ERICO Practical Guide to Electrical Grounding - Manual (Page 76)

Practical Guide to Electrical Grounding
SOME IMPORTANT POINTS ABOUT
GROUNDING
(1) Typically the safety grounding of equipment is
exactly the same for electronic equipment as it is for
any other kind of apparatus, whether it is a refrig-
erator or a printing press. The “green wire” and
conduit/raceway system’s grounding which is well
documented in the NEC and other codes, defines
these requirements completely. This chapter is not
primarily concerned with this form of grounding.
Safe equipment grounding requires fast clearing of
circuit breakers or fuses and minimization of
voltage differences on exposed metal surfaces of
equipment to levels that are safe for people. This is
called the control of “touch potential.” There is
absolutely no conflict between NEC defined
grounding and the more specialized grounding and
bonding practices described in (2) below. An
unnecessary conflict can be created however, such
as when someone attempts to create a “separate”,
“dedicated” or “clean” grounding connection that is
not permitted by the NEC!
(2) Protection of data circuits generally requires
additional considerations beyond the intent of the
NEC, but not in violation of it. Protection of data
circuits from disruption or even damage does not
always involve grounding,
although good
grounding makes this protection a lot easier.
Aircraft have no earth grounds while they are
flying. The airplane carries its own “grounding”
system for its ac and dc systems, and signal
grounding purposes. This grounding system is
entirely metallic in nature and it is often called a self
contained power and signal reference system, which
is a more accurate description. Even direct lightning
“hits” are not likely to cause equipment damage or
even disruption to signals.
(3) The circuits of most electronic systems are almost
always sensitive to voltages of a few tens of volts
or even to as little as one or two volts. As a result,
these systems are designed with great care to keep
transients out of the actual circuitry and off of the
signal paths between interconnected units of a
system. To accomplish this, some equipment uses
electrostatically shielded isolation transformer
techniques and ac-dc power supplies designed to
reject transients. However, for these techniques to
be fully effective, good grounding and bonding
practices exceeding those required in the NEC,
must often be employed.
(4) Data signals inside most electronic systems consists
of bits of information processed as square waves or
impulses at about 5 volts in amplitude and clock
speeds which can exceed 200 MHz. Data
transferred between equipment often has a
magnitude of 12-18 volts, and the speed of transfer
is lower than that of the signal processing speed
available inside of the equipment. In any case, the
signal rise-times of the clock and most other signal
pulses such as those used to transfer bits, are far
faster than the typical lightning strike. Yet, even at
these speeds the systems can be made to have high
reliability and to be relatively immune to
interference if good grounding and bonding
practices are followed.
(5) Lightning related waveforms are usually the “worst
case” situation for transients on most ac power
system wiring and related grounding systems. This
makes lightning the principal threat. More
information about lightning and its typical
waveforms may be obtained by consulting
ANSI/IEEE Std C62.41-1992
(6) Fast electrical transients are created in some
equipment with electromechanical contactors. The
interference problem from these items could be
serious, but it is easy to solve by installing RC
snubbers (consisting of resistors and capacitors)
across the contacts, coils, or both items of the
offending device. This kind of interference with
electronic circuits can sometimes be controlled by
more stringent shielding, or grounding and bonding
practices. However, the root cause of this kind of
problem is really not a shielding, or grounding and
bonding related problem. Instead it is an equipment
circuit modification problem and this is the kind of
thing which typical electrical contractors should
normally not be expected to identify or to solve.
HARMONICS
Note that by itself, harmonic current and voltage generation
is not a grounding problem unless due to a miswired circuit
or a component’s failure in which some of the harmonic
current gets impressed onto the equipment grounding
system. In this case, the effort is not to stamp-out the
harmonics, but to find the miswire or failed component and
to effect the repair.
Harmonics are often an important safety concern on the
neutral conductor of a three-phase, wye-connected ac
system where it is supporting line-to-neutral connected
nonlinear loads- such as computers, etc. In this case the
entire neutral path must be increased in ampacity to as
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Grounding Book 4/14/99 10/5/99 6:01 PM Page 68 (Black plate)