Patent Publication Number: US-10788542-B2

Title: Detection of deteriorated electrical connections in a meter using temperature sensing and time variable thresholds

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
     This is a continuation of U.S. patent application Ser. No. 15/719,095, filed Sep. 28, 2017, now allowed, and is also related to co-pending U.S. patent application Ser. No. 15/719,086, filed Sep. 28, 2017, the entirety of both being incorporated herein by reference 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to electricity meters, and more particularly, to electricity meters having temperature sensing. 
     BACKGROUND OF THE INVENTION 
     Utility meters are devices that, among other things, measure the consumption of a utility-generated commodity, such as electrical energy, gas, or water, by a residence, factory, commercial establishment or other such facility. Utility service providers employ utility meters to track customer usage of the utility-generated commodities. Utility service providers track customer usage for many purposes, including billing and demand forecasting. 
     Electricity meters that measure energy consumption or power consumption typically connect between a utility power line and a load. For example, an electricity meter for a residential customer is often connected at the point at which the electrical system of the residence connects to the utility line. The meter may thereby perform measurements regarding the energy consumed by the load. 
     Electricity meters often include one or more electrical contacts across which the load voltage and a significant amount of current may be found. For example, meters often have blades that connect to the power line to enable the measurement of load current and load voltage from within the meter. The blades are received by the jaws of a meter mounting device of the building. Spring compression within the jaws retains the blades securely. If the meter is to be replaced or repaired, then the meter may be pulled out of the mounting device, and hence the blades out of the jaws. Although the blades and jaws are usually mechanically robust, they are nevertheless subject to wear, and possibly corrosion, particularly if the meter has been removed or replaced several times. If wear on the jaws is significant, or if the jaws have corrosion, then there is a possibility of introducing a non-trivial resistance at the jaw/blade connection, which is undesirable. In some cases, the jaw/blade connection can undesirably deteriorate to a condition in which arcing occurs. 
     Likewise, certain meters have switches that allow for disconnection of electrical service to a load. For example, many meters allow for remote disconnection of the load. Such switches necessarily must have substantial contacts because they carry the entire current of the load when the switch is closed. If these switches are used with some frequency, then there is a potential for degradation. Degradation of the switch contacts increases the resistance over the switch contacts. As with the meter blades and jaws, resistance creates additional power loss within the meter, and potentially arcing, both undesirable. 
     It is known to detect the possible deterioration of meter switch contacts by measuring the resistance and/or current through the contacts within the meter. If the resistance exceeds a threshold, then an indication of potential need for maintenance is displayed or transmitted. Such a method is taught, for example, in U.S. Pat. No. 7,683,642, issued Mar. 23, 2010. One limitation of this technique is that it can require extra elements to carry out the resistance measurement, thereby adding material cost and manufacturing complexity. 
     It is also known to monitor the temperature of the sockets and jaws of the meter for overheating. The detection of an overheat condition in the sockets and jaws of the meter can indicate an arcing condition, or other condition, such as increased resistance, requiring maintenance. Such a method is discussed, for example, in U.S. Pat. No. 7,513,683. This method, however, requires that the temperature sensing device be attached to a mass in thermal contact with the electrical connection. This technique, though simple, cannot be applied in meter designs where the temperature sensing device is isolated from the electrical connection by some sort of significant insulator, such as an air gap. In such meter designs, the measured temperature can be distorted to a significant extent by ambient temperature and other normal operations within the meter. As a consequence, the threshold must be high enough to avoid false positives due to other conditions causing a temperature rise in and around the meter blades. 
     Another known method of detecting the presence of arcing or other meter blade/socket malfunction includes monitoring RF noise within the meter. In particular, arcing between the meter blades and the meter socket causes emission of certain RF noise that may be monitored. Such a method is described in U.S. Patent Publication No. 2014/0327449. Such a solution, however, cannot readily distinguish arcing from other sources of RF noise. Thus, sometimes such a method includes monitoring other meter phenomena, such as internal temperature, so that multiple phenomena can confirm the condition. However, such a solution requires an RF receiver and has the complexity associated with monitoring multiple factors to determine if arcing is present. 
     Thus, a continuing need exists to detect possible issues due to deterioration of high-power switch contacts in a meter or a meter socket that can reliably and efficiently determine the presence of a maintenance issue. 
     BRIEF SUMMARY OF THE INVENTION 
     Different embodiments described herein address the above-cited need, as well as others, by using a compensated temperature measurement to determine if an overheat condition exists. Moreover, other embodiments determine whether a measured temperature (with or without compensation) is outside of normal ranges based on the time of year and the time of day. 
     A first embodiment is a utility meter that includes a meter housing that supports at least one current coil, a temperature sensor, and a processing circuit coupled to both. The current coil can be coupled to receive heat energy from a meter socket. The temperature sensor disposed generates a sensor signal based on a temperature within the meter housing. The processing circuit obtains the sensor signal and generates meter temperature information based at least in part thereon. The processing circuit also obtains a first predetermined threshold based on at least one of time of day information and date information. The processing circuit also determines whether an abnormal condition exists by comparing the meter temperature information to a value based on the first predetermined threshold, and generates an output signal to a memory, display or communication circuit responsive to determining that the abnormal condition exists. 
     The above-described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic block diagram of a metering system for a facility that includes a mounting device and a utility meter; 
         FIG. 2  shows an exemplary set of operations carried performed by the processing circuit of the meter of  FIG. 1  to generate metering information; 
         FIG. 3  shows a flow diagram of an exemplary meter heat monitoring routine that may be performed by the processing circuit of the meter of  FIG. 1 ; 
         FIG. 4  shows in further detail an exemplary set of operations that may be performed as part of the meter heat monitoring routine of  FIG. 3 ; 
         FIG. 5  shows in further detail an exemplary set of operations that may be performed as another part of the meter heat monitoring routine of  FIG. 3 ; 
         FIG. 6  shows an exemplary timeline of the values of a sample operation of the routine of  FIG. 3 ; 
         FIG. 7  shows an exemplary timeline of the values of a sample operation of an alternative embodiment of the routine of  FIG. 3 ; and 
         FIG. 8  shows in further detail an exemplary set of operations that may be performed as part of the alternative embodiment of the meter heat monitoring routine of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that this disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one skilled in the art to which this disclosure pertains. 
     As shown in  FIG. 1 , a metering system  100  for a facility  104  includes a mounting device  108  and a utility meter  112  associated with electrical power distribution lines  116  that distribute electrical energy from a utility  120 . In the exemplary arrangement of  FIG. 1 , the mounting device  108  includes two line-side sockets  124  electrically connected to the distribution lines  116 , and two load-side sockets  128  electrically connected to the facility  104 . The sockets  124 ,  128  are formed from metal and are configured to withstand high currents and voltages. In other embodiments, the mounting device  108  includes any suitable number of sockets  124 ,  128  formed from any suitable material. 
     The utility meter  112  includes a housing  136 , at least one primary coil or current coil  140  (two shown in  FIG. 1 ), at least one secondary coil  144  (two shown in  FIG. 1 ), at least one voltage sensor  141  (two shown in  FIG. 1 ), a temperature sensor  160 , and a metrology circuit  152 . In this embodiment, the utility meter  112  also includes memory  180 , a transceiver  184 , and a display  188 . The housing  136  is an electricity meter housing, as is known in the art, which supports and protects meter components from tampering and harmful environmental conditions. In this embodiment, the voltage sensors  141 , the secondary coils  144  and memory  180 , the transceiver  184 , the display  188  and the metrology circuit  152  are all contained within an interior of the housing  136 . Preferably, a printed circuit board  137  supports the memory  180 , the transceiver  184 , the display  188  and the metrology circuit  152 . The printed circuit board  137  is spaced apart from the high current components, i.e., the current coils  140 , by at least two inches. 
