Abstract:
Detecting a discontinuity or impedance irregularity in an active line, a neutral return line and/or an earth return path of a power distribution network is disclosed. In one aspect, the apparatus is configured to measure a change in voltage and/or current associated with a deliberate switching of a known impedance and/or a naturally occurring random switching of an impedance in the electrical network wherein the change in voltage and/or current flow results from a discontinuity or impedance irregularity in the active line, neutral line and/or earth return path. The apparatus includes an algorithm for identifying discontinuity or impedance irregularity in the presence of allowable variation in voltage and/or current that may mimic or hide a discontinuity or impedance irregularity in the active line, neutral return line and/or earth return path including a reverse current flow through the earth return path.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application, and claims the benefit under 35 U.S.C. §§120 and 365 of PCT Application No. PCT/AU2009/001657, filed on Dec. 18, 2009, which is hereby incorporated by reference. PCT/AU2009/001657 also claimed priority from Australian Patent Application No. 2009901651, filed on Apr. 17, 2009, which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The described technology generally relates to monitoring and/or detecting faults in a supply line of an electrical power distribution network. 
         [0004]    2. Description of the Related Technology 
         [0005]    The electricity power supply industry generally has an earthed return system to provide a protected electrical return path in the case of faults. Flow of current in the system is mostly between the active and neutral return lines. The system allows for current to flow between the active line and the earth return path when a fault occurs in equipment connected to the system. 
         [0006]    Because current can flow in one or two circuits (neutral and earth), a discontinuity or impedance irregularity in one circuit (neutral or earth) can go undetected for a period of time without any indication of danger until the second circuit (neutral and earth) also becomes defective. 
         [0007]    For example, a high impedance or discontinuity in a neutral line or wire may allow current to flow between active and earth. However, the earth return path may become ineffective or defective over time due to a number of factors including drying out of the soil, a faulty connection or cable damage following work carried out on plumbing or the like. When a sound earth return path is not in place current may flow to earth through other paths such as water pipes and storm drains or it may not flow at all. The latter may cause a rise in voltage potential above earth and create a danger of electric shock to persons with a potential for injury or death. 
         [0008]    In addition, a high impedance in an active line or path can result in electrically induced heating and/or electrical arcing that may result in a potential for fire, property damage, injury and/or death. 
       SUMMARY 
       [0009]    One inventive aspect is a method of detecting a fault such as a discontinuity or impedance irregularity in a supply line including an active line, a neutral return line, or an earth return path of an electrical network, wherein presence of a voltage potential may result in a danger or risk of electric shock to persons with a possibility of injury or death or wherein presence of electrically induced heating and/or electrical arcing may result in a danger or risk of fire. 
         [0010]    One embodiment may detect a discontinuity or impedance irregularity in an active line (or lines), a neutral return line (or lines) and/or an earth return path (or lines) of an electrical power distribution network. One embodiment may detect the discontinuity or impedance irregularity at a consumer site. One embodiment may detect the discontinuity or impedance irregularity by monitoring and/or measuring a property or properties associated with the supply lines. The property or properties may include a change in loop impedance, current flow in the active and neutral supply lines and the earth return path and/or a voltage change in an electrical circuit associated with the network. Changes in current flow in the active and neutral lines and the earth return path as well as in voltage may be caused by naturally occurring random and/or deliberate changes in impedance of the network including the active line, neutral return line and earth return path. One embodiment may include one or more algorithms which may distinguish allowable variations in current flow as well as variations resulting from changes in “nominal supply voltage” and other voltage changes including steps, sags, spikes, etc. attributable to normal network operations that may either mimic or hide a discontinuity or impedance irregularity in an active line, neutral return line and or earth return path. The algorithm(s) may also distinguish current flow in the earth return path at a consumer site resulting from discontinuities or impedance irregularities in an active line, neutral return line or earth return path occurring at other installations that may mimic or hide a discontinuity or impedance irregularity in an active line, neutral return line and/or earth return path at the consumer site. 
         [0011]    The electrical properties as well as physical dimensions and characteristics of electrical circuits that develop a discontinuity or impedance irregularity in an active line, neutral line or earth return path may differ from those present in electrical circuits that retain an intact active line, neutral line or earth return path. 
         [0012]    Given that the sum of the magnitude and direction of current flow in the active and neutral lines and the earth return path of an electrical network under normal circumstances should equal zero, and given also that the current flow is dependant upon serial and parallel impedances of the active and neutral lines and the earth return path as well as supply voltage magnitude and phase, a measurement and comparison of changes in the current flow and supply voltage, may reflect changes in impedances of the active and neutral lines and the earth return path of an electrical network. Under a condition of a discontinuity or impedance irregularity in the neutral line, loop impedance of the active neutral/earth return path will increase as will the current flow in the earth-return path. Under a condition of a discontinuity or impedance irregularity in the active line, loop impedance of the active neutral/earth return path will increase while the currents in the neutral and active lines will remain substantially unchanged under similar load conditions. Under a condition of a discontinuity or impedance irregularity in the earth return path, loop impedance of the active neutral/earth return path will increase while the current in the neutral line will also increase. 
         [0013]    However, current flow in both the earth return path as well as the neutral line can result from discontinuities or impedance irregularities in a neutral return line or earth return path occurring at other installations. A phase dependant current flow originating at other installations may mimic or hide a discontinuity or impedance irregularity in the neutral return line or earth return path at the consumer site where the current is being measured. 
         [0014]    Given a stable supply voltage, an expected voltage drop in a circuit may depend upon series and parallel impedances in the circuit, impedance of the active line, neutral line return, and impedance of the earth return path. Under a condition of a discontinuity or impedance irregularity in the neutral line the expected voltage drop may depend primarily on the value of the earth return path impedance, which will generally be measurably greater than in the case of an intact neutral. 
         [0015]    Measurement of a change in current flow and a drop in line voltage resulting from a change in impedance in a network may be used to indicate a discontinuity or impedance irregularity in a supply line of an electrical power distribution network. A measurable change in current flow and voltage drop may result from naturally occurring random switching of impedances within an electrical network, or may result from deliberate or planned switching of impedance in the electrical network. 
         [0016]    Measurement of a change in current flow and voltage drop resulting from naturally occurring random switching of impedances within an electrical network, or from deliberate or planned switching of impedance in the electrical network can be used to estimate magnitude and condition of serial and parallel impedances of the active and neutral lines and the earth return path. 
         [0017]    As the impedance of a neutral return line is generally less than that of an earth return path, the presence of a voltage potential under conditions of high neutral return impedance may result in a danger of electric shock to persons with a possibility of injury or death. 
         [0018]    Another aspect is an apparatus for detecting a discontinuity or impedance irregularity in a supply line and or earth return path of an electrical power distribution network. The discontinuity or impedance irregularity may be present anywhere between a supply transformer and a point of connection of the apparatus to the power distribution network. The apparatus may be installed in a customer&#39;s premises at a convenient location such as a switchboard or it may be associated with metering equipment. 
         [0019]    The apparatus may be adapted to differentiate between a circuit having an intact neutral return line and or earth return path, and a circuit having a discontinuity or impedance irregularity in an active line, a neutral return line and or earth return path. The apparatus may measure a change in current flow and line voltage resulting from a change in impedance in a network that may be used to indicate a discontinuity or impedance irregularity in a supply line of an electrical power distribution network. A measurable change in current flow and voltage drop may result from naturally occurring random switching of impedances within an electrical network, or may result from deliberate or planned switching of impedance in an electrical network. The apparatus may measure a change in current flow and voltage resulting from naturally occurring random switching of impedances within an electrical network, or from deliberate or planned switching of impedance in an electrical network, in order to estimate magnitude and condition of serial and parallel impedances of the active and neutral lines and the earth return path. 
