Digital System Grounding - Cisco MGX MGX 8800 Hardware Installation Manual

Appendix
Bonding Networks
The purpose of a BN is to shield people and equipment from the adverse effects of electromagnetic
energy from DC to low RF range. Lightning and both AC and DC power faults are the energy sources
that cause the greatest concern. Of less concern are quasi-steady-state sources such as AC power
harmonics and function sources, such as clock signals from digital equipment.
The energy sources that cause concern are referred to as emitters. The people and equipment that can
suffer adversely from these emitters are referred to as susceptors.
The coupling between an emitter and a susceptor can be characterized as a transfer function. The purpose
of a BN is to reduce the magnitude of the transfer function to an acceptable level. Reducing the
magnitude of the transfer function is achieved through the design of the BN; specifically, in the way that
MBNs and IBNs are attached to the CBN. The practical aspects of this design are discussed below.
A BN can also function as a return conductor for signaling applications, as a connection to earth for
ground return signaling, and as a path for power fault currents. A BN that can handle large currents can
rapidly de-energize faulted power circuits.

Digital System Grounding

For the Cisco MGX switch, Cisco policy has been to ground the return of the 48 VDC directly to the
frame at the backplane. This method of grounding prevents transient currents caused by lightning or
power surges from entering the system through the backplane, upsetting system performance and
possibly damaging components.
Isolating grounds like this one, using only analog methods, does not address the current high-speed
digital system requirements. Digital systems today have such high speeds and large bandwidths that they
now produce frequencies with harmful effects. Consequently, digital systems now require multipoint
grounding.
Isolation using analog methods provides 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 48 VDC 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, you 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 multipoint ground the 48 VDC 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 (150 M ohms or greater) grounding wires. These standard grounding
conductors have a very high impedance at frequencies greater 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 multipoint, meshed bonding networks be used to control
these excitation currents.
Cisco MGX 8800/8900 Series Hardware Installation Guide
C-6
Releases 2 - 5.2, Part Number OL-4545-01, Rev. H0, May 2006