     The current coils  140 , also referred herein to as primary coils  140 , are electrical conductors (e.g. copper conductors) that are located at least partially within and supported by the housing  136 . The current coils  140  each include two blades  156 , which are configured to partially extend from the housing  136 . The blades  156  are configured to provide a mechanically and electrically sound connection between the current coils  140  and the sockets  124 ,  128 . Specifically, the blades  156  are configured to be received by the sockets  124 ,  128  to operably connect the current coils  140  to the sockets such that electrical current may be transferred through the utility meter  112 . In other words, the electrical current drawn by the facility  104  passes through the current coils  140  when the blades  156  are received by the sockets  124 ,  128 . In addition, the current coils  140  and the blades  156  may also mechanically support the meter  112  in a mounted position (as shown in  FIG. 1 ) on the mounting device  108 . Also, heat energy generated within or on the sockets  124 ,  128  is transferred to the current coils  140  through the blades  156 , since the current coils  140  and the blades  156  are typically formed from metal and are positioned in contact with the sockets  124 ,  128 . 
     The secondary coils  144 , which may suitably be part of a so-called current transformer, are disposed in a current sensing relationship with respect to the current (primary) coils  140 . As is known in the art, a current transformer includes at least one secondary coil (e.g. the secondary coils  144 ) wrapped about a toroidal core, not shown. At least one of the primary coils  140  passes through the center of the toroidal core. Accordingly, the primary coils  140  and the secondary coils  144  are configured as an electrical transformer. Regardless of the specific embodiment, the secondary coils  144  are configured to generate a scaled down version of the current passing through the primary coils  140 . Each of the secondary coils  144  is operably coupled, e.g. through a corresponding burden resistor  145 , to provide a current measurement signal representative of the current passing through the primary (or current) coils  140  to the metrology circuit  152 . 
     The voltage sensors  141 , each of which may suitably comprise a resistive divider, are disposed in a voltage sensing relationship with respect to the current coils  140 . The voltage sensors  141  are configured to generate a scaled down version of the line voltage on the current coils  140 , which is representative of the voltage delivered to the load. The voltage sensors  141  are operably coupled, to provide a voltage measurement signal representative of the voltage on the current coils  140  to the metrology circuit  152 . 
     The metrology circuit  152  is any suitable circuit(s) configured to generate metering data or consumption data by detecting, measuring, and determining one or more electricity and/or electrical energy consumption values based on electrical energy flowing from the line-side sockets  124  to the load-side sockets  128 . Specifically, the metrology circuit  152  uses at least the voltage measurement signal and the current measurement signal to determine the metering data. The utility  120  typically accesses the metering data for billing purposes as well as other purposes. 
     In this embodiment, the metrology circuit  152  includes an analog-to-digital converter (“ADC”)  162 , a processing circuit  164 , all packaged within a single integrated circuit (“chip”) package  170 . The integrated circuit package  170  may also include all or part of the memory  180 . In general, the ADC  162  is operably coupled to receive the voltage measurement signals from the voltage sensors  141 , and to generate digital voltage measurement signals therefrom. The ADC  162  is likewise operably coupled to receive the current measurements signals from the secondary coils  144  and generate digital current measurement signals therefrom. 
     The processing circuit  164  includes one or more processing devices and accompanying support circuitry, configured to carry out program operations and processing. The processing circuit  164  is operably coupled to receive the digital voltage and current measurement signals from the ADC  162 , and is programmed and/or otherwise configured to generate metering information therefrom. For example, the processing circuit  164  may suitably use known computational methods to determine energy consumption (e.g. kw-hrs, VARS, etc.) using the digital voltage and current measurement signals. The processing circuit  164  also executes software instructions to perform control operations, and other operations described hereinbelow. The processing circuit  164  is operably connected to receive the software instructions from the memory  180 . 
     With reference still to  FIG. 1 , the temperature sensor  160  is a sensor device and accompanying circuit that are configured to generate a sensor signal based on a temperature within the meter housing  136 . The temperature sensor  160  is configured to provide the sensor signal to the processing circuit  164 . To reduce manufacturing costs, the temperature sensor  160  can be disposed on or supported by the printed circuit board  137 . Specifically, in this embodiment, the temperature sensor  160  is included in the integrated circuit package  170  in which the processing circuit  164  is disposed. Examples of commercially available meter processing packages that include a suitable processing circuit and temperature sensor include the model 71M6513, 71M6521, and 71M6533 metering ICs available from Silergy Corp. 
     In particular, since the current coils  140  and the sockets  124 ,  128  are configured to conduct heat energy, the current coils  140  have a temperature that is in part based on the temperature of the sockets  124 ,  128 . The current coils  140  are largely disposed within the interior of the meter housing  136 , and thus heat conveyed from the sockets  124 ,  128  cause the temperature within the meter housing  136  to rise. Consequently, the temperature sensor  160  is configured to indirectly sense the temperature within or on the sockets  124 ,  128  by sensing the temperature within the meter housing  136 . 
     Because the temperature sensor  160  in this embodiment is located on the circuit board  137  away from current coils  140  and blades  156 , the measurement of the temperature sensor  160  less directly reflects heat generated within the connection between the meter jaws  124 ,  128  and the meter blades  156 . To this end, normal operational heat of the metrology circuit  152 , as well as environmental heat (weather), and other factors can affect the temperatures within the meter housing  136 . Accordingly, the processing circuit  164  in this embodiment is configured to determine a temperature adjustment value that is intended to approximate the ambient heat or meter self-heating, or in other words, heat that is present due to factors other than a malfunctioning connection between one or more of the sockets  124 ,  128  and blades  156 . 
     In general, the temperature adjustment value is calculated based on the current passing through current coils  140 , and includes a constant value that approximates the self-heating of the metrology circuit  152 . As will be discussed below in detail, the processing circuit  164  is further configured to process the temperature adjustment value using an infinite impulse response filter (or other low pass filter), to account for the fact that changes in current do not immediately result in temperature change. As will also be discussed below in detail, the processing circuit  164  is further configured to determine if the difference between the temperature sensor value and the temperature adjustment value, which approximates heating due to abnormal conditions, exceeds a threshold, thus indicating a heat-generating malfunction. 
     With continued reference to  FIG. 1 , the utility meter  112  in this embodiment further includes an optional disconnect unit  172 . The disconnect unit  172  is operably coupled to the current coils  140  and the metrology circuit  152  and is configurable in a closed state (first state) and an open state (second state). In the closed state, a closed circuit is formed in the current coils  140 , which enables electrical power transfer from the utility  120  to the facility  104  (i.e. the load) through the distribution lines  116 . In the open state, an open circuit is formed in the current coils  140 , which prevents electrical power transfer from the utility  120  to the facility  104  through the distribution lines  116 . Specifically, in the open state electrical current is prevented from flowing from the line-side sockets  124  to the load-side sockets  128 . The disconnect unit  172  includes a relay or any other suitable device that controllably disconnects and re-connects electrical power to the facility  104 . Because the disconnect unit  172  contains switch contacts, not shown, that carry the full load current to the facility  104 , such contacts can also degrade to a point at which they create excess heat within the meter housing  136 . Any overheat conditions caused by degradation of the disconnect unit  172  will also be detected by the sensor  160  and processing circuit  164  performing the operations described herein. 
     Moreover, as described below, the metrology circuit  152  may be configured to control the state of the disconnect unit  172  based on the detection of overheat conditions within the meter housing  136 . 
