Power: How to Choose a PoE solution

June 19, 2012
How to power video, access control and more over the Ethernet network

First used for voice-over-IP (VoIP) telephones, power over Ethernet (PoE) technology has gained popularity among system designers and installers as a means of introducing power safely to other network devices through the same Category 5 or higher cable that transmits Ethernet data.

In security and surveillance systems, power up to 15.4 watts can be supplied to network cameras and other devices using the original IEEE 802.3af standard. The newer IEEE 802.3 at standard, also known as PoE+, can supply up to 25.5 watts to power heaters and blowers for outdoor cameras and other devices. An even higher-power PoE standard is on the horizon that will provide 60 watts using all four wire pairs in the cable instead of the traditional two.

When a PoE device is plugged into a PoE source, a simple “handshake” authentication is performed, which ensures the proper power level and minimizes the risk of damaging non-PoE devices if accidentally connected to a PoE source.

Rather than using standard-type power supplies, integrators are increasingly using PoE network switches or incorporating PoE midspan devices, which inject power into an Ethernet cable after it leaves the switch and before it reaches the network device.

How to choose

Selecting the right PoE switch for a job can be challenging. For example, PoE switches that were initially developed for VoIP applications do not provide enough current when used with video surveillance and access control devices; and pan-tilt-zoom (PTZ) and infrared (IR) cameras require even more power than conventional IP cameras.

Here are some factors to consider when choosing a PoE switch:

  • Is the power supply in the switch designed for constant and consistent current draw?
  • Does the switch have PoE on all ports?
  • Is there enough current to supply every device?
  • Will it power 25.5 watt devices?
  • Does the unit provide over-current protection for each port?
  • Does the security of the switch management software need to be upgraded?
  • Is the PoE switch UL listed for information technology and UL294 for access control?

 The midspan advantage

The power consumption for PoE security devices can add up quickly when there are multiple IP cameras, access control devices, outdoor heater/blowers, etc., and a common pitfall is underestimating how much power is needed. It might be difficult to find a PoE switch that will achieve the necessary power consumption while meeting the PoE+ standard. Using PTZ and/or infrared cameras can aggravate the challenges further. The answer is a PoE midspan, which simplifies power management and ensures system functionality, along with adding scalability, flexibility (such as cascading for longer distances) and the ability to easily match the power requirements for an edge device.

PoE midspans also ensure power if a network connection is lost. For example, many IP cameras now can record images to an optional SD card as a backup in the event of a lost network connection, but the feature does not work if the camera's power supply is interrupted. With PoE midspans, cameras keep recording.

Midspans also enable power to be allocated to devices based on their specific requirements. Managed PoE midspans display how much current a device is using and the total power consumption of all the devices.

Here are some considerations when choosing a PoE midspan:

  • How many total ports for power do you need?
  • What is the total power consumption of all PoE devices?
  • Are there devices that will draw PoE+ 25.5 watts or 60 watts?
  • Is your PoE midspan managed?
  • What agency approvals does the product carry?