Abstract:
A backup power system for household or structural appliances that normally receives power from a primary power source, and is not an uninterruptible power system. Included are batteries, a battery charger, a power interface interconnecting the primary power source and the backup power system, power relays that comprise a switching matrix of individually operating relays that connect and disconnect the sources and appliances one and only one at a time, sensing devices to monitor the currents and voltages, conversion of the DC voltage of the batteries to AC voltage for backup power to the appliances, circuitry for preventing peak power demands by delivering sequentially selected appliances, one and only one at a time and only when power is demanded by each individually selected appliance, thereby prolonging the operating lifetime of fully charged batteries. Short circuit protection is provided to protect the backup power system from appliance short circuits, and to protect the batteries in the event of a short circuit within the system.

Description:
Continuation in part for application Ser. No. 11/179,842, originally filed on 12 Jul. 2005. 

   CROSS REFERENCE TO RELATED APPLICATIONS 
   Not Applicable 
   FEDERALLY SPONSORED RESEARCH 
   Not Applicable 
   SEQUENCE LISTING OR PROGRAM 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to a backup electrical power system for household and structural appliances such as, but not limited to, refrigerators, freezers, furnaces and ancillary equipment, well pumps, and other like appliances. 
   2. Prior Art 
   All of the aforementioned appliances rely upon electrical power for operation. Primary electrical power, generated at a remote power generating plant, is provided to households and structures along power lines. On occasion, power outages occur for a variety of reasons including failure of a power grid or power transformer, power line damage resulting from vehicles colliding into poles carrying power lines, lightning strikes, and in many other ways. In the event of a power outage, household and structural appliances are deprived of their operating power and are rendered useless. 
   Through the years, many systems have been proposed, some of which have been, and are, presently marketed, to operate as a temporary replacement of the primary electrical power system during power outages. For example, a popular system has been gasoline or propane driven motor-generator sets. Solar energy and wind-driven generators have also been used, as have battery-operated inverters. 
   Gasoline operated motor-generator sets present serious problems, such as safety, fuel supply, and environmental pollution. For example, fuel storage is a serious safety consideration and may be in violation of fire codes under certain circumstances. Also, when the fuel supply is depleted, replenishing the fuel supply may not be possible due to the fact that sources of gasoline may not be able to pump gasoline because of the power outage. 
   Environmental considerations include the excessive amount of acoustical noise generated, and the exhausting of combustion products into the atmosphere from the engine. This is also true for propane fuel systems. 
   Except for the most expensive systems, most motor-generator systems require an external transfer switch that adds additional labor and material costs for installation. In addition, if no one is available during an outage to effect the transfer from primary power to backup power, and to start the motor-generator set, the advantage becomes moot. There are motor-generator sets that incorporate these necessities but their cost is prohibitive for most households. 
   Harvesting solar and wind energy are other sources, however both require an enormous physical structure in order to serve in a viable manner. Storage of energy by these generators requires expensive batteries and extensive electronic control equipment. And, these sources are very expensive, eliminating their practicability for the average homeowner. 
   The use of battery-operated DC to AC inverters is a viable alternative. Two directions have been taken to implement batteries. The uninterruptible power supply is one such direction. An uninterruptible power supply provides electrical power by means of a battery-driven DC to AC inverter that is always on line, that is, the uninterruptible power supply continuously generates power, even when primary electrical power is available, while a battery charger is continuously charging the battery. In this manner, in the event of a power outage, the uninterruptible power supply continues to provide power. This system is valuable to computer users since there cannot be loss of data during switchover that may take several milliseconds. 
   While this system eliminates the drawbacks of the systems described above, there are four disadvantages for using this system. First of all, batteries have a predictable lifetime depending upon the number of charge/discharge cycles and the depth of discharge, resulting in relatively early replacement. Secondly, for practical reasons of cost, size, and weight, the length of time an uninterruptible power supply can provide electrical power during a power outage is severely limited, certainly less than 15 to 30 minutes, at best, just long enough to save work in process before shutting down. The third reason is that the size and cost of such systems to endure a typical outage is enormous. Lastly, only limited power can be generated within the constraints of size, weight, and cost. 
   Unlike the uninterruptible power supply, a different type of battery-operated system has been proposed wherein the battery is on standby when primary power is available, and provides energy only during power outages. Heretofore, all these systems begin to generate backup power in the event of a power outage, providing electrical energy to all household and structural appliances simultaneously. This system, however, has a very limited operating time also, and requires greater electrical power to be generated in order to supply the peak power demands of appliances when supplied simultaneously. Peak power demands severely stress both the batteries and the backup power system, resulting in a shorter available operating lifetime of fully charged batteries. 
   None of the patents searched provide a means of maximizing the length of time batteries can provide power since they all begin generating power when a power failure occurs, and power is supplied to all appliances simultaneously. 
   3. Objects And Advantages 
   The above clearly defines the need for a system that can supply electrical power in the event of failure of the source of primary power, and doing so in a cost effective and viable way. Therefore, in the present invention, backup electrical power is delivered to household or structural appliances in a manner that circumvents all of the disadvantages cited above, and does so efficiently and effectively, maximizing the operating lifetime of fully charged batteries. 
   SUMMARY OF THE INVENTION 
   Whereas in presently available backup power systems all appliances receive backup power simultaneously when primary power fails, the uniqueness of the present invention is that this backup power system delivers power to household or structural appliances sequentially, one and only one of a plurality of appliances receiving power at a time, circumventing peak power demands. 
   The dwell time during which backup power is delivered to one and only one of a plurality of appliances is pre-settable and depends upon tests performed on the current drawn, at the time of selection, of each one and only one of a plurality of appliances. 

