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The purpose of this appendix is as follows:
1. Introduce the techniques and policies Cisco practices with regard to DC power, earthing, and bonding of Cisco equipment.
2. Explain why these techniques are the best practice.
3. Describe techniques for DC power and earth bonding to the BPX 8600, MGX 8220, and MGX 8850 product range.
This appendix discusses the principles of mesh bonding. It also shows the earth return systems in the Cisco BPX 8600, Cisco MGX 8220, and Cisco MGX 8850, identifies the earthing points, shows how they are to be earthed, and how the DC power connections are to be connected to the equipment. Extracts are drawn from Cisco internal documentation and engineering material to support the earthing and power techniques described.
The standards from which this document have been drawn are as follows:
The bonding and earthing principles adopted by Cisco are as follows:
To fulfill this purpose, this document:
The following definitions apply to terminology found in IEC 50 [3].
Earth---The conductive mass of the earth, whose electric potential at any point is conventionally taken as equal to zero. In some countries the word "ground" is used instead of "earth."
Earth electrode---A conductive part or a group of conductive parts in intimate contact with and providing an electrical connection to earth.
Earthing network---The part of an earthing installation that is restricted to the earth electrodes and their interconnections.
Main earthing terminal---A terminal or bar provided for the connection of protective conductors, including equipotential bonding conductors and conductors for functional earthing, if any, to the means of earthing.
Earthing conductor---A protective conductor that connects the main earthing terminal or bar to the earth electrode.
Equipotential bonding---Electrical connection putting various exposed conductive parts and extraneous conductive parts at a substantially equal potential.
Equipotential bonding conductor---A protective conductor for ensuring equipotential bonding.
Neutral conductor (N)---A conductor connected to the neutral point of a system and capable of contributing to the transmission of electrical energy.
Protective conductor (PE)---A conductor required by some measures for protection against electric shock by electrically connecting any of the following parts:
Bonding network (BN)---A set of interconnected conductive structures that provide an electromagnetic shield for electronic systems and personnel at frequencies from DC to low RF. The term "electromagnetic shield" denotes any structure used to divert, block, or impede electromagnetic energy. In general, a BN need not be connected to earth, but all BNs in this recommendation have an earth connection.
Common bonding network (CBN)---The CBN is the primary way to create effective bonding and earthing inside a telecommunication building. It is the set of metallic components that are intentionally or unintentionally interconnected to form the principal BN in a building. These components include:
The CBN always has a mesh topology and is connected to the earthing network.
Mesh-BN (MBN)---A bonding network in which all associated equipment frames, racks and cabinets, and usually the DC power return conductor are bonded together, as well as at multiple points to the CBN. Consequently, the mesh-BN augments the CBN.
Isolated bonding network (IBN)---A bonding network that has a single point of connection (SPC) to either the common bonding network or another isolated bonding network. All IBNs in this document will have a connection to earth via the SPC.
Single point connection (SPC)---The unique location in an IBN where a connection is made to the CBN. In reality, the SPC is not a mere "point" but has sufficient size to accommodate the connection of multiple conductors. Usually, the SPC is a copper bus-bar. If cable shields or coaxial outer conductors are to be connected to the SPC, the SPC could be a frame with a grid or sheet metal structure.
Mesh-IBN---A type of IBN in which the components of the IBN (equipment frames) are interconnected to form a mesh-like structure. This may, for example, be achieved by multiple interconnections between cabinet rows, or by connecting all equipment frames to a metallic grid (bonding mat) that extends away from beneath the equipment. The bonding mat is, of course, insulated from the adjacent CBN. If necessary the bonding mat could include vertical extensions that result in an approximation to a Faraday-cage. The spacing of the grid depends upon the frequency range of the electromagnetic environment.
Star IBN---A type of IBN comprising clustered or nested IBNs sharing a common SPC.
System block---All the equipment whose frames and associated conductive parts form a defined BN.
Isolated DC return (DC-I)---A DC power system in which the return conductor has a single point connection to a BN. More complex configurations are possible (see "Mesh Bonding" later).
Common DC return (DC-C)---A DC power system in which the return conductor is connected to the surrounding BN at many locations. This BN could be either a mesh-BN (resulting in a DC-C-MBN system) or an IBN (resulting in a DC-C-IBN system). More complex configurations are possible (see "Mesh Bonding" later).
Figure D-1 shows examples of star and mesh topologies.
Bonding and earthing refer to the construction and maintenance of Bonding Networks (BNs) and their connection to earth. In this recommendation, the acronym BN implies that a connection to earth exists. Also, BN is used to refer to CBNs and IBNs collectively.
The primary purpose of a BN is to help shield people and equipment from the adverse effects of electromagnetic energy in the DC to low RF range. Typical energy sources of concern are lightning and AC and DC power faults. Of generally lesser concern are quasi steady-state sources such as AC power harmonics and function sources, such as clock signals from digital equipment. All of these sources will be referred to generically as "emitters." People and equipment that suffer adversely from the energy from the emitters will be referred to as "susceptors." The coupling between a particular emitter and susceptor may be characterized by a transfer function. The purpose of a BN is to reduce the magnitude of the transfer function to an acceptable level. This may be achieved by appropriate design of the CBN and the MBNs and IBNs attached to that CBN. Practical aspects are discussed below. Other purposes of a BN are to function as a "return" conductor in some signaling applications and as a path for power fault currents. The capability of the BN to handle large current helps rapidly de-energize faulted power circuits. In addition, the BN and its connection to earth are used in "ground return" signaling.
