Cisco MGX 8230 Specifications download pdf (Page 128)

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Cisco MGX 8230 Edge Concentrator Installation and Configuration

Release 1.1.31, Part Number 78-11215-03 Rev. B0, May 2001

Chapter 3 Site Preparation

Power and Grounding

As Figure 3-3 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.

Conductor Characteristics for Carrying Current and Ensuring Low Voltage Drops

To prevent signal degradation, a conductor must be large enough to prevent its impedance from creating

a voltage drop equal to 2 percent of the reference voltage. Also, the protective earth conductor must be

large enough to carry all the current if the 48 VDC return fails. This latter requirement is for safety. Full

fault redundancy is achieved by having equal size conductors for the protective earth ground and the 48

VDC return of the switch.

For wire gauges that prevent unacceptable voltage drops over different lengths of copper wire, see

Table 3-2. For the resistance of 1000 feet of copper wire for each gauge of wire, see Table 3-3. These

references are for planning purposes and may be further subject to local laws and practices.

Table 3-2 Wire Gauge for Current Loads Over Copper Wire Lengths

DC Current Distance in Feet

25 feet 50 feet 75 feet 100 feet 150 feet 200 feet 400 feet

5A 18 14 14 12 10 8 6

10A1412108862

15A141088642

20A12886420

25A12864420

30A108642200

35A1064221000

40A862220000

45A8642100000

50A8442100______

55A8422000______

60A8422000______

65A64210000______

70A642100000______

75A642100000______

100A42100000____________

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