first sensing resistance contacting a first line and a neutral; a second sensing resistance contacting a second line and the neutral having a value greater than the first sensing resistance; and a control switch contacting the first line and the neutral configured to switch from a first state to a second state when the voltage drop across the first sensing resistance is equal to the voltage drop across the second sensing resistance. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a diagram of a utility power source;  
         [0009]      FIG. 2  is a diagram of a change over box;  
         [0010]      FIG. 3A  is a diagram of a closed neutral circuit;  
         [0011]      FIG. 3B  is a diagram of an open neutral circuit;  
         [0012]      FIG. 4  is a diagram of a change over box according to one aspect of the invention;  
         [0013]      FIG. 5A  is a diagram of a closed neutral circuit with a change over box according to another aspect of the invention; and,  
         [0014]      FIG. 5B  is a diagram of an open neutral circuit with a change over box according to another aspect of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Turning now to  FIGS. 3A and 3B ,  FIG. 3A  is a diagram of a closed neutral circuit  50  inside the RV. Four lines, L 1   52 , L 2   54 , neutral  56 , and ground  58  provide power to the RV. As an example, a TV  60  and a microwave  62  are in the RV and powered by a portion of the circuit  50 . In this example, the TV  60  is connected between the L 1  line  52  and the neutral  56 . The microwave  62  is connected to the L 2  line  54  and the neutral  56 . The resistive loads for the TV  60  and the microwave  62  are 1 kW and 10 kW respectively. When the neutral is properly closed, the TV  60  and the microwave  62  are on different circuits, and the relative differences in the resistive loads are inconsequential to the operation of the TV  60  and the microwave  62 .  
         [0016]     Turning now to  FIG. 3B , an open neutral circuit  70  inside the RV. The same four lines, L 1   72 , L 2   74 , neutral  76 , and ground  78  provide power to the RV. However, because of the problem with the connection to the utility power, the neutral  76  is open. A TV  80  and a microwave  82  are in the RV and powered by the circuit  70 . In this example, the TV  80  is connected between the L 1  line  72  and the open neutral  76 . The microwave  82  is connected to the L 2  line  54  and the neutral  56 . The resistive loads for the TV  80  and the microwave  82  are 1 kW and 10 kW respectively. When the neutral is open, the TV  80  and the microwave  82  are on the same circuit, and the relative differences in the resistive loads change the voltage drop across the loads.  
         [0017]     Power cannot flow from the L 1  line  72  and the L 2  line  74  to neutral  76 . Instead, the power flows from L 1  line  72  to the L 2  line  74 , and the voltage drop between L 1  line  72  to the L 2  line  74  will be 240 VAC. Because the loads are in series, the voltage drop across the TV  82  and the microwave  84  will total 240 VAC. Thus, the drop across the TV  80  will be 24 VAC while the drop across the microwave  82  will be 216 VAC. The voltage across the microwave  82  may overload the microwave  82  and the voltage drop across the TV  80  may also damage the TV  80 .  
         [0018]     Turning now to  FIG. 4 ,  FIG. 4  is a diagram of a change over box  86  according to an aspect of the invention. The change over box  86  of  FIG. 4  is similar to the changeover box of  FIG. 2 , except an open neutral sensing circuit  88  is placed between the power sources and the RV. The operation of switching from utility power to generator power is the same.  
         [0019]     The switch over box  86  includes L 1  line  90 , L 2  line  92 , and neutral lines  94 . Line contactors C 1   96  and neutral contactors C 2   98  mechanically couple the power sources (utility source and generator source, respectively) to an AC main panel  100 . A pair of secondary neutral contactors  101  couple the neutral lines  94  to the generator source. A transfer delay board  102  delays power to the main panel from the generator source and converts the AC power from the generator source to a DC current. A line coil  104  actuates the line contactors  96 . An auxiliary contactor  106  is also controlled by the line coil  104 . A neutral coil  108  actuates the neutral contactors  98 .  
         [0020]     In one embodiment, the line contactors  96  are 50 Amp rated contactors while the line coil  104  which actuates the line contactors  96  is a 125 VDC coil. The neutral contactors  98  are 50 Amp rated contactors while the neutral coil  106  which actuates the neutral contactors  98  is a 125 VDC coil.  
