Domestic Electric Circuits - UNSOLVED PRACTICE SET

Q7. Name the three wires in a domestic electric circuit. State the colour code for each (Indian standard) and describe the function of each wire.

Q8. What is earthing? Why is it essential for metal-cased appliances like refrigerators, washing machines, and ACs in Indian homes?

Q9. What is a short circuit? What causes it? Why is it dangerous? How does a fuse or MCB protect the circuit from short circuit damage?

Q10. Explain what is meant by 'overloading' a domestic circuit. Give one real-life Indian example of overloading and its consequences.

Q11. Why are separate circuits used for lighting and heavy appliances (like ACs and geysers) in Indian homes?

Q12. The electricity meter in a home measures consumption in kWh. A family in Delhi reads 1240 units on their meter at the start of the month and 1355 units at the end. Calculate the bill at a slab rate of Rs. 6 for first 100 units and  Rs. 8 for additional units.

Q13. Describe the domestic electric circuit in an Indian home in detail. Your answer must cover:
(a) the supply from the electricity board — voltage, frequency, and type (AC),
(b) the energy meter and main fuse box,
(c) the three wires: live, neutral, and earth — their colours, potentials, and functions,
(d) why all appliances are connected in parallel, and
(e) the distribution of circuits: separate lines for lighting, fans, and heavy loads like ACs and geysers.

Q14. A family in Pune is experiencing repeated MCB tripping. Investigate the possible causes:
(a) Define overloading — what current rating MCBs are typically used in home circuits?
(b) The family has a 5 kW AC, 2 kW geyser, and 800 W washing machine all on the same 20 A circuit at 220 V. Calculate the total current drawn. Does this exceed the MCB rating?
(c) Define short circuit and explain how it differs from overloading.
(d) What is the proper solution: add a new separate circuit or use a higher-rated MCB? Justify.
(e) What safety device protects the user from electric shock even if the MCB has not tripped yet?

Q15. Explain the importance of earthing in domestic circuits with detailed reasoning.
(a) What is earthing — describe the physical connection to the ground.
(b) Why is the metal casing of a washing machine earthed? Trace what happens when a live wire accidentally touches the casing without and with earthing.
(c) What is a Residual Current Device (RCD) / Earth Leakage Circuit Breaker (ELCB) and how does it provide additional protection?
(d) What is the difference between a fuse (protecting the circuit) and earthing (protecting the user)?
(e) How does the three-pin plug in India connect live, neutral, and earth — and why is the earth pin the longest?

Q16. A house in Hyderabad is on a 230 V, 20 A main circuit. The family uses: AC (1.8 kW), geyser (2 kW), refrigerator (150 W), TV (80 W), and lights + fans (300 W).
(a) Calculate the total load in watts.
(b) Calculate the total current drawn.
(c) Is the 20 A MCB sufficient?
(d) If they add a 1500 W microwave, what is the new current?
(e) What must the family do to safely add the microwave?

Q17. A family's electricity meter reads as follows: Start of month: 4521 kWh; End of month: 4756 kWh. The tariff is: First 100 units @ Rs. 4.50; Next 100 units @  Rs. 6.00; Above 200 units @  Rs. 8.50.
(a) Calculate total units consumed.
(b) Calculate the bill in three slabs.
(c) Calculate total bill amount.
(d) The family wants to reduce their bill by 20%. By how many units must they reduce consumption?
(e) Suggest two specific appliance changes to achieve this.

Q18. A short circuit occurs in a 2 m long section of copper wire (ρ = 1.7×10⁻⁸ Ω·m, area = 1 mm² = 10⁻⁶ m²) carrying 30 A (fault current).
(a) Calculate the resistance of the wire section.
(b) Calculate the heat generated per second using H = I²Rt (t = 1 s).
(c) Explain why this rate of heat production is dangerous.
(d) A 15 A fuse is in series with this wire — how quickly will it blow?
(e) If the wire is behind a wooden wall and the MCB does not trip fast enough, describe the hazard.