(Please refer to ELECTRICAL CONTRACTOR magazine for referenced figures).
On a single-phase circuit, the origin of a voltage sag is usually downstream if the neutral-to-ground voltage significantly increased during the sag. Usually, Neutral-to-Ground swell will be about half of the Line-to-Neutral sag, as sag is the sum of the voltage drops in both the line (hot) and neutral (grounded) wires, whereas the swell shows only in the neutral wire.
The Line (or hot) black wire and Neutral (or grounded) white wire in single-phase circuits are often thought of as having zero impedance. As Figure 2 shows, each wire has an inductance, resistance and capacitance value, which results in a complex impedance that varies over the frequency range. Often at the frequencies of the higher-order harmonics and transients, this impedance can be orders of magnitude larger than at the fundamental power frequency (50 or 60 Hz).
But even at the lower frequencies, the impedance is large enough so that when current increases from loads that turn on periodically, such as the heating element in laser printers and copy machines, a significant voltage drop will be created in the wires from the source to the load, and in the wires back to the source. In Figure 1, the voltage sag is approximately 12 Vrms, with 6 Vrms drop occurring in each wire. This means that the Line and Neutral circuits have the same impedance, which isn’t always true when connections loosen up over time. However, very little, if any, current flows in the Equipment Grounding (green) wire. Hence, there is very little voltage drop across it. If we measure the voltage difference between the Neutral and the Grounding conductors at the load, we will see the voltage swell or increase, with a similar waveshape as the current flowing in the neutral conductor.
Hence, if you can’t measure the current in a single-phase circuit, it is useful to measure the Neutral-to-Ground voltage as a means of determining if the current increased when the sag at the load occurs. If the current goes up significantly (or the L-G voltage increases) when the L-N voltage decreases (or sags), it is most likely caused by a source downstream or away from the electrical supply (or generator). EC
BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at 732.287.3680.