Fluke 179 Fluke Multimeters - ABCs of Multimeter Safety Multimeter Safety and - Page 4

Protection against two major electrical hazards Transients-the hidden danger Let's take a look at a worst-case scenario in which a technician is performing measurements on a live three-phase motor control circuit, using a meter without the necessary safety precautions. Here's what could happen: 1. A lightning strike causes a transient on the power line, which in turn strikes an arc between the input terminals inside the meter. The circuits and components to prevent this event have just failed or were missing. Perhaps it was not a CAT III rated meter. The result is a direct short between the two measurement terminals through the meter and the test leads. 2. A high-fault current-possibly several thousands of amps- flows in the short circuit just created. This happens in thousandths of a second. When the arc forms inside the meter, a very high-pressure shock wave can cause a loud bang!-very much like a gunshot or the backfire from a car. At the same instant, the tech sees bright blue arc flashes at the test lead tips-the fault currents superheat the probe tips, which start to burn away, drawing an arc from the contact point to the probe. 3. The natural reaction is to pull back, in order to break contact with the hot circuit. But as the tech's hands are pulled back, an arc is drawn from the motor terminal to each probe. If these two arcs join to form a single arc, there is now another direct phase-to-phase short, this time directly between the motor terminals. 4. This arc can have a temperature approaching 6,000 °C (10,000 °F), which is higher than the temperature of an oxy-acetylene cutting torch! As the arc grows, fed by available short circuit current, it superheats the surrounding air. Both a shock blast and a plasma fireball are created. If the technician is lucky, the shock blast blows him away and removes him from the proximity of the arc; though injured, his life is saved. In the worst case, the victim is subjected to fatal burn injuries from the fierce heat of the arc or plasma blast. In addition to using a multi­ meter rated for the appropriate Overvoltage Installation Category, anyone working on live power circuits should be protected with flame resistant clothing, should wear safety glasses or, better yet, a safety face shield, and should use insulated gloves. A lightning strike causes a transient on the power line, creating an arc between the meter's input terminal and resulting in loud noises. 1 Then, a high current flows in the closed circuit which is formed. An arc starts at the probe tips. 1 2 3 When you pull the probes away, as a reaction to the loud noise, arcs are drawn to the motor terminals you're probing. Figure 2. A worst-case scenario-potential arc blast sequence.  Fluke Corporation ABCs of multimeter safety If those arcs join, the resulting high-energy arc can create a lifethreatening situation for the user. 4