Total Power Control with Comtrol RocketLinx® Power over Ethernet Switches

PoE Devices

– Joe House

As the number of network connected devices continues to increase exponentially, an uninterruptible network continues to get more valuable.  Without power, you have no network – without a network you may as well have no devices!

While there are many methods a system can employ to create redundancy, the base of redundancy often relies on three letters, UPS (Uninterruptible Power Supply).  Regardless of the electrical chaos that may occur in every area around a system, the battery backup UPS maintains power to the parts of the system that need it most.  If you plan to keep a series of PoE devices alive on a UPS, Comtrol’s RocketLinx switches have two key features that can help keep your UPS running like the Energizer Bunny®.

PoE Device

Before we get started: Where are the DC Power inputs on Comtrol’s Power over Ethernet switches?  Most of Comtrol’s PoE switches support redundant DC power inputs (accepting various input voltages depending on model, but shown with +48VDC here) via screw terminal connectors as shown below.  Taking some artistic liberties, you can see how two power supplies could be wired up!


PoE Devices

PoE switch first line of defense: Comtrol’s managed RocketLinx Power over Ethernet switches offer the ability to set a hard limit on the power taken by attached PoE devices.  By using the Power over Ethernet Control tab and focusing on the “PoE system” heading, users can input a distinct level of power (in watts) that each power input will draw.

Power over Ethernet Control

For argument’s sake, we will say that the main power grid is capable of supplying enough power at 48VDC (IEEE standard PoE output) to keep every PoE device running within their power demands.  The UPS, however, only has enough power allocated to keep 40 watts of PoE devices running before battery drain becomes subject to concern.  Keep in mind the power budget being set here applies ONLY to PoE devices; the switch will also consume additional power just to keep itself running. The amount of power needed will depend on the model.

In this case we see that with power input DC1 (main power) the switch is powering eight IEEE 802.3af PoE devices at their maximum level of 12.95 watts (8 x 12.95 = 103.6 watts, say 105 to be safe).

Now, in the event of a power failure, (a blackout or a more likely construction project mishap causes the main power circuit to fall) power input DC2 (the backup) takes over.  In order to maintain battery backup power for an extended period of time it has been determined that the PoE devices should only draw 40 watts as a whole.  By limiting the DC2 input to a 40 watt maximum power draw you can be assured that the eight power-hungry PoE devices will not cut your UPS life short.

PoE switch second line of defense: You may have noticed that a maximum allotted power draw of 40 watts allows for each IEEE 802.3af PoE device to draw 5 watts (8 devices x 5 watts = 40 watts), which could be far below the needs of an IEEE 802.3af Class 3 device.

By focusing on the next menu down – also in the Power over Ethernet Control tab on Comtrol’s web interface – users have the ability to prioritize the importance of keeping each port powered to keep the most important PoE devices alive… even if all eight may seek to draw more than 5 watts.

Let’s say all eight devices are cameras, but the cameras connecting to ports 1,2, and 3 are watching a vault and the cameras on ports 4-8 are watching broom closets… arguably less important than the vault. By allocating a “critical” power priority to ports 1-3 (the cameras watching the vault) and a “low” power priority to ports 4-8 (the cameras watching the broom closet), you can rest assured that should the 40 watts of available power start to fall near, the first 3 ports will occupy the power first.

Power over Ethernet

PoE device prioritization can encompass any number of PoE devices.  Perhaps the IP camera focusing on the building entrance ranks as critically important or maybe even the new PoE bar code scanner at the exit of the repair line is absolutely imperative.  Comtrol’s PoE prioritization can be done on a per-port basis, so any device that is important can be configured that way!

By employing these two power management methods on a managed Comtrol RocketLinx Power over Ethernet switch you can be certain that your power will be monitored and prioritized in a fashion that promotes the longevity of your entire system.

For more information, contact Joe House [email protected] or 763.957.6127


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