Introduction
Neutral to ground voltage is a commonly discussed subject anytime power quality issues are the topic. What are neutral to ground voltages? Why do they occur? Why do they receive so much attention, and how can they be mitigated? These questions are all important in the operating environment of today’s sophisticated electronic systems. Understanding neutral to ground voltages is a first step in assuring that modern technology operates reliably and economically.
The Problem Defined
Common mode (CM) voltage is another term popularly applied to the neutral to ground phenomena. Neutral to ground voltage is any potential measured between the neutral (white) conductor and the safety ground (green or conduit) conductor of a building electrical system. This somewhat broad definition means that neutral to ground voltages can occur over a wide range of both frequencies and voltage amplitudes – a fact that is quickly substantiated after only a few observations.
The characteristics of neutral to ground voltage in any given environment are often unpredictable and as dynamic as the electrical environment itself. Neutral to ground voltages occur for several reasons. This discussion will examine three explanations.
Theory and Reality Collide
As is often the case, reality doesn’t quite agree with theory. The previous example assumes the electrical system is powering loads that are linear in nature, that the system is resistive in nature, that the system is operating at unity power factor, and further, that the system operates in a state of complete equilibrium. In the real world, three phase systems are not as tidy. While electricians do their best to assure that currents in each leg are equal, it is not realistically possible to perfectly balance any three-phase system.
Elevators, compressors, and air handlers cycle in their operation. Computers, lights, copy machines, etc. are constantly being turned on and off. These changing conditions create imbalances in the system. The electrical environment is very dynamic and guaranteed to make a balanced three-phase system an electrician’s “Holy Grail”.
As soon as currents become unbalanced in the example of Figure 1, phase currents no longer cancel and neutral current begins to flow. At this point, the laws of physics take over. As return current flows through the impedance of the neutral conductor, a voltage occurs. Since no return current occurs in the safety ground, a comparable voltage drop does not occur in safety ground. The result is neutral to ground voltage flow and the impedance of the neutral conductor.
