Energy meters assume the great importance with the ever increasing energy prices. Any fault in energy registration will lead to a loss either to the consumer or to the electricity supplier. Latest development in the electronics has significantly improved the metrological performance of the meters but functional expectation & data reliance on the meter has increased many folds. At the same time the electrical environment under which meter is operating has worsen also. Thus to ensure the quality, reliability and to check proper compliance of the meter with international standard IEC & IS, an accredited laboratory has to follow the stringent requirement of Calibration & Testing Laboratory with standards confirming to the state of art technology.
This paper describes the laboratory experience and what need to be done to maintain various standards & test facilities to check the compliance of Static Energy Meter.
1.0 Important consideration for an accredited lab
Various environmental conditions are playing important role in the laboratory while calibrating Static Energy Meters. Figures given below in various reference conditions are as per Indian standards.
Temperature & Relative Humidity
The environmental conditions maintained in the laboratory shall be such that it does not adversely affect the required accuracy of measurement. Laboratory should maintain temperature 25 ± 2.5°C and Relative Humidity should be in between 45% to 65%. To maintain the above temperature & Relative Humidity laboratory should have good hygrometer with one degree resolution. It should be properly calibrated in between 22°C to 28°C. Specifically Alcohol based glass thermometer gives very fast response to change in temperature & easier to read due to red colour.
Sound & Vibration
The calibration area should be adequately free from vibrations generated by central air conditioning plants, vehicular traffic and other sources to ensure consistent and uniform operating conditions .Continuous vibration in the lab leads to malfunctioning of the sensors.
Acoustic noise level in the laboratory should be maintained to facilitate proper performance of calibration work. A threshold noise level of 60 dB is recommended.
The calibration & testing area should have adequate level of illumination. The fluorescent lighting is preferred in order to avoid localized heating and temperature drift. The recommended level of illumination is 450-700 lux on the working table.
Various mains supply (input supply to equipment) parameters are affecting performance of test equipment and measurement uncertainty.
Line Voltage: 230V ± 1.0 % or better for calibration bench
Frequency: 50 Hz ± 0.1 Hz
Better power supply condition can be achieved by using dedicated power supply from transformer to test equipment. All other heavy duty machinery, non-linear load, air conditioners should be connected on the separated.
Keeping the mains supply frequency different (e.g. 52Hz) then test output frequency (e.g. 50 Hz) will help to
- Improve the setting accuracy of parameter (specially frequency)
- Minimize the swinging(hunting) effect
Poor design of the output transformers (or amplifiers) generates the harmonics or distortion or DC component in the output circuit of the test voltage & current circuit. In the transformer operated test bench (if operates directly on mains supply), input supply harmonics/distortion will not be filtered out and transferred to test output circuit. These harmonics will result in a wrong calibration results.
Energy Meter Test bench should not have distortion in voltage circuit more than 0.5%. This should be measured by Harmonic Analyzer.
Spikes & Surges
The spikes and surges over a period of time damage the insulation of the test bench which may cause small leakages & can lead to wrong calibration in extreme cases. A proper filtering/protection from these spikes, surges, harmonics, transients, and swells need to be taken care at the input side.
All equipment should be properly connected to effective earth and all should be at the same level/point in order to avoid loop current. The guard terminal of the instrument should not be used as an earth point for other instruments. The earth resistance value should be always better than 0.1W. Voltage between earth & neutral should be less than 1 volt. This should be periodically checked & stray couplings should be minimized.
All meters under test and reference standard should be switched on at reference voltage & at laboratory temperature. Duration of warm-up should be minimum for 1 hour or as specified in respective technical specification of the instrument. This will be necessary to bring all devices in thermal equilibrium state and helps to achieve consistent measuring results.
Suitable action need to be taken, if reference standard was already used in previous testing and meter under test are newly connected which are under cold state. This can also cause difference in error due to different thermal equilibrium states of both devices.
Test Uncertainty Ratio (TUR)
The test uncertainty ratio =
The TUR should be 1:10 or if not possible then at least 1:5. This can be achieved by using high precision reference standard and other components which are contributing in the overall measurement uncertainty.
All measurements are affected by SWIPE
S = Standard Energy Meter (accuracy)
W = Work Piece (resolution)
I = Instrument (other instrument in the measurement process)
P = Process of Measurement
E = Environment of laboratory
Total uncertainty of measurement can be reduced by monitoring SWIPE. This also decides the TUR.
Laboratory accreditation provides a means of evaluating the competence of laboratories to perform specific type of testing, measurement & calibration. It enhance the technical competence of laboratory and provides systematic feed backs (internal/external) for further improvements. Manufacturing organization may also use laboratory accreditation to enhance the testing of their products by their own in-house laboratories.
Laboratory accreditation provides formal recognition to laboratories for their technical competency & providing a ready means for customers to find reliable testing & calibration services which are able to meet their needs.
Criteria for Selection of the Laboratory
Throughout the world today, customer seeks reassurance that the products or materials they purchase meet their expectations or confirm to specific requirements. This often means that the product needs to be tested or measured by a laboratory to determine its characteristics against a standard or specification.
It is crucial that the laboratory performing these tests or calibrations is able to provide accurate & reliable data on the product. This depends on the laboratory having:
- Qualified & Experienced Staff
- Properly Calibrated Testing /Measuring Instruments
- Traceability of Measurements to National /International Standards of SI Units
- An adequate Quality Assurance System
- Proper Sampling Procedure
- Valid Test methods
- Accurate Recording & Reporting Procedures
- Suitable testing facilities
- Proper Accommodation & Environment
All above factors contribute to a laboratory’s technical competence to do your testing or calibration.
Mr Pradeep Gujarathi is well known metrologist, scientist, NABL Lead auditor in India. He is having more than 25 years of experience in the field of testing & calibration of various products including energy meters. He is representing MSME(formerly known as IDEMI) at various technical & regulatory committees such as BIS & MERC.
IDEMI is Govt. of India Society of Micro, Small and Medium Enterprises. The aim of the Institute is to provide qualitative services to Micro Scale, Small Scale, and Medium Scale Industries. IDEMI has been rendering services in the following areas.
- Calibration & Testing of Electrical Measuring Instruments
- Process Control Instruments
- Calibration of Mechanical Instruments
- Also the testing of electrically operated Domestic appliances.
The IDEMI is also engaged in Design, Development and Batch production of Transducers, Microprocessor based Equipment and Critical Instrumentation. Tool Design & Manufacturing of Mechanical Components, Press Tools, Jigs & Fixtures, Die Casting dies, Prototype Fabrication. Also undertakes manufacturing of Critical components and Sheet-Metal components. Technical training for wide range of practicing professionals from Industries and Laboratories on Instrumentation and Calibration. National Seminars, Workshops on Electrical, Electronic and Process Control Instrumentation. Special courses on Laboratory Quality System, Management and Internal Audit for Laboratories