     The memory  180  is operably coupled to the processing circuit  164  and is configured to store metering data generated by the metrology circuit  152 . The memory  180  may include separate devices within or external to the integrated circuit package  170 . Additionally, the memory  180  is configured to store look-up tables and program data for operating the temperature sensor  160  and the processing circuit  164  according to the methods described herein, as well as storing any other electronic data used or generated by the metrology circuit  152 . The memory  180  is a non-transitory machine readable storage medium. While the memory  180  is shown in the drawing as being external to the integrated circuit package  170 , the memory  180  shall be considered to encompass data and program storage both internal to and external to the integrated circuit package  170 . In a preferred embodiment the data values discussed below in connection with  FIGS. 3 through 8  may be programmed externally, through the transceiver  184 , to allow for the values to be location-specific, as well as meter-configuration specific. 
     The transceiver  184  is operably coupled to the processing circuit  164  and is configured to send electric data to the utility  120  and/or to an external unit (not shown), and to receive electric data from the utility and/or the external unit. In one embodiment, the transceiver  184  is a radio frequency (“RF”) transceiver operable to send and to receive RF signals. In another embodiment, the transceiver  184  includes an automatic meter reading (AMR) communication module configured to transmit data to an AMR network and/or another suitable device. The transceiver  184  may also be configured for data transmission via the Internet over a wired or wireless connection. In other embodiments, the transceiver  184  is configured to communicate with an external device or the utility  120  by any of various means used in the art, such as local optical communications, power line communications, telephone line communications, or other means of communication. 
     The display  188  is operably coupled to the processing circuit  164  and is configured to display data associated with the utility meter  112  in a visually comprehensible manner. For example, the display  188  may be configured to display consumption data, the state of the disconnect unit  172 , and the sensed temperature within the meter housing  136 , for example. The display  184  may be any suitable meter display device, such as a liquid crystal display unit, for example. 
     In operation, the metering system  100  operates to measure and quantify electrical energy provided to the facility  104  from the utility  120  via the power distribution lines. To this end, line voltages and line currents flowing from the distribution lines  116  to the facility  104  pass through the sockets  124  and blades  124  to the current coils  140   a ,  140   b . The secondary coils  144  detect the line currents and generate scaled down versions of the line current on the current coils  140   a ,  140   b . The secondary coils  144  provide to the ADC  162 , via corresponding burden resistors  145 , a current measurement signal representative of the current passing through each of the current coils  140   a ,  140   b . The voltage sensors  141  obtain the line voltage from each of the current coils  140   a ,  140   b , and generate a scaled down version of each line voltage, which is representative of the voltage signals delivered to the load. The voltage sensors  141  are operably coupled to provide a voltage measurement signal representative of each of the line voltages to the ADC  162 . 
     The ADC  162  digitizes the voltage and current measurement signals to generate streams of samples (digital waveforms) having values approximating the voltage and current measurement signals. The ADC  162  provides the digital voltage and current measurement signals to the processing circuit  164 . 
     The processing circuit  164  uses the digital voltage and measurement signals to generate the metering data. Such metering data can includes and accumulated energy consumption value (e.g. kw-hrs, VARS, etc.), RMS voltage values, RMS current values, and instantaneous energy values, among other things. To this end, the processing circuit  164  in this embodiment performs the operations of  FIG. 2 . It will be appreciated that many other types of operations and calculations may be performed in addition to those in  FIG. 2 , and not all of the calculations of  FIG. 2  need be performed to carry out at least some embodiments of the invention. 
     In step  205 , the processing circuit  164  receives digital voltage and current measurement values (e.g. samples) for the current time, t. In step  210 , the processing circuit  164  calculates at least one energy consumption value, for example, accumulated value AEC(t) as a function of the previous value, AEC(t−1), and the newly received digital voltage and current measurement values. Various sample-based energy and power calculations are known to those of ordinary skill in the art. For example, it is not known to multiply contemporaneous voltage and current samples, and to accumulate the totals over time. 
     In step  215 , the processing circuit  164  generates one or more current magnitude values representative of the magnitude of current flowing through the current coils  140 . For example, the processing circuit  164  can generate root-mean-square (“RMS”) current calculations for each of the current coils  140   a  and  140   b . For example, the value IA may suitably be the RMS current for the current detected on the current coil  140   a  at time t, and the value IB may suitably be the RMS current for the current detected on the current coil  140   b  at time t. The processing circuit  164  generates such RMS values in any traditional manner based on the samples of the digital current measurement signal from the prior several AC cycles of the line current. For example, in a 60 Hz system, the samples for the last one second cover sixty AC cycles, and thus provide a reasonable RMS voltage calculation. Fewer or more samples maybe used. It will be appreciated that step  215  need not be performed every time the energy consumption value AEC(t) is updated in step  210 . It may be sufficient to update the current measurement values every second, every few seconds, or even every minute or longer. 
     In step  220 , the processing circuit generates other energy-related values, which can include RMS voltage per line, and/or power factor related values. The calculation and use of such values would be known to those of ordinary skill in the art. 
     In step  225 , the processing circuit  164  stores the current magnitude values in the memory  180 , and may also store in the memory  180  any of the values generated in steps  210  or  220 . In particular, because meter  112  includes two current sensors, the processing circuit  164  may suitable calculate and store the two current magnitude values IA and IB in step  225 . In steps  230 , the processing circuit  164  causes the display  188  to display one or more of the values calculated in steps  210 ,  215  or  220 . Typically, the processing circuit  164  will cause the display to display the energy consumption value AEC(t) generated in step  210 . It will again be appreciated that steps  225  and  230  need not be executed every time a new energy consumption value is calculated. 
     In addition to such ongoing functions, the processing circuit  164  can contemporaneously perform other tasks. In this embodiment, the processing circuit  164  performs a heat monitoring routine that determines whether there is a heat-related issue with the meter  112 , such as from arcing between any of the sockets  124 ,  128  and any of the meter blades  156 . 
       FIG. 3  shows a flow diagram of an exemplary meter heat monitoring routine  300  that may be performed by the processing circuit  164 . As is known in the art, the processing circuit  164  may be contemporaneously perform other steps of other routines, such as those of  FIG. 2 , while executing the steps of monitoring routine  300 . The processing circuit  164  is preferably configured to execute the routine  300  at regular intervals t, such that each iteration a predetermined time period from the previous iteration. An exemplary interval may be within the range of t=0.1 seconds to 1.0 second. 
     In step  305 , the processing circuit  164  obtains at least one current magnitude value I SUM  that is representative of the sum of the current magnitudes through the current coils  140   a ,  140   b.    
     In the embodiment described herein, the processing circuit  164  obtains the most recently stored RMS current values IA and IB (see step  215 , discussed above) from the memory  180 . The processing circuit  164  then adds the two numbers together to yield I SUM . In other words, the processing circuit  164  performs the following calculation: I SUM =IA+IB. Alternatively, other values representative of the total current magnitude delivered through the current coils  140   a ,  140   b  may be used. It will be appreciated that in an embodiment in a three-phase meter having three current coils, the processing circuit  164  would obtain a value representative of the sum of the current magnitudes in all three current coils. 
     In step  310 , calculation constants A, B, C, and a are retrieved from the memory  180 . The calculation constants A, B, and C represent the coefficients of the equation that models the self-heating temperature component of the meter  112 . In accordance with the embodiments described herein, the self-heating temperature value, CSH, is a measure of the internal heating of the meter that occurs due to normal meter operations. As will be discussed below, the value CSH is calculated using the generalized equation:
 