   
     DRAWINGS—FIGURES 
     The preferred embodiment of the present invention will be described by referring to  FIGS. 1 through 8 : 
       FIG. 1  illustrates the various component parts comprising the backup power system. 
       FIG. 2  illustrates the preferred embodiment of the power interface. 
       FIG. 3  illustrates the preferred embodiment of a portion of the control and timing logic. 
       FIG. 4  illustrates the preferred embodiment of the battery status logic. 
       FIG. 5  illustrates the preferred embodiment of the appliance selection logic. 
       FIG. 6  illustrates the preferred embodiment of the inverter. 
       FIG. 7  illustrates the preferred embodiment of the battery charger. 
       FIG. 8  is a flow chart illustrating the operation of the backup power system. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 DRAWINGS-REFERENCE NUMERALS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 01 
                 Mains Power Panel 
               
               
                 02 
                 Backup Power System 
               
               
                 03 
                 Power Interface 
               
               
                 04 
                 Battery Charger 
               
               
                 05 
                 Batteries 
               
               
                 06 
                 Control and Timing 
               
               
                 07 
                 Inverter 
               
               
                 08 
                 Current Sensing 
               
               
                 09 
                 Voltage Sensing 
               
               
                 10 
                 Display 
               
               
                 11 
                 Primary Power Input Lines 
               
               
                 12 
                 Backup Power Output Lines 
               
               
                 13 
                 Relay Select Lines 
               
               
                 14 
                 Primary Power Failed Signal 
               
               
                 15 
                 Backup Load Current Sense Lines 
               
               
                 16 
                 Battery Charging Voltage 
               
               
                 17 
                 Primary Power Sensing Line 
               
               
                 18 
                 Battery Output Voltage 
               
               
                 19 
                 Inverter Enable/Disable Signal 
               
               
                 20 
                 Batteries Depleted Signal 
               
               
                 21 
                 Backup Power Voltage 
               
               
                 22 
                 Battery Status Signal 
               
               
                 23 
                 AC Interconnect Terminals 
               
               
                 24 
                 Power Relays 
               
               
                 25 
                 Main DC Circuit Breaker 
               
               
                 26 
                 DC Source Terminals 
               
               
                 27 
                 Battery Positive 
               
               
                 28 
                 Switched Battery Positive 
               
               
                 29 
                 Inverter DC Voltage Input 
               
               
                 30 
                 Inverter Frequency Signal 
               
               
                 31 
                 Oscillator and Divider Chain 
               
               
                 32 
                 Dwell Time 
               
               
                 33 
                 Time Slot Generator 
               
               
                 34 
                 Relay Select Lines 
               
               
                 35 
                 Relay Drivers 
               
               
                 36 
                 System Clock 
               
               
                 37 
                 Battery Status 
               
               
                 38 
                 Slot Duration Clock 
               
               
                 39 
                 Slot Advance Clock 
               
               
                 40 
                 Slot Counter Gate 
               
               
                 41 
                 Inhibit 
               
               
                 42 
                 Current Comparator 
               
               
                 43 
                 Backup Power Voltage Lines 
               
               
                 44 
                 Voltage Reference 
               
               
                 45 
                 Voltage Standard Signal 
               
               
                 46 
                 Voltage Comparator 
               
               
                 47 
                 Display Status Lines 
               
               
                 48 
                 LED Drivers 
               
               
                 49 
                 Voltage Level Signal 
               
               
                 50 
                 Inhibit Operation Gate 
               
               
                 51 
                 Disable Inverter 
               
               
                 52 
                 System Clock 
               
               
                 53 
                 Current Sensor Output 
               
               
                 54 
                 High-Gain Amplifier/Rectifier 
               
               
                 55 
                 Sensed Current DC Level 
               
               
                 56 
                 Current Comparator 
               
               
                 57 
                 Voltage Regulator 
               
               
                 58 
                 Current Calibration Standard 
               
               
                 59 
                 Class D Amplifier 
               
               
                 60 
                 60 Hz Square Wave Signal 
               
               
                 61 
                 Low-Pass Filter 
               
               
                 62 
                 60 Hz Sine Wave Signal 
               
               
                 63 
                 Mosfet Drive Signals 
               
               
                 64 
                 Power Mosfets 
               
               
                 65 
                 Power Drive Lines 
               
               
                 66 
                 Inverter Output Transformer 
               
               
                 67 
                 Bridge Rectifier 
               
               
                 68 
                 Rectified Line Voltage 
               
               
                 69 
                 EMI/RFI Filter 
               
               
                 70 
                 Current and Voltage Controls 
               
               
                 71 
                 Buck Drive Signals 
               
               
                 72 
                 Pulse Width Modulator 
               
               
                 73 
                 Output Mosfet Drive 
               
               
                 74 
                 Charger Output Mosfet 
               
               
                 75 
                 Flyback Drive Lines 
               
               
                 76 
                 Flyback Transformer and Rectifier 
               
               
                   
               
             
          
         
       
     
   

