The multimeter measures voltage, continuity and resistance; the clamp meter reads current without breaking the circuit; the insulation tester (megger) proves cables have not been damaged during installation — 500 V test, expect >1 MΩ. Before energising any new work: continuity of every conductor, insulation resistance, polarity (switch in the line, not the neutral), and earth continuity to every point. Then, and only then, close the main breaker.