34 CHAPTER 2: INSTALLING THE SWITCH
WARNING: When powering any Switch 5500 from an RPS, the unit must
be earthed (grounded). This can be achieved by either connecting the
power cord to the unit or by connecting the earth terminal on the rear of
the unit to a reliable electrical earth, or by connecting both. You must
ensure that the earth connection is made before connecting the DC
supply from the RPS.
3Com Switches which support -48V DC RPS inputs, that are PoE enabled,
can only be powered by an RPS which complies with the isolation
requirements of IEEE-Std 802.3af. Non PoE enabled switches do not have
this restriction.
WARNING: A standard 'positive-earthed' -48V redundant power system
suitable for use with telecommunications equipment should not be used
with the 3Com Power-over-Ethernet (PoE) network switches. In order to
meet the IEEE 802.3af (PoE) specification, the -48V output must be
isolated from earth (ground) and meet the isolation requirements in that
specification.
WARNING: Any RPS must be approved as a SELV output in accordance
with IEC 60950-1/UL 60950-1/EN 60950-1.
WARNING: The characteristics of the Switch 5500 DC supply input are
given in Appendix C
on page 127.
The Switch 5500 can be powered in three different ways:
■ AC Mains only — does not offer any power redundancy. If the AC
mains supply or the AC power supply fail, the Switch will power off.
■ AC Mains and -48V DC (primary supply) — the internal AC supply
acts as the backup in the event of a DC power failure.
■ DC only — the Switch does not need an AC supply and the resiliency
is provided by the DC supply. This is useful in an environment where
only DC power is available.
The RPS provides three main benefits to the customer:
■ Power Redundancy — if a Switch is powered from the mains supply
unit, a failure of the internal power supply will cause the Switch to fail.
This can be overcome by connecting both the AC and DC RPS supplies
to the Switch. Additional redundancy can also be added to the DC
power by using (N+1) DC power supplies to further increase the
availability of the system.
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