   DETAILED DESCRIPTION 
   The present invention relates to a backup power system primarily for delivering backup power to household or structural appliances typically, but not limited to, a furnace, refrigerator, freezer, or water pump. 
   In the event of failure of the source of primary power, appliances become inoperative. As a result, essential utilities are not available for the duration of the power failure. Inconveniences may include loss of refrigerated or frozen food, lack of heat, loss of water where water is pumped locally, and other similar services. 
   The purpose of the present invention is to reduce the inconveniences resulting from the loss of electrical power. The stated purpose is met by supplying an alternate source of electrical power during a power outage. Normal power is generally provided by a commercial power generating station with power lines connecting the generating station with households and structures. These external power lines generally terminate in a mains power panel located at each household or structure. The present invention provides a viable alternative for use by the most essential appliances. 
   The preferred embodiment of the present invention will be described by referring to the attached drawings,  FIGS. 1 through 8 . 
     FIG. 1  illustrates the embodiment of the present invention that comprises the components of the backup power system. These components are power interface  3 , timing and control logic  6 , batteries  5 , battery charger  4 , current sensing logic  8 , display  10 , voltage sensing logic  9 , and inverter  7 . Batteries  5  may, or not, be external to the physical backup power system although they are an integral, and necessary, part of its operation. 
   Mains electrical panel  1  serves as a distribution point between the primary power source and the normal household or structure. Primary power input lines  11  provide 120 volt and 240 volt AC power to the backup system. Backup power output lines  12  provide 120 volt and 240 volt backup power output to the selected appliances during a power failure, on a one and only one of a plurality of appliances at a time basis. 
   In the present invention, power interface  3  serves a dual purpose. First of all, wires from and to mains electrical panel  1  are preferably terminated in blocks. The second purpose of power interface  3  is to serve as a switching matrix to switch household or structural appliances between the source of primary power and the generated backup power when a power failure is detected. 
   The switching matrix comprises “n” number of single-pole, double-throw (SPDT) power relays, where “n” is the number of appliances to receive backup power. In the normal configuration, the wiper of each individual relay is wired to an individual appliance, the normally-closed (NC) contact is wired to the circuit breaker assigned to that individual appliance, and the normally-open (NO) contact of that individual relay is wired to the backup power voltage. 
   During normal operation, i.e., no primary power failure, all individual relays are de-energized and each individual circuit breaker provides power through its associated individual relay&#39;s NC contacts to its associated appliance. 
   In the event of failure of primary power, one and only one individual relay, of a plurality of relays, is energized and backup power is delivered through the NO contacts (closed during power failure) of the selected relay out to the appliance associated with that relay. 
   Relays are individually selected by relay select lines  13  under control of control and timing logic  6 , and are further described in  FIGS. 2 and 3 . 
   In the present invention, control and timing logic  6  comprises circuitry necessary to control and synchronize the occurrence of events in a timely fashion and to generate the frequency at which inverter  7  operates. 
   Battery charger  4  provides failure of primary power notification to the control and timing logic  6  by means of primary power failed signal  14 , derived from one of the primary power input lines  11 , which, under normal operation, is at 120 volts but reduces to zero (0) volts when primary power fails. Battery charger  4  serves to re-charge batteries  5  through battery charging voltage  16  when normal primary power is available 
