Fluke 179 Fluke Multimeters - ABCs of Multimeter Safety Multimeter Safety and - Page 1

ABCs of multimeter safety Multimeter safety and you Application Note Don't overlook safety- your life may depend on it Where safety is a concern, choosing a multimeter is like choosing a motorcycle helmet- if you have a "ten-dollar" head, choose a "ten-dollar" helmet. If you value your head, get a safe helmet. The hazards of motorcycle riding are obvious, but what's the issue with multimeters? As long as you choose a multimeter with a high-enough voltage rating, aren't you safe? Voltage is voltage, isn't it? Not exactly. Engineers who analyze multimeter safety often discover that failed units were subjected to a much higher voltage than the user thought he was measuring. There are the occasional accidents when the meter, rated for low voltage (1000 V or less), was used to measure medium voltage, such as 4160 V. Just as common, the knock-out blow had nothing to do with misuse-it was a momentary high-voltage spike or transient that hit the multimeter input without warning. 189 TRUE RMS MULTIMETER TEMPERATURE A mA A COM V 10A MAX FUSED 400mA FUSED CAT 1000V Voltage spikes-an unavoidable hazard As distribution systems and loads become more complex, the possibilities of transient overvoltages increase. Motors, capacitors and power conversion equipment such as variable speed drives can be prime generators of spikes. Lightning strikes on outdoor transmission lines also cause extremely hazardous high-energy transients. If you're taking measurements on electrical systems, these transients are "invisible" and largely unavoidable hazards. They occur regularly on lowvoltage power circuits, and can reach peak values in the many thousands of volts. In these cases, you're dependent for protection on the safety margin already built into your meter. The voltage rating alone will not tell you how well that meter was designed to survive high transient impulses. Early clues about the safety hazard posed by spikes came from applications involving measurements on the supply bus of electric commuter railroads. The nominal bus voltage was only 600 V, but multi-meters rated at 1000 V lasted only a few minutes when taking measurements while the train was operating. A close look revealed that the train stopping and starting generated 10,000 V spikes. These transients had no mercy on early multimeter input circuits. The lessons learned through this investigation led to significant improvements in multimeter input protection circuits. New safety standards To protect you against transients, safety must be built into the test equipment. What performance specification should you look for, especially if you know that you could be working on high-energy circuits? The task of defining new safety standards for test equipment was recently addressed by the IEC (International Electro­ technical Commission). This organization develops international safety standards for electrical test equipment. For a number of years the industry used IEC 348 in designing equipment. That standard has been replaced by IEC 1010. While well-designed IEC 348 meters have been used for years by technicians and electricians, the fact is that meters designed to the new IEC 1010 standard offer a significantly higher level of safety. Let's see how this is accomplished. From the Fluke Digital Library @ www.fluke.com/library