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FAQs

 

 

Why would a contractor want to use a flat sheathed cable instead of a circular cable?
Flat sheathed cable has one less operation in manufacture, contains less PVC than a circular cable and is therefore cheaper and being relatively flat it can be easier to install where space is confined. The disadvantage of flat sheath is that any twists makes neat installation difficult and it is very difficult to handle as conductor sizes increase.

PVC is available at various temperature ratings, ie 75°C, 90°C and 90°C-HT. As XLPE is rated at 90°C what's the difference between that and the higher rated PVC?
All grades of PVC are nominally rated at a 75°C operating temperature for normal installations however the higher PVC rated cables can operate at higher temperatures under certain conditions. These conditions are outlined in AS/NZ 3000:2000, but generally limit the time at which they can operate at those temperatures and restrict their usage where there is no potential for mechanical damage (PVC softens considerably at these temperatures). XLPE does not soften at 90°C and hence does not have the installation and operation restrictions of PVC.

Does the change of V105 to V90-HT mean that this grade of PVC has been down rated?
Some contractors interpreted the V105 rating as a continuous operating temperature of 105°C, which is incorrect. The V105 grade should only be operated at this temperature for limited periods under certain conditions. In order to avoid this confusion, the name has been changed to V90-HT, ie indicating that the cable can operate at high temperatures for limited times. The actual composition of the PVC has not changed.

What does the .6/1kV designation mean?
This is the method adopted generally by Australia, UK, Asia and Europe in indicating the voltage rating of a cable. The first number indicates the voltage rating of the insulation to ground and the second number indicates the voltage rating from one insulated conductor to another insulated conductor. Generally if a cable is only referred to by one voltage, the second number is used. For example a 19/33kV cable is often referred to as a 33kV cable.

Why are generally used cables designated as 450/750V when only 240 volts are being carried?
A minimum thickness of insulation is required to offer a reasonable level of resistance to mechanical damage. If only enough insulation was used to insulate to 240 volts, the covering would be too easily damaged.