         [0020]    Electricity distribution supply networks generally provide electricity at a defined “nominal supply voltage” that may vary between allowable high and low bounds. In addition to these allowable variations in “nominal supply voltage” are voltage changes, (steps, sags, spikes, etc.) resulting from normal network operations. These may include voltage rises or drops due to various factors including loads imposed on the local or distribution network, overloading of transformers, switching, lightning strikes, re-closer operation, etc. 
         [0021]    As naturally occurring voltage sags and spikes in a supply voltage may result in voltage drops or rises that may mimic or hide a discontinuity or impedance irregularity in an active line, a neutral supply line and/or earth return path, and naturally occurring changes in local and distribution network impedances may result in a change to current flow that may mimic or hide a discontinuity or impedance irregularity in an active line, a neutral supply line and or earth return path, the apparatus may include an algorithm that may minimise impact of such anomalous events on reliable detection of the discontinuity or impedance irregularity in an active line, a neutral supply line and or an earth return path. Thus the algorithm may facilitate identification of a discontinuity or impedance irregularity in an active line, a neutral supply line and/or an earth return path under anomalous voltage or current flow conditions. 
         [0022]    A reverse current flow through the earth return path from other installations may also alter current flow between the active and neutral lines and the earth return path. Thus the algorithm may allow for estimation of the magnitude and condition of serial and parallel impedances of the active and neutral lines and the earth return path under conditions of reverse current flow in the earth return path and may identify potential discontinuities or impedance irregularities at other customer sites. 
         [0023]    The apparatus may include means such as an audible or visual signal or an alarm or an electronic signal to communicate to the consumer and/or a third party that an active line, a neutral return line or earth return path may contain a discontinuity or impedance irregularity. 
         [0024]    The apparatus may include means to interrupt current flow in the active line suppling a consumer site in the event that an active line, a neutral return line or earth return path may contain a discontinuity or impedance irregularity. 
         [0025]    Another aspect is an apparatus for detecting a discontinuity or impedance irregularity in an active line, a neutral return line and/or an earth return path of an electrical power distribution network including the active line, neutral return line and earth return path, the apparatus including: means for measuring a change in voltage and/or current flow associated with a deliberate switching of a known impedance and/or a naturally occurring random switching of an impedance in the electrical network wherein the change in voltage and/or current flow is due to a discontinuity or impedance irregularity in the active line, neutral line and/or earth return path; means for implementing a first algorithm for identifying the discontinuity or impedance irregularity in presence of allowable variation in voltage and/or current flow that may mimic or hide a discontinuity or impedance irregularity in the active line, neutral return line and/or earth return path including a reverse current flow through the earth return path; and means for comparing a result of the measuring with a reference to provide an indication of the discontinuity or impedance irregularity. 
         [0026]    The apparatus may include means for measuring a voltage change in the electrical network associated with the deliberate switching, wherein the voltage change is due to the discontinuity or impedance irregularity in the neutral return line and/or earth return path. 
         [0027]    The first algorithm may be adapted for identifying the discontinuity or impedance irregularity in presence of allowable variation in nominal supply voltage to the electrical network including a voltage change resulting from network operations that mimic or hide a discontinuity or impedance irregularity in the active line, neutral return line and or earth return path. 
         [0028]    The apparatus may include means for implementing a second algorithm that includes an estimation of magnitude and condition of serial and parallel impedances of the active and neutral lines and the earth return path in presence of allowable variation in nominal supply voltage to the electrical network including a voltage change resulting from network operations that mimic or hide a discontinuity or impedance irregularity in the active line, neutral return line and/or earth return path. 
         [0029]    The second algorithm may include an estimation of variation in current flow that may mimic or hide a discontinuity or impedance irregularity in the active line, a neutral supply line and/or earth return path including a reverse current flow through the earth return path that may alter current flow between the active and neutral lines and the earth return path. 
