Abstract:
A branch circuit monitor module for monitoring electrical properties of electrical branch circuits comprises one or more detectors, each detector for measuring an electrical property on a respective branch circuit and outputting a measurement signal; processing means for processing the measurement signal to generate digital output data based on the measurement signal; and means for outputting the output data. By using a modular approach to branch circuit monitoring, the user can tailor the selection of modules to his requirements; by providing digital processing means directly at the branch circuit monitor module instead of remotely, accuracy and reliability can be improved. The invention extends to a system comprising a plurality of branch circuit monitor modules. The invention also extends to a method of synchronising first and second measurement modules as well as a corresponding system.

Description:
[0001]    The present invention relates to branch circuit monitoring. Particular aspects relate to a branch circuit monitor module for monitoring electrical properties of electrical branch circuits. The branch circuit monitor module comprises processing means for processing the measurement signal to generate digital output data. The present invention further relates to a method of synchronising measurement modules, for the purpose of accurate voltage based measurements. The present invention further relates to a system of measurement modules. 
         [0002]    In a first aspect of the invention, there is provided a branch circuit monitor module for monitoring electrical properties of electrical branch circuits comprising: one or more detectors, each detector for measuring an electrical property on a respective branch circuit and outputting a measurement signal; processing means for processing the measurement signal to generate digital output data based on the measurement signal; and means for outputting the output data. 
         [0003]    The module is preferably arranged to be used with additional such modules, to provide a modular branch circuit monitoring system. By using a modular approach to branch circuit monitoring, the user can tailor the selection of modules to his requirements. Furthermore, by providing digital processing means directly at the branch circuit monitor module instead of remotely as may be done in some prior art branch circuit monitors, accuracy and reliability can be improved, for example by avoiding the distortion due to interference that an analogue signal might be subject to. Preferably the processing means are on-board (or on the same board as the rest of the parts of the branch circuit monitor module), and/or in close proximity to the detector(s). Preferably, the detectors are arranged on a first side of a circuit board, and processing means are arranged on a second (opposing) side of the circuit board. A single processing circuit/processor preferably provides for processing of signals from multiple detectors. 
         [0004]    The means for processing may include one or more of: means for data acquisition, means for analogue/digital conversion, means for performing mathematical operations on signal data, means for data storage, means for signal sampling, and/or means for signal filtering. 
         [0005]    Preferably the detectors are arranged to measure current. The detectors may alternatively or additionally be arranged to measure voltage. 
         [0006]    Preferably the detectors are current transducers, yet more preferably solid core current transducers. For example, the detectors may be powered solid core current transducers, Hall Effect transducers, or self-powered solid core current transducers. The detectors may also be split core current transducers. Different types of transducers may also be combined in any suitable manner. 
         [0007]    Preferably the branch circuit monitor module comprises at least two detectors, more preferably between two and ten detectors, and yet more preferably six or eight detectors. This can enable a module size with optimal system design freedom and hence user convenience, while maintaining reasonable levels of hardware resource requirement per sensor and hence cost. 
         [0008]    Preferably the branch circuit monitor module comprises a connector portion for connection to a further such branch circuit monitor module, said connector portion preferably including connector pins protruding from an area of the connector portion forming a recess. This arrangement can enable serial connection of modules (similar or different in nature) for efficiency and convenience of connection. 
         [0009]    Preferably the connector portion comprises a tessellation feature capable of tessellation with a further such branch circuit monitor module. The tessellation feature can enable electrical connection for minimisation of communication pathways, and at the same time mechanical connection for stability and formation of a composite unit. For simplicity and convenience the tessellation feature may comprise at least one step shape. 
         [0010]    The branch circuit monitor module may comprise at least two rows of detectors and two step shapes. The two step shapes may be located at opposite ends of the branch circuit monitor module. A first of the rows may be offset to a second of the rows in a direction of one of the rows. This arrangement can enable spacing of detectors at a desired pitch, especially if the detector width is wider than the desired pitch width. Preferably the height of the step shape of the tessellation feature corresponds approximately to the offset distance. 
         [0011]    For measurement accuracy, the branch circuit monitor module may be arranged to receive input data from a further monitor module, and to generate output data based on the measurement signal and the input data. 
         [0012]    For measurement accuracy, the branch circuit monitor module may be arranged to receive a synchronisation signal from the further monitor module and to synchronise data obtained from the measurement signal and the input data based on the synchronisation signal. 
         [0013]    Preferably the input data comprises voltage data from a voltage monitor module. 
         [0014]    Preferably the output data relates to at least one of: current; and power. Preferably the power is computed from locally measured current data and received voltage data, or locally stored, pre-configured voltage data. 
         [0015]    Preferably the branch circuit monitor module further comprises means for connecting to (and/or communicating with) a further such monitor module. The connection may include a data communication connection and/or a mechanical connection. The connection may enable transmission of data via the branch circuit monitor module. The connection may be a push-together connection. 
         [0016]    Preferably the branch circuit monitor module further comprises means for receiving a power-up signal, and performing power-up in response to the signal; preferably wherein the power-up signal is received in from another branch circuit monitor module. This can allow a power-up sequence to be implemented to avoid multiple modules powering up at the same time. The branch circuit monitor module preferably performs a power-up process in an externally controlled sequence with other modules, based on the power-up signal. The module may also signal another module to perform power-up, e.g. as part of or after completing its own power-up sequence. 
         [0017]    Preferably the branch circuit monitor module further comprises means for establishing a private connection with a further such monitor module. This may be of use to communicate a message directly from one module to a next module, (one example is the power-up signal mentioned above). 
         [0018]    Preferably the branch circuit monitor module further comprises means for transmitting data to the further such monitor module. 
         [0019]    Preferably the branch circuit monitor module further comprises means for transmitting output data to a controller. The controller preferably communicates with multiple modules of the module system. 
         [0020]    For ease of installation, a plurality of detectors may be arranged at a regular pitch. This may enable convenience in the installation of the branch circuit monitor module to a plurality of regularly spaced branch circuits. 
         [0021]    Preferably the branch circuit monitor module further comprises means for synchronising the module based on a received synchronisation signal. The synchronisation may address inherent signal characteristics (in particular phase lag or difference between voltage and current, e.g. to enable computation of a power factor), communications lag between separate measurement modules (e.g. between a remote voltage measurement module and current measured locally by the module), or both. 
         [0022]    Preferably synchronising comprises setting a phase counter in response to the synchronisation signal, the phase counter preferably used to synchronise the module to the phase of a signal detected by a remote detector, preferably wherein the synchronisation signal is received from the remote detector. 
         [0023]    Preferably the output data comprises power data, preferably wherein the power data is determined using a phase value calculated using the phase counter. 
         [0024]    Preferably the power data is determined based on current data measured by the module and based on voltage data, preferably wherein the voltage data is preconfigured voltage data or is received from a voltage measurement module. 
         [0025]    The module is preferably configured to participate in a synchronisation method as set out below. 
         [0026]    In a further aspect of the invention, there is provided a system comprising a plurality of branch circuit monitor modules as described herein, each branch circuit monitor module preferably associated with a respective plurality of branch circuits. Preferably the branch circuit monitor modules are serially connected to one another. 
         [0027]    Preferably in the system the plurality of branch circuit monitor modules measure current and at least one of the branch circuit monitor modules is adapted to measure voltage and provide voltage data to the other branch circuit monitor modules. 
