Electrical Measuring Instruments and Testing
What is tested
Operating principle, connection and reading of ammeter, voltmeter, wattmeter, energy meter, multimeter, megger and clamp meter, plus extension-of-range calculations.
Principles
PMMC instruments use the force on a current-carrying coil in a magnetic field; they read DC only and have a uniform scale. Moving-iron instruments read AC and RMS values but have a cramped scale. A dynamometer wattmeter has a fixed current coil and a moving pressure coil; the current coil is in series with the load, the pressure coil in parallel. An induction energy meter is a small induction motor whose disc speed is proportional to power.
Range extension
A low-resistance shunt in parallel extends an ammeter's range; a high-resistance multiplier in series extends a voltmeter's. A 1 mA movement of 50 Ω to read 10 mA needs Rsh = Rm/(m − 1) = 50/9 = 5.56 Ω.
Energy meter test
Meter constant 1200 revolutions/kWh. If the disc makes 5 revolutions in 40 seconds, power = (5 × 3600)/(1200 × 40) = 0.375 kW = 375 W. Compare this with the connected load to decide whether the meter is fast or slow.
Instruments you must know
Megger for insulation resistance - a healthy 230 V circuit reads at least 1 MΩ on a 500 V test with the supply off and loads disconnected. Earth tester for electrode resistance, clamp meter for current without breaking the circuit, test lamp and continuity tester for dead testing.
Traps
Connecting the pressure coil in series; using a PMMC meter on AC; meggering with the supply on or lamps connected; and forgetting that an energy meter's constant is revolutions per kWh, not per revolution.