Basic Electrical Interview Questions

Q:How tube light circuit is connected and how it works?

A :A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided ,the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000 volts. This volt is capable to break the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part.

Q:Why Human body feel Electric shock ? Why we dont get shock in Electric train in motion?

A :Our body is a good conductor of electricity. Current always flow through the lowest resistant path. If we have insulation to our feet example rubber shoes, the circuit does not get complete. Hence we dont get shock. The electric train is well insulated from its electrical system So we dont get shock.

Q:Why, when birds sit on transmission lines or current wires doesnāt get shock?

A :Its true that if birds touch the single one line (phase or neutral) they donāt get electrical shockā¦ if birds touch 2 lines than the circuit is closed and they get electrical shockā¦ so if a human touch single one line(phase) then he donāt get shock if he is in the air. If he is standing on the ground then touching the line (phase) he will get a shock.

Q:what happens if we give 220 volts dc supply to the bulb or tube light?

A: Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element. Low dc voltage will not damage the appliances

Q:1 ton is equal to how many watts?

A: 1 ton = 12000 BTU/hr and to convert BTU/hr to horsepower,12,000 * 0.0003929 = 4.715 hp therefore 1 ton = 4.715*.746 = 3.5 KW.

Q:Why in a three pin plug the earth pin is thicker and longer than the other pins?

A : It depends upon R= Ļl/a where area(a) is inversely proportional to resistance Ā®, so if AREA increases, RESISTANCE decreases & if RESISTANCE is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because it is the first to make the connection and last to disconnnect. This assures Safety for the person who uses the electrical instrument.

Q:If one lamp is connected between two phases will it glow or not?

A: If the voltage between the two phase is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.

Q:Two bulbs of 100w and 40w respectively are connected in series across a 230v supply which bulb will glow bright and why?

A : Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the bulb(P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.

Q : Why should be the frequency 50 Hz 60Hz only why not others like 45, 95 56 or anything , why should we maintain the frequency constant if so why it is only 50 Hz 60Hz?

A :We can have the frequency at any frequency you like, but than you must also make your own motors,high voltage transformers or any other equipment you want to use.We maintain the frequency at 50hz or 60hz because the world maintains a standard at 50 /60hz and the equipments are are made to operate at these frequency.

Q:Battery banks are in connected in series or parallel and why?

A : Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remaining same. Ex : 24 nos. 2V,200Ah batteries connected in series will give 48V,200Ah output (AH = Ampere hours)

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