Automation Canada - Cables & Connectors Issue - 13

SEIZE THE POE OPPORTUNITY
WITH THE RIGHT TESTING AND
TROUBLESHOOTING
BY MARK MULLINS
As an electrical professional, there's no doubt that you're now being
exposed to Power over Ethernet (PoE) as a means of delivering
low-voltage DC power over network cabling to LED lights and a variety
of other components - from nurse call and building automation
devices, to cameras and access controllers. And with a 90% growth in
the next 10 years from PoE lighting alone, it's no wonder that the IBEW
is experiencing huge demand as a training delivery agent for the
Network Cabling Specialist (NCS) designation they're now referring to
as " the trade of the future. "
Whether you're already designing and installing PoE systems, or
gearing up to seize this amazing opportunity, you need to ensure that
your PoE projects go smoothly. And that comes down to understanding
how PoE works and how it's classified - and then learning the best
practices to ensure it will work and will keep working.
A SAFE HANDSHAKE WITH CLASS
You're already familiar with how DC power works and are likely used
to implementing conversion technology and step-down transformers to
power LED lighting. But PoE is a bit different in that it eliminates the
need for any AC power or conversion, delivering DC power directly
over the same twisted-pair cable used to connect devices to a local area
network and enable data transmission.
In a PoE circuit, the DC power is delivered to a powered device (PD)

C A N A D I A N A U TO M AT I O N

from power sourcing equipment (PSE), which is typically a network
switch located in a telecom room. It can also be a midspan PoE injector
for scenarios where a PoE switch is not available. In all PoE
implementations, power is delivered by the PSE only after it is
requested by the PD, and if the PD is disconnected, the PSE will remove
power. This " handshake " that happens between the PD and PSE makes
PoE safer than other power delivery methods that always have power
flowing, regardless of whether you've got a device plugged in. Plus,
we're only talking about voltages in the range of 43 to 57 VDC so it's
considered safety extra low voltage (SELV).
In a PoE installation, power can be delivered differently depending on
the power level and type of PoE being used, which is defined by its IEEE
standard. Early PoE standards, like IEEE 802.3af Type 1 that delivers a
maximum of 15.4 W (13 W available at the device) and IEEE 802.3at
Type 2 (PoE+) that delivers a maximum power to 30 W (25.5 W
available at the device), deliver power over just two pairs of a four-pair
cable. For more power hungry devices, IEEE 802.3bt can also deliver
power across all four pairs, either via Type 3 that delivers a maximum of
60 W (51 W available at the device) or Type 4 that delivers a maximum of
90 W (71.3 W available at the device). Regardless of which type is being
used, DC power is transmitted over the pairs by applying
common-mode voltage, meaning that current is split evenly between
each pair and between the two conductors of a pair.

VOLUME 3, ISSUE 2

13
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