IEEE Std C37.26-2014 pdf download – IEEE Guide for Methods of Power-F actor Measurement for Low-Voltage (1000 V AC or lower) Inductive Test Circuits.
4.1 General Devices such as current-limiting fuses, fused circuit breakers, and similar fast-clearing devices may have total interrupting times of 0.5 cycles or less. The ratio method permits measurement to be made within the operating time of these devices. The ratio method is generally not suitable for use on test circuits with a power factor above 30%. Because this method requires closing the test circuit to produce the maximum current asymmetry, the resulting high electrical loading and mechanical forces on the bus supports and circuit components may jeopardize the test plant equipment. In such cases, the test laboratory may request to determine the power factor of the test circuit using a reduced current level (such as 50% of the rated test current), with the agreement of the manufacturer and the witnessing test inspector. 4.2 Procedures for determining the power factor The power factor of the test circuit is determined at an instant of time one-half cycle (based on the fundamental frequency timing wave) after the initiation of the test current flow by determining the symmetrical and asymmetrical (total) currents at this point of time (see Figure 1, Figure 2, and Table 2). Both the rms symmetrical current and the total rms asymmetrical current are to be measured, and the ratio MA (for three-phase test circuits) or MM (for single-phase test circuits) is to be calculated as follows:a) Construct the envelope of the test current wave as shown in Figure 1. b) Construct a vertical line at the one-half cycle time after the initation of the test current. c) Measure the deflection of the trace from the zero line to the maximum point of the envelope at the one-half cycle line (this is the value shown as A’). d) Measure the deflection of the trace from the zero line to the minimum point of the envelope at the one-half cycle line (this is the value shown as B’). e) Calculate the value for the rms symmetrical current, I, using Equation (1). . f) Calculate the value for the rms asymmetrical current, I’, using Equation (2).