WCA July 2015

From the 2D plot, and as expected, the maximum temperature of the arrangement 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. 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).

Temperature (°C)

❍ ❍ Figure 2 : Cross-sectional temperature plot

Cable sample

Thermo couple

DC power supply leads

❍ ❍ Figure 3 : Measurement setup 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 .

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. For the Cat6A 26 AWG U/FTP cable, this approximation was found to be: (INSERT IMAGE/CALCULATION 1 HERE)

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Wire & Cable ASIA – JulyAugust 2015