As mentioned earlier, Ethernet is also being deployed as a means for delivering power. This is very attractive to designers who also need to power the end equipment. Terminal equipment, VoIP phones and security video cameras are examples. The basic Power over Ethernet or “PoE” standard is IEEE 802.11af. This standard describes a nominal 48V system that is capable of delivering up to 13W of power to the end equipment. In the standard, the powered equipment is referred to as the “Powered Device” or “PD” for short. The source is referred to as the “Power Supply End” or “PSE”. There are two wiring options allowed. The first, referred to as “Mode A”, places the power on the signal pairs of a 10Base-T cable. While PoE can be applied to Gig-E systems, Mode A assures that power will be delivered in a 10Base-T or 100Base-TX system using only two pairs of the RJ45 connector. The power is applied in a scheme known as “Phantom Powering” that takes advantage of the fact that all Ethernet interfaces use a coupling transformer that essentially “floats” the pairs from ground. Power is fed to the centertaps of two transformers. This way, there is no voltage between the conductors in a pair, the voltage only appears between the pairs. Also, this scheme splits the current between both conductors in each pair, maximizing the available power transfer to the PD. Another option for the power wiring is called “Mode B”. Mode B uses the spare or alternate pairs in a 10Base-T system but is otherwise very similar. Because the standards committees often lag behind the practice in the industry, Cisco, 3Com and others have implemented non-standard PoE systems in the past.  Also, designers of some proprietary systems have ignored the standard and created “homebrew” versions to meet their specific needs. Now IEEE 802.11at is currently under development as the new standard for “PoE+” which will allow up to 24W to be delivered to the PD. The bottom line here is that most designers do not know exactly what will get plugged into the equipment and their protection strategy should be capable of handling any situation. It is recommended that, at a minimum, all pairs be protected at a 64 to 90V protection level.