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.
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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
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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.
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.
1 Grounding for Electrical Distribution Systems, ALLTEC
Corp., Cohasset, MA
®
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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