 CSH=A*I   SUM   2   +B*I   SUM   +C   (1)
 
     As will be discussed below, the self-heating temperature value CSH is used as an adjustment to the measured temperature to yield an approximation of the heating within the meter  112  that is due to abnormal conditions, as opposed to normal internal self-heating. In this embodiment, the value CSH is furthermore filtered to account for the time lag between current swings and the corresponding temperature change in the current coils  140 . The value a is a coefficient for that filter, which is in the form of an infinite impulse response filter. 
     As can be seen in equation (1), the self-heating within the meter  112  includes a portion that is dependent on load current (I SUM ), and a portion that is independent of load current. To this end, normal self-heating occurs due to load current flowing through the current coils  140 , and other factors, such as the operation of the processing circuit  164  and/or communications circuits. The portion of self-heating due to load current has been found in one typical type of meter to be 0.1° to 0.15° C./amp, with an exponential increase after the current reaches 160 amps. These values will vary from meter to meter and may be determined empirically. 
     The value of A represents the exponential coefficient, and therefore will be well below unity for temperatures below that which the self-heating becomes exponential with current. Thus, the example where the current increases exponentially after 160 amps, the value A should be below 1/160 or 0.00625, assuming the value of CSH is in amps. The value of B, however, represents the proportional portion of the load current based-self heating, and therefore will in the range 0.1 to 0.15, assuming the value of CSH is in amps. 
     The value of C is not related to load current, and represents an approximation of all other normal self-heating within the meter  112 . Such other self-heating will be present even when little or no current is flowing through the current coils  140   a ,  140   b . It has been found that the load current independent self-heating in one exemplary meter is in the range of 4° C. to 10° C. 
     The values of A, B, and C can be determined empirically for every meter configuration. In particular, temperature readings can be taken from the temperature sensor  160  at various load current levels, in a facility with a known ambient (environmental temperature). The load current would presumably be applied through the current coils  140   a  and  140   b  in a controlled condition with properly conducting electrical contacts. Each temperature reading should be taken after a delay from the application of the load current in order to ensure that the steady-state temperature is reached for each load current level. The differences between the temperature readings and the known ambient temperature would then be plotted as a function of load current level. Traditional curve-fitting techniques can be used to develop the best fit coefficients A, B, C for the function CSH=A*I SUM   2 +B*I SUM +C. 
     The value of a is the decay factor of the IIR filter. The filter many suitably have the form of:
 
