Paul DeWinter, Wavetek Corp.
Of all the features to consider when purchasing a digital multimeter (dmm), safety is certainly at the top of the list. When selecting a dmm, be cognizant of the application or environment in which the meter will be used most often. Although the topic of safety is wide-ranging, be aware of safety issues related to current measurement and meter ratings that will protect both the device and the user.
Measuring current requires taking safety precautions because the dmm joins the circuit in series. Fusing, therefore, becomes a critical safety factor. To ensure safety when taking a current measurement, look for a meter with either fused amperage circuits or a specific current clamp.
Fused amperage circuits protect against the risk of shock or fire. Although fusing adds a slight burden voltage to current measurements, the safety factor vastly outweighs the calculation inconvenience. Current clamps are typically either meters with current probes or meters with clamp-on heads; clamp-on meters do not actually join the circuit in series but instead clamp around the conductor and use the Hall effect to measure the current flow.
Ratings can often be confusing, so a basic rule of thumb advises the selection of meters with an International Electrotechnical Commission (iec) 1010 Category III rating if the meter will ever be used to measure commercially supplied power at electrical connections, outlets, or fuse panels. Be sure to choose the rating that applies to the highest voltage range in that particular application (up to 1000 volts for Category III). This will provide the user assurance that the meter`s construction can withstand transient impulses, which can occur without warning on commercial power lines.
The most dangerous power-line fluctuations result from lightning strikes, which can cause thousands of volts to be felt over a wide region of the power grid. Keep in mind that as the distance increases between the point of measurement and the entry distribution box through the building`s wiring or commercial power lines, the hazard decreases due to attenuation. By considering these and other safety features, you help protect the life of the meter--and the user.
Paul DeWinter is product support specialist at Wavetek Corp. (San Diego, CA).