Filter
Top Products
C-10
Cisco MGX 8950 Hardware Installation Guide
Release 3, Part Number 78-14147-02 Rev. A0, November 2002
Appendix C Earthing and Bonding Recommendations
Bonding and Grounding the Cisco MGX 8950
As Figure C-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.
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% 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 C-2. For the resistance of 1000 feet of copper wire for each gauge of wire, see Table C-3. These
references are for planning purposes and may be further subject to local laws and practices.
Note Table C-2 is for reference, Cisco recommends using 50A or greater. Table C-3 is for reference, Cisco
recommends using 6 gauge or greater.
Ta b l e C-1 Ground Point Descriptions for Mixed Grounding
Connection Description
Z1 –48 VDC return.
Z2 Protective earth or safety ground (green/yellow).
Z3 Equipment ground for non-isolated equipment.
Z4 Equipment ground for isolated equipment.
Z5 Equalizing frame ground. This ground creates low-impedance equalization
between frames.
B Battery ground.
1, 2, 3, 4 Connection points to the building’s ground system: a potential can exist
between these points within the ground system.
T Common-mode EMI filters.