Question 4
High-voltage transmission cable insulation would most likely be manufactured from
Not quite. PVC insulates the wiring in your house, but that is low voltage. At high voltage PVC wastes energy as heat inside the insulation, so transmission cables do not use it.
Not quite. Polypropylene insulates well, but it is stiffer than polyethylene and turns brittle in the cold. A transmission cable has to bend in any weather without cracking.
That is it. Step through the hints below to see why.
Not quite. Polystyrene is a good insulator, but it is rigid and brittle. A cable is coiled, pulled and bent, and polystyrene insulation would crack.
Turn the stem into demands. “High-voltage” means the insulation must hold back a very large voltage without wasting energy. A transmission cable is also coiled, pulled through conduit and left outdoors or underground, so it has to bend without cracking in any weather.
All four options keep water out, so that demand cannot separate them. Spend your time on the other two.
Polystyrene is rigid and brittle, so it fails the bend. That rules out (D).
Polypropylene insulates well, but it is stiffer than polyethylene and turns brittle in the cold. That rules out (B) and leaves (A) and (C).
PVC is the plastic on the wiring in your house, which is why it tempts you. At high voltage it wastes energy as heat inside the insulation.
Polyethylene does not, and it stays flexible in the cold. High-voltage cables use cross-linked polyethylene, XLPE. That is (C).
Polyethylene is the only option that insulates at high voltage and stays flexible in any weather. The 2019 syllabus that 2020 students sat listed polyethylene for “plastic containers, water pipes and cable insulation”, and it gave cable insulation to none of the other three.
With no 2020 data, my bet for the most chosen wrong answer is (A). PVC is the insulation you see on household wiring, so it feels familiar, but “high-voltage” is the word that rules it out.
The 2020 subject report says all ten multiple choice questions were simple familiar, and this one is a single recall step from the job to the plastic. For 2026, none of the four options is named in Unit 4. Polyethylene, polypropylene, PVC and polystyrene now appear only in Unit 2, which the external exam does not assess. The method transfers: list what the job needs, then keep the one option that meets every demand. If a 2026 question asks which Unit 4 plastic insulates best, the syllabus gives “excellent electrical insulation” to ABS alone.
If I had to guess where you might lose marks, it would be choosing (A) because PVC is the plastic you have seen on wires. The stem adds a condition, “high-voltage”, and that condition is what rules PVC out.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording and any figures are from the 2020 QCAA Engineering external assessment, © State of Queensland (QCAA) 2020, licensed under CC BY 4.0, and have been adapted, except where a third-party source is named. Tangent Tuition is not affiliated with the QCAA.
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