         [0030]    Another aspect is a method for detecting a discontinuity or impedance irregularity in an active line, a neutral return line and/or an earth return path of an electrical power distribution network including the active line, neutral return line and earth return path, the method including: measuring a change in voltage and/or current flow associated with a deliberate switching of a known impedance and/or a naturally occurring random switching of an impedance in the electrical network wherein the change in voltage and/or current flow is due to a discontinuity or impedance irregularity in the active line, neutral line and/or earth return path; implementing a first algorithm for identifying the discontinuity or impedance irregularity in presence of allowable variation in voltage and/or current flow that may mimic or hide a discontinuity or impedance irregularity in the active line, neutral return line and/or earth return path including a reverse current flow through the earth return path; and comparing a result of the measuring with a reference to provide an indication of the discontinuity or impedance irregularity. 
         [0031]    The method may include measuring a voltage change in the electrical network associated with the deliberate switching, wherein the voltage change is due to the discontinuity or impedance irregularity in the neutral return line and/or earth return path. 
         [0032]    The method may include the step of implementing a second algorithm that includes an estimation of magnitude and condition of serial and parallel impedances of the active and neutral lines and the earth return path in presence of allowable variations in nominal supply voltage to the electrical network including a voltage change resulting from network operations that mimic or hide a discontinuity or impedance irregularity in the active line, neutral return line and/or earth return path. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  shows a simplified diagram of a typical installation. 
           [0034]      FIG. 2  shows a simplified diagram of a faulty installation. 
           [0035]      FIG. 3  shows a representation of a local network including an intact neutral return line. 
           [0036]      FIG. 4  shows a representation of a local network including a discontinuous neutral return line. 
           [0037]      FIG. 5  shows a representation of normal variations in “nominal voltage” including randomly occurring voltage sags and spikes. 
           [0038]      FIG. 6  shows a block diagram of an apparatus for detecting a discontinuity in an electrical power distribution system. 
           [0039]      FIG. 7  shows a diagram of one form of apparatus according to one embodiment. 
           [0040]    
         5 
       
           [0041]      FIG. 8  shows a flow diagram of one form of algorithm. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    Embodiments will be described with reference to the accompanying drawings. Reference herein to a supply line including an active line, a neutral return line and/or an earth return path of an electrical power distribution network includes a reference to a plurality of supply lines including a plurality of active lines, neutral return lines and/or earth return lines of the electrical power distribution network as the case may be. 
         [0043]      FIG. 1  shows a simplified example of a domestic electrical power supply installation including overhead transmission line  10  between house  11  and distribution transformer  12 . The installation has an intact neutral return line  13  between house  11  and distribution transformer  12 . 
         [0044]      FIG. 2  shows the same domestic power supply installation including a break  14  in the neutral return line  13  to house  11 . In this case the earth and the water-pipe bond form a secondary connection with the neutral connection of house  15  next door and/or with an earth return connection of distribution transformer  12 . 
         [0045]      FIG. 3  shows a representation of a local network  40  including a plurality of naturally switched loads Z L  made up of Z L1 , Z L2 , Z L3  connected between active line  41  and neutral line  42 . A local current I A  flows between the active line and the neutral line/earth return path determined by voltage V 1  across the local network and the total local network impedance Z L . The impedance Z N  represents the neutral impedance associated with neutral line  42  while local earth impedance is represented by Z E . The equivalent impedance Z NE  of parallel impedances Z N  and Z E  may be represented by 
         [0000]        Z   NE =1(1/ Z   N +1/ Z   E ) 
         [0046]    The voltage V 2  at a common connection point of the neutral line and the earth return path is dependant upon the voltage across the local network, V 1  the total network impedance Z L  and the equivalent parallel impedance Z NE . 
