Common Mode Voltage Problems
Traditionally, this power virus hasn’t received much attention.
But detection of common mode voltage problems is easier and, as a result, more system problems are being traced to its existence.
The condition is characterized by unwanted voltage measured between neutral and ground (the white wire and the green wire or conduit) in the electrical system.
In fact, the common mode voltage virus is probably the most serious power virus infecting electronic systems today.
It occurs as a result of high impedance safety grounds, neutral conductors shared with other circuits, and branch circuit lengths that are excessive.
When the electrical noise virus (already mentioned) appears between the neutral and ground conductors it becomes a common mode virus with the ability to cause lost files, system lockups or re-boots, communication errors, and “no problem found” service calls.
This virus is characterized by abnormal variations in the electrical circuit’s nominal operating voltage (120 volts, for example). These variations are generally greater than ±10 percent of nominal voltage and may last for several line cycles or more.
Traditionally this virus has been referred to as the “sag” or “swell,” and it is typically caused by large loads turning on and off, and overloaded branch circuits or distribution transformers. In some cases, voltage regulation viruses can be the responsibility of the power utility.
If an electronic system requires tightly regulated voltage (most of today’s systems don’t) the voltage regulation virus is likely to cause system lock-ups and unreliable operation in addition to damaged or destroyed components.
Blackouts are the most visible and easily identifiable of all the power viruses. And they have the most obvious cause and effect relationship.
One moment power is present—the next moment it’s not and your system is dead in its tracks as a result. The effects of unanticipated power loss are obvious. This is especially true if the system is a network or some other “fault intolerant” architecture.
Fortunately, in spite of what most UPS manufacturers advertise, blackouts account for comparatively few occurrences of all the power viruses.