How to avoid a PoE disaster

How to avoid a PoE disaster

Power over Ethernet (PoE) is a great way to power remote equipment. Typical examples of PoE include powering VoIP phones and IP video cameras. However, it may come as a shock to realize that the first PoE standard (IEEE 802.3af) was only released in 2003, and prior to that equipment manufacturers used various ad hoc methods to remotely power their equipment. This equipment is still in use today, and technicians working on existing systems need to be aware of their potential to damage network equipment.

PoE History

There are two PoE standards, the IEEE 802.3af with a 12.95 watt maximum power output and the 802.3at Type 2 that has a 25.5 watt power output. Both systems are inherently safe in that they will only power up when they detect that a PoE-capable device is connected and will automatically shut down if the device is disconnected. PoE voltages vary between 37 and 57 volts.

Power is supplied through standard Cat 3, Cat 5 or Cat 6 cables, although the higher power PoE is only available on Cat 5 or Cat 6 cables. Power is supplied in two ways. Lower-speed Cat 5 networks that only use 4 cores can source positive power on pins 4 and 5 and negative power on pins 7 and 8. Alternatively, PoE can be multiplexed with active data pairs providing positive power on pins 1 and 2 and negative power on pins 3 and 6.

Legacy Systems

Before the IEEE PoE standards were introduced, several manufacturers went ahead with their own PoE schemes using the spare cores in Cat 5 cables. Usually, DC power was injected into the spare cores by a power supply, which was then tapped off at the powered device. Some used a nominal 48 volt power supply, but others used 24 and even 12 volts DC. In addition, none of these schemes used the PoE signature detection process and circuits were always energized.

DIY Power Injection

Another hazard to be aware of is that amateurs sometimes deliberately injected a DC voltage into a Cat 5 cable to power remote devices. Although more common in domestic situations, it may also be found elsewhere. Just like the legacy systems mentioned earlier, no PoE protection is provided and these injected voltages can easily damage newer equipment.

Faulty PoE Device

The third pitfall to watch out for is faulty equipment. It is not unknown for stray voltages to be found on Ethernet cables due to poor circuit grounding. Another risk is that unusual DC voltages may be present on a device that has suffered damaged as a result of a lightning strike or power surge.

Avoid a Disaster by Using a Digital Network Cable Tester

To avoid surprises when working on existing networks, it is always best to check for wrong connections and the presence of unexpected voltages. The easiest way to do this is with a digital network cable tester, such as the network cable testers manufactured by T3 Innovation. Using the Cable Prowler™, you can detect and measure PoE as well as identify cable faults such as split wiring, open circuits and shorts. Another useful feature is the ability to accurately measure the distance to a fault, making it easy to locate and fix a faulty cable. The built-in tone generator helps you locate the right cable or even the location of an individual wire in the cable. The Net Prowler™ and Net Chaser™ digital cable testers incorporate similar capabilities, plus a host of features to help with network mapping, protocol discovery and measuring link speed. Avoid a PoE disaster: check your circuits with a digital tester.