         [0000]        V   2   =V   1   *Z   NE ( Z   NE   +Z   L ) 
         [0047]    Local current I A  flows through parallel impedances Z N  and Z E  as currents I N  and I E  respectively based upon their relative impedances such that 
         [0000]        I   N   *Z   N   =*Z   E  or  I   N   =I   E   *Z   E   /Z   N    
         [0048]    Since the sum of magnitude and direction of current flows in the active and neutral lines and the earth return path of an electrical network under normal circumstances should equal zero this implies that 
         [0000]        I   A   =I   N   +I   E  or  I   E   =I   A   −I   N    
         [0049]    It follows that I N =I A *Z E /(Z N +Z E ) 
         [0050]    Under normal circumstances when both the neutral line and earth return path are continuous and regular, the magnitude of and relative difference between impedances Z N  and Z E  is generally such that when
   Z NE →0 hence Z NE /(Z NE +Z L )→0 and V 2 →0   Z E &gt;&gt;Z N  hence I N &gt;&gt;I E  and I N →I A      
 
         [0053]    Under various conditions, a reverse earth return current I RE  may be present in the network originating from other networks. This current may distort measured values of I N  and I E , and may be in-phase with networks active current or may be out of phase with the active current. 
         [0054]      FIG. 4  shows local network  40  of  FIG. 3  including a discontinuity  43  in neutral return line  42 . Discontinuity  43  or an impedance irregularity may give rise to a change in neutral impedance Z N . 
         [0055]    Under these circumstances the magnitude and relative difference between impedances Z N  and Z E  is generally such that when
   Z N →∞ hence Z NE =1/(1/Z N +1/Z E )→Z E      Z N &lt;&lt;∞ hence V 2 →0   Z N → hence Z NE  increases and V 2  increases   Z E &lt;&lt;Z N  hence I N &lt;&lt;I E  and I N →0   
 
         [0060]    In  FIG. 4  the local current I A  flows exclusively or primarily via earth impedance Z E  resulting in a change in relative magnitude of current flows I N  and I E  which can be compared to a reference or standard relative magnitude to provide an indication of the discontinuity  43  or an impedance irregularity in neutral line  42 . 
         [0061]    Under a condition of discontinuity  43  or an impedance irregularity in a neutral line voltage V 2  is greater than it would be under a condition of a continuous or regular neutral line. The increase in voltage V 2  may be detected by comparing V 2  to a reference or standard voltage to provide an indication of the discontinuity  43  or impedance irregularity in neutral return line  42 . 
         [0062]    Under these circumstances, an increased earth return current I E  may appear on one or more other networks as a reverse earth return current I RE . This current may distort measured values of I N  and I E , the ratio of I N /I A , on these other networks, and may be in-phase with other networks active current, or may be out of phase with other networks active current. 
         [0063]      FIG. 5  provides an example of line voltage variations that may be present in a typical electrical distribution network. The variations include variations in “nominal supply voltage” and voltage changes such as steps, sags, spikes, etc. due to normal network operations, including voltage changes or drops due to loads imposed on a local or distribution network, overloading of transformers, switching, lightning strikes, re-closer operations, etc. 
         [0064]      FIG. 6  shows a conceptual diagram of one form of apparatus for detecting a discontinuity or impedance irregularity in an electrical power distribution system. The apparatus includes switchable impedance block  60  for applying an impedance to a line voltage supply. Impedance block  60  includes means for controlled switching of impedance to a circuit associated with the line voltage supply. 
         [0065]    The apparatus includes voltage measurement block  61  including a means for converting the voltage input from an analog into a digital representation by using an analog to digital converter. 
         [0066]    The apparatus includes current measurement block  62  including current sensing means such as transformers or shunts in series with the active and neutral supply lines for measuring current flow and a means for converting the voltage input from the current sensing transformers or shunts from an analog into a digital representation by using an analog to digital converter. The current measurement block  62  may include means to measure current flow in a single or multiple active phase lines either individually or as a single current flow. 
