EuroWire May 2015

Technical article

May 2015

www.read-eurowire.com

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 perfor-

mance 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.

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]

The model was set up to replicate the

proposed measurement method

[3]

, which

allowed for a comparison between theory

and practice.

In order to achieve this, a five-cable

linear configuration was set up with the

intention of providing a good prediction of

the thermal behaviour at the centre cable

without the need for including additional

cables in a model requiring higher

computational resource.

Heat capacity at constant pressure,

density and thermal conductivity material

properties were applied to represent the

constituent parts of the Cat6A 26 AWG U/

FTP cable. These properties were applied

to the copper (Cu) conductor, aluminium/

PET (Al/PET) tape, Low Smoke Zero

Halogen (LSZH) jacket, and polyolefin

insulation, see

Figure 1

. Conduction,

convection and radiation heat transfer

mechanisms

[5]

were accounted for in the

model.

Simulated electric energy was applied

to one pair of each cable in the model.

A stationary solver was used to determine

the thermal behaviour for (a), a point at the

centre of one of the energised conductors

(see probe position in

Figure 1

) and (b), a

2D temperature plot of the cross-section,

Figure 2

Air

Energised

pairs

LSZH

jacket

Probe

Polyolefin

AI/PET tape

▲

Figure 1

Simulation setup in COMSOL Multiphysics