Juniper EX8208 Hardware Manual page 177

Figure 52: Removing the Plastic Cable Cover on a DC Power Supply in an EX8200 Switch
4. Remove the washers and nuts from each DC power input terminal, using the 3/8-in. (9.5 mm) nut driver
or socket wrench to loosen the nuts. Leave the bolts installed on the input terminals.
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. Verify that the DC power cables are correctly labeled before making connections to the power supply.
In a typical power distribution scheme where the return is connected 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:
The power source cables might be labeled (+) and(–) to indicate their polarity. There
is no standard color coding for DC power cables. The color coding used by the
external DC power source at your site determines the color coding for the leads
on the power cables that attach to the DC power input terminals on each power
supply.
You must ensure that power connections maintain the proper polarity.
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