Abstract:
A diagnostic system for a power distribution circuit including a line, neutral and ground may include a switch configured to electrically connect the line and neutral, a first sensor configured to sense a line to neutral electrical parameter, and a second sensor configured to sense a neutral to ground electrical parameter. The system may also include a processor configured to close the switch, to observe at least some of the sensed electrical parameters before and after the switch is closed, and to identify a fault condition in the line or neutral based on the observed electrical parameters.

Description:
BACKGROUND 
     Residential homes and commercial buildings are typically wired to provide electrical power via electrical outlets. Inspecting this wiring to identify faults may be time-consuming, expensive and/or impractical depending on the extent or layout of the wiring. 
     Existing diagnostic units may electrically connect a light source across the line to neutral, line to ground, and neutral to ground of a power distribution circuit. If properly wired, the light source turns on when connected across the line to neutral or line to ground. If improperly wired, the light source turns on when connected across the neutral to ground. 
     SUMMARY 
     A power distribution circuit including a line, neutral and ground may be characterized by first collecting line to neutral voltage information or line to neutral current information, and collecting neutral to ground voltage information or neutral to ground current information. At least one of a length of the neutral, a gauge of the neutral, and a fault condition in the line or neutral may then be determined based on the collected information. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an example power distribution circuit and an embodiment of a power distribution circuit test system. 
         FIG. 2  is a block diagram of an embodiment of a vehicle. 
     
    
    
     Like numerals of  FIGS. 1 and 2  may share similar, although not necessarily identical, descriptions. As an example, numbered elements  10 ,  110  may share similar descriptions. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an example power distribution circuit  10  for a building  12  is arranged in a known fashion to supply power from a power source  14  via an electrical outlet  16 . The electrical outlet  16  includes a line terminal  18 , a neutral terminal  20  and a ground terminal  22 . The terminals  18 ,  20 ,  22  are respectively electrically connected with a line  24 , a neutral  26  and a ground  28 . The line  24  and neutral  26  are electrically connected with the power source  14  via a fuse box  30 . Of course, other arrangements are also possible. 
     The power source  14  has an associated resistance, R S , the line  24  has an associated resistance, R 1 , and the neutral  26  has an associated resistance, R 2 . 
     An embodiment of a power distribution test system  32  may include a line terminal  34 , a neutral terminal  36  and a ground terminal  38 . The terminals  34 ,  36 ,  38  may be arranged as a plug to be plugged into the electrical outlet  16 . A voltage sensor  40  is electrically connected between the terminals  34 ,  36 , and may thus sense the voltage drop from the line  24  to the neutral  26  (a line to neutral electrical parameter). Another voltage sensor  42  is electrically connected between the terminals  36 ,  38 , and may thus sense the voltage drop from the neutral  26  to the ground  28  (a neutral to ground electrical parameter). A load  44  having a known resistance, R L , and a switch  46  are also electrically connected in series between the terminals  34 ,  36 . 
     Alternatively, current sensors may be used. For example, a current sensor (not shown) may be electrically connected between the terminal  34  and load  44  to sense the current through the line  24  and neutral  26  (a line to neutral electrical parameter); a current sensor (not shown) and an associated voltage source (not shown) may be electrically connected between the terminals  36 ,  38  to sense the current through the neutral  26  and ground  28  (a neutral to ground electrical parameter), etc. 
     The test system  32  may also include a display  48 , and microprocessor  50  in communication and/or operative control with the sensors  40 ,  42 , the switch  46 , and the display  48 . 
     In certain embodiments, the test system  32  may be arranged as a wall wart to provide stand-alone diagnostic capability. Referring to  FIG. 2 , the test system  132 , in other embodiments, may be integrated (or operatively arranged) with an on-board (or off-board) battery charger  133  of a vehicle  135  to analyze the power distribution circuit  110  before charging the vehicle&#39;s battery  137 . Other arrangements and scenarios are also possible. 
     As explained in more detail below, the microprocessor  50  may analyze characteristics of the power distribution circuit  10  based on the voltages reported by the sensors  40 ,  42  (and/or currents as the case may be). For example, the test system  10  may determine whether there is an additional load  52  electrically connected between the line  24  and neutral  26 , determine whether there is a fault in the line  24  or neutral  26 , determine the length of the neutral  26  and/or determine the gauge of the neutral  26 . This analysis and/or fault identification may, for example, be reported via the display  48 , stored in memory for later access, or transmitted via a suitable wireless/wired transmission system (not shown). 
     Additional Load on Circuit 
     To determine whether there is another load  52  on the power distribution circuit  10 , the microprocessor  50  may determine V LN@t=0  (the line to neutral voltage drop before the switch  46  is closed) and V NG@t=0  (the neutral to ground voltage drop before the switch  46  is closed) based on information from the sensors  40 ,  42  respectively. The microprocessor  50  may then cause the switch  46  to close and determine V LN@t=1  and V NG@t=1  (respectively, the line to neutral and neutral to ground voltage drops after the switch  46  is closed) based on information from the sensors  40 ,  42 . Next, the microprocessor  50  may determine the current, I, through the line  24  and neutral  26  as well as the resistance, R 2 , associated with the neutral  26  via the following relationships
 