         [0021]     When power is supplied from the generator source, the transfer delay board  102  delays the closing of the neutral contactors  98 , generally for 15 to 20 seconds. Once the delay has passed, the line coil  104  is energized, and switches the state of the line contactors  96 , opening the normally closed line contactors  96  and closing the normally open line contactors  96 . The coil  104  also closes the auxiliary contactor  106  so the power will flow to the neutral coil  108 . When the neutral coil  108  is energized, the neutral contactors  98  switch states, opening the normally closed neutral contactors  98  and closing the normally open neutral contactors  98 . Power flows from the generator to the AC main power panel  100 . When the generator is turned off, the coils  104  and  108  are de-energized and power flows from utility to the AC main power panel  100  as the line contactors  96  and the neutral contactors  98  which are normally closed are returned to the closed state and the de-energized neutral contactors  98  and the line contactors  96  which are normally open are returned to the open state.  
         [0022]     Regardless of the source of power, the power flows from the power source to the AC main power panel  100  through the open neutral sensing circuit  88 . The open neutral sensing circuit  88  includes a control relay  110 , a power relay  112 , a neutral relay  113 , a pair of normally open power actuators  114 , a normally open control actuator  116 , a high load resistor  118  and a low load resistor  120 . The low load resistor  120  is placed between the neutral  94  and the L 1  line  90 . The high load resistor  118  is placed between the neutral  94  and the L 2  line  92 . One of the normally open power actuators  114  is placed on the L 1  line  90 , and the other normally open power actuator  114  is placed on the L 2  line  92 . The control relay  110  is placed between the neutral  94  and the L 1  line  90 , and is parallel to the low load resistor  118 . The normally open control actuator  116  and the power relay  112  are placed in series between the neutral  94  and the L 2  line  92 , and are parallel to the high load resistor  118 . The neutral relay  113  is placed parallel to the power relay  112 .  
         [0023]     The open neutral sensing circuit  88  senses an open neutral by measuring the voltage drop across the load resistors  118  and  120 . When the neutral  94  is closed (i.e., properly attached to the power source and AC main panel  100 ), the voltage drop across both load resistors  118  and  120  is 120 VAC. The control relay  110  is energized and the relay closes the normally open control actuator  116 . When the normally open control actuator  116  closes, then power flows to the power relay  112  and the normally open power actuators  114  close. Power then flows from the power source to the AC main panel  100 .  
         [0024]     When the neutral  94  is open, the voltage drop across the low load resistor  118  is too small to actuate the control relay  110 . Because the control relay  110  does not close the normally open control actuator  116 , the power relay  112  does not energize and power does not flow to the AC main panel  100 . The neutral relay  113  is placed in parallel to the power relay  112  to close the neutral lines  94  to the generator when the neutral is properly sensed. The neutral relay  113  keeps the neutral  94  of the generator from interfering with the neutral of the generator.  
         [0025]     As an example, the resistive loads  118  and  120  may be set to 270 kOhms and 2.7 MOhms. The ten fold difference in resistance between the two loads  118  and  120  allows the voltage drop difference between the low load resistor  118  and the high load resistor  120  to be markedly different when the neutral  94  is open. Moreover, by setting the resistances  118  and  120  relatively high to the resistances inside the RV, the current to the AC main panel  100  will be minimally effected by the open neutral sensing circuit  88 .  
         [0026]     The control relay  110  may be chosen as a 110 VAC relay. This allows the control relay  110  to close when the voltage drop across the low load resistor  118  is at least 110 VAC. Such a condition exists only when the neutral  94  is closed. The power relay  112  and neutral relay  113  may also be a 110 VAC relay. Because the power relay  112  closes contactors  114  that are placed along the L 1  line  90  and the L 2  line  92 , the power relay should be rated to high currents. In this example, the relay  112  is rated to 50 Amps. The neutral relay  113   
         [0027]     Turning now to  FIGS. 5A and 5B ,  FIG. 5A  is a diagram of the closed neutral circuit with a change over box according to an aspect of the invention. Using the values from the example of  FIG. 4 , the voltage drops across the resistances  118  and  120  are 120 VAC when the neutral is closed. The control relay  110  will close, and operation to the AC main panel  100  is normal. In  FIG. 5B , an open neutral circuit with the change over box according to an aspect of the invention is shown. When the neutral  94  is open, then the voltage drop across the low load resistor  118  is approximately 218.2V while the drop across the high load resistor is 216 VAC. The small voltage drop across the low load resistor  118  is not enough to energize the control relay  112 , and power will not flow from the power source to the AC main panel  100 .  
         [0028]     While the invention has been shown in embodiments described herein, it will be obvious to those skilled in the art that the invention is not so limited but may be modified with various changes that are still within the spirit of the invention.