 CSH′=α*CSH +(1−α)*PREV_ CSH′,   (2)
 
where n is the index, discussed above, that is incremented each time the routine  300  are executed. In general, a represents the decay factor for increasing or decreasing heat radiated by the current coils  140   a ,  140   b  responsive to a change in current. For example, a current coil that transitions from carrying only one (1) amp to carrying one hundred sixty (160) amps will not have the same temperature immediately as it will if it remains at one hundred sixty (160) amps for twenty minutes. The value of a will vary based on the physical characteristics of the current coils  140   a ,  140   b , and the time period between successive executions of the routine  300 . The factor α may also be determined empirically by applying transitions in load current and taking temperature measurements at various intervals after the transition.
 
     It will be appreciated that in lieu of retrieving the values A, B, C, and a separately from memory  180 , the values may be embedded within the actual software executed by the processing circuit  164 , for example, when carrying out the calculations of steps  315  and  320  below. It will be appreciated that such program itself would be stored in the memory  180 , and thus that the values A, B, C, and a would thus be stored in the memory  180  either way. However, the embodiment described herein stores the values A, B, C, and α in a table in the memory  180  separate from the software code to promote flexibility. For example, separate storage of the values A, B, C, and a can allow the same software program to be used in meters that may have different features, and thus different constants A, B, C, and α. 
     In any event, after obtaining the values A, B, C, and a in step  310  in this embodiment, the processing circuit  164  proceeds to step  315 . In step  315 , the processing circuit performs the calculation to obtain the raw adjustment value CSH for the current iteration. As discussed above, this value is equal to:
 
 CSH=A*I   SUM   2   +B*I   SUM   +C,   (1)
 
