Measuring Instruments and Their Safe Use
What this domain tests
Measuring Instruments and Their Safe Use tests selection, connection and interpretation of multimeters, clamp meters, insulation testers, earth testers and test lamps.
Core rules
- Select the function and range before connecting. For voltage, connect in parallel; for current, connect in series or use a clamp meter around one conductor only.
- Resistance and continuity are measured only on a de-energised circuit. A live resistance measurement can damage the meter and is unsafe.
- An insulation resistance tester (megger) applies a test voltage, commonly 500 V for 230 V circuits. Isolate, discharge, then test between live conductors and earth. A low-voltage installation should normally read at least 1 MΩ; a good new circuit reads much higher.
- An earth tester measures electrode resistance. Domestic earth resistance is typically required to be low, commonly below 5 Ω, and the value depends on soil and electrode condition.
- A clamp meter must close around a single conductor. Clamping the whole cable reads near zero because phase and neutral currents cancel.
- Prove the tester on a known live source before and after use.
Worked example
To test a lighting circuit, isolate the DB, lock it, prove the tester, and confirm dead. Set the megger to 500 V. Test live-to-neutral, live-to-earth and neutral-to-earth. If all readings exceed 1 MΩ and the values are steady, insulation is acceptable. Record the readings.
Traps that cost marks
Measuring resistance on a live circuit; clamping a two-core cable; using a 1,000 V megger on a 230 V circuit without need; failing to discharge a capacitor; and trusting a tester that has not been proved.