         [0028]    A further aspect of the invention provides a system comprising a plurality of branch circuit monitor modules as described herein, with each module having one or more detectors arranged to measure current at respective branch circuits, the branch circuits being connected to a main power supply line; and a voltage monitor module arranged to measure voltage at the main power supply line and to supply voltage data to the branch circuit monitoring modules or to a controller. 
         [0029]    The system is preferably arranged to perform a synchronisation method as set out below. 
         [0030]    In a further aspect, the invention provides a method of synchronising first and second measurement modules measuring characteristics of respective first and second electrical connections, comprising: at the first measurement module, detecting a synchronisation event in a measured signal from the first connection; in response to detection of the synchronisation event, transmitting a synchronisation signal to the second measurement module; and synchronising the second measurement module to the measured signal based on the synchronisation signal. 
         [0031]    By detecting a synchronisation event at the first measurement module and transmitting a synchronisation signal to the second measurement module, synchronisation is enabled without the second measurement module being required to replicate the measurement of the first measurement module. This enables design efficiency and can reduce hardware costs. 
         [0032]    As described above, the synchronisation may address inherent signal characteristics, communications lag between the measurement modules, or both. 
         [0033]    Preferably the synchronisation updates a time reference (such as a counter, a timer, or a clock). 
         [0034]    Preferably at least one of the measurement modules is a branch circuit monitor module as described herein. 
         [0035]    Preferably the synchronisation event corresponds to the value of the measured signal meeting a predetermined criterion. 
         [0036]    Preferably the first measurement module is adapted to measure a voltage of the first connection, in which case the synchronisation event may relate to the phase of the measured voltage. To improve detection accuracy, the synchronisation event preferably corresponds to a zero-crossing of the measured voltage (or other measured quantity). 
         [0037]    Preferably the second measurement module is adapted to measure a current of the second connection. 
         [0038]    Preferably the method comprises detecting a priming event in the measured signal prior to the synchronisation event. Detection of a priming event prior to the synchronisation event may assist in ensuring that a current, on-going event is complete before a synchronisation cycle is commenced, to enable clean detection of the event. Further, if the event occurs regularly, awaiting the priming event can enable a full period between events for the synchronisation to operate. 
         [0039]    Preferably the method comprises, in response to the priming event, entering, at the first measurement module, a readiness state for responding to the synchronisation event. Entering a state of readiness may enable fast transmission and synchronisation in response to occurrence of the synchronisation event. 
         [0040]    Preferably entering a readiness state comprises activating a synchronisation module configured to transmit the synchronisation signal in response to the synchronisation event. The synchronisation module is preferably dedicated to enabling fast transmission of the synchronisation signal with minimal delay. 
         [0041]    For minimising delays, activating a synchronisation module preferably comprises activating a hardware circuit configured to transmit the synchronisation signal in response to detection of the synchronisation event. 
         [0042]    Preferably the method comprises detecting the priming event at the first measurement module in response to a received synchronisation initiation command, preferably received from a controller. The controller may be incorporated in one of the measurement modules, or it may be incorporated in a different unit, or it may be an independent unit. Preferably the controller is arranged to issue a synchronisation initiation command periodically, preferably between 10 times per second and once every five seconds, yet more preferably approximately once every second. 
         [0043]    Preferably the method comprises entering, at the second measurement module, a readiness state for responding to the synchronisation signal in response to a received synchronisation initiation command, preferably received from a controller. Entering a state of readiness may enable fast synchronisation in response to receipt of the synchronisation signal with minimal delay. 
         [0044]    Preferably the priming event relates to at least one of: the value of the measured signal meeting a predetermined criterion; the phase of a voltage of the first connection; a zero-crossing of the measured signal. 
         [0045]    Preferably the priming event corresponds to a first zero-crossing detected in the measured signal (e.g. voltage), and wherein the synchronisation event corresponds to a second zero-crossing detected in the measured signal after the first zero-crossing event, preferably wherein the second zero-crossing is a next zero-crossing after the first zero-crossing, or a next zero-crossing after the first zero-crossing having the same phase or direction. 
         [0046]    Preferably the method comprises, in response to receipt of the synchronisation signal at the second measurement module, triggering an interrupt at the second measurement module, preferably a high-priority interrupt, to process the synchronisation signal and/or perform the synchronising step. 
         [0047]    Preferably the method comprises generating output data based on measurement data from the first and second measurement modules using the established synchronisation. For example to output data that quantifies the real power based on measured current data, the phase (difference) between the measured current data and the real voltage may be estimated by means of a time reference that is periodically synchronised with measured voltage data. 
         [0048]    Preferably synchronising the second measurement module comprises resetting a counter at the second measurement module. This allows the counter at the second measurement module to become synchronised with the synchronisation event. The counter can then provide an accurate time reference relating to the event that can be used in conjunction with subsequent measurements at the second module. In particular if the counter is susceptible to drift, and/or if the event is susceptible to drift and irregularity, the synchronisation can enable greater accuracy. 
         [0049]    Resetting of the counter may comprise setting the counter to zero, or setting the counter to another predetermined value. The counter may be an (instantaneous) phase counter. The counter may be a period (duration) counter. A plurality of counters may be reset. Preferably the counter is periodically incremented based on a clock signal. The clock signal is preferably provided by a clock associated with or incorporated within the second module (which preferably also maintains said counter). 
         [0050]    Preferably the method comprises using the counter to determine a phase (difference) relationship between (or a factor relating to the phase between) the measured signals of the first and second measurement modules. 
         [0051]    Preferably the counter is a phase counter (or a phase time reference or a phase clock) synchronised to the signal measured at the first measurement module, or to a phase of the signal measured at the first measurement module. Preferably the counter indicates the (instantaneous) phase of the signal measured at the first measurement module. 
         [0052]    Preferably the method comprises generating output data based on the counter. 
         [0053]    Preferably the method comprises, at the second measurement module, determining a phase of the signal measured at the first measurement module using the counter, and generating output data based on the determined phase. Preferably the method comprises, at the second measurement module, determining a phase of the signal measured at the first measurement module using the counter, determining the phase of the signal measured at the first measurement module, determining a phase difference, and generating output data based on the determined phase difference. 
         [0054]    Preferably the phase is determined based on the phase counter and a signal period of the signal measured at the first measurement module, preferably wherein the signal period is determined based on phase signals received from the first measurement module, preferably wherein the phase signals correspond to zero crossings (or other events) of the signal measured at the first measurement module. Alternatively the signal period may be determined at the first measurement module, and period information transmitted to the second measurement module. 
         [0055]    Preferably the output data comprises power data, preferably wherein the power data is determined using a power factor, the power factor preferably calculated using the counter or the determined phase. 
         [0056]    The factor may be a power factor. The power factor may be determined by calculating the cosine of the phase difference between the measured signals of the first and second measurement modules. The power factor may be determined by retrieving from a look-up table the cosine of the phase difference between the measured signals of the first and second measurement modules. 
         [0057]    Preferably the method comprises detecting a further event in the measured signal from the first connection; in response to detection of the further event, transmitting a further signal to the second measurement module; and recording at the second measurement module occurrence of the further event. 
         [0058]    Preferably the further event is the completion of a period in the measured signal following the synchronisation event. 
         [0059]    Preferably the synchronisation event corresponds to a zero-crossing of the measured signal, and the further event corresponds to a subsequent zero-crossing of the measured signal, and wherein preferably the zero-crossings have the same phase or direction. 
