Insulators and Underground Cables Questions and Answers

Answer: D

Pin insulators carry the line conductor on a pin fixed to the cross arm, and they must be made of an electrical insulating material that is also strong in compression and weatherproof. Porcelain, other ceramics and silicon rubber (a polymer used in composite insulators) are insulators and are suitable.

Copper is a very good conductor of electricity, so it would short the line to the cross arm and cannot be used. The answer is copper, option D.

Answer: C

The loop test, such as the Murray loop test, is used to locate a fault in an underground cable. The faulty cable and a sound cable are joined to form a loop, and a bridge with two known ratio arms and the cable's resistance is balanced, so the test works on the principle of the Wheatstone bridge. Faraday's law is the basis of induced voltage, magnetic attraction and repulsion is the basis of moving-iron instruments, and hysteresis is a property of magnetic materials. The answer is Wheatstone bridge, option (C).

Answer: C

Impregnated paper is a cable insulation with high insulation resistance, high dielectric strength and low capacitance, which are its advantages. Its disadvantage is that it is hygroscopic: it absorbs moisture from the air if the lead sheath is damaged, and the moisture lowers its insulation level, so the cable must be sealed. The other three options are advantages and not disadvantages. The answer is hygroscopic nature, option (C).

Answer: D

In a power cable the layers from the conductor outwards are the insulation, the metallic sheath, the bedding, the armouring and the serving. The layer of steel wire or steel tape laid over the bedding to protect the cable from mechanical damage is the armouring, option (D). The serving is the outer covering of jute or compounded fibre over the armour, the metallic sheath keeps moisture out, and the insulation separates the conductors.

Answer: D

Cables are classed by the voltage they are made for: low voltage cables are up to 1 kV, high tension cables up to 11 kV, super tension cables up to 33 kV, extra high tension cables up to 66 kV and extra super voltage cables beyond 132 kV, as the key gives. So extra super voltage cables are made to operate beyond 132 kV, option (D). The values of 11 kV, 33 kV and 66 kV are the upper limits of the lower classes of cables.

Answer: A

Underground cables are buried to protect them from damage, and where they cross a road they have to be deeper because of the weight and the vibration of the traffic. The key gives the desired minimum depth from the ground level to the top of the cable at road crossings as 1 metre, option (A). The greater depths of 2, 3 and 4 metres are more than needed and would add to the cost of digging without improving the protection.

Answer: D

A ground fault in an underground cable is a fault in which current flows from the core of the cable to the earthed lead sheath because the insulation between them has failed. A flow of current between two cores of the same cable is a short circuit or a fault between cores, and an open circuit in the core or in the lead sheath is a break in the conductor caused by damage. The answer is flow of current from the core of the cable to the lead sheath due to insulation failure, option (D).

Answer: C

Pin insulators are normally used up to about 33 KV.

A pin insulator must carry the whole line voltage in one piece. Above about 33 kV it becomes too large, heavy and costly to make reliably, so suspension (disc) insulators, built from a string of identical units, are used instead.

66 KV and 100 KV are suspension-insulator voltages. 25 KV is within the pin insulator's range, but it is not the upper limit the question asks about.

Answer: C

High-voltage insulators are tested by Both A and B: power frequency tests and impulse tests.

In service an insulator must withstand the normal power-frequency voltage, including temporary overvoltages, under dry and wet conditions, which is what power frequency withstand and flashover tests check. It must also survive lightning and switching surges, which impulse tests reproduce with a standard steep-fronted voltage wave.

Either test alone covers only one kind of stress, so neither single option, and certainly not 'None of the above', is complete.

Answer: D

Breakdown strength depends on All of the above.

The breakdown strength of an insulating material is not a fixed number. It depends on the nature of the applied voltage (DC, AC or impulse, and how long it is applied), on imperfections such as voids, cracks and impurities where local discharges start, and on the environment: pressure, temperature and humidity all change it, the last especially for gases and surfaces.

Each of the first three options names a genuine factor, so none alone is the full answer.