Wiring Experiments — Lamp Circuits, Staircase Wiring and the Distribution Board

What this topic is for. These are the experiments you will actually perform. Read the relevant section before the session so you arrive knowing what you are building and why, rather than copying the person beside you.

Experiment 1 — identification and ratings

Before wiring anything you must be able to name what you are holding and read its rating.

Cables and wires. Note the conductor cross-section in mm² — 1.0, 1.5, 2.5, 4.0 — since this determines current capacity. As a rough guide for domestic PVC-insulated copper: 1.0 mm² for lighting, 1.5–2.5 mm² for 6 A sockets, 4 mm² for a 16 A power point.

Switches: one-way, two-way, intermediate; rated 6 A or 16 A. Sockets: 6 A (2-pin/3-pin) and 16 A (power). Fuse and fuse carrier: rewirable (kit-kat) type, rated in amperes. MCB: rated current (6, 10, 16, 32 A) and tripping curve — type B trips at 3–5 times rated current for domestic use, type C at 5–10 times for inductive loads such as motors. ELCB/RCCB: rated current and sensitivity — 30 mA for personal protection, 100–300 mA for fire protection only. MCCB: higher current, adjustable settings.

In the viva you will be handed a device and asked what it is, what it protects and what its rating means. Practise reading the printing on each one.

Experiment 2 — one light point with a 6 A socket, PVC conduit

The simplest circuit, and the pattern for everything after.

Circuit sequence: supply → MCB → switch in the LIVE → lamp → neutral back to supply. Earth runs continuously to the socket's earth pin and to any metal enclosure.

Procedure:

1. Mark out the board and fix the conduit and boxes.
2. Draw the wires through the conduit — do not join wires inside a conduit run; joints belong only in boxes.
3. Connect the live through the switch to the lamp holder's centre contact.
4. Connect the neutral directly to the lamp holder's outer contact.
5. Connect the earth to the socket earth pin.
6. For the socket, connect live, neutral and earth to the correct pins. Looking at a 3-pin socket face-on: earth is the top, larger pin; live is on the right; neutral on the left.
7. Check continuity with a multimeter before energising.
8. Energise only with the teacher present.

The two errors that recur: switching the neutral rather than the live (dangerous, see the safety topic), and connecting the lamp's centre contact to neutral so that the screw shell stays live.

Experiment 3 — staircase wiring with two-way switches

One lamp controlled from two places — the classic experiment.

Principle. Each two-way switch has a common terminal and two travellers, and connects the common to one traveller or the other. Wire the two travellers of switch A to the two travellers of switch B. Whichever position each switch is in, the circuit is complete only when both select the same traveller — so operating either switch changes the state.

Connections:

• Live → common of switch A.
• Traveller 1 of A → traveller 1 of B; traveller 2 of A → traveller 2 of B.
• Common of switch B → lamp.
• Lamp → neutral.

Check before energising: with a continuity meter across the lamp terminals, operating either switch alone must change the reading from continuity to open. If one switch does nothing, the travellers are crossed with the common.

For three or more control points, an intermediate switch is added between the two two-way switches. The syllabus asks only for the two-switch case.

Experiment 4 — fluorescent lamp and 16 A power socket

The choke (ballast) limits current, and the starter provides the initial surge that strikes the arc. Sequence: supply → switch → choke → lamp → starter across the lamp's two filaments → neutral.

A fluorescent lamp is an inductive load with a poor power factor, around 0.5, which is why a power factor correction capacitor is often fitted across the supply — connecting directly to Module 2 of the Electrical and Electronics theory course, where the same correction is calculated.

The 16 A socket takes a larger conductor, typically 4 mm², because it supplies heavy appliances.

Experiment 5 — the distribution board

Wiring a single-phase DB with energy meter, main switch, ELCB and MCBs.

Order from the supply inward — and the order matters:

Supply → energy meter → main switch → ELCB/RCCB → MCB busbar → individual MCBs → final circuits

• The energy meter comes first so that everything consumed is measured.
• The main switch isolates the whole installation.
• The ELCB protects every circuit downstream, which is why it is placed before the MCBs rather than on one circuit.
• Individual MCBs protect each final circuit at its own rating.

Neutral and earth are separate throughout the installation and must never be interchanged. The neutral is a current-carrying conductor; the earth carries current only during a fault.

Experiments 6 to 8 — measurement and familiarisation

Transformers. Step-up and step-down, using low-voltage transformers. Measure input and output voltage; the ratio should match the turns ratio. Use only the low-voltage transformers provided.

CRO. Display the waveform and measure amplitude and frequency by counting divisions: $V_{p-p} = \text{divisions} \times \text{volts/div}$ and $T = \text{divisions} \times \text{time/div}$, with $f = 1/T$. Sketch the waveform to scale on graph paper — the syllabus asks specifically for this.

Rheostats and relative potential. Measure the potential at points along a resistance chain relative to a chosen reference, demonstrating that potential is meaningful only relative to a datum — the idea introduced in Module 1 of the theory course.

Batteries. Compare lead-acid, Li-ion and NiCd: nominal cell voltage, capacity in Ah, and rechargeability. Note that Li-ion is about 3.7 V per cell against 2 V for lead-acid and 1.2 V for NiCd.

Earthing. Examine pipe, plate and mat schemes and the use of ground enhancing material.

The record book — where marks are quietly lost

Continuous assessment is worth 10 of the 50 CIE marks, split between preparation and pre-lab work (2), conduct of experiments (2), lab reports and record keeping (3) and viva (3).

For each experiment record: aim; apparatus with ratings; circuit diagram in BIS symbols; procedure in your own words; observations and readings in a ruled table with units; result; and a note of precautions.

Write the record the same day. Reconstructing a session from memory a fortnight later produces the vague, identical reports that examiners recognise instantly.