In any technical environment and in particular in the electrical sector, in order to make 'workmanlike' devices, all relevant legal and technical standards must be complied with. Knowledge of standards and distinguishing between legal standards and technical standards is thus the basis for a correct approach to measuring instrument questions that involve not only technical aspects relating to accuracy and safety but also tax and accounting matters.
Legal standards are all those governing the behaviour of parties subject to the authority of the state, including the European directives that are normally enacted in national legislation through legislative decrees. Technical standards are all the prescriptions on the basis of which the machines, devices, materials and plants have to be designed, built and tested to ensure their operating efficiency and safety.
The technical standards set by national and international bodies (CEI, CENELEC, IEC) have been drawn up in a very detailed manner and can have legal significance when this is assigned to them by a legislative measure.
IEC standards
Three committees are specifically responsible for measuring instrumentation:
TC85 "Measuring equipment for electrical and electromagnetic quantities"
TC66 "Safety of measuring, control and laboratory instruments"
TC13 "Electrical energy measurement, tariff and load control".
The first committee draws up and publishes the reference standards for all the instruments (voltmeters, ammeters, wattmeters, etc) of both analogue and digital type and provides the prescriptions for the instruments and the sample equipment (batteries, resistors, recording instruments, etc).
Committee 85 is also responsible for a series of standards, all of which are European in origin (from IEC EN 61557-1 to IEC EN 61557-10), dedicated to electrical safety in low voltage distribution systems. These standards also contain some safety prescriptions and the functional features required for the instruments for the tests, measurements and controls of low voltage electric plants such as, for example, earth resistance measuring instruments, instruments for measuring the impedance of fault loop, instruments for testing continuity of protection conductors, insulation measuring instruments, etc.
These are therefore particularly important standards for defining the required characteristics of the measuring instruments to be used for the tests prescribed by standard IEC 60364 that govern low voltage electric plants.
Committee 66 deals with the safety prescriptions for electrical measuring devices that must be met by the manufacturer to ensure the safety of the operator.
Lastly, Committee 13 is entirely dedicated to publishing the standards governing active energy and reactive energy measurements and the relative devices: meters, integrated units, devices of different types. In this connection, the following standards have particular importance for the purposes of tests on energy meters: EN 50470-1, EN 50470-2, EN 50470-3, which set the test prescriptions both for electromechanical active energy meters and for static meters.
Figure1 summarises the standards to which measuring instrumentation is subject.
Figure-1: Reference Standards for measuring instruments
MID Directive
European Directive 2004/22/EC of 31 March 2004 introduced a Community framework law governing devices and systems with measuring functions relating to common consumer goods: water, gas, fluids in general and, in particular, “active electric energy meters and measuring transformers”, which are identified in the directive by MI-003
The directive states that the measuring instrument must conform to 'the particular requirements which are applicable to the instruments in question'; for the active electric energy meters, the annex defines specific requirements in terms of: accuracy, operating conditions, maximum tolerated errors, procedures for ascertaining conformity
The directive applies to all-electric energy meters, whether they belong to the utility company or to private individuals, that are installed for any reason in plants for measuring and/or metering electric energy; it is also specified that the meters can be used in combination with external transformers
The significance of the directive is considerable, not only because it proposes eliminating all unreliable measuring instruments that have not been constructed in conformity to the product standard and sometimes do not even have CE marking, but also because it enables instrumentation to be used (provided that it conforms to the directive) also to meter energy for tax purposes.
Reference: Practical guide to electrical measurements in low voltage switchboards by ABB