     The value CSH represents the steady-state self-heating adjustment if the current remained at the level I SUM  for a long period of time. 
     Thereafter, in step  320 , the processing circuit  164  filters the steady state self-heating estimate CSH through the IIR filter of equation (2), or CSH′=α*CSH+(1−α)*PREV_CSH′. The resulting value CHS′ represents the estimate of the current measure of self-heating within the meter  112  under normal conditions, for example, when there is no arcing in the blades  156  and/or sockets  124 ,  128 , nor any other heat-generating malfunction. The self-heating estimate CSH′ is useful because it makes it easier to determine whether a heat-generating malfunction is present more accurately, particular when the temperature sensor  160  is located away from the current coils or blades  140 , for example, on the printed circuit board  137 , or within the integrated circuit chip  170  itself. 
     In step  325 , the processing circuit  164  stores the current CSH′ value as the value PREV_CSH′ for use in step  320  in the subsequent iteration of the steps  300 . The processing circuit  164  in step  330  obtains the current temperature sensor value TM from the temperature sensor  160 . As discussed above, the value TM will be influenced by the current ambient (external environmental) temperature ET, the meter self-heating under normal conditions CSH′, and any heat generating malfunction. 
     In step  335 , the processing circuit  164  adjusts the temperature sensor value TM by subtracting out the value CSH′. The resulting temperature value HE represents the measured temperature, with the estimated self-heating (for normal operations) removed. Thus, under normal conditions, the temperature value HE should be close to the ambient temperature ET. As such, any significant difference between HE and ET can indicate a heat-generating malfunction within the meter  112 , or in or on the meter blades  156 . 
     In step  340 , he processing circuit  164  obtains a threshold TH, which is a function of date and time, that identifies an overheating threshold. The overheating threshold TH is adjusted for time of day and seasonal temperature trends. Further detail on one useful embodiment for providing the threshold TH as a function of time and date is provided below in connection with  FIGS. 4 through 8 . In an alternative embodiment, if the processing circuit  164  can obtain the true local external temperature ET from an external (e.g. remote) source of temperature information, the value TH can be set to that value plus a predetermined buffer of a few degrees, ET+δ. In any event, after the processing circuit  164  obtains the threshold TH in step  340 , the processing circuit  164  proceeds to step  345 . 
     In step  345 , the processing circuit  164  determines whether the value HE exceeds TH. If so, then the processing circuit  164  proceeds to step  350 . If not, however, then the processing circuit  164  proceeds to step  355 , discussed further below. 
     In step  350 , the processing circuit  164  stores in the memory  180  an indication (e.g. an overheat flag) that a heat-generating anomaly or event has been detected. The indication may be stored within a predetermined position in a predefined data table, such that an external computing device communicating with the meter processing circuit  164  may receive the table data and determine the existence of the heat-generating event has been detected. Similarly, the processing circuit  164  may also cause the communication circuit  184  to transmit a signal indicating the event to an external device, such as a central computer monitored by the utility service provider. The processing circuit may also, or alternatively, cause the display  188  to display an indication of the event, and/or cause the service switch  172  to open, thereby interrupting the current through the meter  112 . It is noted that it could be advantageous to require multiple detections of a heat-generating anomaly (i.e. in subsequent executions of the routine  300 ) before communicating the indication, displaying the indication, and/or opening the service switch  172 . 
     In the present embodiment, the processing circuit  164  maintains two separate overheat flags indicating first and second levels of severity. To this end,  FIG. 4  shows in further detail an exemplary set of operations that may be used as step  350  of  FIG. 3 . 
     Referring to  FIG. 4 , the processing circuit executes step  405  as a result of the processing circuit  164  determining in step  345 , that the HE&gt;TH. In step  405 , the processing circuit  164  determines whether HE&gt;TH+Δ, where A represents the difference between a first level alarm (e.g. an “overheat warning”), and a second level alarm (e.g. an “overheat alarm”). If HE≤TH+Δ, then the processing circuit  164  proceeds to step  410 . If, however, HE&gt;TH+Δ, then the processing circuit  164  proceeds to step  420 . 
     In step  410 , the processing circuit  164  stores two first level alarm flags in the memory  180 , e.g. “overheat warning” flags. One first level alarm flag is persistent, and can only be set once, and only reset by a technician. The other first level alarm flag is a present condition indicator, and can be reset at any time the condition is no longer present. This present condition indicator flag, for example, can be reset in step  355 , discussed further below. Thereafter, in step  415 , the processing circuit  164  causes a visual indication of the setting of the first level alarm flag(s) on the display  188 . The processing circuit  164  thereafter returns to step  360  of  FIG. 3 . 
     By contrast, in step  420 , the processing circuit  164  stores two second level alarm flags in the memory  180 , e.g. “overheat alarm” flags. One second level alarm flag is persistent, and can only be set once, and only reset by a technician. The other second level alarm flag is a present condition indicator flag, similar to the first level present condition indicator flag, discussed above in connection with step  410 . Thereafter, in step  425 , the processing circuit  164  causes a visual indication of the setting of the second level alarm flag(s) on the display  188 . The processing circuit  164  may also open the service switch  172 , or cause communication of the presence of the condition by the transceiver  184 . The processing circuit  164  thereafter returns to step  360  of  FIG. 3 . 
     As discussed above, the first and second level persistent flags can only be cleared by a process that involves interaction with a technician from the utility. Thus, the indication of either or both flags also persists until cleared by a utility technician. 
     Referring again to  FIG. 3 , step  355  occurs if is determined in step  340  that no overheat condition currently exists, or in other words, HE≤TH. The processing circuit in step  355  clears either or both of the present condition indicator flags set, if either had been set per steps  410  and  420 . The processing circuit  164  thereafter proceeds to step  360 . 
     In step  360 , the processing circuit  164  completes the routine  300 . After a predetermined time, the processing circuit  164  returns to step  305  re-execute the routine  300  for the next time period interval. 
     It will be appreciated that the value HE, which approximates the external ambient temperature under normal conditions, may be useful for other functions. For example, some meters perform a load-profiling operation in which energy usage and other values are stored for successive time increments (e.g. every 5 to 30 minute interval) to allow usage patterns and condition patterns to be analyzed. Some load-profiling operations also store ambient temperature, if available for each load profiling time interval. Thus, the processing circuit  164  in this embodiment may store the value HE, or an average of such a value, for each load profiling time interval (along with energy consumption and other information), the load profiling log stored in the memory  180 . However, the processing circuit  164  may be configured to avoid storing the HE value if it exceeds the threshold TH, as that number would be influenced by overheating conditions. 
     It can thus be seen that the processing circuit  164  can use methods to allow a temperature sensor  160 , which may be located some distance from the source of arcing or other heating anomaly, to detect an abnormal conditions that is timely adjusted for meter self-heating under normal conditions, and to otherwise approximate the external air temperature. 
     As discussed above, the threshold TH is preferably based on a current expected or real ambient temperature, plus a margin of a few degrees. For example, if the current ambient temperature ET(n) is known to be 20° C., then the threshold TH may be 25° C. As discussed above, the processing circuit  164  may be configured in some embodiments to receive current, accurate ambient temperature information from external sources via the communication circuit  184 . However, if access to the real ambient temperature ET(n) is not available, then the threshold TH should be set to a maximum expected ambient temperature. 
     In a very simple case, a single threshold TH for the maximum temperature may be used. Thus, for example, the threshold of 65° C. may be used in moderate climates, due to sunlight loading. However, using a single threshold is disadvantageous because for most of the year, average temperatures are far below 65° C., particularly at night. As a result, it will take more potentially damaging internal overheating to cause the HE(n) to exceed TH. 
