Electrical Tools, Instruments and Measuring Techniques
Tools and their use
Insulated screwdrivers, combination pliers, side cutters, wire strippers, crimping tool, hacksaw, hammer, chisel, fish tape and test lamp form the standard kit. Never use a tool with cracked insulation on or near live parts.
Instruments
- Multimeter: voltage in parallel, current in series, resistance and continuity only on a de-energised circuit.
- Clamp meter: encircles one conductor only.
- Insulation tester (megger): 500 V DC for 230/415 V wiring; a healthy LV circuit should read at least 1 MΩ to earth.
- Earth tester: three- or four-terminal method for earth resistance.
Worked example
A 100/5 A CT feeds an ammeter reading 3.2 A on the secondary. Primary current = 3.2 × (100/5) = 3.2 × 20 = 64 A. If a clamp meter is clipped around the whole cable, it reads near zero because line and neutral currents cancel; clip it around the phase conductor alone.
Traps
Measuring resistance on a live circuit damages the meter; using the wrong megger voltage can stress electronics; assuming a clamp meter reads true RMS on distorted loads; and forgetting that a CT secondary must never be open-circuited under load. Before meggering, disconnect electronics and capacitors, and prove the tester on a known good circuit.