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5. Ensure that the power source circuit breaker is open so that the voltage across the
DC power source cable leads is 0 V and that the cable leads do not become active
while you are connecting DC power.
6. Verifythat the DC power cables are correctly labeled before making connections to
thepowersupply.Inatypicalpowerdistributionschemewherethereturnisconnected
to chassis ground at the battery plant, you can use a multimeter to verify the
resistance of the –48V and RTN DC cables to chassis ground:
•
The cable with very high resistance (indicating an open circuit) to chassis ground
is negative (–) and will be installed on the –48V (input) DC power input terminal.
•
The cable with very low resistance (indicating a closed circuit) to chassis ground
is positive (+) and will be installed on the RTN (return) DC power input terminal.
CAUTION: You must ensure that power connections maintain the proper polarity. The
power source cables might be labeled (+) and (–) to indicate their polarity. There is no
standardcolor coding for DC power cables. The color coding used by the external DC
powersourceatyoursitedeterminesthe colorcodingforthe leadson thepowercables
that attach to the DC power input terminals on each powersupply.
7. Install each power cable lug on the DC power input terminal, securing it first with a
lockingwasher,thenwiththenut(seeFigure69onpage179).Applybetween29in.-lb
(3.3 Nm) and 31 in.-lb (3.5 Nm) oftorqueto eachnut. (Usethe 3/8-in. [9.5 mm] nut
driver or socket wrench.)
a. Secure each positive (+) DC source power cable lug to the RTN (return) DC
power input terminal.
b. Secure each negative (–) DC source power cable lug to the –48V (input) DC
power input terminal.
Eachpowersupplyhas twoindependent sets of DC power input terminals(A: –48V
A: RTN and B: – 48V B: RTN ). For feed redundancy,each power supply should be
poweredbydedicatedpowerfeedsderivedfromfeedAandfeedB.Thisconfiguration
provides the commonly deployed A/B feed redundancy for the switch.
NOTE: If the power cord tray is installed, you can thread the DC power cables through
the power cord tray.
Copyright ©2010,Juniper Networks,Inc.178
CompleteHardwareGuide forEX8208 EthernetSwitches