   The charge status of batteries  5  is continuously monitored by the voltage sensing logic  9 , by means of battery output voltage  18 , which is also used to provide operating voltage to all electronic circuitry. Battery status signal  22  is decoded by display  10  so as to provide an indication of the charge level of batteries  5 . 
   When display  10  indicates that the battery voltage is at or near depletion, batteries depleted signal  22  signals control and timing logic  6  to inhibit further operation of inverter  7  by means of inverter enable/disable signal  19 . 
   AC voltage generated by inverter  7  is coupled through current sensing logic  8  which measures the current flowing through backup power voltage  21  out to power interface  3  along backup load current sense lines  15 . 
     FIG. 2  illustrates the preferred embodiment of power interface  3  in the present invention. Terminations for primary power input lines  11  and backup power output lines  12  are provided by AC interconnect terminals  23 . 
   Power relays  24  comprise the preferred embodiment of the switching matrix. These relays are preferably electro-mechanical relays, however other types of relays may be used interchangeably, such as solid-state relays. Under normal conditions, primary power input lines  11  provide power through the normally-closed contacts of the relays and during power failure, backup power output lines  12  provide power through the normally-open (closed during power failure) contacts of power relays  24 . At any given time one and only one of a plurality of power relays  24  is energized by the relay select lines  13 . Thus, only one and only one of a plurality of appliances receives backup power. 
   In the present invention, main DC circuit breaker  25  serves a dual purpose. First of all, it provides batteries  5  protection from a possible short circuit within backup power system  2 . Secondly, it serves as a shut off switch, stopping operation of the backup system. This is for safety concerns, such as during wiring external connections, testing components, and cleaning the apparatus and changing internal filters. The load side of main DC circuit breaker  25  provides DC source terminals  26  with inverter DC voltage input  29  to operate inverter  7 . 
     FIG. 3  illustrates the circuitry that comprises control and timing logic  6 . The basic components are oscillator and divider chain  31 , time slot generator  33 , battery status  37 , slot counter gate  40  current comparator  42 , (in  FIG. 4 ) voltage reference  44 , voltage comparator  46 , inhibit operation gate  50 , timing generator  31 , LED drivers  48 , and display  10 . 
   Oscillator and divider chain  31  provides inverter frequency signal  30  (typically 60 Hz but may be any other standard frequency) and is controlled by inhibit operation signal  36  from battery status logic  37  which determines that the measured battery voltage is at or near depletion as determined by measurement of battery output voltage  18 . 
   Outputs from oscillator and divider chain  31  include dwell time  32  and slot duration clock  38 , wherein the dwell time (time allowed for each individual appliance to receive power) is pre-settable to suit individual applications, and the duration time marker is a clock for triggering purposes. 
   Time slot generator  33  determines the actual time period during which each individual power relay  24 , of a plurality of relays, is energized, one and only one at a time, via relay drivers  35 , each of which is selected one and only one at a time by its select one of “n” relays line  34 . 
   All outputs of relay drivers  35  are normally high (logical “1”), and each output is driven low (logical “0”) when time slot generator  35  drives one and only one of a plurality of power relays  24  into its energized state, thereby delivering backup power to its associated appliance. 
   Current comparator  42  preferably utilizes a current transformer to measure the current drawn via each individually selected power relay  24 . Actual current measurement is made by passing backup power voltage lines  43  through the center of the current transformer (line current sensor  8 ,  FIG. 5 ), amplifying the AC voltage developed across the secondary, rectifying it to obtain an equivalent DC value, and using this advance counter clock  41  signal to advance time slot generator  33  via slot counter gate  40 . 
