GENERAL PRECAUTIONS IN ELECTRICAL MEASUREMENTS

Some General precautions should be observed in electrical measurements, and sources of error should be avoided, as detailed below:

1. The accuracy limits of the instruments, standards, and methods used should be known so that appropriate choice of these measuring elements may be made. It should be noted that instrument accuracy classes state the “initial” accuracy. Operation of an instrument, with energy applied over a prolonged period, may cause errors due to elastic fatigue of control springs or resistance changes in instrument elements because of heating under load. ANSI C39.1 specifies permissible limits of error of portable instruments because of sustained operation.

2. In any other than rough determinations, the average of several readings is better than one. Moreover, the alteration of measurement conditions or techniques, where feasible, may help to avoid or minimize the effects of accidental and systematic errors.

3. The range of the measuring instrument should be such that the measured quantity produces a reading large enough to yield the desired precision. The deflection of a measuring instrument should preferably exceed half scale. Voltage transformers, wattmeters, and watthour meters should be operated near to rated voltage for best performance. Care should be taken to avoid either momentary or sustained overloads.

4. Magnetic fields, produced by currents in conductors or by various classes of electrical machinery or apparatus, may combine with the fields of portable instruments to produce errors. Alternating or time-varying fields may induce emfs in loops formed in connections or the internal wiring of bridges, potentiometers, etc. to produce an error signal or even “electrical noise” that may obscure the desired reading. The effects of stray alternating fields on ac indicating instruments may be eliminated generally by using the average of readings taken with direct and reversed connections; with direct fields and dc instruments, the second reading (to be averaged with the first) may be taken after rotating the instrument through 180°. If instruments are to be mounted in magnetic panels, they should be calibrated in a panel of the same material and thickness. It also should be noted that Zener-diode-based references are affected by magnetic fields. This may alter the performance of digital meters.

5. In measurements involving high resistances and small currents, leakage paths across insulating components of the measuring arrangement should be eliminated if they shunt portions of the measuring circuit. This is done by providing a guard circuit to intercept current in such shunt paths or to keep points at the same potential between which there might otherwise be improper currents.

6. Variations in ambient temperature or internal temperature rise from self-heating under load may cause errors in instrument indications. If the temperature coefficient and the instrument temperature are known, readings can be corrected where precision requirements justify it. Where measurements involve extremely small potential differences, thermal emfs resulting from temperature differences between junctions of dissimilar metals may produce errors; heat from the observer’s hand or heat generated by the friction of a sliding contact may cause such effects.

7. Phase-defect angles in resistors, inductors, or capacitors and in instruments and instrument transformers must be taken into account in many ac measurements.

8. Large potential differences are to be avoided between the windings of an instrument or between its windings and frame. Electrostatic forces may produce reading errors, and very large potential difference may result in insulating breakdown. Instruments should be connected in the ground leg of a circuit where feasible. The moving-coil end of the voltage circuit of a wattmeter should be connected to the same line as the current coil. When an instrument must be at a high potential, its case must be adequately insulated from ground and connected to the line in which the instrument circuit is connected, or the instrument should be enclosed in a screen that is connected to the line. Such an arrangement may involve shock hazard to the operator, and proper safety precautions must be taken.

9. Electrostatic charges and consequent disturbance to readings may result from rubbing the insulating case or window of an instrument with a dry dustcloth; such charges can generally be dissipated by breathing on the case or window. Low-level measurements in very dry weather may be seriously affected by charges on the clothing of the observer; some of the synthetic textile Fibers such as nylon and Dacron are particularly strong sources of charge; the only effective remedy is the complete screening of the instrument on which charges are induced.

10. Position influence (resulting from mechanical unbalance) may affect the reading of an analogtype indicating instrument if it is used in a position other than that in which it was calibrated. Portable instruments of the better accuracy classes (with antiparallax mirrors) are normally intended to be used with the axis of the moving system vertical, and the calibration is generally made with the instrument in this position.