 I=V   LN@t=1   /R   L   (1)
 
 R   2 =( V   NG@t=1   −V   NG@t=0 )/ I   (2)
 
Finally, the microprocessor  50  may determine the current, I al , associated with the load  52  (if present) via the following relationship
 
 I   al   =V   NG@t=0   /R   2   (3)
 
An I al  greater than some predetermined threshold value may indicate that a load  52  of significance is present on the power distribution circuit  10 . A threshold value greater than σ (0.1 A for example) may be used to account for errors in the above process, and may be based on testing, simulation, etc.
 
Length of Neutral Wiring
 
     To determine the length, L, of the neutral wire  26 , the microprocessor  50  may first retrieve resistance per length information, β G , from, for example, a table stored in memory for various wire gauges. The microprocessor  50  may then determine the length, L, associated with each gauge via the following relationship
 
 L=R   2 /β G   (4)
 
If the wire gauge is already known or otherwise determined, the microprocessor  50  may need only to retrieve resistance per length information for that particular gauge.
 
     If it is assumed that R 1 ≈R 2 , the length of the line wire  24  may be approximately equal to the length, L, of the neutral wire  26 . 
     Gauge of Neutral Wiring 
     To determine the gauge, G, of the neutral wire  26 , the microprocessor  50  may first determine R 2  at two instances of time (e.g., R 2@t , R 2@t+250 ; that is, two R 2  values that are 250 seconds apart. Other time periods, however, may also be used) via (2). The microprocessor  50  may then determine a change in wire temperature based on the following relationship
 
Δ T =( R   2@t+250   −R   2@t )/(α* R   2@t )  (5)
 
where α is the temperature coefficient of the metal used in the wire (assumed to be copper). This change in temperature, αT, may be used to determine the mass, m, of the neutral wiring  26  based on the following relationship
 
 m=Q   w /( c*ΔT )  (6)
 
where c is the specific heat capacity that is a material (in this example, copper) specific constant and Q w  is the energy stored in the wire:
 
 Q   w =∫( I   2   *R   2 ) dt−Q   rejected   (7)
 
where Q rejected  is any heat transferred from the wire to the surroundings. This term is assumed to be positive.
 
     The mass, m, of the neutral wiring  26  may also be determined based on the following relationship
 
 m=ζ*A*L   (8)
 
where ζ is the density of the wire, and A and L are the cross-sectional area and length of the wire respectively. The cross-sectional area is necessarily known for a given gauge. The length is determined via (4) for a given gauge.
 
