Juniper EX9253 Hardware Manual page 97

NOTE: Grounding is required for models that use DC power supplies.
You connect DC power to the switch by attaching power cables from the DC power source to the
terminals on the power supply faceplate.
CAUTION: A licensed electrician must attach the ring lugs to the DC power cables.
WARNING: Before connecting the switch to a DC power source, ensure that the cable
leads will not become active while you are connecting DC power and that the voltage
across the DC power source cable leads is 0 V. Locate the input circuit breaker on the
panel board that provides DC power, set the circuit breaker to the off position, and tape
the switch handle of the circuit breaker in the off position. We recommend that you use
a dedicated customer-site circuit breaker rated for 40 A (–48 VDC) minimum, or as
required by local code. If you plan to operate a DC-powered switch at less than the
maximum configuration and do not provision a 40 A (– 48 VDC) circuit breaker, we
recommend that you provision a dedicated customer-site circuit breaker for each DC
power supply rated for at least 125 percent of the continuous current that the system
draws at – 48 VDC.
Before you connect power to the switch, wrap and fasten one end of the ESD wrist strap around your
bare wrist, and connect the other end of the strap to the ESD point on the switch.
To connect power to a DC-powered switch (for each power supply):
1. Switch off the dedicated customer-site circuit breakers. Ensure that the voltage across the DC
power source cable leads is 0 V and that the cable leads do not become active during installation.
2. 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 large resistance (indicating an open circuit) to chassis ground is – 48V.
• The cable with very low resistance (indicating a closed circuit) to chassis ground is RTN.
CAUTION: You must ensure that power connections maintain proper polarity. 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
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