AC Frequency, Domestic Circuits and Fuse Ratings | Class 10

Quick answer: In India, household electricity is alternating current (AC) at 220 V and 50 Hz. The current changes direction twice in every cycle, so it reverses 100 times per second, i.e. every $\frac{1}{100}$ second. A fuse protects circuits from overloading and short-circuiting. To choose a fuse, find the normal current with $I = \frac{P}{V}$ and pick the next standard rating above it: for a $1\text{ kW}$ iron at $220\text{ V}$, $I \approx 4.5\text{ A}$, so a 5 A fuse is used.

The electricity supplied to homes is alternating current (AC): its direction keeps reversing. A battery, by contrast, gives direct current (DC), which always flows in one direction.

Household circuits also need protection against dangerously large currents. This is done with an electric fuse.

In CBSE Class 10 Science, Chapter 12 (Magnetic Effects of Electric Current), AC frequency in India, domestic circuits, fuse ratings and the difference between overloading and short-circuiting are frequently asked board exam topics.

Syllabus note: In the rationalised NCERT textbook, the explanation of AC and its frequency was part of the section on the electric generator, which has been removed from Class 10. Domestic circuits, fuses, overloading and short-circuiting may still be in your syllabus. Check the current CBSE syllabus before your exam.

What You Will Learn

• The difference between AC and DC
• What 50 Hz means and why AC reverses direction 100 times per second
• The two types of domestic circuit: 5 A and 15 A
• How a fuse works and where it is connected
• How to choose the correct fuse rating
• The difference between overloading and short-circuiting

1. AC vs DC

2. What Does 50 Hz Mean?

In India, household electricity is supplied at 220 V with a frequency of 50 Hz, which means 50 complete cycles per second.

Step 1: In one cycle, the current flows one way for the first half and the opposite way for the second half, so it changes direction twice per cycle.

Step 2: Number of reversals per second:

$50 \times 2 = 100$

Step 3: Time between reversals:

$\frac{1}{100}\text{ s} = 0.01\text{ s}$

Step 4: Time for one complete cycle:

$T = \frac{1}{50}\text{ s} = 0.02\text{ s}$

Key point: AC in India changes direction every $\frac{1}{100}$ second, which gives a frequency of $50\text{ Hz}$.

3. Domestic Circuits: 5 A and 15 A

Houses usually have two separate circuits:

4. The Electric Fuse

Definition: An electric fuse is a safety device connected in series in the live wire. When the current becomes too large, the fuse wire heats up (heating effect of current, $H = I^2Rt$), melts and breaks the circuit.

• Fuse wire is made of a metal or alloy with a suitable melting point, so that it melts at the rated current.
• It is placed in the live wire, so that when it blows, the appliance is cut off from the high-voltage supply.

5. Choosing the Correct Fuse Rating

Question

An electric iron of $1\text{ kW}$ is operated at $220\text{ V}$. What current does it draw, and which fuse should be used: $1\text{ A}$, $2\text{ A}$, $3\text{ A}$, $5\text{ A}$ or $10\text{ A}$?

Solution

Step 1: Write the given values. $P = 1000\text{ W}$, $V = 220\text{ V}$.

Step 2: Find the current.

$I = \frac{P}{V} = \frac{1000}{220} \approx 4.5\text{ A}$

Step 3: Choose the fuse. A $1\text{ A}$, $2\text{ A}$ or $3\text{ A}$ fuse would blow in normal use. A $10\text{ A}$ fuse would let too much current flow before blowing. So the best choice is the next rating above $4.5\text{ A}$.

Answer: The iron draws about $4.5\text{ A}$, so a 5 A fuse should be used.

6. Overloading vs Short-Circuiting

A fuse protects against both, by melting when the current becomes too large.

7. Summary Table

8. Exam Tips and Common Mistakes

Exam Tip: For fuse questions, always calculate $I = \frac{P}{V}$ first, then choose the next standard rating above it.

Common Mistake: Saying 50 Hz means the current reverses 50 times per second. It reverses twice per cycle, so 100 times per second.

Common Mistake: Connecting the fuse in the neutral wire. It must be in the live wire.

Common Mistake: Confusing overloading with short-circuiting. Overloading is too much load; short-circuiting is direct contact between the live and neutral wires.

Frequently Asked Questions

What is the frequency of AC in India?

50 Hz, which means 50 complete cycles per second.

How many times does AC change direction per second in India?

100 times, because the current changes direction twice in every cycle.

What fuse should be used for a 1 kW iron at 220 V?

The current is about 4.5 A, so a 5 A fuse should be used.

What is the difference between overloading and short-circuiting?

Overloading happens when too many appliances draw current from one circuit. Short-circuiting happens when the live and neutral wires touch directly.

Why is a fuse connected in the live wire?

So that when it blows, the appliance is disconnected from the high-voltage supply and is safe.