         [0060]    Preferably synchronising the second measurement module comprises resetting a phase counter and a period counter at the second measurement module, and recording at the second measurement module comprises stopping the period counter at the second measurement module. In this way, the second module can measure the period length of the signal, whilst synchronising its phase counter. However, in other examples, measuring the period and synchronising the phase counter need not necessarily both be performed as part of synchronisation, or with the same frequency. Instead the period could be determined based on phase signals generated from zero-crossings (or other events) detected independently of the synchronisation process. 
         [0061]    Preferably the method comprises using the phase counter and the period counter to determine a factor relating to the phase difference between the measured signals of the first and second measurement modules. Preferably the period counter quantifies the period of the measured signal from the first connection. Preferably the (instantaneous) phase is determined by the modulo of the (instantaneous) phase counter divided by the period. Preferably the (instantaneous) phase difference is determined by calculating the difference between the (instantaneous) phase of the measured signal of the first measurement module and the (instantaneous) phase of the measured signal of the second measurement module. Preferably the factor is determined by calculating the cosine of the (instantaneous) phase difference. The factor may be determined by retrieving from a look-up table the cosine of the (instantaneous) phase difference between the measured signals of the first and second measurement modules. The factor may be a power factor, and may be used to calculate the real power by multiplication with an (instantaneous) current value and a voltage value. 
         [0062]    Preferably the synchronisation signal is transmitted to a plurality of second measurement modules. This allows efficiency, as a single first measurement module can serve a plurality of second measurement modules. 
         [0063]    Preferably a/the synchronisation initiation command is issued from a controller and transmitted to the first measurement module and a plurality of second measurement modules. 
         [0064]    Preferably the first measurement module is a voltage measurement module arranged to measure voltage on a main power supply line, and wherein the or each second measurement module is a current measurement module arranged to measure current on one or more branch circuits connected to the main power supply line. 
         [0065]    The method may further comprise transmitting a correction signal to the second measurement module; and correcting the synchronisation of the second measurement module based on the correction signal. This can enable correction of errors in the synchronisation while taking advantage of the low latency of the synchronisation. Preferably the correction signal is representative of the time difference between occurrence of a synchronisation event and transmission of a synchronisation signal. A plurality of correction signals may be transmitted, each correction signal relating to a particular power input line. 
         [0066]    In a further aspect, the invention provides a method comprising those steps that are performed by or at the first module in the synchronisation method above. Separately, the invention also provides a method comprising those steps performed by or at the second module in the method above. 
         [0067]    According to a further aspect of the invention there is provided a system for synchronising first and second measurement modules measuring characteristics of respective first and second electrical connections, comprising: at the first measurement module, means for detecting a synchronisation event in a measured signal from the first connection, and means for transmitting a synchronisation signal to the second measurement module in response to detection of the synchronisation event; and at the second measurement module, means for synchronising to the measured signal based on the synchronisation signal. Preferably the measurement module further comprises means for determining correction signal indicative of the time difference between occurrence of a synchronisation event and transmission of a synchronisation signal. 
         [0068]    According to further aspects of the invention there are provided a measurement module and corresponding method comprising:
       (means for) measuring a characteristic of an electrical connection;   (means for) detecting a synchronisation event in a measured signal from the connection; and   (means for) transmitting a synchronisation signal to another measurement module in response to detection of the synchronisation event.       
 
         [0072]    According to further aspects of the invention there are provided a measurement module and corresponding method comprising:
       (means for) measuring a characteristic of an electrical connection;   (means for) receiving a synchronisation signal from another measurement module; and   (means for) generating output data based on the measured characteristic and the synchronisation signal.       
 
         [0076]    In the above aspects, the measurement module may comprise means for performing a synchronisation method as set out above, and the methods may comprise the relevant steps of the synchronisation method as set out above. The measurement module may be a measurement module as set out in the first aspect above and as described elsewhere herein. 
         [0077]    The invention extends to methods and/or apparatus substantially as herein described with reference to the accompanying drawings. 
         [0078]    The invention also provides a computer program and a computer program product for carrying out any of the methods described herein and/or for embodying any of the apparatus features described herein, and a computer readable medium having stored thereon a program for carrying out any of the methods described herein and/or for embodying any of the apparatus features described herein. 
         [0079]    The invention also provides a signal embodying a computer program for carrying out any of the methods described herein and/or for embodying any of the apparatus features described herein, a method of transmitting such a signal, and a computer product having an operating system which supports a computer program for carrying out any of the methods described herein and/or for embodying any of the apparatus features described herein. 
         [0080]    Any apparatus feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure, such as a suitably programmed processor and associated memory. 
         [0081]    Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination. 
         [0082]    It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently. 
         [0083]    Furthermore, features implemented in hardware may generally be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly. 
     
    
     
         [0084]    These and other aspects of the present invention will become apparent from the following exemplary embodiments that are described with reference to the following figures in which: 
           [0085]      FIG. 1  is an illustration of a branch circuit monitor; 
           [0086]      FIG. 2  is an illustration of a modular branch circuit monitor; 
           [0087]      FIG. 3   a  is a top view of modular branch circuit monitors; 
           [0088]      FIG. 3   b  is a side view of modular branch circuit monitors; 
           [0089]      FIGS. 4   a  and  4   b  are top views of modular branch circuit monitors with alternative layouts; 
           [0090]      FIG. 5  is a schematic diagram of a modular branch circuit monitor; 
           [0091]      FIG. 6  is an illustration of a modular branch circuit monitor with split core detectors; 
           [0092]      FIG. 7  is an illustration of a first example of a modular voltage monitor; 
           [0093]      FIG. 8  is an illustration of a second example of a modular voltage monitor; 
           [0094]      FIG. 9  is a flow diagram of the synchronisation; 
           [0095]      FIG. 10  is a schematic diagram of the synchronisation; and 
           [0096]      FIG. 11  is a schematic diagram illustrating the measurement error within a zero crossing detector circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0097]    Branch circuit monitors allow monitoring of branches of an electrical circuit. A monitor typically determines for a number of branch circuits the current, voltage, energy and/or power consumption relating to each branch. Knowledge of the consumption of each branch permits operations such as the allocation of costs to a branch based on usage; protection from overloading at individual branches; and management of the load distribution between different branches. 
         [0098]      FIG. 1  shows an example of an electrical distribution board  10  (also known as circuit breaker box or panel box) with a main power line  18 . The power is distributed by power distribution units  12  to a number of branch circuits  14 . A branch circuit monitor  02  comprises a number of detectors  04 , typically solid core current transducers, each of which is capable of monitoring a branch circuit  14 . The branch circuit monitor  02  passes signals from the detectors  04  via a ribbon cable  06  on to a data acquisition board  08 , where the data from the detectors  04  is processed. The processed data is passed further via a communication link  16  (for example using the Modbus communication protocol) to a controller or a computing device for storage, display and evaluation of the data; optionally, the communication link  16  may pass via a one or more further data acquisition boards (for a daisy-chain arrangement of data acquisition boards, if desired). 
         [0099]    Solid core current transducers as detectors  04  have the advantage of higher accuracy. Solid core current transducers as detectors  04  are mounted on a branch circuit monitor  02  (typically on a printed circuit board) at a pitch which corresponds to the pitch of the branch circuits  14  in the distribution board  10 . 