     Respective masses may be determined via (8) for each potential gauge. Each of these masses may be compared with the mass determined via (6). By assuming Q rejected ≧0, the gauge yielding a mass equal to or less than the mass determined via (6) may be assumed to be the gauge for the neutral wire  26 . As an example, if the mass is found to be 6 kg via (6), and is found to be 8 kg for 12 gauge via (8) and 5 kg for 14 gauge via (8), the gauge of the neutral  26  may be assumed to be 14 gauge, the smaller of the two. This may offer the advantage of selecting the wire with the lower rated current capacity when determining the maximum safe current capacity of the circuit. 
     Fault in Line and/or Neutral 
     To determine whether there is a fault in the line  24  and/or neutral  26 , the microprocessor  50  may examine the resistance, R S , associated with the power source  14 . As this resistance is not directly measured, the microprocessor  50  may be determine it via the following relationships
 
 V   LN@t=1   −V   LN@t=0   =I *( R   1   +R   2   +R   L   +R   S )  (9)
 
rearranging (9) to yield (10)
 
 R   1   +R   S =(( V   LN@t=1   −V   LN@t=0 )/ I )−( R   2   +R   L )  (10)
 
 R   S =( R   1   +R   S )− R   2  (assuming  R   1   ≈R   2 )  (11)
 
     Assuming that the typically industry-stated average of 95% of the power supplied by the power source  14  reaches the electrical outlet  16 , and that the test system  32  presents a load of 6 A (i.e., R L =20Ω at 120 V) to the power distribution circuit  10 , 720 W (120 V*6 A) should be consumed by the test system  32 . In order for the power source  14  to provide 720 W, however, it needs to produce 758 W (720 W/0.95). As apparent to those of ordinary skill
 
 P=I   2   *R   S (12)
 
In this example, R S =1Ω (P=38 W, I=6 A). Similarly, if the test system  32  presents a load of 12 A to the power distribution circuit  10 , R S =0.5Ω. Because the load presented by the test system  32  is known, the microprocessor  50  may determine reasonable values for R S .
 
     If from (11) the microprocessor  50  finds R S  to be negative (or small relative to its expected value), the microprocessor  50  may report via the display  48  a potential fault condition, such as a loose connection, broken wire, etc., within the neutral  26 . (Under normal circumstances, R 1 ≈R 2 . A negative R S  would indicate that R 2 &gt;R 1 +R S .) If from (11) the microprocessor  50  finds R S  to be large relative to its expected value, the microprocessor  50  may report via the display  48  a potential fault condition within the line  24 . (A relatively large R S  may indicate that R 1 &gt;&gt;R 2 .) 
     Referring to (5) and (6), those of ordinary skill will recognize that the large long wires typically used between the power source  14  and fuse box  30  will experience an insignificant temperature rise due to the current through R L , so any change in R S  will be due to changes in R 1  and R 2 . 
     The microprocessor  50  may also examine if the determined value of R S  changes significantly over time to identify a potential fault condition within the line  24  or neutral  26 . A wire&#39;s resistance increases as it heats up. According to (11), if the line  24  and neutral  26  are heating up at approximately the same rate, R S  will remain relatively constant. If the line  24  is heating up much faster than the neutral  26  (once the switch  46  has been closed) because of a fault within the line  24 , R S  will increase in value (because R 1  will increase in value faster than R 2 ). If the neutral  26  is heating up much faster than the line  24  because of a fault within the neutral  26 , R S  will decrease in value (because R 2  will increase in value faster than R 1 ). 
     As apparent to those of ordinary skill, the algorithms disclosed herein may be deliverable to a processing device, which may include any existing electronic control unit or dedicated electronic control unit, in many forms including, but not limited to, information permanently stored on non-writable storage media such as ROM devices and information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media. The algorithms may also be implemented in a software executable object. Alternatively, the algorithms may be embodied in whole or in part using suitable hardware components, such as Application Specific Integrated Circuits (ASICs), state machines, controllers or other hardware components or devices, or a combination of hardware, software and firmware components. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.