Page 51 - Asterisk™: The Future of Telephony

relation to something, we are in danger of assuming things that may not be so. If you
measure the voltage between two grounding references, you’ll often find that there is
a voltage potential between them. This voltage potential between grounding points can
be significant enough to cause logic errors—or even damage—in a system where more
than one path to ground is present.

One of the authors recalls once frying a sound card he was trying to
connect to a friend’s stereo system. Even though both the computer and
the stereo were in the same room, more than 6 volts of difference was
measured between the ground conductors of the two electrical outlets
they were plugged into! The wire between the stereo and the PC (by way
of the sound card) provided a path that the voltage eagerly followed,
thus frying a sound card that was not designed to handle that much
current on its signal leads. Connecting both the PC and the stereo to the
same outlet fixed the problem.

When considering electrical regulations, the purpose of a ground is primarily human
safety. In a computer, the ground is used as a 0V logic reference. An electrical system
that provides proper safety will not always provide a proper logic reference—in fact,
the goals of safety and power quality are sometimes in disagreement. Naturally, when
a choice must be made, safety has to take precedence.

Since the difference between a binary zero and a binary one is represen-
ted in computers by voltage differences of sometimes less than 3V, it is
entirely possible for unstable power conditions caused by poor ground-
ing or electrical noise to cause all kinds of intermittent system problems.
Some power and grounding advocates estimate that more than 80 per-
cent of unexplained computer glitches can be traced to power quality.
Most of us blame Microsoft.

Modern switching power supplies are somewhat isolated from power quality issues,
but any high-performance system will always benefit from a well-designed power en-
vironment. In mainframes, proprietary PBXes, and other expensive computing
platforms, the grounding of the system is never left to chance. The electronics and
frames of these systems are always provided with a dedicated ground that does not
depend on the safety grounds supplied with the electrical feed.
Regardless of how much you are willing to invest in grounding, when you specify the
electrical supply to any PBX, ensure that the electrical circuit is completely dedicated
to your system (as discussed in the next section) and that an insulated, isolated ground-
ing conductor is provided. This can be expensive to provision, but it will contribute
greatly to a quality power environment for your system. ‖

It is also vital that each and every peripheral you connect to your system be connected
to the same electrical receptacle (or, more specifically, the same ground reference). This

Environment | 23