         [0067]    The apparatus includes an audible and/or visual signal or alarm  63  and/or an electronic alarm signal block  64  to communicate to a consumer and/or a third party that an active line, a neutral return line and or earth return path may contain a discontinuity or impedance irregularity. 
         [0068]    The apparatus may include an active current flow breaker block  65  that may be used to interrupt active current flow in the event of a discontinuity or impedance irregularity in an active line, a neutral return line and or earth return path. Active breaker block  65  may be controlled by a microprocessor and memory block  66 . 
         [0069]    Microprocessor and memory block  66  may be adapted for controlling impedance block  60 , voltage measurement block  61 , current measurement block  62 , alarm signal block  63 , and active current flow breaker block  65  as well as for determining and/or confirming whether the supply line has a discontinuity or impedance irregularity in an active line, a neutral return line and/or earth return path. The apparatus may include a communications channel block  67  to allow the apparatus to communicate with an external third party and may provide to the third party information on a discontinuity or impedance irregularity in an active line, a neutral return line and/or earth return path. 
         [0070]      FIG. 7  shows a schematic diagram of one form of apparatus for detecting a fault in an active line, neutral line or earth return path. The apparatus includes a power supply  70  which provides power for operation of microprocessor  71 , alarm lights  72 , and audible alarm  73 . The apparatus includes switchable impedance  74  consisting of power resistor R 1  switched by means of triac T 1  under control of microprocessor  71 . Microprocessor  71  includes a software implementation of an algorithm as described below. Microprocessor  71  measures line voltage and current by means of an inbuilt analog to digital converter, controls operation of switchable impedance  74  via triac T 1  and controls operation of alarm lights  72 , audible alarm  73  and communications to external devices and third parties. 
         [0071]      FIG. 8  shows a flow diagram for an example algorithm that may allow for identification of a discontinuity or impedance irregularity in an active line, neutral supply line and or earth return path in a network or associated networks:
       Under anomalous voltage conditions such as allowable variations in “nominal supply voltage” and voltage changes, (steps, sags, spikes, etc.) resulting from normal network operations.   Under anomalous changes in local and distribution network impedances that may result in changes to current flows resulting from normal network operations.   Under reverse current flow through the earth return path from other installations that may alter the current flow between the active and neutral lines and the earth return path.       
 
         [0075]    The algorithm includes a start-up and self-test routine  80  that checks whether the user interface and apparatus hardware is operating within defined parameters, and is enabled upon apparatus power-up or restart and/or on a timed basis during operation of the apparatus. In the event that the algorithm detects that there is a problem with the user interface and/or apparatus hardware a “failed self-test” alarm condition will signalled and the algorithm will hold this alarm until the device is reset. 
         [0076]    If no problems during self-test routine  80  are detected, the algorithm will proceed to routine  81  in which the algorithm will pause for a random period prior to proceeding to “desynchronise” the device from other simultaneously started devices. Voltage and current measurements may be used to determine 5-second average values of V A5 , I A5  and I NS  using a ¼ filter. 5-minute average values of V A300 , I A300  and I N300  may be determined using a filter for a 300-second time constant. The device will measure current in the active phase conductor I A , current in the neutral conductor I N , and active-neutral voltage V A . Values may then be calculated for V A5 , I A5 , and I NS  and V A300 , I A300  and I N300 , and for I N /I A  and I NS /I A5 . 
         [0077]    Following initial measurement of voltage, active and neutral line currents the algorithm will proceed to routine  82  and the following initial conditions will be tested. If I N /I A &lt;5% then signal “broken neutral” alarm. If I N /I A &lt;30% then signal “impaired neutral” alarm. 
         [0078]    The algorithm will next enter a primary operational routine loop. The first subroutine  83  will initiate a program hold for a random period prior to proceeding in order to “desynchronise” the device from other simultaneously started devices. 
