ERICO Practical Guide to Electrical Grounding - Manual (Page 75)

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INTRODUCTION TO ELECTRONIC
SYSTEM “GROUNDING”
Grounding electronic equipment for personal safety and
clearing of faults is no different than that of any other
equipment. Safe grounding requires fast opening of circuit
breakers or fuses and minimization of voltage differences
between exposed metal surfaces on all of the involved
electrical system and equipment, to levels that are safe for
people.
What makes electronic systems different is the sensitivity of
their circuit components to relatively small transient
currents and voltages. It is also inherent in the nature of
solid state devices to be very fast, so they are affected by
equally “fast” electrical disturbances. Even lightning is a
slow transient compared to the response of almost any
electronic device. Typical threats to proper operation of
electronic devices and systems include:
1. Lightning - Direct strikes, but the effects also
include overhead cloud-to-cloud, and nearby strikes
causing induced voltages
2. Switching transients from power network
operations and power factor capacitor switching,
lightning arrestor operation, and fault clearing
activities- especially on nearby power circuits.
3. Static electricity - Directly applied arcs to the
equipment, but sometimes arcs near the equipment
will also affect the equipment.
4. Electrical fast transients - Typically as caused by
arcing contacts or collapsing magnetic fields in the
coils of contactors in equipment - usually very near
the affected equipment
BASICS OF TRANSIENT
PROBLEM SOLVING
Solving transient problems is never easy. They may be
random or repetitive. In general, they have waveshapes
which are not easily analyzed. Transients though are
capable of being tamed by:
1. Limiting overvoltages (surge voltages) on the ac
power conductors with surge protective devices
(SPDs)
2. Reducing the chances of electrical noise getting on
power circuits connected to electronic equipment
and the data signal circuit cables that interconnect
the units of equipment. This can often be
accomplished by observing the requirements for
proper routing and grounding of branch circuits
including their conduits, and ensuring adequate
separation of power and data signal wiring.
3. Proper grounding involving the correct installation
of equipment grounding conductors of all types,
and neutral terminal grounding and bonding at the
service entrance and for separately derived ac
systems.
While the above are all within the scope of the contractors’
job, we want to emphasize that the equipment supplier can
and must provide equipment that can “live within” practical
levels of transients as are known to exist on the typical
commercial and industrial site. Otherwise, extensive effort
and great expenditures may be needed in order to get this
kind of too-sensitive equipment to work in an acceptable
way.
The kinds of power and data Surge Protective Devices,
(SPDs) that are needed for the control of transients are
discussed in Chapter 7 and will not be discussed in detail
in this chapter. It is assumed that properly selected devices
are used where we suggest one be applied.
INTERCONNECTED ELECTRONIC
EQUIPMENT SYSTEMS
This section deals with grounding of electronic systems that
are interconnected by signal, data, or telecommunications
cables. It is helpful to think in terms of two kinds of
grounding with this kind of equipment:
1. Safety grounding for fire and personnel protection.
This kind of grounding also helps to provide for the
protection of equipment to minimize damage from
electrical system faults and transients such as
lightning.
2. Performance grounding for the protection of data
circuits and solid-state components within various
items of interconnected equipment making up an
electronic system. Sometimes this is called
“computer” or “electronic” grounding but these are
not very accurate terms. Note that the protection of
data circuits does not have to involve earth grounding
electrode connections, although good grounding to
the building service equipment’s grounding electrode
system makes this protection a lot easier.
For example and as mentioned above, airplanes flying
through lightning storms have no earth grounds connected
to them but, while experiencing lightning hits, are probably
safer than many land-based systems. And after a lightning
strike all of the electronic equipment within the aircraft is
expected to continue to work in flawless fashion.
Chapter 4: Transients & Other High Frequency “Grounding” & Bonding
Grounding Book 4/14/99 10/5/99 6:01 PM Page 67 (Black plate)