         [0100]    Instead of solid core current transducers, split core current transducers may be used. Split core current transducers allow opening and closing of the core so as to enclose a circuit connection. This is necessary for example if the detector is to be fitted to an existing circuit connection, in which case the detector can be installed without interrupting the circuit. 
         [0101]    The above system presents several shortcomings. Passing the analogue detector data from the current transducers  04  via a ribbon cable  06  (or any other cable) on to a data acquisition board  08  introduces electrical noise due to the cable. The analogue signal from the detector can be relatively low and subject to a relatively high signal-to-noise ratio. Electrical interference to which the analogue signal is subjected en route to the data acquisition board  08  can degrade the signal. For highly accurate measurement it is preferable to minimise cabling. Further, the branch circuit monitor  02  typically comprises tens of detectors  04 . If, as in the illustrated example, only a subset of the power distribution units  12  have a branch circuit  14 , then some (or even many) of the detectors  04  are superfluous. 
         [0102]    The shortcomings of the above system are overcome by a system with modular branch circuit monitors with on-board signal processing. As referred to herein, a module is a self-contained unit that is connectable to another such module (or to a different type of module), the modules (of whatever type) having a common communication link. 
         [0103]      FIG. 2  shows an alternative electrical distribution board  10  (or circuit breaker box) with modular branch circuit monitors  20  with on-board signal processing (like reference numerals denote like components). Each modular branch circuit monitor includes a number of detectors  04 . Modular branch circuit monitors  20  may be physically linked or attached to one another by a connection to form a composite branch circuit monitor with the appropriate number of detectors  04  for a given number of branch circuits  14 . As seen in  FIG. 2 , the number of modular branch circuit monitors  20  in a composite branch circuit monitor can be chosen to minimise the superfluous detectors  04 : on the left of the distribution board there are only two branch circuits  14 , and a single three-detector module  20  is sufficient. On the right there are six branch circuits  14 , and two three-detector modules  20  are connected to form a composite six-detector monitor. 
         [0104]    To enable freedom in the choice of connection of modules, each module  20  includes means for processing (at the module itself) the analogue signal detected by the detectors  04  to produce digital output data. This ensures that operation of a composite monitor is independent of the number of modules forming the composite monitor. To achieve processing at the module  20 , the module may be provided in the form of a printed circuit board on which are mounted detectors as well as components for signal processing (including analogue and/or digital processing and/or A/D conversion), and/or components for other functions (such as communication). 
         [0105]      FIGS. 3   a  (top view),  3   b  (side view) and  5  (schematic diagram) show modular branch circuit monitors  20  with detectors  04  as well as further components  34  mounted on a printed circuit board  36 . The further components are mounted on the opposite side from the detectors  04  on the printed circuit board  34  for compactness and space-efficiency; if compactness is not important, then some or all of the components  34  can be mounted on the same side as the detectors  04 . 
         [0106]    The components  34  include a processor  40  and a memory  42  to provide processing capability. Processing in particular includes conversion of analogue measurement data into digital data that can be communicated via a digital communication link (in the described example the Modbus link). A communication module  44  provides an interface to the communication link. 
         [0107]    By enabling on-board localised processing of measurement data, electrical noise associated with cabling is minimised and the accuracy of the system is improved. 
         [0108]    A terminal unit  30  may be designed to attach to a modular branch circuit monitor  20 ; if two or more modular branch circuit monitors are serially connected, one to the next, then the terminal unit  30  is attached to the end-most module. The terminal unit provides a connection to other components such a system interface, controller or a computing device for storage, display and evaluation of the data. 
         [0109]    The number of detectors  04  per modular branch circuit monitor  20  may vary depending on requirements. In  FIG. 2  an example is illustrated with three detectors  04  per modular branch circuit monitor  20 . In  FIGS. 3   a  and  3   b  an example with six detectors  04  per modular branch circuit monitor  20  is shown. In  FIG. 4   a  an example with eight detectors  04  per modular branch circuit monitor  20  is shown. In  FIG. 4   b  an example with five detectors  04  per modular branch circuit monitor  20  is shown. Modules with different numbers of detectors may also be combined. 
         [0110]    The detectors  04  on a modular branch circuit monitor  20  are mounted at a pitch which corresponds to the pitch of (or in other words the spacing between) the branch circuits on the distribution board the modular branch circuit monitor  20  is intended for use with. In the examples illustrated in  FIGS. 3   a  to  4   b , the detector width is wider than the branch circuit pitch, and the detectors are arranged in two rows. The detectors in a first row are offset against the detectors in the second row by an offset distance  46 . This staggered detector arrangement allows a branch circuit pitch that is less than the detector width. 
         [0111]    The detectors  04  on a modular branch circuit monitor  20  are shown to be solid core current transducers. In one example, the detectors are self-powered solid core transducers. While these detectors require no power supply and are relatively inexpensive, they can exhibit relatively low accuracy in detecting low currents. In another example the detectors are powered solid core transducers, such as Hall Effect transducers. Although these detectors require a power supply, they provide current measurements that are more accurate than their self-powered equivalent. The provision of more accurate measurements enables more detailed analysis of the current signal. For example harmonic analysis can evaluate the wave form to establish whether the current drawn by the load is linear or not. This can be important in identifying loads that are affecting the supply power quality, or to sense power supply fluctuations which might be indicative of failure. 
         [0112]    A connector portion  38 - 1  of each modular branch circuit monitor  20  can be connected to a connector portion  38 - 2  of a neighbouring such modular branch circuit monitor  20 . The connector portions  38 - 1   38 - 2  establish an electrical as well as a mechanical connection between the modular branch circuit monitors  20 . The electrical connection can enable a serial connection of the modules, which can provide a convenient route for communication. The mechanical connection can enable formation of a composite unit comprising a plurality of modular branch circuit monitors  20 . To form a composite unit, the connector portions  38 - 1   38 - 2  in the examples illustrated in  FIGS. 3   a  to  4   b  comprise a tessellation feature that is capable of tessellation with a further such branch circuit monitor module. The tessellation feature is not only advantageous for establishing a mechanical connection; it can also permit modules with the staggered detector arrangement described above to be combined to form a composite unit. In the illustrated examples the tessellation feature is a step shape for simplicity and convenience regarding the staggered detector arrangement. Other tessellation shapes can be used. The connector portions  38 - 1   38 - 2  can include pins and sockets, clips, and other fixation features. For protection of male connector parts such as pins, the location in a recessed area of connector portions (such as connector portions  38 - 1  in  FIG. 4   a ) is particularly suitable; corresponding female connector parts can be located in a projecting area of the connector portions (such as connector portions  38 - 2  in  FIG. 4   a ). 
         [0113]    In a preferred embodiment, the modular branch circuit monitors  20  are connected in series via a communications link, for example a Modbus link. In normal operation a modular branch circuit monitor  20  routes communications to the system interface via the other modular branch circuit monitors. A modular branch circuit monitor issues a communication onto the Modbus line; the Modbus line passes from one monitor to the next, until the terminal unit  30  at the end of a series of monitors. At the terminal unit  30  the Modbus line continues on via a communication link  16  to the system interface. The communications are digital, not analogue. If necessary the modules may address one another, but generally the modules communicate with the system interface. 
