ERICO Practical Guide to Electrical Grounding - Manual (Page 79)

an isolation transformer is installed adjacent to the
served loads and is positioned between the output of
a transfer switch and the input of the served
electronic loads. In these cases the neutral terminal
on the secondary of the isolation transformer is
solidly grounded and both the transformer and
electronic load equipment are made common to one
another for broadband grounding purposes, if they
are also connected to an SRG that has been installed
in the equipment room and just beneath the
equipment.
(18) More than one isolation transformer may be used in
the above manner if the site is large. For example,
multiple isolation transformers installed and
grounded to an SRG in an equipment room are a
recommended practice for larger sites. Also,
multiple, separated, but SRG equipped rooms may
each be provided with its own isolation transformer
and grounded as above.
(19) Specially designed, “original” forms of grounding
which are not in literal compliance with NEC
requirements are not recommended. This includes
approaches to grounding called “clean”,
“dedicated”, “single point” and other forms of
“isolated” grounding not permitted by the NEC.
The authors are aware of instances where all
grounds are initially properly connected together
with a jumper which the owner or operator can later
remove at his discretion. Since removal of this
connection creates both an NEC violation and
fire/shock safety hazard, the authors do not
recommend this approach!
(20) Surge Protective Devices (SPDs) are described in
Chapter 7. SPDs provide overvoltage protection at
various points for power and data circuits wherever
they are properly applied. Proper use of SPDs is
highly recommended.
(21) After the electrical installation is complete, a
careful inspection of the wiring is needed to ensure
safety and performance criteria have all been met.
Regarding grounding, the following should be part
of the inspection process:
(a) Misidentification of conductors such as the
neutral and “green wire” safety grounding
conductors, often occurs. The problem shows
up at the point where they terminate. A
mistake of this kind is a serious violation of
NEC Section 250-21, and others. Cross-
connection between neutral and ground
conductors results in unwanted current flow in
the equipment grounding system, but will
normally not cause an overcurrent protection
device to operate. Hence, there is often no
immediate indication of a problem such as
when the power is first applied. Therefore,
these conductors and connections need to be
verified before power is applied.
(b) All metallic conduit, wireway, raceway and
other metallic enclosures, must be well-
bonded along their length to ensure end to end
continuity. They should also be well grounded
at multiple points along their length to
building steel and SRGs within 6 feet to
provide effective high frequency grounding.
Effectively grounded, end terminations to and
from served equipment are most important.
(c) Ensure that the shortest possible lead length
has been used to connect SPDs to the
conductors they are protecting. Ideally, the
SPD would be mounted directly on or inside
the equipment it protects. External mounting
in a separate enclosure and a conduit
connection to the protected equipment creates
longer distances between the SPD and the load
it protects. This decreases the effectiveness of
the protection.
(d) Any connection that is not a good electrical
connection over the life of the installation is
potential trouble. Such a poor connection can
be a cause of noise or of a total interruption of
the signal process or power continuity. Either
a connection is made properly, or it must be
reworked to bring it within specifications.
71 Chapter 4: Transients & Other High Frequency “Grounding” & Bonding
Grounding Book 4/14/99 10/5/99 6:01 PM Page 71 (Black plate)