Testing and Test Instruments 57
can be made by the user to determine whether calibration is necessary by comparing readings with an instrument known to be accurate, or by measurement of known values of voltage, resistance, etc., available on units now on the market and advertised as ‘check boxes’. It may be the case that readings are incorrect simply because the instrument is not zeroed or nulled before use, or because the internal battery needs replacing. Most modern instruments have battery condition indication, and of course this should never be ignored. Always adjust any selection switches to the off position after testing. Too many instrument fuses are blown when, for example, a multimeter is inadvertently left on the ohms range and then used to check for mains voltage. The following procedure may seem rather basic but should ensure trouble-free testing: 1. Check test leads for obvious defects. 2. Zero/null the instrument (where required). 3. Select the correct range for the values anticipated. If in doubt, choose the highest range and gradually drop down. 4. Make a record of test results, if necessary. 5. Return switches/selectors to the off position. 6. Replace instrument leads in carrying case.
CONTINUITY OF PROTECTIVE CONDUCTORS All protective conductors, including main protective and supplementary bonding conductors must be tested for continuity using a low-resistance ohmmeter. For main protective bonding there is no single fixed value of resistance above which the conductor would be deemed unsuitable.
