Help us improve your experience.

Let us know what you think.

Do you have time for a two-minute survey?


Connecting DC Power to a QFX5100 Device

Before you begin connecting DC power to the switch:

  • Ensure that you have taken the necessary precautions to prevent electrostatic discharge (ESD) damage (see Prevention of Electrostatic Discharge Damage).

  • Ensure that you have connected the switch chassis to earth ground.


    Before you connect power to the switch, a licensed electrician must attach a cable lug to the grounding and power cables that you supply. A cable with an incorrectly attached lug can damage the switch (for example, by causing a short circuit).

    To meet safety and electromagnetic interference (EMI) requirements and to ensure proper operation, you must connect the chassis to earth ground before you connect it to power. For installations that require a separate grounding conductor to the chassis, use the protective earthing terminal on the switch chassis to connect to the earth ground. For instructions on connecting earth ground, see Connecting Earth Ground to a QFX5100 Device.

  • Install the power supply in the chassis. For instructions on installing a power supply in a QFX5100 device, see Installing a Power Supply in a QFX5100 Device.

Ensure that you have the following parts and tools available:

  • DC power source cables (14–16 AWG) with ring lug (Molex 190700069 or equivalent) (not provided)

  • Phillips (+) screwdriver, number 2 (not provided)

  • Multimeter (not provided)

The QFX5100 is shipped from the factory with two power supplies. Each power supply is a hot-removable and hot-insertable field-replaceable unit (FRU) when the second power supply is installed and running. You can install replacement power supplies in the two slots next to the fan modules without powering off the switch or disrupting the switching function.


DC-powered QFX5100 devices are intended for installation only in a restricted access location.


The battery returns of the DC power supply must be connected as an isolated DC return (DC-I).

To connect DC power to a QFX5100 device:

  1. Attach the grounding strap to your bare wrist and to a site ESD point.
  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 low resistance (indicating a closed circuit) to chassis ground is positive (+) and will be installed on the V+ (return) DC power input terminal.

    • The cable with very high resistance (indicating an open circuit) to chassis ground is negative (–) and will be installed on the V– (input) DC power input terminal.


    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 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.

  3. Ensure that the input 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.

    The V+ terminals are referred to as +RTN, and V– terminals are referred to as –48 V in DC Power Wiring Sequence Warning and DC Power Electrical Safety Guidelines.

  4. Ensure that the power supplies are fully inserted in the chassis.
  5. Remove the terminal block cover. The terminal block cover is a piece of clear plastic that snaps into place over the terminal block (see Figure 1).
  6. Remove the screws on the terminals using the screwdriver. Save the screws.

    Ensure that the power cables do not block access to device components or drape where people can trip on them.

  7. Connect each power supply to the power sources. Secure power source cables to the power supplies by screwing the ring lugs attached to the cables to the appropriate terminals by using the screw from the terminals (see Figure 1 and Figure 2 ).

    The QFX5100 is designed to operate with a DC power supply that has a single, non-redundant, feed input. For source redundancy, two DC power supplies must be installed in QFX5100; connect source (A) to one power supply and connect source (B) to the second power supply. This configuration provides the commonly deployed A/B feed redundancy for the system.

    The terminal block of the power supply has four terminals labeled V+, V+, V–, and V– for connecting DC power source cables labeled positive (+) and negative (–). The V+ terminals are shunted internally together, as are the V- terminals.


    The connection between each power source and power supply must include a circuit breaker.

    Do not connect two sources to a single power supply because doing so can potentially cause circulating current in feed wires whenever there is any difference in the voltage of the two sources.


    For QFX5100-96S installations using battery backup, a single 15 A circuit breaker is recommended.

    1. Secure the ring lug of the positive (+) DC power source cable to the V+ terminal on the DC power supply.
    2. Secure the ring lug of the negative (–) DC power source cable to the V– terminal on the DC power supply.
    3. Tighten the screws on the power supply terminals until snug using the screwdriver. Do not overtighten—apply between 5 in-lb (0.56 Nm) and 6 in-lb (0.68 Nm) of torque to the screws.
    Figure 1: DC Power Supply Faceplate for a QFX5100 DeviceDC Power Supply Faceplate for a QFX5100 Device
    Shunt negative input terminals (-48V)
    ESD grounding point
    Shunt positive input terminals (+RTN)
    Fault LED
    Terminal block
    Output LED
    Ejector lever
    Input LED

    The V+ terminals are shunted internally together, as are the V- terminals. The same polarity terminal can be wired together from the same source to provide an additional current path in a higher power chassis. Do not connect the terminals to different sources.

    Figure 2: Securing Ring Lugs to the Terminals on the QFX5100 DC Power SupplySecuring Ring Lugs to the Terminals on the QFX5100 DC Power Supply
  8. Replace the terminal block cover.
  9. Close the input circuit breaker.

    The switch powers on as soon as power is provided to the power supply. There is no power switch on the device.

  10. Verify that the IN and OUT LEDs on the power supply are lit green and are on steadily.

    A system reboot with Routing Engine FPGA version 7.1 might not successfully boot the Junos OS software. In case of a system reboot failure, you need to power cycle the switch. To check the current FPGA version, issue the show chassis firmware command.