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The current capacity of conductors is designed by limiting the value of current to that which would not result in too large a temperature rise of the conductor and insulation. Given the insulation is in contact with the conductor, the conductor temperature and maximum insulation temperature are the same value.
Standard practice is to limit the operating temperature of thermoplastic material (i.e. PVC) to 70°C and thermosetting (i.e. XLPE) to 90°C. This provides an acceptable balance between conductor size and the life expectancy of the cable.
As a rule of thumb, for every 10°C rise in temperature, the life of the insulation is halved. For example, an XLPE cable designed to operate for 40 years at 90°C, will have a statistical life expectancy of around 20 years. Conversely lowering the temperature increases the life expectancy. For more accurate assessments, the Arrhenius equation can be used.
By monitoring the cable temperature, particularly at anticipated hot-spots, the overall health and functioning of the cable can be assessed. Cables running cooler than design can be expected to have an increased life, while those running hotter a decreased life.
Knowledge of the temperature can also be used for decisions on if the cable loading can be increased, or if there is a need to reduce the loading. Cables running slightly hotter can be flagged for regular review, and those reaching limits for remedial action. Actual temperatures can be fed back into current and future designs to optimise cable selection and installation costs.
Measuring the outer sheath temperature is not the same as measuring the conductor temperature. Due to heat transfer through the insulation, bedding and armour, the sheath temperature will always be less than that of the conductor.
However, this does not mean we cannot use the sheath temperature. The sheath temperature still provides valuable information about the cable, albeit not as accurate as measuring the conductor temperature itself. We can still assess the cable, look for over or under loading, and potential abnormal conditions. For example, iF we want conductor temperature alerts at 80°C and 90°C, but only have access to the sheath temperature, we may set the alarms at 60°C and 70°C.
The advantage of sheath temperature measurement is this it does not interfere with the constructional integrity of the cable. The temperature sensor is attached only to the outside of the cable. To measure the conductor temperature directly, it is necessary to embed sensors at the time of manufacture or open up the cable to do this.
As discussed, the cable can be assessed by the use of sheath temperature and without knowledge of the conductor temperature. However, in large or commercially sensitive installations, there may be a requirement to estimate the conductor temperature based only on a knowledge of the sheath temperature.
The standard approach to this issue is to use a thermal model of the cable. In a thermal model, the sheath temperature and conductor current can be used to calculate the expected conductor temperature. Some things to note: