ERICO Practical Guide to Electrical Grounding - Manual (Page 78)

to ensure good longitudinal coupling. In addition to
being well grounded/bonded to the equipment at the
ends of the run, the conduit or raceway should also
be bonded to any nearby structural steel along the
run.
(7) All metallic piping, ducting, conduit/raceway,
wireway and cable tray located within 6 feet
(horizontal or vertical) of any installed Signal
Reference Grid (SRG) must be bonded to the SRG.
This is especially important where these conductors
enter or leave the area defined by the SRG. If this is
not done, then lightning side flash may occur from
the above or any nearby grounded metal items to the
SRG. A side flash can cause a fire, electronic circuit
damage, or both. More about the subject of side
flash may be obtained by reference to ANSI/NFPA-
780-1995, the National Lightning Protection Code.
(8) In addition to any NEC requirements, the neutral
terminal, such as the Xo terminal on a wye-
secondary connected transformer of a separately
derived system, should be connected to the SRG
and if available, also to the closest building steel.
(9) Be sure to bond the SRG to any nearby accessible
building steel so as to create many points of
grounding/bonding. This is important to do along
the SRG’s perimeter and for any steel that
penetrates the SRG’s surface.
(10) Grounding for ac systems and equipment must
conform completely to NEC requirements. Also, if
the electrical or electronic equipment has been
tested and listed by an NRTL (Nationally
Recognized Testing Laboratory, such as UL), then
there may be additional or special
grounding/bonding requirements which must also
be met if proper operation is to be obtained. Again,
any use of a “dedicated”, “clean” or other non-NEC
allowed connection, such as one which is separated
from the building’s service grounding electrode and
the associated equipment grounding conductor
system, is totally against the intent of this book.
Only grounding systems and connections which
meet National Electrical Code requirements are
suitable.
(11) Special care must be used to assure proper
grounding if NEC permitted isolated grounding is
specified. “Isolated/Insulated grounding” (IG) must
be per NEC Section 250-74; Connecting Receptacle
Terminal to Box; exception No. 4; and Section 250-
75, Bonding Other Enclosures for field wired (e.g.,
direct) branch circuit connections to electronic
equipment.
(12) In particular, no attempt must be made during or
after installation to separate the electronic system’s
equipment grounding conductors from the ac power
system’s equipment grounding conductors and its
associated earth electrode grounding connections.
Such separations would violate the NEC and
produce potential electrical fire and shock hazards.
They would also be likely to damage circuits inside
the related electronic equipment, or to at least
degrade the operation of it.
(13) Note that the use of the IG method even if it follows
NEC requirements, does not always improve the
performance of equipment. In fact, the use of the
IG wiring method is just as likely to make things
worse or to result in no observable change to the
operation of the equipment. There is usually no way
to predict the benefits if any, of isolated ground
circuits except by direct observation and
comparison between solid grounding (SG) and IG
methods in each case.
(14) It is relatively easy to convert existing IG circuits to
SG circuits on an as-needed basis. On the other
hand, it is generally both impractical and not cost
effective to convert an existing SG circuit to an IG
style that conforms to NEC requirements.
Accordingly, circuits used to supply power to
electronic equipment can be designed and first
installed as IG types, so that they may later be
converted back and forth between IG and SG as
needed.
(15) The equipment grounding conductors in a feeder or
branch circuit must always be routed within the
same conduit or raceway containing that circuit’s
associated power circuit conductors. This also
applies to flexible cord and cable assemblies.
(16) Where transfer switches (including those found in
UPS systems) are used, the possibility of common
mode noise is not removed. Proper grounding
between alternate sources of power is required,
usually by solid interconnection of the two system’s
neutrals, but with only one of the two ac systems
being the one with the neutral grounded. Unless
the two involved ac systems are installed
physically adjacent to one another, a ground
potential shift disturbance may occur during
transfer operations on the switch. This shift in
ground potential can then unwantedly introduce
common-mode noise into the load being served by
the switch.
(17) Ground potential-shift problems and common-
mode noise problems in general are avoided when
Practical Guide to Electrical Grounding 70
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