It has been the policy of the Cisco to ground the 48V DC return directly to the frame at the backplane of the Cisco BPX 8600, MGX 8220, and MGX 8850 product range. This policy is in keeping with the latest requirements to eliminate lightning and power surge related transients from entering through the backplane, upsetting system performance and possibly damaging components.
Isolating the grounds through analog requirements of years past does not address the related problems of current high-speed digital system requirements. The digital requirements, due to increased speeds and system bandwidth, now include frequencies with harmful effects that could previously be mitigated but now need multi-point grounding to control them.
Isolation is being provided at the physical level of our interfaces and not at the power-supply end. The bus currents and isolation parasitic capacitance that are represented by the 48V DC side of the system create much greater threat levels to the backplane of our systems, which have embedded communication buses distributed through them. To mitigate these effects, we must bond and provide the lowest possible impedance to ground at the backplane. Capacitors used to isolate the DC common paths are inadequate at RF frequencies outside the backplane structure. Therefore, isolation must be kept to multi-point ground the 48V DC return to chassis and logical ground at the backplane level of the Cisco equipment.
Bellcore GR-1089, 1997 edition, speaks of these recent challenges in Chapter 9. This new thinking is the outgrowth of the ITU-T K.27 recommendations released in 1991. The bonding of meshed bonding networks and the digital high speeds dictate the eventual acceptance of this new philosophy on a universal basis. The CE-Mark requirements for the induced effects of transient and power surge lightning cannot be met with large, high impedance (greater than 150 MOhms) grounding wires. These standard grounding conductors have a very high impedance at frequencies grater than 10 MHz.
The grounding of the frames and the mesh bonding network must be effective over a frequency range of 60 Hz to 100 GHz according to Bellcore requirements. 30 cm of wire represents 30 nH of inductance. This represents 2 ohms of reactance at a frequency of 30 Mhz. This high impedance would be a large change from earth reference if earth were several stories below the equipment installation. A four-story building would represent 1000 ohms above ground during a 30 MHz-frequency disturbance in this example. Therefore it is required that multi-point, meshed bonding networks be used to control these excitation currents.
Equipment backplane speeds are in the category above 800 MHz. Because we must design for the worst case scenario; our concerns about RF damage are much greater. At 800 MHz only 10 inches of wire will represent 500 ohms reactance. For the average coaxial cable shield integrity to be maintained, the termination of the shield must see a ground reference of no more than 50 ohms. The importance of this relationship is that although the 800 MHz speed is not the data speed of E1/T1, we must mitigate frequency susceptibility issues that will upset the 800 MHz operation. Therefore, we must use the multi-point grounding techniques supported by the K.27 recommendations. Although K.27 is designed around lightning and transient issues, the same theory applies to the higher frequency problems; they are just smaller in scale. As frequency increases, the wave length becomes smaller, and the reactance of a fixed length of wire goes up.
The need is to multi-point ground our backplane, and the 48V DC return directly to the frame at frequent intervals that represent at least 1/20 of a wavelength. The frame, in turn, will be bonded to the isolated mesh-bonding mat. At 800 MHz, 18.8 mm represent a 20th of a wavelength, so grounding/bonding must be done at these intervals to maintain backplane-to-cabinet integrity for its full perimeter. Using capacitors to achieve the necessary bonding becomes extremely difficult at these frequencies in addition to the added cost due to the isolation breakdown voltage requirements of 2.1 Kvolts, should the old philosophy be insisted upon.
The theoretical concepts are confirmed by practical experience and lead to the general principles listed below. A consequence of applying these principles is that the number of conductors and interconnections in the CBN is increased until adequate shielding is achieved. Concerning the important issue of electric shock, the following implementation principles apply to mitigation of electric shock as well as to equipment malfunction.
1. All elements of the CBN shall be interconnected. Multiple interconnections, resulting in a three-dimensional mesh, are especially desirable. Increasing the number of CBN conductors and their interconnections increases the CBN shielding capability and extends the upper frequency limit of this capability.
2. It is desirable for the egress points for all conductors leaving the building (including the earthing conductor), to be located close together. In particular, the AC power entrance facilities, telecommunications cable entrance facilities, and the earthing conductor entry point should be close together.
3. The facility should have a main earthing terminal located as close as possible to the entrance to the AC power and telecommunications cable entrance facilities. The main earthing terminal shall connect to the following:
4. The CBN shall be connected to the main earthing terminal. Multiple conductors between CBN and the main earthing terminal are recommended.