   The control logic for advancing time slot generator  33  is covered in detail in  FIG. 5  where all factors for selecting one and only one of a plurality of power relays  24  at a time are discussed in greater detail. 
     FIG. 4  is the present embodiment of the measurement and display logic for determining the state of charge of batteries  3 , and the disable inverter signal  51  detected by LED drivers  48 , used to inhibit further operation of inverter  7  when batteries  3  are at or near depletion. 
   Battery status signal  22  is applied to voltage comparator  46 , which compares the battery voltage to a stable and accurate voltage reference  44  that generates voltage standard signal  45 . The result of this comparison is voltage level signal  49  which is decoded by LED drivers  48  and displayed along display status lines  47  by display  10 . 
   In the present embodiment of this invention, four LEDs are used to display the charge level of batteries  3  by means of bi-color (red/green) LEDs. This approach is by no means the only way to indicate battery voltage; this method was chosen because of simplicity and ease of use in typical installations. For example, other means considered are: 
   a. an analog DC voltmeter to read the voltage directly, or, 
   b. analog to digital conversion of voltage level signal  49  for display on a bar graph or LCD panel. 
   Since the client base for a backup power system most likely is typically a non-technical user, in this present embodiment four bi-color LEDs, representing charge levels of 100%, 75%, 50%, and 25% are used. With fully charged batteries, all LEDs glow green and when each lower level is reached, its LED changes from green to red. Batteries  3  are assumed to be at or near depletion when all LEDs glow red. 
   When the charge level drops below 25%, batteries depleted signal  20  is generated and used by inhibit operation gate  50  to inhibit inverter  7  by means of disable inverter signal  51 . 
   Timing generator  31  provides system clock  52  to continually pulse LED drivers  48  in order to update the display when required by voltage level signal  49 . 
     FIG. 5  displays the logic used to select one and only one of a plurality of appliances at a time for backup power, and to enable or disable inverter  7 . 
   Central to this logic is slot counter gate  40 ; all other logic is peripheral to the decision making process. 
   As illustrated in  FIG. 3 , time slot generator  33  selects relay drivers  35  via relay select lines  34 . In turn, slot counter gate  40  clocks time slot generator  33  via slot advance clock  39 . Therefore, each time slot advance clock  39  occurs, time slot generator selects the next one and only one of a plurality of power relays  24  in sequence, such that one and only one power relay  24  is selected at a time, thereby delivering backup power only to the selected relay&#39;s assigned appliance. 
   Timing generator  31  provides synchronizing signals to slot duration clock  38  and system clock  52  to slot counter gate  40 . These are housekeeping signals and unrelated to criteria dependent upon current drawn by each selected appliance. 
   Backup power voltage  21  from inverter  7  is preferably passed through current transformers that comprise line current sensor  8 . This method was chosen since there is virtually no voltage drop across the primary of the transformers. 
   However, current sensor output  53  is normally in the 20 millivolt AC (minimum) range that must be amplified by high-gain amplifier/rectifier  54  to obtain sensed current DC level  55  signal for comparison by current comparator  56 . Note that a gain of at least 100, minimum, must be obtained by high-gain amplifier/rectifier  54  and at an extremely low noise level to be usable. High gain is necessitated by the fact that forward conduction diode drops of typically 750 millivolts across diodes must be overcome in order to obtain usable measurements. 
   Voltage regulator  57  must be a stable and accurate voltage reference source in order to provide current calibration standard signal  58  for reference by current comparator  56 . 
   Current comparator  56  is responsible for making three decisions, namely:
         1. that the selected appliance is requiring power to be delivered,   2. that the power required is reasonable, and,   3. that no short circuits exist in backup power output line  12 .
 
