1. Basic electrical quantities

QuantitySymbolUnitBasic meaning
VoltageVVolt (V)Electrical pressure
CurrentIAmpere (A)Flow of electric charge
PowerPWatt (W)Rate of electrical energy use
ResistanceROhm (Ω)Opposition to current flow
EnergyEWh / kWhElectrical energy consumed
FrequencyfHertz (Hz)AC cycles per second

Important formulas

Ohm’s Law:

V = I × R

Therefore:

I = V / R

R = V / I

Power:

P = V × I

Therefore:

I = P / V

V = P / I

For a resistive load:

P = I² × R

P = V² / R

Example

A 100 W lamp connected to 230 V:

I = P / V

I = 100 / 230

I ≈ 0.43 A

So the lamp draws approximately 0.43 A.

2. Common domestic electrical supply

For typical Indian domestic installations:

  • Supply voltage: approximately 230 V AC
  • Frequency: 50 Hz
  • Single-phase supply normally has phase/live (L) and neutral (N)
  • Protective earth (E) is provided for safety.

Actual voltage can vary, so equipment should be rated for the supply.

3. Switch

A switch is used to open or close an electrical circuit.

Basic switch terminals

A simple switch may have:

  • COM — common terminal
  • L / L1 — switched terminal

The switch should normally interrupt the live/phase conductor, not the neutral.

4. One-way switch

A one-way switch controls one light/load from one location.

Typical arrangement:

Phase → Switch → Lamp → Neutral

Example:

Phase (L)
   │
   │
 [SWITCH]
   │
   │
 [LAMP]
   │
 Neutral (N)

A common one-way switch is SPST (Single Pole, Single Throw).

5. Two-way switch

A two-way switching circuit allows one light to be controlled from two locations.

Commonly used for:

  • Staircases
  • Corridors
  • Rooms with two entrances

A two-way switch generally has:

  • COM
  • L1
  • L2

Conceptually:

       L1 ───────── L1
      /              \
Phase COM            COM → Lamp → Neutral
      \              /
       L2 ───────── L2

The two switches change the path between the two travellers.

6. Stair light

A stair-light circuit commonly uses two two-way switches.

For example:

  • Switch 1 at the bottom of stairs
  • Switch 2 at the top of stairs
  • One lamp between them

Either switch can change the lamp’s ON/OFF state.

For actual mains wiring, follow the applicable electrical code and have a qualified electrician make the connections.

7. Lamp / light

A lamp converts electrical energy into light.

Example:

LED lamp: 9 W, 230 V

Approximate current:

I = P / V

I = 9 / 230

I ≈ 0.039 A

So it uses about 39 mA under the stated conditions.

Older incandescent lamps commonly had much higher wattage for similar light output.

8. MCB

MCB = Miniature Circuit Breaker

It protects a circuit against conditions such as:

  • Overcurrent
  • Short circuit

Common ratings include:

  • 6 A
  • 10 A
  • 16 A
  • 20 A
  • 25 A
  • 32 A
  • 40 A
  • 63 A