Most people believe that “power problems” start at the power company or within the transmission network. It’s true that brownouts do occur and cars occasionally careen off into a power pole; however, in the grand scheme of things, this is super rare. The most common power issue is caused by neutral to ground voltage and it’s coming from inside your facility. So what exactly is it? Where does it come from and how can we prevent it? Let’s get into the details.

Defining the Problem

Neutral to ground voltage is most often called Common Mode (CM) Voltage.  It’s measured between the neutral (white) conductor and safety ground (green or conduit) conductor of the electrical system. Common mode voltage can occur over a wide range of both frequencies and voltages. Neutral to ground events can cause some really serious disruption to the operation of microprocessor based equipment. In the old days, microprocessors used to be fed by large linear power supplies that did a fantastic job of eliminating Common Mode voltage. The tiny switch mode power supplies of today are great at regulating voltage but do very little to suppress Common Mode voltage. Microprocessors are constantly measuring logic voltages against the “zero voltage reference” of safety ground. Since all of a computer’s decisions are the result of discriminating one rapid changing voltage from another, ultra-clean and quiet electrical safety grounds are essential. The microprocessor expects to see very low (less than .5 volts) of neutral to ground voltage. When common mode voltages get out of this range you’ll see system lockups, communication errors, reduced operating throughput, unreliable test data, fragmented hard drives, and operational problems that cannot be explained or duplicated. Software developers and equipment manufacturers get fingers pointed at them, but the facility power is the source. Let’s look at from where in the facility these transients are coming.

Shared Neutral Conductors

Electrical Codes, let electricians “share” the neutral conductor… so they do. This practice allows a neutral conductor to serve three different circuits. On paper this look like the voltages would cancel out and everything would work in a state of total equilibrium. In real life, three-phase systems are not so tidy. Electricians may do their best to try and balance the currents in each leg, but it is nearly impossible to balance correctly.

Equipment like elevators, compressor and air handlers cycle in their operation while computers, lights, copy machines etc. are continuously turned on and off. These changing conditions create imbalances in the system. An electrical environment is very active and is guaranteed to make the balanced math fall apart.

So, what we get is neutral to ground voltage flow.

Load Balancing Difficulties

While changing load conditions make load balancing difficult, all the switch mode power consuming current in nonlinear “gulps’ from the power line makes it even worse. Even if an electrician managed to balance all three RMS phase currents, he will discover that current is still flowing in the neutral conductor.

This circumstance will occur in a modern facility even when good wiring practice and load balancing techniques have been observed.

Branch Circuit Length

Sue’s blood gas analyzer is on the opposite side of the building from the electrical panel. The 240V branch circuit feeding his device shares a neutral conductor with the refrigerator in the break room. Every time that DC compressor motor kicks on, a frenzy of transients are transmitted back to her Analyzer. The additional circuit impedance of the long branch circuit makes Sue’s issues even worse. Sue gets inconsistent results and blames the OEM. Three months later, the support staff is pulling their hair out trying to figure out the issue.

Induced/Conducted Voltages.

An induced disturbance happens through electromagnetic fields. That fancy inductive iPhone charger of yours is creating an electromagnetic field. Lightning, close physical proximity to motors or other devices with electrical windings can all cause issues. The common mode voltage disturbances that affect systems are produced by the systems themselves. “It’s coming from inside the house!”

Personal computers, copy machines, fax machines, laser printers, medical instrumentation, telephone switches, the point of sale systems etc. all are contributors to this effect of conducted neutral to ground voltages.

So what do we do about it?

Finding Solutions

Microprocessors are getting smaller, more sensititve and ubiquitous. Reduce the impact of common mode voltage is imperative. Here’s how to do it:

  • Use oversized conductors to lower impedance
  • Run individual neutral conductors to each circuit
  • Perfectly balance each circuit


  • Use an Isolation transformer at the point of use

The most effective tool for control of neutral to ground and common mode disturbances is an isolation transformer. These allow the bonding of neutral to ground on the transformer secondary. That just means there’s full isolation from the building’s electrical system. This creates predictable impedance (almost zero) it is impossible to cause the voltage drop associated with long branch circuits. Isolation transformers eliminate the problems associated with common mode voltage. Service calls are reduced, uptime is increased and users are happy.


This is why there’s an isolation transformer in most every device we carry.


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