Electrical Measuring Instruments and Testing
This domain tests instrument connection, scaling through CTs and PTs, insulation testing and the commissioning sequence for a new installation.
Instruments and connections
- Voltmeter in parallel, ammeter in series, wattmeter with current and voltage coils, energy meter in kWh.
- CT-operated ammeters: actual current = meter reading × CT ratio. A 200/5 CT has a multiplier of 40.
- PT/VT steps high voltage down for metering; the voltmeter reading is scaled by the PT ratio.
- A clamp meter reads current without breaking the circuit; a multimeter covers volts, amps, ohms, continuity and diode test.
- A megger applies DC at 250, 500 or 1000 V; new low-voltage installations are typically expected to exceed 1 MΩ at 500 V.
Testing sequence
- Visual check, earth continuity and polarity, insulation resistance between phases, phase–neutral and phase–earth, then functional tests of RCCB trip and starter logic.
- Earth resistance by the fall-of-potential method with an earth tester; record results on the commissioning sheet.
- Calibrate instruments periodically against a reference and retain certificates.
Worked example
A 200/5 CT ammeter reads 3.2 A. Actual line current = 3.2 × 40 = 128 A, about 64% of the CT primary rating - a healthy metering point. If the same meter read 1.1 A, the actual current would be 44 A, close to the lower accuracy limit, so a 100/5 CT would measure that load better.
Traps
- Connecting an ammeter in parallel; its near-zero resistance short-circuits the supply.
- Leaving a CT secondary open-circuited, which develops dangerous voltage.
- Using a megger on a live circuit or before discharging capacitors.
- Trusting an average-sensing meter to read a distorted waveform as true RMS.