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Connecting DC Power to a QFX5110

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 Connect the QFX5110 to Earth Ground.

    On DC systems, you can also ground the chassis using the DC protective earthing terminal on the DC power supply as an alternate method. This method is useful if you are unable to access the chassis ground point by using the two-holed grounding lug, see Alternate Method to Ground QFX5110-48S-DC and QFX5110-32Q-DC Systems.

  • Install the power supply in the chassis. For instructions on installing a power supply in a QFX5110, see Installing a Power Supply in a QFX5110.

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 QFX5110 is shipped from the factory with two 650 W 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.


A DC-powered QFX5110 is 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 QFX5110:

  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. Install heat-shrink tubing insulation around the power cables.

    To install heat-shrink tubing:

    1. Slide the tubing over the portion of the cable where it is attached to the lug barrel. Ensure that tubing covers the end of the wire and the barrel of the lug attached to it.

    2. Shrink the tubing with a heat gun. Ensure that you heat all sides of the tubing evenly so that it shrinks around the cable tightly.

    Figure 1 shows the steps to install heat-shrink tubing.


    Do not overheat the tubing.

    Figure 1: How to Install Heat-Shrink Tubing How to Install Heat-Shrink Tubing
  4. 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.

  5. Ensure that the power supplies are fully inserted in the chassis.
  6. 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 2).
  7. 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.

  8. 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 2 and Figure 3).

    The QFX5110 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 the QFX5110; 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.

    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 2: DC Power Supply Faceplate for a QFX5110DC Power Supply Faceplate for a QFX5110

    Shunt negative input terminals (+RTN)


    Protective earthing terminal


    Shunt positive input terminals (-48V)


    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 3: Securing Ring Lugs to the Terminals on the QFX5110 DC Power SupplySecuring Ring Lugs to the Terminals on the QFX5110 DC Power Supply
  9. Replace the terminal block cover.
  10. 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.

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