3-20
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
10A
14
12
10
8
8
6
2
15A
14
10
8
8
6
4
2
20A
12
8
8
6
4
2
0
25A
12
8
6
4
4
2
0
30A
10
8
6
4
2
2
00
35A
10
6
4
2
2
1
000
40A
8
6
2
2
2
0
000
45A
8
6
4
2
1
0
0000
50A
8
4
4
2
1
00
______
55A
8
4
2
2
0
00
______
60A
8
4
2
2
0
00
______
65A
6
4
2
1
0
000
______
70A
6
4
2
1
00
000
______
75A
6
4
2
1
00
000
______
100A
4
2
1
00
000
______
______