Basic Electric

1. Basic electrical quantities

Quantity	Symbol	Unit	Basic meaning
Voltage	V	Volt (V)	Electrical pressure
Current	I	Ampere (A)	Flow of electric charge
Power	P	Watt (W)	Rate of electrical energy use
Resistance	R	Ohm (Ω)	Opposition to current flow
Energy	E	Wh / kWh	Electrical energy consumed
Frequency	f	Hertz (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.

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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.

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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.

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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).

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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.

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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.

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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.

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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