## 1 Introduction
For example, the:  
* Effective value of current and voltage,
* Active, reactive, and apparent power and energy,
* Frequency,
* Power factors: cos\(\phi\), tg\(\phi\), or \(\lambda\),
* rms value of given harmonics for voltage and current,
* Total harmonic distortion (THD) coefficients for voltage and current. In [15], the results of an interesting experiment relating to the designation of power quality parameters were shown. Ten instruments used in the experiment (Table 1), owned by various institutions, measured the same set of parameters describing the electrical power quality. \"Own design\" means the prototype instrument, developed in the scientific institution and taking part in the experiment. The measurements included the network frequency, voltage rms values, total harmonic distortion, rms values of the dominant harmonics, and also the voltage fluctuation parameters: short and long term flicker severity. The measurements were carried out at bus-bars of the medium voltage 6 kV, by means of voltage transformers. A similar set of ten instruments was used for the following measurements [16]. A schematic diagram of the measurement site is shown in Figure 1. The measurement points were located on the secondary side of a supply transformer, at the substation denoted as GSTS-30 kV (phase-to-phase voltage level \(U_{rms}\) = 30 kV). Ten measuring instruments were connected via the voltage transformers. All instruments used in both experiments were declared as complying with the terms of the respective IEC 61000-4-15 standards [17], according to the statements of the designers and manufacturers. Each of the two measurements was carried out by independent experiments. They were held at the same point of the supply network at the same time, for each experiment. Although the instruments measured the same voltage signals at the same time, their indications differed significantly, independently for each experiment. In [18], the analysis of possible reasons for the different indications of instruments observed in the aforementioned experiments is presented. As a source error, the authors of the paper indicate the human factor (erroneous set-up of the instrument's parameters) or design drawbacks of the manufacturer's equipment. After an estimation of the relations between the results obtained during the measurements, they formulate the simplified linear formulas showing the dependencies between the obtained results and bring the result sets from all of the instruments to a common course, within the accepted range of deviations. They concluded that the reason for the deviations in the measurements results could have originated in variable amplification and could be due to the constant offset of the measurement characteristics of the individual instruments. \begin{table}
\begin{tabular}{c l l} \hline
**No.** & **Type of Instrument** & **Owner** \\ \hline
1 & Panensa MEF & Technical University of Radom, Radom, Poland \\
2 & Memobox 686 & Technical University of Radom, Radom, Poland \\
3 & Own design & Technical University of Lodz, Lodz, Poland \\
4 & Own design & Electrotechnical Institute, Gdansk, Poland \\
5 & Memobox 800 & Semicon, Warszawa, Poland \\
6 & Power recorder 1650 & University of Mining and Metallurgy, Krakow, Poland \\
7 & Own design & University of Mining and Metallurgy, Krakow, Poland \\
8 & Own design & Institute of Power Engineering, Katowice, Poland \\
9 & Memobox 686 & Energopomiar-Elektryka, Gliwice, Poland \\
10 & Own design & Gdynia Maritime Academy, Gdynia, Poland \\ \hline \end{tabular}
\end{table} 
Table 1: List of instruments used in the measurements [15]. Figure 1: Schematic diagram of the measurement site in the electric steelworks power supply system [16]. The description of effects related to the deviations of the characteristics of the measurement channel (i.e., lost performance or improper calibration) of various devices, causing the deterioration of the operation performance, can be found, e.g., in [19; 20].