Electrical Contracting News (ECN) December 2016 | Page 38
SPECIAL
FEATURE
DATA CABLING
CAUSE 4 THOUGHT
Power over Ethernet (PoE) combines energy and data transmission, allowing network devices to be
powered using copper data cabling. This helps reduce installation costs, simplifies UPS implementation
to raise QoS levels and makes ongoing maintenance easier and cost effective. The latest version –
4PPoE – increases available power levels and allows more demanding terminal equipment to be
powered via network cables. There are, however, some things to consider when introducing 4PPoE.
Matthias Gerber of Reichle & De Massari explains.
W
hen the PoE
standard was
introduced a
decade ago it
supported up
to 15W. PoEP,
introduced in in
2009, supported
up to 25W. The latest version, as described
in the IEEE 802.3bt standard, uses all four
twisted-pairs in the network cabling for
energy transfer, hence its name: 4-pair
Power over Ethernet (4PPoE).
4PPoE can power devices over 100m
of data cable, and has the potential to
make separate power cabling to sizeable
devices obsolete. It can supply at least 55W
of power in level 3 mode and up to 90W in
level 4 – three to four times more than the
current rate.
Defeating the heat
For 4PPoE level 3 and level 4 every
twisted-pair in the network cabling carries
a current of between 650 and 1,100mA. A
significant load compared to using cables
exclusively to transfer data. This increase
in power does cause twisted-pair copper
cables and bundles to heat up, reducing
the possible transmission range. The
resulting temperature increase is not
without side effects. As a rule of thumb,
a long term 10ºC temperature rise halves
the life expectancy of the materials used in
the cables. Also, the temperature increase
must be monitored and managed for fire
and occupational safety reasons. This
potential temperature increase needs to be
taken into account from the very beginning
of a cabling project. That introduces new
challenges when installing data networks.
The elevated temperatures increase
resistance. In turn, the signal is increasingly
attenuated, reducing the possible length of
a link. In the worst case, data transmission
may even become altogether impossible at
high temperatures even though it would work
fine at room temperature. Therefore, the
cabling has to be planned in accordance with
the expected temperatures, should 4PPoE
be used, and the overall link length has to be
adjusted to the assumed conditions.
Thick cable bundles heat up the cables
at the centre of the bundle. What’s more
small diameter cables lead to higher
temperature increase. We recommend
using cables with larger conductor crosssections and avoiding large cable bundles
during installation. Shielded cables
can support longer cabling links as the
cable shield helps to dissipate the heat
from inside the bundle and reduces the
temperature increase.
For example: Category 5e installation
cables with an AWG 24 (0.22mm2) conductor
cross-section reach their ‘natural limits’
quite quickly. Temperature grows rapidly,
especially when they are used in large
cable bundles. A reduction of the link
length is often necessary under these
conditions. With Categorty 6 cables (AWG 23)
temperature increase is not so much of an
issue and the length of the permanent link
can normally reach the full 90m length.
Small sparks, big problems
Cables should be terminated to modules
and plugs as securely as possible to
prevent heat up under 4PPoE use.
Insulation displacement technology,
for example, is ideally suited for this
application. The widely used piercing
technology, on the other hand, penetrates
only the insulation of the copper cables
and creates a loose contact. Over time,
the termination can grow less reliable and
in the worst case this can result in the
destruction of the connection.
‘Using larger
diameter
cabling
for 4PPoE
has clear
benefits in
the area of
diminishing
cable
resistance,
thereby
preventing
power
losses.’
It’s worth noting, that disconnecting
‘live’ contacts may damage them. The
small spark resulting from disconnection
can destroy part of the contact when
under increased load, potentially
permanently impairing its quality
of contact (spark erosion). Plug and
connection modules should be designed
in such a way, that sufficient distance
between breaking point (where the
spark and potential damage occurs) and
nominal contact area are ensured.
Energy savings,
IoT and cooperation
Using larger diameter cabling for
4PPoE has clear benefits in the area of
diminishing cable resistance, thereby
preventing power losses. This not only
reduces the environmental impact of PoE,
but can also deliver savings to operators
of large networks. As the number and size
of data facilities and LANs continues to
grow exponentially, every option for energy
saving, however small, can help.
Soon, structured cabling systems will no
longer be used exclusively to transport data
at up to 10G, but also to power numerous
IP terminal devices, from monitors to
wireless access points. This is in line with
developments around the Internet of Things
– lots of sensor equipped devices can be
hooked to the structured cabling backbone
and powered at the same time.
Creating a cabling system for PoE and
PoEP using realistic framework conditions
is a straightforward task, since transmission
length restrictions rarely need to be taken
into account. However, if structured cabling
is also to be suitable for future 4PPoE
planners/consultants, installers and
suppliers need to closely cooperate in order
to ensure 4PPoE compliance.
38 | December 2016
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11/11/2016 16:42