Modern measurement techniques for verifying electricity meters and measuring loads on site
This paper provides an insight into the measurement techniques available today.
Measurements in situ are carried out for various reasons, and this article looks at available means according to user groups. We must establish whether the measurement will simply be carried out on the basis of a summary verification of the meter and its connections, or whether it will be complete verification at many load points, as in a calibration. It is obviously not necessary to use expensive precision equipment to test a standard household electricity meter of class 2, for example
Two basic types of procedure are used for on-site measurements. The first uses a reference standard meter to carry out a verification, using the existing customer load. This implies that measurements can only be carried out at values which are defined by the at-the-time loading which may or may not be typical values for the user. The second procedure involves a power source a phantom load to create the several desired load points at which the meter will be tested. In this case, the meter can be tested under exact, defined conditions.
Treatment of customer complaints
The proportion of a family’s income which is spent on energy is growing. This is largely due to the increasing number of comfort units available today, such as microwave ovens and clothes’ dryers. The energy they consume is often unknown, or not considered, and the resulting high billing is often blamed on a defective meter. In such cases it is usually enough for the electricity authority, using straightforward and uncomplicated measurements, to show the user that his installation is within the required limits of accuracy.
Various types of inexpensive measurement equipment exist to perform this function reference standard meters, power sources, or a combination of both within a single unit. They are characterised by their small dimensions and low weight, unlike the equipment of the past. Some earlier units were limited to a single load point for example, using a one kilowatt heater and measurements were carried out for a limited number of turns of the meter disc. The meter was judged accurate by such limited means!
Measurements using the available on-site loading
In domestic installations it is usual to find both single-phase and polyphase meters with currents limited to 60 A, or exceptionally to 80 A or even 100 A. The techniques require that measurements are carried out with the standard voltage, +- 15% and from 5% of the nominal current up to the maximum current value. It is also important to know whether the meter is running under no-load conditions.
Such meters are of accuracy class 2, although national meter regulations often allow the meters to be double the calibration error limit.  If it is necessary to carry out a measurement of these meters within the usual technical norms, a reference standard to the accuracy of 0.5% or 0.2% is required.
Many countries do not allow a meter to be calibrated outside the state authorised test laboratory. It is possible, however, to carry out a series of measurements with a reference standard of the required precision, in order to decide whether the meter reaches the precision defined in the local regulations.If it does not, or if the user contests the results, the meter can be removed for extensive examination. In this case, the results of laboratory tests will probably be identical to those found in the field.
Reference standards are offered in several models. Some can be connected directly to the meter via the voltage terminals, after removing the covers. The current circuits are opened and the reference standard is inserted in series into the current circuit. The mains power supply fuses must be removed beforehand to ensure the operator’s security.
Measurements can start immediately after the connection to the supply. To carry out measurements at various load values, different domestic appliances can be switched on and off. Meter error is calculated by using a scanning head to detect meter revolutions or impulses, or by manual operation of a hand switch. The display of the reference standard will show the required values, which could include the voltages, the currents, the power and power factor and the error of the meter being tested.
An alternative procedure uses a reference standard which operates with transformer clamps in the current circuit. The great advantage is that it is unnecessary to open the current circuits. While error measurement using the current clamps is not as accurate as measurement using a direct connection, since the transformer clamps themselves have their own inherent error, the results in most cases reach the required level of accuracy.
A third option involves the measurement range adapters on transformer connected meters. Measurement transformer meters are often installed in such a way that a secondary circuit of the current transformer can be opened, allowing the higher measurement accuracy of the directly connected meter to be used.
There is a unit which combines the advantages of the units discussed above. It can be used for measurements with direct current connections in the range of 10 mA up to 10 A. In addition, further measurements may be made using error compensated transformer clamps in the range of 50 mA to 100 A. This unit weighs less than 2.5kg, including the current clamps and scanning head.
It can be used for a whole series of measurement operations: measurements of active power, reactive power, apparent power, voltage, current, power factor, energy measurements for register tests and error measurements for calculation of the relative error of the meter under test. The measurement precision lies within 0.2% or 0.5% accuracy.
Measurements with defined loads
In certain cases it is important to test the meter under defined load conditions, or to be able to define its complete load curve. Here it is necessary to combine the use of the reference standard with a power source – a load transformer. A single phase unit already exists, and the three phase version will soon be available.