         [0079]    The algorithm will next move to subroutine  84  which will perform a series of active impedance tests involving switching of a know impedance in and out of the network circuit so as to minimise impact on measurement of V, I A  and I N , of changes in current flows and voltages that may result from naturally occurring random switching of impedances within an electrical network, or that may result from deliberate or planned switching of impedance in an electrical network. 
         [0080]    With the impedance switched out and then switched in, measurements of the line voltage, active and neutral line currents are performed under each condition with each measurement averaged over a defined interval. 
         [0081]    This series of measurements may be repeated to determine a reliable average value over a short time period and the resulting averages logged to memory. 
         [0082]    Calculate active-return path loop impedance Z ANE  using measured voltage and currents. 
         [0000]        Z   ANE =( V   1   −V   2 )/( I   A2   −I   A1 ) 
         [0000]      confirm 
         [0000]        Z   ANE =( V   2   −V   1 )*(230/ V   1 ) 
         [0083]    Calculate active-neutral path loop impedance Z AN  and active-earth return path loop impedance Z AE  when impedance switched in. 
         [0000]    
       
      
       Z 
       AN 
       =Z 
       ANE 
       *I 
       A2 
       /I 
       N2  
      
     
         [0000]        Z   AE   =Z   ANE   *I   A2 /( I   A2   −I   N2 ) 
         [0084]    Calculate covariance of loop impedances vs active currents. 
         [0085]    Covariance C AN  of Z AN  vs I A , 
         [0086]    Covariance C AE  of Z AE  vs I A , 
         [0087]    Covariance C ANE  of Z ANE  vs I A    
         [0088]    The measured and calculated values may be compared with previously logged values and programmed limits subroutine  85 , as follows and actions taken as required. 
         [0089]    If Z AN &gt;1.0 Ohm then signal “broken neutral” alarm. 
         [0090]    Z AE &gt;10 Ohm then signal “impaired earth” alarm. 
         [0091]    Z ANE &gt;1.0 Ohm then signal “impaired active” alarm. 
         [0092]    C AN &gt;2.0 then add 1 to counter CO A    
         [0093]    C AE &gt;5.0 then add 2to counter CO A    
         [0094]    C ANE &gt;2.0 then add 3 to counter CO A    
         [0095]    CO A =5 then signal possible hot joint on active 
         [0096]    CO A =1 or 4 then signal possible hot joint on neutral 
         [0097]    CO A =2 or 5 then signal possible hot joint on earth 
         [0098]    The algorithm will now enter a passive monitoring loop subroutine  86  and will perform measurements of line voltage, active and neutral currents with each measurement averaged over a defined interval. These values will be used in the calculation of the running averages, V 05 , I A05 , I N05  and V 0300 , I A0300 , I N0300 . 
         [0099]    Following these measurements of voltage, active and neutral line currents subroutine  87  will make the following calculations, log the specified values to memory and take any required actions. 
         [0100]    Calculate the value for I N /I A  and I N5 /I A5    
         [0101]    If I N /I A &lt;5% then log “broken neutral” alarm 
         [0102]    If I N /I A &lt;30% then log “impaired neutral” alarm 
         [0103]    If I N /I A &gt;105% then log “reverse current flow” alarm 
         [0104]    If 1−{(I N /I A )/(I N5 /I A5 )}&gt;25% or &lt;−25% then run Active Test 
         [0105]    Subroutine  88  will make the following calculations, and take any required actions. 
         [0106]    If V A300 &gt;270 V then signal “voltage higher than regulatory maximum” and proceed. 
         [0107]    If V A300 &lt;200 then signal “voltage lower than regulatory minimum” and proceed. 
         [0108]    Subroutine  89  will determine if a scheduled self test is required. 
         [0109]    Subroutine  91  will determine if a scheduled active test is required. 
         [0110]    Return to start of passive monitoring loop, subroutine  86 . 
         [0111]    Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts values and/or parameters previously described without departing from the spirit or ambit of the following claims.