If the decision process yields an affirmative answer, then, and only then, will power be delivered to the one and only one of a plurality of selected appliances via its selected one and only one of a plurality of power relays  24 .
       

   Criteria 1 and 2, above, may be preset depending upon each particular installation. In the present embodiment, and depending upon empirical observations, it is assumed that at least 2 amperes AC, rms, must be drawn by any selected appliance before power will be delivered. This conclusion is based upon the fact that this is a backup power system for emergency use only and is not designed to power unnecessary items. The reasonable current draw capability is based upon typical appliances surveyed and found to be in the range of &lt;10 amperes AC at 120 volts, rms, and &lt;5 amperes AC at 240 volts, rms (typical for well pumps). 
   Criteria 3 pertains to possible short circuits detected at &gt;10 amperes at 120 volts AC, rms, and &gt;5 amperes at 240 volts AC, rms. This criteria is not subject to change (by increasing it) since inverter  7 , in this present embodiment, is designed for 1200 VA, rms. Of course, the maximum power available can be increased at a corresponding increase in inverter size and battery capability. 
     FIG. 5  also shows the interconnections that control the generation of backup power, as follows:
         1. Battery output voltage  18  is measured by battery status logic  37  which inhibits operation of inverter  7  when batteries  3  are at or near depletion as measured by batteries depleted signal  20 .   2. Battery charger  4  provides primary power status information via primary power failed signal  14  that forms a second input to inhibit operation gate  50 .   3. The output of inhibit operation gate  50  is inverter enable/disable signal  19  that controls generation of backup power from Class D Amplifier  59  that comprises the active element of inverter  7 .       
     FIG. 6  displays the circuitry of inverter  7 , and is comprised of four major components.
         1. Timing generator  31  generates a 60 Hz square wave signal  60  that passes through low-pass filter  61 , producing a 60 Hz sine wave signal  62 .   2. Class D Amplifier  59 , when enabled by inverter enable/disable signal  19 , drives power mosfets  64  in a push-pull configuration, generating backup power.   3. Power mosfets  64  are selected for lowest gate to source resistance (typically &lt;5 ohms) and high current capability, typically &gt;100 amperes.   4. Drain connections of each power mosfet  64  are connected to opposite ends of a bi-filiar wound toroidal transformer  66  primary to ensure balance between each half of the primary and also for minimal flux leakage.   5. The secondary winding(s) of transformer  66  comprise the backup output voltage of inverter  7 .       
     FIG. 7  illustrates battery charger  4 . This buck regulator is connected to primary power input lines  11  at AC interconnect terminals  23 . Bridge rectifier  67  produces rectifier line voltage  68  that is then filtered by EMI/RFI Filter  69  so as to reduce or eliminate conducted spurious signals. 
   Current and Voltage Controls  70  examine battery output voltage  18  and rectified line voltage  68  and derive buck drive signals  71  in order to control pulse width modulator  72  that serves to provide output mosfet drive signal  73  to properly drive charger output mosfet  74 , driving flyback transformer and rectifier  76  via flyback drive lines  75 . 
   The purpose of battery charger  4  circuitry is to rapidly charge depleted batteries  5  and then float batteries  5  at the rated terminal voltage recommended by the battery manufacturer. 
   Inhibit operation gate  50  provides an on/off function, the state of which depends upon the status of the primary power source. This control is effected by primary power failed signal  14 ; under normal conditions, its voltage is at a logical “1” and goes to a logical “0” when a power failure is detected. 
     FIG. 8  is a flow chart of operations, depicting how the backup power system operates. Diamond shapes indicate tests performed; each test has a “yes” or “no” output. Rectangles represent action taken as a result of each test. 
   Main power is applied to the household power panel; shown are 4 utilities, N, N+1, N+2, N+3, although the system is not limited to 4 utilities. The terms “utility” and “appliance” are interchangeable in the context of this discussion. 
   The “Has power failed” test is performed continuously. Under normal conditions, the “no” output re-circulates back to the main panel. In the even a failure occurs, the test yields a “yes” answer and backup power is enabled, and simultaneously, one and only one of a plurality of appliances is selected. 
   The selected utility is further tested to ensure that power is being demanded. Consider a typical refrigerator wherein the ambient temperature is lower than the thermostat setting. In this case, no power is demanded, the test result is “no” and so the next utility is then selected. Had the ambient temperature been higher than the thermostat setting, then the test result would have been “yes” and power would be demanded. 
   Once a demand for power is sensed, a test is performed to ensure that at least 2 amperes of current is flowing to the utility. This is to ensure that only a necessary load is receiving power. If the test results in “no”, then next utility is selected. A value of 2 amperes has been determined empirically and is easily resettable for a particular installation. 
   If the test result is “yes”, then the current drawn is measured to ensure that there is no short circuit, in which case backup power is supplied, otherwise the next utility is selected. In the even a short circuit exists in the appliance being tested, then a protective device within the backup power system permanently opens the circuit to the utility. 
   Once power is being supplied to a selected utility, a test is continually performed to determine if power is still required. A “no” answer can occur if the ambient temperature, as in the previous example, becomes lower that the thermostat setting, or if the presettable power delivery time has expired. In either case, the next utility is selected and the entire process repeats, at least until primary power returns, or if batteries  5  become depleted, in either case of which the backup power system is turned off.