5. As contributors to the shielding capability of the CBN, interconnection of the following items of the CBN are important:
6. The coupling of surges into indoor signal or power cabling is reduced, in general, by running the cables in close proximity to CBN elements. However, in the case of external surge sources, the currents in the CBN will tend to be greater in peripheral CBN conductors. This is especially true of lightning down-conductors. Therefore, it is best to avoid routing cables in the periphery of the building. When this is unavoidable, metallic ducts that fully enclose the cables may be needed. In general, the shielding effect of cable trays is especially useful, and metallic ducts or conduit that fully enclose the cables provide nearly perfect shielding.
7. In steel frame high-rise buildings, the shielding effects that the steel frame provides against lightning strikes can help. For cables extending between floors, maximum shielding is obtained by locating the cables near the center of the building. However, as stated above, cables enclosed in metallic ducts may be located anywhere.
8. If the facility has over-voltage primary protection on telecommunication wires, it should have a low impedance connection to the cable shield, if it exists, and to the surrounding CBN.
9. Over-voltage protectors are advisable at the AC power entrance facility if the telecommunication building is located in an area where power lines are exposed to lightning. These protectors should be bonded with low impedance to the CBN.
10. Mechanical connections in a protection path of the CBN whose electrical continuity may be insufficient shall be bypassed by jumpers that are visible to inspectors. These jumpers shall comply with IEC requirements for safety. However, for EMC applications, the jumpers should have low impedance.
11. The CBN facilitates the bonding of cable shields or outer conductors of coaxial cables at both ends by providing a low impedance path in parallel and in proximity to the cable shields and outer conductors. Thus, most of the current driven by potential differences is carried by the highly conductive members of the CBN. Disconnection of one cable shield for inspection should minimally affect the current distribution in the CBN.
The main feature of a mesh-BN is the interconnection at many points of cabinets and racks of telecommunications and other electrical equipment as well as multiple interconnections to the CBN.
Telecommunication techniques sometimes use circuits for signaling with earth return, for example, lines with ground start, three wire inter-exchange connection. Equipment interconnected by these circuits needs functional earthing. The signaling range is normally determined by the resistance of the current path. Most of this resistance is contributed by the earth electrodes. The performance provided by the earthing network via the main earthing terminal is generally sufficient for this signaling purpose.
To maintain the full EMI and EMC integrity of this equipment, it must be bonded to an integrated ground plane or a non-isolated ground plane network. The purpose is to mitigate the damaging effects of electrostatic discharge or lightning. Refer to the latest edition of ITU-T Recommendation K.27 or Bellcore GR-1089-CORE to ensure that the correct bonding and grounding procedures are followed. As recommended in these documents, a frame bonding connection is provided on the Cisco cabinet for rack-mounted systems. To see how to make a connection, see "Making the Frame Bonding (Ground) Connection (Cisco Supplied Rack)" in Chapter 3.
Except for the AC power supply modules, every module in a rack-mount system uses the rack for grounding. Therefore, the rack must connect to protective earth ground, and the equipment must be secured to the rack so as to ensure good bonding.
A DC-powered node must have grounding conductors that connect at two separate locations:
For DC-powered systems, Cisco has designed the MGX 8850 node and other WAN switches to connect to a non-isolated ground system. In contrast, routers and other LAN equipment often use an isolated grounding scheme. If properly wired together through an equalization connection as described in ITU-T recommendation K.27, the isolated and non-isolated ground systems can form a mixed grounding system. The potential between any points in the ground system---whether or not the ground system is mixed---must not exceed 2% of the referenced voltage (2% of 48 volts is 960 millivolts).
A mixed ground system appears in Figure D-2. This figure shows safety and earth grounds and the primary and redundant DC sources Battery A and Battery B. Individual ground conductors are labeled Z1, Z2, ..., Z5. The Z represents the impedance of the ground conductor between a chassis, for example, and a connection to the building's ground system. The numbers 1, ..., 4 represent building ground points and indicate that an impedance can exist between different points in the ground system of the building. Each of these symbols indicate that a voltage drop may result (but must not exceed 2% of the referenced voltage). See Table D-1 for a definition of each Z1-Z5.
As Figure D-2 shows, the non-isolated system has a 48 VDC return that internally connects to the backplane. (This design calls for a hard-wired return and so does not allow for an optional or alternate ground connection.) The internal connection provides a low-impedance connection between 48 VDC return and frame ground. This grounding scheme protects the signals on the backplane from corruption by transients that can result from lightning or electrostatic discharge.
To improve protection against transients, the loop area (and resultant loop impedance) should be made as small as possible by locating the -48 VDC supply, 48 VDC return, and protective earth conductors as close to each other as possible.
As recommended in ITU-T K.27, the multi-point grounding in a mesh bonding network provides the best protection for equipment by providing the lowest impedance in the ground system. For more detailed information, refer to the recommendation itself.
For wire gauges that prevent unacceptable voltage drops over different lengths of copper wire, see Table D-2. For the resistance of 1000 feet of copper wire for each gauge of wire, see Table D-3. These references are for planning purposes and may be further subject to local laws and practices.
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Note Table D-2 is for reference, Cisco recommends using 60 Amps or greater. Table D-3 is for reference, Cisco recommends using 6 gauge or greater. |
Posted: Mon Jul 31 09:57:11 PDT 2000
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