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Tech Tips
Telecommunications
and the Single Point Ground
by Jeff Jowett
Megger
F
or effective grounding of a telecommunications facility, the most essential
element is the master ground bar (MGB). This is the means by which the
site becomes protected as a single point ground.1 The utilization of a single
point ground is recommended practice in order to eliminate voltage gradients
around the electrical system. During electrical disturbances, sensitive equipment
that has been randomly connected at various physically convenient points around
the grounding system can experience damaging current flow through signal
cables as potential differences develop. This problem is minimized or eliminated
by terminating all grounding elements at a common point.
The MGB provides this function and can be augmented by auxiliary ground
bars at convenient locations provided they are connected at sufficiently low impedance to the MGB. The MGB is a copper bar at least 18 by 3 inches and ¼ inch
thick. Typically, it is wall-mounted directly above the grounding conductor (the
conductor to the grounding electrode) in the central office. The bar is mounted
on a bracket by means of an insulator. Terminations are attached by exothermic
welds or compression clamps.
The grounding system must be implemented with the goal of eliminating extraneous current flow that might go unnoticed in another facility but is injurious
to sensitive telecomm switching. Accordingly, the sequence of connections to
the MGB is critical and must be carefully observed. From left to right across the
grounding bar, the sequence is generators of overvoltages, absorbers, nonisolated
zone, and isolated zone.
Generators of overvoltages are conductive metallic paths offering minimal impedance for atmospheric discharges or transients. These include components such
as radio and microwave towers and cable shields. Connection within this zone
is a necessary step in eliminating radio frequency noise. Auxiliary ground bars
such as those for the main distribution frame (MDF) and entrance cables are
connected in this section of the MGB. Various auxiliary bars can be established
to protect discrete groups of equipment. So long as they in turn are connected
only to the MGB, the single point concept is maintained. Also connected are
the generator ground frame, emergency generator chassis, telephone protector
terminals, and window entrance of wave guides. Multicoupler receptors must
each have their own connection.
www.netaworld.org Absorbers of overvoltages are elements such as the central office
grounding electrode or ground field
and the metallic water pipe system. The
nonisolated zone is the connection point
for equipment with exposed metal
surfaces that could become energized.
This includes the frames of equipment
and racks, MDF, power room frames
not grounded with green cables, battery racks, and the bus for battery
return (positive). Connections are
made in this zone to prevent voltage
gradients between all cabinets outside
the isolated ground zone.
Finally, the isolated zone is the
point of connection of a separate
grounding bar, the ground window
bar (GWB). This is an isolated copper bar similar to and installed like
the MGB. Equipment in the isolated
zone is not connected to other grounds
but bears a unique connection to the
GWB. Typically, this zone has the
least voltage variation. This allows all
the equipment to float to a potential
equal to the GWB, because the GWB
is a single point ground. With sensitive electronics operating at the same
potential, there are no overcurrents.
All equipment must be isolated from
floor, walls, and ceiling, taking care to
include bolts that hold items to the
floor. The GWB in turn is connected
Winter 2008-2009 NETA WORLD
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to the MGB by a 2/0 AWG or larger connector, following
the shortest route. Also, two conductors in parallel may be
used.
The isolated zone includes equipment such as multiplexors, inverters, digital switches, fiber optic transmission
equipment, digital telephone equipment, and cable racks.
Inverters, used to convert dc to ac power, must be physically located within this zone and connected to ground.
Equipment such as teletypes, printers, modems and video
terminals, requiring ac current but connected to equipment
within the isolated zone, must operate from receptacles fed
by inverters. The MGB and MDF bar are not to be physically located in the isolated zone.
Separate requirements exist for equipment in the nonisolated zone. All cabinets are isolated from all grounds
except the connection to the designated section of the MGB.
The main distribution frame is grounded to the generators
of overvoltage section (left side) of the MGB. Entrance
cables are connected to their own auxiliary copper bar, cable
entrance ground bar (CEGB), again similar to the MGB.
The shields are grounded to this bar so that the individual
grounding conductor for the shield is as short and direct as
possible. The CEGB also connects to the generator section
of the MGB.
The single point ground is only as effective as its termination in the earth. Careful attention must be paid to the
grounding electrode, the point of final contact with earth.
Towers and buildings will have an exterior ground ring
buried around the structure, interconnected, and supplemented by ground rods. The MGB is connected to this
system, and also may connect to the ac power ground or
to building steel.
Additionally, an interior ring, called a halo, may be extended within the building, elevated or around the walls
(15 cm below interior ceiling is recommended). This
provides an equipotential ground plane protecting against
electromagnetic pulses of high frequencies. It connects
noncritical metallic parts and inactive elements like the
frames of metallic doors and HVAC ductwork. Properly
connected to the exterior ground ring at the four corners
of the structure, the halo functions as a faraday shield.
Only inactive metallic parts should be connected to the
halo. Electrical equipment should never be connected to
the halo. Such practice interferes with the goal of diverting
currents developed by electromagnetic voltages through the
shortest path to earth. Offering an alternate path around
the halo can lead to potential differences between cabinets
and promote equipment damage. Accordingly, equipment
is grounded directly to the MGB, and the halo is not paralleled between the MGB and the exterior ground. If the
halo were connected to both the building ground and the
MGB, a parallel condition would exist and the single point
concept would be violated If the halo is connected to the
MGB without parallel connection to the exterior ground,
the single point ground is maintained.
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NETA WORLD Winter 2008-2009
An additional word on lightning protection is in order.
Antennae on towers must be grounded. The National Electrical Code® (NEC®) calls for two down conductors. The
tower structure can serve as one of these. If ungrounded,
an antenna can develop an arc between the central conductor and shield of coaxial cable. The propagation difference
creates destructive high frequency noise that will circulate
toward equipment. If the antenna is mounted on a building, a #2 AWG conductor can be extended down to the
grounding electrode, and building steel can serve as a parallel
connection.
Prevailing standards are in effect for the required resistances of the key elements in such a system. The buried
electrode, whether it be ring ground, grid, or several such
structures connected in parallel, should be no more than five
ohms. However, the telecommunications industry prefers a
one ohm ground. The resistance of the grounding conductor
from the MGB to the central office grounding electrode
should be less than 0.005 ohm, as should the conductor to
the ac power ground, which is typically 2/0 AWG or greater.
The connection between the GWB and the MGB must also
be no more than 0.005 ohm.
Careless ground connections made principally on the
basis of physical proximity can establish voltage gradients
that damage sensitive telecommunications equipment.
Studious attention to the specific requirements of the single
point concept will eliminate this problem.
Grounding for Electrical Distribution Systems, ALLTEC®
Corp., Cohasset, MA
1
Jeffrey R. Jowett is Senior Applications Engineer for Megger in Valley
Forge, Pennsylvania, serving the manufacturing lines of Biddle®, Megger®, and Multi-Amp® for electrical test and measurement instrumentation. He holds a BS in Biology and Chemistry from Ursinus College.
He was employed for 22 years with James G. Biddle Co. which became
Biddle Instruments and is now Megger.
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