         [0114]    One example where the modules communicate with one another is when the modules are being powered up. When the modules are serially connected in a daisy-chain arrangement, and are all powered up at once, the power source to the modules is required to supply more power than during normal usage. This necessitates selection of a power supply of higher performance than is required during usual operation. In order to overcome this drawback, the modules are preferably powered up sequentially. An end module, for example the module with the terminal unit, powers up first; once it is ready for operation, it issues a signal to its neighbouring module, which then commences to power up, and so on. In one example the signal is a digital signal transmitted via the common Modbus link, in which case addressing information for the neighbouring module is included. Alternatively there is a link between neighbours that is private between the two neighbouring modules; in this case no addressing information is required. The signal in this case can be digital or analogue. 
         [0115]      FIG. 6  shows modular branch circuit monitors  20  with split core detectors  50 . The split core detectors  50  are in this example connected to the modular branch circuit monitors  20  by a cable  52 . The cable  52  carries the analogue detector data to the modular branch circuit monitors  20  for processing (including digitising). Subsequent operation is the same as for the modular branch circuit monitors  20  with solid core detectors  04 . Split core current transducers are generally of a self-powered type. 
         [0116]    As mentioned above, split core current transducers provide the advantage of enabling retrofitting of detectors. The analogue signal transmitted from the detectors  50  to the modules can suffer degradation due to electrical interference in the cable  52 . So although some noise may be introduced, the signal is nevertheless not transmitted in its analogue form from the branch circuit monitor to a data acquisition board (as in the system described with reference to  FIG. 1 ). Minimising the distance the signal travels in its analogue form can reduce the noise incurred. Further, the number of modules can be selected depending on requirements, as with the solid core modules. The use of the modular branch circuit monitors hence provides the same advantages for use with split core detectors as it does with solid core detectors. Modules with split core detectors can also be combined with modules with solid core detectors. 
         [0117]    An example of a modular branch circuit monitor design is now described in more detail. 
         [0118]    In this example, communication between modules and the controller is provided via the Modbus protocol. 
         [0119]    To provide power to the modular branch circuit monitor, in the embodiment described here the modular branch circuit monitor operates from a nominal +24 Volts supply (on- or off-board). The supply voltage input can range from +18.6 Volts to +36.6 Volts without compromising operation. For a configuration with six solid core current transducers (as shown in  FIGS. 3   a  and  3   b ) the current consumption at +24 Volts input is 103 mA. There are four power rails on the board derived from the incoming 24 Volt supply. The four power rails carry voltages of +24 Volts, +5 Volts, +15 Volts and −15 Volts from three DC-DC converters. Microcontroller and level shifting and filtering circuits are powered by a 3.3 Volts supply derived from the +5 Volts line. If split core current transducers are used instead of solid core current transducers the current consumption at +24 Volts input is 30 mA, and +15 Volts and −15 Volts power lines can be omitted. 
         [0120]    Each of the detection channels (six for the six-detector arrangement shown in  FIGS. 3   a  and  3   b ) of the modular branch circuit monitor is scanned sequentially in a ‘round robin’ order. Each channel is measured in order and when the last channel has been measured, the board immediately returns to the first again. There is a short gap of approximately 3 milliseconds after a new channel is switched to. This allows the amplifiers to settle at a new value before a new measurement commences. 
         [0121]    The signal from each channel is attenuated and level shifted to bring the peak-to-peak signal in to the appropriate range for the 3.3 Volt, 10-bit analogue-to-digital converter of the microcontroller. 
         [0122]    Each channel is low-pass analogue filtered with a 2 pole active filter to remove any harmonics above 3.1 KHz. This frequency is chosen so that negligible attenuation is brought about on the 7 th  and 9 th  harmonics of the fundamental frequency of the signal for later harmonic analysis. 
         [0123]    The sinusoidal signal from the current transformers is sampled at 12.8 KHz. This sampling frequency is chosen because it enables capture of 256 samples over one cycle of the 50 Hz mains waveform. 
         [0124]    Digital filtering is performed once the signal is sampled. Digital filtering is desirable at this stage: the peak signal-to-noise ratio is approximately 54 dB, as some broadband noise is present on the signal presented to the inputs of the microcontroller; additionally, some high frequency signals are induced by the DC-DC converters on the board. Within the microcontroller a single pole Exponentially Weighted Moving Average Filter (EWMA) reduces any high frequency noise that the signals might have acquired on the modular branch circuit monitor. 
         [0125]    To measure the signal the 256 samples of the processed signal are summed to establish the area under the curve. To establish the current consumption the resultant area value is multiplied by a factory calibration. This value is added to an array of collected current measurements to be used for averaging. The averaged current consumption is stored to Modbus registers. 
         [0126]    With the oversampling the noise is reduced, resulting in the coherent signal from the continuous-time channel being measured to an accuracy of 2 in 10,000 (equivalent to −74 dB) RMS noise. The modular branch circuit monitor resolves to an accuracy of 20 mA in a full scale range of 100 A. 
         [0127]    Once a new current value has been established, this is multiplied by a user-configured voltage to establish the instantaneous power consumption (in Watt or kilowatt). The instantaneous power consumption is saved to Modbus registers. Additionally, a cumulative record is kept of the power consumption, alongside a value quantifying the cumulative power consumption of the foregoing half hour. 
         [0128]    Instead of referring to a user-configured voltage parameter, the system may include a module for monitoring voltage (as described in more detail below). In this case, the actual measured voltage can be reported to the modular branch circuit monitors. 
         [0129]    To enable even better accuracy in calculation of the power consumption, the phase difference between the actual voltage and current (which may deviate from the ideal situation with voltage and current perfectly in phase) may be determined by means of the module for monitoring voltage. A method of synchronising a time reference so that the actual phase difference between the voltage and current can be determined is described in more detail below. 
         [0130]    A power factor quantifies the effect of the phase difference between voltage and current on the real power consumption. Assuming perfectly sinusoidal waveforms, the power factor PF is: 
         [0000]        PF= cos φ
 
         [0000]    where φ is the phase difference between voltage and current (this may be determined using a phase counter as discussed in more detail below). The real (instantaneous) power RP is: 
         [0000]    
       
      
       RP=PF×V×I  
      
     
         [0000]    where V is the voltage and I is the current. Taking the power factor into account enables more accurate calculation of real power consumption. 
         [0131]    To enable determination of the power factor, the modular branch circuit monitors are supplemented by a modular voltage monitor. The modular voltage monitor detects the voltage signal and communicates it to the modular branch circuit monitors in order for the modular branch circuit monitors to be able to determine the phase difference between the current signal and the voltage signal. 
         [0132]    Rather than each current board having the ability to measure the amplitude and phase of voltages and currents, the voltage information can be measured by a single central modular voltage monitor and communicated to all modular branch circuit monitors. 
         [0133]      FIGS. 7 and 8  show examples of a system with modular branch circuit monitors  20  and a modular voltage monitor  60 . In the example illustrated in  FIG. 7 , the modular voltage monitor  60  has a voltage detector  62 , with which it measures voltage. The modular voltage monitor  60  is positioned at the main power line  18  and connected by data cables  24  to the modular branch circuit monitors  20 . The communication between the modules, including between the modular voltage monitor  60  and the modular branch circuit monitors  20 , uses the Modbus protocol. The modular voltage monitor  60  includes on-board signal processing (using a processor or microcontroller with associated memory), akin to the modular branch circuit monitors  20 . In particular, the modular voltage monitor  60  may include means for voltage reduction, data acquisition, analogue-to-digital conversion, sampling, filtering, and/or signal analysis. 
