ERICO Practical Guide to Electrical Grounding - Manual (Page 77)

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much as 200% of the ampacity used for the related line
conductors. This is regularly done in order that a fire be
avoided due to current overload from third harmonic and
other odd multiple harmonics called “triplens”.
Other steps may be required to prevent harmonics from
interfering with proper system operation. However, the
exact method and point chosen for grounding of the neutral
conductor at the ac supply source, will not improve any
problems associated with harmonics. Ungrounding of the
neutral is likely to be an NEC violation in almost all
designs, and would decrease personnel safety. Solving
problems related to harmonics is beyond the scope of this
book, however something can be said in this regard.
HARMONIC CURRENT FILTERS
(TRAPS)
Harmonic filters commonly called “traps” are not
grounding problems unless they are miswired to direct the
current through them into the equipment grounding system.
This is an unusual situation and involves an NEC violation
which would need correction. Typically, the trap is
connected line-to-line, line-to-neutral, or both, but never to
equipment or any other ground.
SURGE PROTECTIVE DEVICES (SPDs)
AND GROUNDING CONNECTIONS
In addition to line-to-line and line-to-neutral connections,
surge protective devices (SPDs) are also connected to the
circuit’s equipment grounding conductor. Any transient
voltage which then operates the SPD and causes current
flow through it and to the equipment grounding conductor,
raises the ground potential as measured at the installation
point of the SPD and to the remote “ground” used as a zero
voltage reference. Because SPDs may be subject to very
high voltages with steep (e.g., fast rise time) wavefronts,
the concurrent effects on the grounding system may be
very severe.
SOME PRACTICAL
RECOMMENDATIONS
These are some of the practical electrical installation
considerations we recommend:
(1) Field installed electrical grounding/bonding
conductors routed between the metal frame or
enclosures of separate units of electronic equipment
should be connected to the NEC “green wire”
grounding system at both ends, not isolated or
insulated from it.
(2) Isolation transformers with electrostatic shielding
between the windings are readily available and
should be employed to interface the electrical
system to the panelboard used to supply branch
circuit power to the electronic equipment. The
installation of both the transformer and
panelboard(s) should occur as physically close to
the served electronic equipment as is possible. Note
that the electrostatic shielding can provide useful
attenuation of most types of common mode
transients up to about 1000:1 (e.g., -60 dB).
Attenuation figures above this value are generally
unrealistic and are not likely to be provided by a
transformer that is installed into a real-world instal-
lation and in conformance with the NEC. In any
case, follow the transformer manufacturer’s
recommendations closely to achieve the maximum
benefit, but only if the instructions conform to the
NEC.
(3) Interconnecting cables between electronic system
enclosures in equipment rooms should be routed in
close proximity to the structural subfloor. This is
especially the case if it contains substantial metal
structures that are well grounded such as steel
decking, etc. The best results however, are obtained
when these cables are laid in close proximity to a
specially installed signal reference grid, such as is
recommended to be installed under a raised floor
normally used in a computer room. If intercon-
necting cables are routed between locations in a
cable tray or wireway, then the use of random lay is
preferred rather than “neat” bundling in these forms
of raceway. (This is recommended as random lay
decreases the coupling of noise from one adjacent
conductor into the other when they are laid parallel
to one another for any significant length.)
(4) If wireways are used to route cables, they should be
made from metal, be well and continuously
grounded and bonded, and be equipped with a tight
cover such as one fastened by screws. Ladder tray is
less desirable than solid-bottom tray.
(5) Field installed data cables should normally be
separated from power cables and conduits to the
greatest practical distance. This reduces unwanted
coupling between the two circuits. To avoid noise
coupling problems where one circuit crosses over or
under the other, try to make the crossover at right-
angles.
(6) Where metal raceways or conduits are used to
contain interconnecting data cables, it is
recommended that additional bonding connections
be made at several points along their entire length
Chapter 4: Transients & Other High Frequency “Grounding” & Bonding
Grounding Book 4/14/99 10/5/99 6:01 PM Page 69 (Black plate)