EuroWire May 2015

Technical article

May 2015

www.read-eurowire.com

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.

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

), equation (1),

and after the temperature had stabilised

), equation (2). Change in (or delta)

conductor temperature (Δt) was then

calculated using initial (R

) and stabilised

) resistance, equation (3).

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

, 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)

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

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)

AWG

DC loop

resistance (Ω)

Cat6A

23.3

CCA

28.4

Cat5e

18.2

Cable sample

Thermo couple

Temperature (ºC)

DC power supply leads

▲

Figure 2

Cross-sectional temperature plot

▲

Figure 3

Measurement

setup

▲

Table 1

DC loop resistance of pair under

investigation for each cable type