EuroWire May 2015

Technical article

site-to-site, caution should be taken when installing PoE systems into uncontrolled and/or warmer environments. Further consideration should be given to the correlation of simulated data and that of the installation environment. On one hand, the simulation may be based on a worst case scenario. However, in reality, the duty cycle may dictate that the power is only supplied for a fraction of the time. Good installation practices should be implemented wherever possible, such as minimising bundle sizes, accounting for temperature rise for maximum cable lengths, and keeping pathways and spaces free of thermally insulating materials. It is important to note that, while excellent correlation was seen between simulated and measured results for a single cable setup, this work was not intended to replicate the behaviour of cables in bundles. However, it is anticipated that good correlation between theory and practice will also apply to bundled configurations in free air and in a variety of cable containment systems, ie tray, trunking, conduit, etc. A comparison between the CCA 24 AWG UTP and Cat6A 26 AWG U/FTP samples show that it is possible for cables with smaller conductors to radiate less heat than those of larger conductors when supplied with identical DC current values. It is also known that the conductive foil in screened cables act as a heat sink which helps to reduce the amount of heat radiated from the cable [10] . Therefore, it is important to take into account the construction of cable, and not only the conductor diameter for PoE system deployment. and therefore,

Measurement Simulation Approximation

Δ Conductor temperature (ºC)

Current (A)

▲ ▲ Figure 4 : Simulated, measured and approximated change in conductor temperature

Individual Value Plot of Differences (with Ho and 95% t-confidence for the mean)

Differences

▲ ▲ Figure 5 : Individual value plot of temperature differences

The results presented in this paper show the temperature rise of one pair energised with DC power using a cable located in a controlled 20°C environment. Realistically, the ambient temperature will vary from

Temperature rise due to the Joule heating effect is known to be proportional to I 2 R losses [8] so, as current is fixed for each measurement point, the resistance of the cable pair under investigation will differentiate temperature rise from one cable to another. Therefore, as expected, the cable with highest DC resistance will have the most temperature rise, and vice versa. Discussion Heating cables is known to increase attenuation [9] which has a limiting effect on cable reach. In relation to PoE, the maximum temperature is likely to be in the proximity of the energised conductors which may be used for data transmission. Therefore, the consequences of DC powering on attenuation of the same pair should be taken into consideration.

▼ ▼ Figure 6 : Measured change in conductor temperature

Cat6A-26AWG CCA-24AWG Cat5e-24AWG

Δ Conductor temperature (ºC)

Current (A)

80

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May 2015