         [0134]    In  FIG. 7  three voltage detectors  62  are shown, which are used to detect the voltage carried in the three circuit conductors of a three-phase power line. In case the main power line  18  to the electrical distribution board  10  is a single-phase power line, then a single voltage detector may be provided to detect the voltage carried in the circuit conductor. To measure the voltage, a circuit conductor is typically wired (parallel to the power line) directly onto terminals on the modular voltage monitor  60  and stepped down to be measured by a detector  62 . 
         [0135]    In the example illustrated in  FIG. 8  the modular voltage monitor is a combined modular voltage/branch current monitor  64 . It carries—like the other modular branch circuit monitors  20 —branch current detectors  04 ; in addition, a voltage detector  62  is included, from which voltage measurements are supplied to the other modules. 
         [0136]    The arrangement with a combined modular voltage/branch current monitor (as seen in  FIG. 8 ) is suitable for a single-pole distribution board, where all branches have the same phase. In a three-pole distribution board, the arrangement with a dedicated modular voltage monitor (as seen in  FIG. 7 ) positioned at the main power line  18  enables convenient detection of all three phases. 
         [0137]    The modular voltage monitor measures voltage information (voltage amplitude and voltage phase information) and communicates it to all modular branch circuit monitors. This allows separation of voltage measurement from current measurement. As there are only few (usually three) channels of voltage information to be collected, whereas there can be hundreds of current channels to be measured, this allows elimination of redundant measurement capability, and can thereby allow efficiency of applied resources and reduction of costs. 
         [0138]    The arrangement with a combined modular voltage/branch current monitor (as seen in  FIG. 8 ) can provide a simpler system with easier installation. 
         [0139]    The arrangement with a dedicated modular voltage monitor (as seen in  FIG. 7 ) can provide better overall system accuracy. By having a modular voltage monitor dedicated to only measuring voltage amplitude and voltage phase, the voltage measurement hardware need do only one set of measurements to a high degree of accuracy, rather than having to carry out the multiple, disparate tasks that would be involved if the voltage measurement were part of the branch monitoring hardware. 
         [0140]    The arrangement with a dedicated modular voltage monitor may also have benefits with respect to safety considerations. Since dangerous voltages are present on the voltage phases, measuring the voltages separately and communicating the voltage information to the modular branch circuit monitors (potentially even by opto-isolated communications) increases safety for the users and service personnel. 
         [0141]    To ensure that the modular system achieves high accuracy, it is important that the communication between the modular voltage monitor and the modular branch circuit monitors occurs with high integrity. 
         [0142]    The communication between the modular voltage monitor and other devices is across the Modbus, a relatively slow medium that typically carries data at between 9600 and 115,200 bits per second. This data is ‘asynchronous’ in that messages can be sent and received at any time without reference to a master, system-wide clock. This is normally sufficient for routine communications. If however for example there is a substantial lag in communicating an instantaneous phase value, then the lag itself may incorrectly be attributed to the phase, and introduce errors. 
         [0143]    A detection, communication and synchronisation procedure as is described below is designed to ensure that the communication is efficient and accurate. 
         [0144]    As described above, one or more modular branch circuit monitors and a modular voltage monitor are connected to one another in a network. The communication links between the modules use the Modbus communication protocol. To ensure efficient and accurate communication, the modular voltage monitor communicates a signal to the modular branch circuit monitors on detection of the occurrence of an event that marks a point of reference in the voltage waveform. The event is advantageously a zero-crossing of the voltage, which may enable the phase to be accurately distinguished and detected. The modular branch circuit monitors receiving the signal are triggered by the signal to synchronise to the detected phase. To ensure that minimal delays occur, an alerting signal precedes and initiates detection, communication and synchronisation to the detected phase, and sets the modular branch circuit monitors into a state of readiness. From the state of readiness, receipt of the communication from the modular voltage monitor immediately triggers synchronisation to the detected phase. 
         [0145]      FIG. 9  is a flow diagram of the detection, communication and synchronisation procedure.  FIG. 10  is a schematic diagram of the phase synchronisation. Briefly, the modular voltage monitor waits until a first zero-crossing is detected, then enters an active state in which the detection of a second zero-crossing triggers an event communication. 
         [0146]    With reference to  FIG. 9 , following issuance of an alerting command  72 , the modular branch circuit monitors are primed to expect an event communication  74 . Also, the modular voltage monitor prepares to issue event communication  76 . To do so, the modular voltage monitor waits until the current or on-going wave period is completed  78 ; with respect to a zero-crossing event, it waits until the next zero-crossing occurs. Thereafter, the modular voltage monitor is primed to generate an event communication at the next following occurrence of the event  80 . The modular voltage monitor waits until the next event (zero-crossing) is complete  82  and immediately thereafter generates event communication  84  to the network. Meanwhile, the modular branch circuit monitors have been awaiting the event communication in a state of readiness. On receipt of the event communication, the modular branch circuit monitors reset their internal counters  86 , and thus become synchronised to the detected voltage phase. Following synchronisation, the modular branch circuit monitors may remove the event communication from their communication buffers  88 . 
         [0147]    The sine-shaped signal has two zero-crossings in each wave period: one with positive gradient at the crossing, and the other with negative gradient at the crossing. In the example illustrated here only the zero-crossings with positive gradients are used. Alternatively only zero-crossings with negative gradients may be used, or zero-crossings regardless of gradients. 
         [0148]      FIG. 10  shows a system interface  92  (or computing device or controller or other interfacing device) that issues the alerting command and initiates the synchronisation procedure. The system interface  92  is connected by a communication link  94  (using the Modbus communication protocol) to a modular voltage monitor  96  and a number of modular branch circuit monitors  98 . The modular voltage monitor  96  monitors the voltage levels and voltage phases of three voltage inputs: L 1 , L 2 , and L 3 , the three lines of a three-phase power supply as mentioned above). 
         [0149]    The modular voltage monitor  96  detects occurrence of zero-crossing events, in the illustrated example for the L 1  voltage input. It generates an event communication output  100  that is communicated to the modular branch circuit monitors  98  to allow them to synchronise with the zero-crossing of a selected phase (L 1  in the illustrated example). Each modular branch circuit monitor  98  has a free running phase counter  102  running within the processor. On receipt of the synchronisation communication, each modular branch circuit monitor  98  synchronises (for a zero-crossing event: resets) its phase counter  102  by means of a reset logic  104 . The free running phase counter  102  is used to reference the phase relationship between the line voltages (L 1  in the illustrated example) and the measured currents. Knowing this relationship allows modular branch circuit monitors  98  to calculate the phase difference, and therewith (using the collected current measurements) the real power. 
         [0150]    By using dedicated hardware for the detection, communication and synchronisation procedure the timing integrity is maintained for measurements that are communicated around the system. Latencies present in a normal Modbus (or other similar) communication protocol are minimised by the detection, communication and synchronisation procedure. This allows using Modbus lines  94  to carry time-critical information, such as the zero-crossing information of a selected phase. 
         [0151]    An example of the detection, communication and synchronisation procedure is now described in more detail and with reference to  FIGS. 9 and 10 . 
