Tools and Measurement
What this domain tests
Selection and correct use of the multimeter, clamp meter, megger, earth tester and energy meter, plus reading vernier and micrometer scales without error.
Core rules
- A voltmeter is connected in parallel, an ammeter in series. A multimeter set to current mode across a supply will blow its fuse.
- A clamp meter reads current by surrounding one conductor only; clamping a cable carrying live and neutral together reads zero.
- A megger tests insulation at 500 V or 1000 V DC on a de-energised circuit; never on a live one.
- Vernier least count = smallest main-scale division ÷ number of vernier divisions. A 1 mm main scale with 20 vernier divisions gives 0.05 mm.
- Micrometer least count = pitch ÷ circular scale divisions: 0.5 mm pitch with 50 divisions gives 0.01 mm.
- The energy meter measures kWh; the earth tester measures earth electrode resistance.
Worked example
A vernier has a 1 mm main scale and 20 vernier divisions. The main scale reads 12 mm and the 7th vernier line coincides with a main-scale line. Least count = 1/20 = 0.05 mm. Vernier reading = 7 × 0.05 = 0.35 mm. Total = 12 + 0.35 = 12.35 mm.
Traps that cost marks
Measuring voltage with the meter in current mode; clamping two wires together and reporting zero current as a fault; testing insulation on a live circuit; reading the vernier from the wrong zero; forgetting to add the main-scale value.