WCA July 2015

In other words, as the temperature and resistance increased, the voltage required (in order to meet Ohm’s Law) was larger than the maximum voltage 60V) of the bench power supply. A current value of 1.95A was chosen in order to generate the fifth data point. Figure 6 shows the change in conductor temperature, versus DC current level, which was calculated from the measurement. For the CCA cable sample, approximated conductor temperature rise was found to be: (INSERT IMAGE/CALCULATION 2 HERE) 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. 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 site-to-site, and therefore, caution should be taken when installing PoE systems into uncontrolled and/or warmer environments. ❍ ❍ Table 1 : DC loop resistance of pair under investigation for each cable type

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 Using the approximation, a current of 3A would provide a temperature rise of 20.7°C for a single cable within an environment fixed at 20°C. The correlation between simulated and measured results was further investigated from a statistical point-of-view using a Paired t-test via Minitab software [7] . Figure 5 shows an individual value plot of the temperature differences between simulation and measurement, which also shows the 95 per cent confidence interval based on these differences. The results shows that 95 per cent of additional simulated and measured values are expected to fall within the ±0.1 difference range, confirming excellent correlation. As such, the null hypothesis of no difference in mean values between the two sets of data is not rejected. Copper clad aluminium A sample of UTP CCA cable with 24 AWG conductor size was acquired and measured as per the Cat6A 26 AWG U/ FTP cable sample in section 3. The DC loop resistance of the pairs under investigation for each cable type are given in Table 1 . For comparison, a Cat5e UTP cable with 24 AWG solid copper conductors was included in the study. Due to the high resistance of the CCA cable under investigation, the high voltage required to provide a current of 2.2A was not possible using the bench power supply.

DC loop resistance (Ω)

AWG

Cat6A

26 24 24

23.3 28.4 18.2

CCA

Cat5e

❍ ❍ Figure 6 : Measured change in conductor temperature

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

Δ Conductor temperature (ºC)

Current (A)

55

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Wire & Cable ASIA – July/August 2015