Electrical Safety and Earthing Questions and Answers

Answer: A

Water can be used as a fire extinguisher if the fire involves ordinary combustibles such as wood and paper, which are class A fires. Water cools the burning material below its ignition temperature and soaks it.

It must not be used on electrical equipment, since water conducts electricity and can give a shock, nor on burning flammable liquids, which float on water and spread the fire, nor on flammable gases, which are not put out by cooling. So the first option is correct.

Answer: C

For a fire from flammable gases, dry powder is the most suitable extinguisher of the options. Dry chemical powder interrupts the chain reaction of the flames and is used for class C (gas) fires, as well as for liquid fires and, in multipurpose form, other classes.

Water and foam do not put out a gas fire, because the flames are not cooled by them and foam cannot cover a gas. Carbon dioxide can smother small fires and is mostly used on electrical equipment, but it is less effective on a gas fire that keeps burning from a leak. So the third option is the one the key gives.

Answer: A

Both statements are true. Foam and water conduct electricity, so they cannot be used on electrical equipment, where they may give a shock to the person using them and damage the equipment. Carbon dioxide or dry powder is used on such fires.

Halon extinguishers have been made with halogenated hydrocarbons such as bromochlorodifluoromethane (halon 1211) and carbon tetrachloride (an older halon-type agent). The statement says they 'may be filled with' these, which is true. The key therefore gives 'True, True', the first option, and 'False' for either statement would be wrong.

Answer: D

KCCB is not a valid type of circuit breaker. ELCB (earth leakage circuit breaker), MCB (miniature circuit breaker) and RCCB (residual current circuit breaker) are all in use, to protect against earth leakage and overload and short circuit.

There is no circuit breaker called KCCB, so the fourth option is the one that is not valid. Of the three real ones, the ELCB is the older voltage-operated earth leakage device and the RCCB is the current-operated one that has replaced it.

Answer: B

MCCB stands for 'moulded case' circuit breaker. It is a breaker for higher currents than an MCB, with its parts sealed in a moulded insulating case, and with thermal and magnetic trips to give protection from overload and short circuit.

The words 'medium current', 'maximum current' and 'minimum current' are not what the letters stand for, so the second option is the answer.

Answer: A

The key gives 'L' series MCBs for the protection of circuits with resistive loads. I could not confirm this classification from a standard I could check, so the answer rests on the key. In older makers' classifications the series letters indicated the tripping characteristic, with one series for lighting and resistive loads and others for motors and inductive loads.

The 'R', 'D' and 'A' series are the other options, and the D type is generally known for loads with high inrush currents such as motors and transformers, so it is not the one for resistive loads. The first option is the answer given.

Answer: B

The key gives 16 mm as the minimum diameter of rod electrodes made of steel or galvanised iron. This is the dimension given for earthing rods in the Indian earthing code. I did not check the clause for this answer, so it follows the key.

A rod of 12 mm would be too thin to be driven into the ground without bending, and the larger sizes 25 mm and 35 mm are bigger than the minimum that is asked. So the second option is the answer.

Answer: A

The earth tester works on the principle of the fall of potential method. A current is sent through the earth electrode to an auxiliary electrode, and the potential drop between the earth electrode and a second auxiliary electrode is measured, from which the earth resistance is found.

Kirchhoff's current and voltage laws are general laws of circuits, and the rise of potential method is not the one used. So the first option is correct.

Answer: B

The key gives once every two years for the testing of earthing systems of the supplier. The rule requires the test to be made on a dry day during the dry season, when the soil resistance is highest, so that the result is the worst case. I could not check the interval in the text of the rules, so the answer follows the key.

'Once every year', 'twice every year' and 'once every five years' are the other intervals offered. The second option is the one given.

Answer: D

The relay in the diagram is a dry reed relay. A reed relay has two thin magnetic metal blades sealed in a small glass tube, which come together when a magnetic field is applied by a coil or a magnet. The picture shows the slim glass capsule with its two leads.

A thermal relay uses a bimetal strip heated by current, an electromechanical relay has a coil, an iron armature and contacts in an open frame, and a hybrid relay combines an electronic circuit with a mechanical relay. So the fourth option is the answer.