         [0152]    The modular voltage monitor  96  is designed to monitor the voltage levels and relative phases of three voltage inputs L 1 , L 2 , L 3 . Each of the three voltage input lines is an approximation of a 250 Volt RMS sine wave with a frequency of 50 Hertz and with the phase of each line offset from each other by 120 degrees. The modular voltage monitor  96  operates autonomously and responds to Modbus requests with the requested information. The modular voltage monitor  96  can measure and report data including the following:
       L 1  Phase RMS Voltage   L 2  Phase RMS Voltage   L 3  Phase RMS Voltage   Power Grid Frequency   L 1 , L 2 , L 3  relative phase   Zero Crossing Pulse   L 1  Voltage Harmonic Content   L 2  Voltage Harmonic Content   L 3  Voltage Harmonic Content
 
The modular voltage monitor  96  can select and monitor any one of the three phases for various measurements. Optionally, the Zero Crossing Pulse is monitored to improve measurement accuracy, as is described in more detail below. These data can be communicated across the Modbus lines  94  in the normal manner, for example to the system interface  92 .
       
 
         [0162]    In addition to carrying out measurement tasks, the modular voltage monitor  96  is able to monitor every zero-crossing of any one of the connected voltage phases. The modular voltage monitor  96  is designed to carry out a special function where it generates an output to allow modular branch circuit monitors  98  to synchronise with the zero-crossing of a selected phase. The output is a special event communication signal that is routed directly to the Modbus line  94 . The event communication signal is a single pulse or step that occurs at the ‘zero-crossing’ of the selected phase. This ‘zero-crossing’ occurs when the voltage signal equals (or is sufficiently close to) 0 Volts. This pulse position of the Zero Crossing Pulse is affected, to a small extent, by the frequency and amplitude of the line phase that is input to the Zero Crossing Detector circuit. To allow the modular branch circuit monitors  98  to be able to correct for any error in the Zero Crossing Detector Circuit, the relative phase of the Zero Crossing Pulse is measured with respect to the 3 power phases, L 1 , L 2  and L 3  with a record taken of which phase is driving the Zero Crossing Detector circuit. Since the relative phase measurements are gathered at the same time, the error of the Zero Crossing Detector is known and can be communicated to the modular branch circuit monitors  98 . 
         [0163]    Within the modular branch circuit monitors  98 , the error of the Zero Crossing Detector circuit on the modular voltage monitor  96  can be corrected for, thus cancelling out variations in the Zero Crossing Pulse brought about by varying voltage or frequency on the input phase driving the Zero Crossing Detector circuit. 
         [0164]      FIG. 11  illustrates the measurement error within a Zero Crossing Detector circuit in more detail. In the lower part of the figure, an enlarged view of a realistic, non-ideal zero-crossing pulse  108  is shown, with a zero-crossing error  124  in the time of the (measured) zero crossing  108 . The zero-crossing detection point can vary in dependence on the amplitude and frequency of the signal driving it. For example, if the driving power line  106  (in the illustrated examples L 1 ) changes amplitude or frequency, it is observed that the zero crossing can vary by up to plus or minus 10 degrees. 
         [0165]    For correction of the zero-crossing error a 2-dimensional correction table can be created by testing the board, allowing characterisation of the error of the zero-crossing detector at a range of voltages and frequencies. Use of this calibration table requires interpolated lookup and a complex algorithm to be useful. 
         [0166]    Instead of using a correction table, the zero-crossing error  124  (or relative phase) of the zero-crossing pulse  108  is measured at the same time as the three power lines L 1 , L 2  and L 3   106   120   122 . At the modular voltage monitor  96 , the occurrence of a zero-crossing event is communicated to the modular branch circuit monitors  98  with a low-latency communication link, as described in more detail below. The zero-crossing pulse  108  is known to have an error. At the same time, the modular voltage monitor  96  measures with high accuracy, but not as part of the low latency communication link, the three power lines L 1 , L 2  and L 3 . By comparing the zero-crossing pulse  108  with the driving power line  106  (in the illustrated example L 1 ), the actual zero-crossing error  124  becomes known. The actual zero-crossing error  124  is communicated by a standard communication to the branch boards, which can then apply the correction to the zero-crossing pulse  108  that was received. Thereby it is possible to correct a zero-crossing error  124  at the modular branch circuit monitors  98 , and enable accurate calculation of power at the modular branch circuit monitors  98 . 
         [0167]    As there are typically three power lines L 1 , L 2  and L 3   106   120   122  at different phases relative to one another, and as at a given modular branch circuit monitor  98  any one of the possible power lines may be in use, a set of correction values, one for each possible power line, is communicated. 
         [0168]    By the correction of errors in the zero-crossing pulse  108  as described above the system can maintain a high level of accuracy, even if the individual monitor boards are subject to drift or settling throughout their lifetime; the correction can also enable accuracy despite an external influence such as an environmental condition. As the correction does not rely on board calibration for a particular drive frequency or amplitude, there is no dependency on the exact calibration conditions. 
         [0169]    The process of communicating the zero-crossing to the modular branch circuit monitors  98  is initiated by the system interface  92  issuing a special alerting command (marked ‘Alert’ in  FIG. 10 ). The alerting command is a string of binary data sent serially (according to the Modbus protocol) along the Modbus lines  94 . This command initiates synchronisation and sets the modular voltage monitor  96  in a primed state and the modular branch circuit monitors  98  in a state in readiness of synchronisation. In the preferred embodiment synchronisation only occurs approximately once a second in order to maintain high accuracy of the calculated real power without clock drift becoming significant. 
         [0170]    In the primed state (marked ‘P’ in  FIG. 10 ), the modular voltage monitor  96  monitors (using a software loop running on the processor) the selected phase  106  and waits until it sees a zero-crossing event  108 . The monitoring may be part of its normal monitoring. The selected phase is usually phase L 1 , since this is the phase that serves as the reference phase for other measurements. The modular voltage monitor  96  waits in this loop until it sees that a zero-crossing has completed. When a zero-crossing event is completed the modular voltage monitor  96  goes on to enter an active state (marked ‘A’ in  FIG. 10 ), in which dedicated on-board logic hardware is enabled to route  110  the next zero-crossing signal directly to the Modbus communication line  94 . The reason the modular voltage monitor  96  waits for the end of a zero-crossing (instead of directly communicating the zero-crossing) is to ensure that when the next zero-crossing occurs, there need be no processor interaction and risk of introducing delays. In this manner the zero-crossing circuit drives the Modbus line  94  directly and without any delays or latency. 
         [0171]    In the active state the modular voltage monitor  96  enables a hardware logic circuit that passes the next full zero-crossing pulse directly to a hardware circuit that issues an event communication to the Modbus line  94 . In this manner dedicated, autonomous hardware on the modular voltage monitor  96  passes the zero-crossing event directly on to the Modbus communication line  94 . At the Modbus line  94  an event communication is issued in the form of an ‘active’ binary state for the length of one single Modbus character (equivalent to the ‘Null’ character ‘00’). The modular voltage monitor  96  persists in the active state for 20 milliseconds, and if no event occurs within this time, then normal operation is resumed. 
         [0172]    The modular branch circuit monitors  98  have, inside their microprocessor, a continuously running 32 bit internal phase counter  102 . The value of the counter  102  indicates the voltage phase of the primary power supply (usually L 1 ) connected to the system. During normal operation the modular branch circuit monitors  98  listen for the special alerting command. When this command is seen, the modular branch circuit monitors  98  enter a state of readiness, in particular a hardware state, in which they expect a zero-crossing event communication. 
