Differential Mode (Floating Neutral or Ground-Neutral Voltage)

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A major source of measurement error is coming right from the Wall Outlet.  Differential Mode Noise or Floating Neutral Voltages are when the Common and the Ground are not at the same potential.  In an ideal case, the Ground and the Common would measure zero volts right at the outlet.  The reality, in most large buildings it is far from it.  By code, the Ground and the Common cannot be tied together at the outlet.  Instead they are tied together at the main electrical panel which could be several hundred feet away.  This allows the Neutral to develop a voltage relative to the Ground.  But why should this be if the Ground and the Neutral are the same length?  The simple answer, electrically they're not.  A Ground is the combination of the wire conductor, the BX casing, the EMT conduit, the Earth, etc. making the Ground less resistive than the Neutral.  The differential of resistances between Ground and Neutral is what induces the Ground-Neutral Voltage. 

So how bad could it be?  It depends on what you were doing.  If you were to have a Ground-Neutral Voltage above 1.0 VAC your communications adapter (USB, GPIB, Ethernet, etc.) might hang up not knowing what logic zero was.  If you were talking current measurements you'd probably see an offset, or currents that were bouncing around, and/or noise.  Most measurement equipment vendor recommend not using the equipment with Ground-Neutral Voltages above 1.0 VAC.  In some cases you could void your equipment warrantees.  In addition, most calibration services certify the equipment to be used within a certain Ground-Neutral range, most being below 1.0 VAC. 

Below are some graphs of an outlet in my work area.  This outlet is shared with a air compressor I use for air tools.  If you were looking at the Ground to Line voltages you wouldn't know anything was wrong.  But a closer look at the Ground to Neutral voltage shows it is elevated about 1.0 VAC.  In addition, you can see something riding on the line AC.  So as an experiment I set up a simple circuit consisting of a 555 timer, a couple of gates, and a LED to see what would happen if this output was used to power an analog/digital circuit.  Using clean power the LED flashed on-and-off as the circuit was designed to do.  Using the outlet with the elevated Ground-Neutral the LED never turned off, the voltage at the LED was sitting on an 0.8V offset.   

The good news, there's a fix.  An Isolation Transformer ties the Ground and the Neutral together at its secondary.  This has several benefits; one is to electrically isolate itself from the building Ground which could be noisy, it also reduces the Ground-Neutral voltages because they're essentially tied together at the secondary rather than at the building's electrical panel, and depending on which type of Isolation Transformer you get Common-Mode Noise would be reduced.  These transformers are basically just a 1:1 windings with an extra dielectric barrier and an electrostatic shield between primary and secondary. The grounded shield prevents capacitive coupling between primary and secondary windings which helps eliminate Common-Mode Noise.  When I get a minute I'll go into what Common-Mode Noise is all about.  Please take note, not all Isolation Transformers are wired the same.  Some actually tie the Earth-Ground from the primary and the secondary together.  Please be sure you know what you are buying and what your needs are. 

Aside from measurement accuracy there are several other reasons why you would want to use Isolation Transformers.  One would be to protect users from faulty equipment.  Could you imagine being hooked up to electrodes in the hospital being directly tied to building Ground?!?!  Ground looping is another which will be covered in the next article.