WCA July 2015

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53
Wire & Cable ASIA – July/August 2015
Measured and simulated
DC powering of data
cables for power over
Ethernet
By Stephen W Simms, Brand-Rex Ltd
Abstract
The increasing demand for higher power levels in Power
over Ethernet (PoE) systems is evident, with a variety of
non-standard products currently available on the market
which provide power levels in excess of those stated in
IEEE 802.3at.
Higher power levels will allow PoE to be used in a wider
range of applications. However, they will also increase
performance risk. With this increase in demand for
more power, and the fact that installations using PoE
technology differ greatly in terms of their configuration
and environment, it is beneficial to mitigate risk by using
numerical simulation.
The work presented here provides numerical simulation
and experimental verification of the thermal properties
of data cables under DC powering which is used in PoE
applications.
Introduction
The supply of DC power to end devices along the same
electrical path used for AC signal communication has been
successfully employed for many years, eg in telephones
and audio equipment.
The technique used to provide this functionality is
commonly known as ‘phantom powering’. In relation to
Ethernet, this technique allows power from the Power
Sourcing Equipment (PSE) to be delivered to the Powered
Device (PD) on the same pair that is used for data.
The DC power is applied to the centre tap of the signal
coupling transformer and does not interfere with data
transfer. This allows PoE to be deployed over 1000BASE-T
systems, in which data is carried on all four pairs. IEEE
802.3at standardisation in 2009 stated the system
parameters required for Type 1 (PoE) and Type 2 (PoE+)[1].
The standard classifies nominal highest DC current
values of 0.35A and 0.60A per pair, for Type 1 and Type 2,
respectively. Some of the most common applications which
use PoE technology include wireless LAN access points,
VoIP telephones and network cameras.
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❍ Figure 1: Simulation setup in COMSOL Multiphysics
Applying electric current to a conductor releases heat
energy, an effect known as Joule heating. In relation to
Ethernet cables and components, this heating effect
causes concern due to the rise in attenuation, which has
a limiting effect on link length. This concern is heightened
for cables with a higher resistance than standard cables,
eg copper clad aluminium (CCA)[2], and smaller diameter
(26 AWG) solid copper conductor cables.
In 2009, IEC subcommittee 46C put forward a test method
(46C/906/NP) entitled ‘Proposal for measuring of heating
of data cables by current’[3] .
In this paper, the aim is to achieve a strong correlation
between simulation and the proposed measurement
method regarding the DC powering of Ethernet cables
for PoE applications. The paper also aims to compare
temperature rise due to DC powering of CCA cable with
cables which have solid copper conductors.
Numerical modelling
A 2D model was set up using COMSOL Multiphysics
4.4, a software package which utilises the Finite Element
method[4].