     To address this issue, one embodiment of the invention employs a threshold TH that is a function of time and date. In this embodiment, the processing circuit  164  maintains a real-time clock and calendar, as is well-known in the metering art. The processing circuit  164  obtains a threshold TH that is based on a maximum expected temperature for the date, and for that date, the maximum expected temperature for the time of day. In this embodiment, the threshold TH is selected from a set of stored thresholds TH M,D , each corresponding to a combination M,D of a time of year M and a time of day D. In this embodiment, the set of stored thresholds TH M,D  include two estimated maximum temperature thresholds (night and day) for each month of the calendar year. Thus, the memory  180  stores twenty-four values TH M,D , with two thresholds, D=0 (night) and D=1 (day) for each month M=1 to 12. 
     By way of example, at 12:08 pm on February 4th, the processing circuit  164  in step  340  would retrieve as the threshold TH the value TH 2,1  from memory. At 11:30 pm on July 18 th , the processing circuit  164  would retrieve as the threshold TH the value TH 7,0 . In addition, in the embodiment described herein, the memory  130  further stores an additional threshold, TH 0,0 , that is used if the time and date is not presently available (e.g. due to a recent power interruption or meter restart). 
     In this embodiment, step  340  of  FIG. 3  may be carried out as illustrated by the operations of  FIG. 5 . Referring to step  5 , the processor  164  in step  505  determines whether the real-time clock has a sufficiently accurate value. As discussed above, the processor  164  and/or other circuits are configured to maintain the real-time clock (including date) during normal meter operation and even during most power outages. However, there are conditions in which the processor  164  loses the real-time clock, such as during very long power outages or other malfunctions. The processor  164  will store a value or flag indicative of a failure/loss of the real-time clock. If the processor  164  in step  505  determines that the real-time clock has a sufficiently accurate value, then the processor  164  proceeds to step  510 . If, however, processor  164  in step  505  determines that the real-time clock does not have a sufficiently accurate value, then the processor  164  proceeds to step  515 . 
     Referring now to step  510 , the processor  164  retrieves as M the month value from the real-time clock. Thereafter the processor  164  in step  520  determines whether the time of day is in or around daylight hours, for example, between 6:00 am and 9:00 pm. In this embodiment, the real-time clock employs a twenty-four hour format. Thus, the processor  164  determines in step  520  whether the hour value HR is greater than a sunrise time value SR, but less than a sunset time value SS. The values SR and SS may be constant, but preferably vary as a function of date, as the number of daylight hours varies throughout the year. If the values SR are based on date, then the memory  180  preferably stores values SR and SS that are also a function of date, for example, the month value M In such a case the processor  164  retrieves the values SR and SS from the memory  180  to carry out the operations of step  520 . 
     If the processing circuit  164  determines that SR&lt;HR&lt;SS, then the processing circuit  164  sets the value D to 1 in step  525 . If not, then processing circuit  164  sets the value D to 0 in step  530 . After either of steps  525  or  530 , the processing circuit  164  executes step  535 . In step  535 , the processing circuit  164  sets the threshold value TM equal to the array value TM M,D . After step  535 , the processing circuit  164  has completed step  340  of  FIG. 3  and can proceed to step  345  as described above. 
     It will be appreciated, however, that other methods of employing the value TM M,D  in the operations of  FIG. 3  may be used. 
     Referring again to step  515 , which occurs when the real-time clock is not accurate, the processor  164  sets both M and D to 0. The processing circuit  164  thereafter proceeds to step  535  to set TM=TM 0,0 . The TM 0,0  value is preferably set to (or at least based on) the maximum temperature threshold in the array TM M,D , to reduce the occurrences of false positives from ambient temperature. 
       FIG. 6  shows an exemplary timeline of the values of that occur in a sample operation of meter  112 , which help illustrate the operations of the exemplary embodiment of  FIGS. 3 and 5  described above.  FIG. 6  shows a timeline graph  600  of temperature versus time over a twenty-seven (27) hour period in the month of July. The timeline values include the measured temperature value TM(n) generated by the temperature sensor  160  (line  602 ), the adjusted temperature HE generated in step  335  (line  604 ). and the true ambient temperature (e.g. environmental temperature) ET, which is not available within the meter  112  (line  606 ). The timeline values also include the first threshold value TH obtained in step  340  of  FIG. 3  (line  608 ) and the second threshold value TH+Δ used in step  405  of  FIG. 4  (line  610 ). 
     The values of  FIG. 6  represent a time in which no heat was generated within the meter  112  due to malfunction or arcing in the current blade  156 . In other words, the values of  FIG. 5  illustrate a normal operation of the meter  112 . As illustrated by the lines  602  and  606 , the difference between the true ambient temperature ( 606 ) and the measured temperature ( 602 ) can vary significantly. This variance is due to the meter self-heating factors discussed further above. It is also noted that the differences are exaggerated in the late afternoon and early evening, for example, from about 6:00 pm (18:00) to 10:00 pm (22:00). Such exaggerated differences are likely due to elevated current usage, which tends to occur in the late afternoon or evening in residences, particularly in summer months. The elevated current usage increases the meter self-heating significantly, which is reflected in the value CSH of equation (1). 
     It is also noted that the measured temperature  602  has higher frequency (e.g. hourly) variance occurring at intervals, particularly during the day. Such high frequency variations can be due, for example, to temporary increases in current from devices that operate periodically. 
     As shown in  FIG. 6 , the compensated temperature HE line  504  does not in this embodiment exactly track the true ambient temperature line  506 . This is due to the fact that some sources of error are difficult to predict. For example, sunlight loading can introduce, depending on the amount of cloud cover, a large degree of difference between the sensed temperature within the meter  112  and the external ambient temperature. In this embodiment, it has been determined that the increase in accuracy in the compensated temperature HE does not improve performance sufficiently to justify the increase in complexity necessary to achieve such accuracy. However, other embodiments can vary and account for different or additional factors to improve accuracy, if desired. 
     Nevertheless, it can be seen that the compensated temperature HE line  604  eliminates a large part of the error in the measured temperature TM line  606 . Also, the compensated temperature HE line  604  does not reflect the higher frequency fluctuations in the measured temperature TM line  606 , which further indicates that the fluctuations are due to periods of heavy current usage. 
     Referring to the first threshold value TH, the line  608  toggles between the daytime level TH 7,1  and the nighttime level TH 7,0 . The second threshold value TH+Δ, the line  610  toggles between the daytime level TH 7,1 +A and the nighttime level TH 7,0 +A. It can be seen that the unadjusted measurement TM would exceed the threshold from the times of 21:00 to 22:00 causing a warning and alarm, if it were compared to the thresholds TH and/or TH+Δ, resulting a false event. In the prior art, the only way to avoid such false events would be to increase the thresholds significantly, which would be undesirable because a true arcing condition could take longer to detect. 
     In this embodiment, the threshold values TH can only have one of two values for a given day. However, in an alternative embodiment shown in  FIG. 7 , the threshold value line  708  has a trapezoidal shape, employing transition areas  712 ,  714  where the threshold varies between the values TH 7,1  and the nighttime level TH 7,0  as a function of time. In this case, the threshold value TH varies as a linear function of time in the transition areas  712 ,  714 . The trapezoidal shape accommodates the transition areas between night and day to further improve the responsiveness to a relatively rapidly developing overheat situation. The upper threshold, TH+Δ, has the same shape, as indicated by line  710 . This feature, combined with daylight transition time values SR and SS can create a robust overheat detection operation. 
       FIG. 8  illustrates an alternative set of operations that can be used as step  340  of  FIG. 3  to implement the trapezoidal shape threshold line  708  of  FIG. 7 . Referring to  FIG. 8 , the processing circuit  164  in step  805  determines whether the real-time clock has a sufficiently accurate value, as per step  505  of  FIG. 5 . If the processing circuit  164  determines that the real-time clock has a sufficiently accurate value, then the processing circuit  164  proceeds to step  810 . If, however, the processing circuit  164  determines that the real-time clock does not have a sufficiently accurate value, then the processor  164  proceeds to step  815 . 
     Referring now to step  810 , the processor  164  retrieves as M the month value from the real-time clock. Thereafter, the processor  164  in step  820  determines which of four conditions exist based on the time of day, the sunrise time value SR, and the sunset time value SS. These conditions relate generally to daylight hours, nighttime hours, a transition time (TTND) from night to day, and a transition time (TTDN) from day to night. Table 1 shows the possible conditions: 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 1. 
                 SR + TTND &lt; HR &lt; SS 
                 (daylight) 
               
