EuroWire May 2015

Technical article

For the Cat6A 26 AWG U/FTP cable, this approximation was found to be: (INSERT IMAGE/CALCULATION 1 HERE) 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. 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) ▲ ▲ Table 1 : DC loop resistance of pair under investigation for each cable type AWG DC loop resistance (Ω) Cat6A 26 24 24 23.3 28.4 18.2 CCA Cat5e

Temperature (ºC)

▲ ▲ Figure 2 : Cross-sectional temperature plot

Temperature and voltage data was logged at 15 second intervals using National Instruments LabVIEW software [6] . The temperature of the cable sample increased due to the Joule heating effect, and after a certain time, the temperature stabilised. At this point in time, the heating due to the DC power input became equal to the radiated power of the sample and the temperature was prevented from rising further. Conductor resistance was calculated based on voltage immediately after the power was switched on (U 0 ), equation (1), and after the temperature had stabilised (U T ), equation (2). Change in (or delta) conductor temperature (Δt) was then calculated using initial (R 20 ) and stabilised (R t ) resistance, equation (3).

Cable sample

Thermo couple

DC power supply leads

▲ ▲ Figure 3 : Measurement setup

From the 2D plot, and as expected, the maximum temperature of the arrange- ment is evident in the proximity of the energised conductors. Test method and results The test method proposed by IEC Subcommittee 46C [3] was followed in order to establish the rise in conductor temperature due to DC powering. This method involved measuring voltage supplied and jacket temperature using a 100-metre sample of cable wound onto a reel and positioned within an environmental chamber fixed at 20°C, see Figure 3 . This method was followed using a sample of Cat6A U/FTP cable with solid copper 26 AWG conductors, as simulated in section 2. The cable sample was conditioned at 20°C for at least 16 hours before testing. A thermocouple of J type was positioned along the jacket at the halfway point of the cable. Using a Keithley 2200-60-2 (60V, 2.5A) bench power supply operating in constant current mode, a current (I) of 0.6A was applied to the pair under test with the far end of the sample short circuited.

This methodology was repeated using four different current (I) values, ie 1.0A, 1.4A, 1.8A and 2.2A. Figure 4 shows the change in conductor temperature versus DC current level simulated at the probe (see Figure 1 ) and calculated from the measurement. Results show a linear relationship with both delta conductor temperature and current plotted on logarithmic scales. Based on this relationship, it was possible to apply an approximation, in the format Δ t = x * I y , which could be used to predict conductor temperature rise for current values outwith the range measured.

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May 2015