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Shelf Installation
EMI (Electromagnetic Interference)
Zone 3
Shielded cables minimize EMI and are required for Zone 3 I/O connections to meet the radiated
EMI limits for FCC Part 15 Class A and EN55022 Class A. These shielded cables are connected to
the OCM chassis and thus to local earth ground. This occasionally causes a problem if the
equipment to which the OCM is connected is at a different ground potential than the OCM, and that
equipment also grounds the cable shields to its earth ground. These shielded cables are connected
to chassis ground via the two mounting screws on the connector hood. See the paragraphs
Signal Ground to Chassis Ground
Telephone Signaling
E&M Types I, V and SSDC5A signaling uses an implicit ground connection between the switching
equipment (PBX) and the transmission equipment (OCM), for the return current in the E and M
signaling leads. On the OCM, this return path is via chassis ground. Make sure that a connection is
made between the OCM chassis ground and the reference ground of the switching equipment when
using E&M Type I, V or SSDC5A signaling. FXS and FXO ground start signaling also uses an
implicit ground connection for current return. On the OCM, this current return path is via chassis
ground. Connect the chassis ground screw post to a good earth ground when using FXS or FXO
ground start signaling.
Tying Signal Ground to Chassis Ground
You are responsible for providing Earth connection to the cabinets or racks that the shelves are
mounted in. This is in addition to any earth conductor that may exist. Continuity between the
shelves and cabinets or racks must be verified.
It is common practice to tie chassis ground and signal ground together. Signal ground is the 0V
reference for the digital circuits in the unit and is also the reference for unbalanced data interfaces
such as EIA/TIA-232-E and RS423. The default (shipped) and preferred position for the Signal
Ground-Chassis ground strap is shorted. This is usually best for AC power line noise immunity, but
a problem can exist if signal grounds on two equipments (at different chassis ground potentials) are
connected together via a data cable. A large current can flow in the signal ground lead (EIA/TIA-
232-E pin 7). To prevent this, make sure the potential difference between grounds is less than 0.25
V RMS. Alternatively, open the connection between chassis ground and signal ground on the unit
and connect a wire from chassis ground to the preferred earth point.
The shelf is shipped from the factory with the signal and chassis ground strapped together at the two
screw terminals located on the rear panels. This is the required position if using any FXS voice
channel card or the DPV FXO ground start card.
To connect common signal ground to chassis ground:
1. Loosen both ground post screws.
2. Slide or rotate the strap to make contact with both ground terminals.
3. Tighten both screws.
To attach separate wire to earth at a remote location:
1. Loosen both ground post screws.
2. Slide the strap up, then rotate it one-half turn to prevent it from contacting the signal ground
terminal.
3. Connect a wire from earth ground to the earth point.
2-8
below.
Office Connunications Manager OCM-1000/OCM-2000
Installation & Operation
Figure 2-5
illustrates shelf ground connections.
Grounding
onTying
036R340-000
Issue 16
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