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What it means and why device specifications still matter
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Power over Ethernet (PoE) is the term used when the ethernet cable to an electrical device is also used to power that device through Power Sourcing Equipment (PSE). Sourcing power in this way is often preferable for surveillance cameras as it means physical camera placement is not restricted by the need for direct, localised power supply.
In the case of PoE-enabled surveillance cameras, the PSE is typically a PoE switch that communicates with the camera to automatically identify power requirements, i.e. how much wattage is required by the individual device. A PoE switch will not deliver power to a device that is not enabled as it does not know how much power to deliver and, therefore, could damage it.
Any device capable of being powered this way will have a PoE classification (see table).
These are globally recognised parameters that specify the maximum wattage range a device could need and the minimum power output a PSE device needs to produce in order to meet that requirement (the latter is higher to allow for power loss in transmission over cable length).
|Class||Usage of Class||Maximum Power Levels Output at the PSE||Power Range Available to Powered Device||Class Description|
|0||Default||15.4W||0.44 to 12.95W||Classification Unimplemented|
|1||Optional||4.0W||0.44 to 3.84W||Very Low Power|
|2||Optional||7.0W||3.84 to 6.49W||Low Power|
|3||Optional||15.4W||6.49 to 12.95W||Mid Power|
|4||Reserved at 802.3af||Treat as Class 0||0.44 to 12.95W||Classification Unimplemented|
|4||Reserved at 802.3af||30W||12.95 to 25.5W||High Power|
Most surveillance cameras have a power requirement of over 7W and are, therefore, at least Class 3. PTZ cameras are typically at least Class 4 because moving the pan and tilt part of the camera requires more power. It is important to recognise that a camera’s PoE classification may have to take into account additional options that might be available, such as IR LEDs, even if they are not present on the actual camera unit installed.
Classification is done as a maximum range, e.g. Class 0 = 0.44 to 12.95W. This means that even if a camera does not need the maximum power specified by its PoE classification all the time, the PoE switch has to ensure the full amount. In this case, 12.95W is available if needed.
Unused power cannot be reallocated to another more power-hungry device on the same switch unless this is done manually using a fully managed switch, which is typically more expensive. It is also worth noting that cameras with dual power sourcing, i.e. able to switch between direct power and PoE for network failure protection, would still be allocated the maximum wattage they might require even if PoE is never used.
PoE classification provides an important international mechanism for network specification, but because it is based on a maximum range basis, the most effective system design takes into account device power specification and PoE classification. This gives the clearest picture of requirements and capabilities.