         [0173]    Each modular branch circuit monitor  98  primed in its state of readiness detects the ‘active’ binary state representing the synchronisation event communication on the Modbus line  94 , and in response its processor resets its internal phase counter  102  to zero. In other words, when primed in the state of readiness the next ‘edge’ of the data present on the Modbus line  94  causes the internal phase counter to be reset to zero. More specifically, the state of readiness allows the next change of binary state on the Modbus line  94  to interrupt the modular branch circuit monitor processor with a primary level interrupt, causing the microprocessor to reset the 32 bit counter  102 . In this manner the processor responds with extremely small and predictable latency to the change of binary state on the Modbus line  94 . Thus, the counter  102  is synchronized with the zero-crossing of the primary voltage phase. 
         [0174]    The modular branch circuit monitors  98  continue to carry out measurements while they are in the state of readiness, but they are configured to carry out actions on the edge of the change of binary state on the Modbus line  94  with high priority. The modular branch circuit monitors  98  remain in the state of readiness for up to 45 milliseconds before returning to their normal operating state. The 45 ms duration of the state of readiness is chosen because it can accommodate two full cycles of the mains that is running at 50 Hertz (2×the period of 20 ms plus a margin of 5 ms=45 ms). In the absence of receipt of an event communication within the window, or after resetting of the internal phase counter  102 , the modular branch circuit monitors  98  then return to normal operation, receiving requests and generating replies. For routine current measurements made by the modular branch circuit monitor  98 , the value of the counter  102  can be used to indicate the exact phase of the voltage lines to a high degree of accuracy. 
         [0175]    Once the modular branch circuit monitors  98  resume normal operation they treat the event communication as a ‘Null’ character (‘00’) on the Modbus and remove it from their Modbus communication buffer. 
         [0176]    The system interface  92  receives and ignores the event communication as it appears in the Modbus communication buffer as a ‘Null’ character (‘00’). 
         [0177]    After the counter of a modular branch circuit monitor is synchronized with the zero-crossing of the primary voltage phase, the modular branch circuit monitor can calculate the real power at the module and pass real power data on to the system interface (or controller or computing device) for storage, display and evaluation. 
         [0178]    The instantaneous phase between the current and the voltage is calculated on the modular branch circuit monitor by referring to the instantaneous value of the internal phase counter  102  (also referred to a local phase clock). In addition, the period of the sine wave is required. Determination of the period is described in more detail below. The modulo of the instantaneous phase counter value divided by the period quantifies the instantaneous phase. As used herein, the modulo is the fractional (non-integer) part of the real-number result of the division. 
         [0179]    The phase counter (which represents the voltage waveform, as it was synchronised with a zero-crossing of the voltage) for example operates at 10,000,000 Hz and the period is 200,000 counts. If the phase counter has an instantaneous value of 900,000 (taken at the instant a current measurement has finished), then dividing the counter value by the period value give a result of 4.5. The modulo (here: 0.5) indicates the proportion of a full 360 degree cycle that the voltage waveform is at when that current measurement is taken. The value in this example is 0.5 which indicates 360* 0.5=180 degrees. 
         [0180]    The phase of the current signal is also required to determine the phase difference between the phase of the measured current waveform and the phase of the voltage waveform. Provided the current measurement samples the current signal at a sufficient frequency, the instantaneous phase of the measured current waveform can be determined with high precision. This allows accurate calculation of the phase difference between the current and voltage in the branch monitor. 
         [0181]    As described above, the power factor is a cosine function of the phase difference between the current and voltage waveforms. Once the phase difference is determined, the power factor is determined by accessing a sine wave look-up table. The modular branch circuit monitor has a suitable look-up table in its local memory. Once the power factor is determined, the real power is calculated, using the voltage amplitude obtained from the voltage board and the measured current. 
         [0182]    As mentioned, the calculation of the instantaneous phase difference between the current and the voltage requires determination of the wave period (or frequency, the frequency being the inverse of the period). In one example, the period is determined from the current data measured on the modular branch circuit monitor. 
         [0183]    In another example the period is determined at the modular voltage monitor from the voltage data measured there, and the period information is communicated to the modular branch circuit monitor. To measure the period the processor on a module has a period counter that measures the time between 2 successive zero-crossings (with the same gradient orientation) of a voltage phase. For example in the case of a 50 Hz waveform and with a clock frequency of 10,000,000 Hz, then a period has a duration of 10,000,000/50=200,000 counts. This value is communicated as a standard message across the Modbus communication line informing the modular branch circuit monitors that the period is 200,000 counts. 
         [0184]    This method of determining the period has a disadvantage relating to clock variations. Each board in the system uses its own clock. Although the clocks are typically very accurate, they are not infinitely accurate, and the different clocks are not exactly the same. For example, two modules with clocks with a nominal frequency of 10,000,000 Hz and an accuracy of 30 parts per million can have an actual frequency of 10,000,300 Hz on one module, while the other module has an actual frequency of 9,999,700 Hz. Similarly a period that on one module has a duration of 200,010 counts has—on the other module—a period of 199,990 counts. Hence if the modular voltage monitor reports a period—of for example 200,000 counts—to a modular branch circuit monitor with a clock running slightly faster or slower, an error can be introduced in the period value, in the illustrated example +/−10 counts. 
         [0185]    To overcome the disadvantage relating to clock variations in a further example the period is determined at the modular branch circuit monitor, using the local clock, from signals transmitted from the modular voltage monitor in response to voltage measurements. An arrangement to extend the synchronisation method so as to enable determination of the period at the modular branch circuit monitor is now described in more detail. 
         [0186]    As described above, the synchronisation method uses the Modbus communication line to communicate time-critical information. The same arrangement is extended to measure the period (and/or frequency) of the voltage waveform. The same hardware is used, with no changes necessary. 
         [0187]    At the modular voltage monitor, instead of measuring the time between two successive zero-crossings of a voltage phase, the zero-crossings cause the modular voltage monitor to issue a signal to the Modbus line. The issuing of a signal follows the same procedure as the synchronisation signal. Each modular branch circuit monitor receiving the signal stores two successive snapshots of the value of its clock (rather than resetting the phase counter as with the synchronisation command described above). The period is the difference between the two time stamps. As with the synchronisation, the period signal may cause an interrupt at the modular branch circuit monitor processor with a primary level interrupt, causing the microprocessor to store a period start timestamp and a period end timestamp. Instead of using time stamps, a period counter could be reset to zero at the first signal, and stopped at the second signal. 
         [0188]    By collecting critical data (including instantaneous current amplitude and phase, instantaneous voltage phase and period) locally and calculating the real power locally each module measures to its maximum accuracy, and errors introduced due to not all clocks running at exactly the same rate can be minimised. 
         [0189]    In the preferred embodiment determination of the period occurs approximately once a second in order to maintain high accuracy of the calculated real power without clock drift becoming significant. 
         [0190]    In some examples the modular branch circuit monitor supplies more data than just the real power, and the system interface undertakes some further calculations with the data. Alternatively, the modular branch circuit monitor may supply current data to the system interface, while the modular voltage monitor supplies voltage data to the system interface, allowing the power calculation to be made by the system interface. 
         [0191]    In a daisy chain arrangement where modules connect serially one to another, data passes through a communication channel on the modules. In this way the modules are physically connected to one another and form a network. Data from a first module may pass through a second module without the second module requiring or making use of the data from the first module. The network arrangement at the same time enables the modules to share data, as in the case of the synchronisation described above. 
         [0192]    It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention. 
         [0193]    Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.