               
                 2. 
                 (HR &gt; SS + TTDN) or (HR &lt; SR) 
                 (nighttime) 
               
               
                 3. 
                 SR ≤ HR ≤ SR + TTND 
                 (transition) 
               
               
                 4. 
                 SS ≤ HR ≤ SS + TTDN 
                 (transition) 
               
               
                   
               
            
           
         
       
     
     As noted above in connection with  FIG. 5 , the values of SR and SS may vary throughout the calendar year. Thus, step  810  may further include retrieving from memory  180  the values of SR and SS, based on the date information in the real-time clock. For example, the values SR and SS may be retrieved based on the month value of M In addition, the transition time values TTND and TTDN may be constant, and be the same. In the example of  FIG. 7 , the value of each of TTND and TTDN is two (2) hours. 
     The processing circuit  164  then proceeds based on which of the conditions 1 to 4 exists. If condition 1 exists, then the processing circuit  164  executes step  825 . If condition 2 exists, then the processing circuit  164  executes step  830 . If condition 3 exists, then the processing circuit  164  executes step  835 . If condition 4 exists, then the processing circuit  164  executes step  840 . 
     In step  825 , the processing circuit  164  sets the threshold value TH to TH M,1 , and then proceeds to step  845 . In step  830 , the processing circuit  164  sets the threshold value TH to TH M,0 , and then proceeds to step  845 . In step  835 , the processing circuit  164  sets the threshold value TH to: 
                   TM   =       TM     M   ,   0       +       (         TM     M   ,   1       -     TM     M   ,   0         TTND     )     ⁢     (     CV   -   SR     )                 (   3   )               
where CV is the current clock value. The equation represents the linear slope between 6:00 and 8:00 on  FIG. 7 . After step  835 , the processing circuit  164  proceeds to step  845 . In step  840 , the processing circuit  164  sets the threshold value TH to:
 
                   TM   =       TM     M   ,   1       +       (         TM     M   ,   0       -     TM     M   ,   1         TTDN     )     ⁢     (     CV   -   SS     )                 (   4   )               
where CV is the current clock value. The equation represents the linear slope between 19:00 and 21:00 on  FIG. 7 . After step  840 , the processing circuit  164  proceeds to step  845 .
 
     In step  845 , the processing circuit  164  has completed step  340  of  FIG. 3  by obtaining the proper value TH, and can proceed to step  345  as described above. 
     Referring again to step  815 , which occurs when the real-time clock is not accurate, the processing circuit  164  sets TH=TH 0,0 . The processing circuit  164  thereafter proceeds to step  845 . The TH 0,0  value is preferably set to (or at least based on) the maximum temperature threshold in the array TH M,D , to reduce the occurrences of false positives from ambient temperature. 
     The operations of  FIG. 8  thus provide an improved, but more complex version of the variable, time dependent thresholds discussed above in connection with  FIGS. 5 and 6 . It will be appreciated that the variable thresholds described herein have utility even in meters that do not employ temperature adjustments. For example, physical location of the temperature sensor  160  in another part of the meter  112 , for example, near the blades  156 , may obviate the need for temperature compensation, or may only require minor current-based temperature compensation. In such a case, however, the time-dependent thresholds discussed in connection with  FIGS. 5 to 8  would nevertheless provide advantages. 
     It will also be appreciated that the system may be configured to use more than twelve sets of daily thresholds (one set per month) as taught herein. For example, it may be sufficient to include as few as four sets, particularly in warm climates, and it may be advantageous to include more than twelve sets in some cases. 
     Moreover, it will be appreciated that the temperature measurement compensation operations described further above have utility even in cases where a single threshold is used regardless of time or date. In addition, it will be appreciate that the combination of temperature adjustment and threshold comparisons may be carried out in multiple, mathematically equivalent ways. Additionally, it will be appreciated that although the meter  112  is described as using current sensors in the form of current transformers, it will be appreciated that he inventions described herein are readily applicable to meters that employ other types of current measurement devices (e.g. shunts) that work with or include a conductor (referred to herein as a current coil or primary coil) carrying large magnitude currents. 
     In addition, it has been experimentally determined that the effects of attenuated heat transfer from a meter current coil (i.e. load carrying conductor) to a board mounted sensor are significant at air gaps as low as 0.1 inches. Thus, the embodiments described above can offer advantages when the temperature sensor is as little as 0.1 inches away from the current coil. 
     Accordingly, in other configurations in which at least the same amount of thermal insulation separates the temperature sensor from the current coil, the inventive techniques described here would demonstrate advantages. 
     It will therefore be understood that the above-described embodiments are merely illustrative, and that those of ordinary skill in the art may readily devise their own modifications that incorporate the principles of various aspects of the present invention and fall within the spirit and scope thereof.