Safety and Measuring Instruments
This domain combines electrical safety with the instruments used to measure electrical quantities. Expect questions on how each instrument is connected, what it reads, and how ranges are extended.
Safety essentials
Current, not voltage, harms the body: roughly 1 mA is perceptible and 50 mA can be fatal. Precautions include proper earthing, insulated tools, rubber mats, avoiding work on live circuits, and switching off plus tagging before maintenance. A shock victim must be freed using an insulated object, never bare hands, and given artificial respiration if breathing has stopped.
Instruments and their connections
- Ammeter: very low resistance, connected in series; range extended by a parallel shunt.
- Voltmeter: very high resistance, connected in parallel; range extended by a series multiplier.
- Wattmeter: current coil in series with the load, potential coil in parallel across it.
- Energy meter: records kWh, connected like a wattmeter.
- Megger: measures insulation resistance; never used on a live circuit.
- PMMC: DC only, reads average, uniform scale. Moving iron: AC or DC, reads RMS.
Worked example
A PMMC meter has full-scale current 1 mA and coil resistance 50 Ω. To read 10 V full scale, add a series multiplier R: total resistance needed = 10/0.001 = 10000 Ω, so R = 10000 − 50 = 9950 Ω.
Traps
- Connecting an ammeter in parallel across a supply - it short-circuits and blows the fuse.
- Using a megger on a live circuit.
- Expecting a PMMC instrument to read AC directly.
- Forgetting to subtract the meter's own resistance when calculating a multiplier.
- Treating an ELCB as an overload protector.