After accidentally damaging his expensive, namebrand digital multimeter beyond repair, an electrician went to a local electronics store to purchase a replacement. The salesman showed him meters with essentially the same specifications, voltage ranges, and accuracy, but at one-fourth the cost of his original! At this price, he decided to buy one.
Within a few days of his purchase, the electrician went to troubleshoot an inoperative gantry crane. He quickly found a suspect motor contactor on the crane's bridge and prepared to test the voltages. As he applied the probes from the meter in a "chopstick" style, a blinding flash occurred as the inrush current began vaporizing the meter's brass probe tips. Instinctively, the electrician withdrew his hand, but the large energy release vaporized the probe tips so quickly that half an inch of each probe's tip was consumed. As a result, the superheated vaporized brass caused serious burns to his hand, arm, and eyes. Temporarily blinded, the electrician had to feel his way down off the crane.
Following the accident, the electrician (plaintiff) hired an attorney to sue the electronic store where he purchased the meter. In the resulting lawsuit, the electrician's attorney hired three forensics engineers (one specializing in explosions) to support his case. The defense attorney also hired an electrical engineer to support the storeowner.
After separate inspections of the damaged meter, each engineer found one of the circuit board's copper traces completely vaporized. They also determined this trace belonged to the meter's "milliamps" range. Unfortunately, these findings confirmed the electrician had not set the meter's switch to "AC V" as he'd thought. Instead, he selected "AC mA."
The engineers found the meter's design protected the milliampere circuits with a small, glass-encased fuse, rated 250V at 0.5A. They also found the inrush current had completely vaporized the small copper fuse wire and caused the fuse's glass tube to explode. This led them to believe the vaporized trace from the circuit board contributed to the current flow through the meter.
After learning these facts, the plaintiff's engineers felt compelled to discuss the electrician's responsibility with the plaintiff's attorney. Undaunted, the attorney encouraged a review of industry standards for multimeter design. The search revealed the fuse installed in the meter as a safety device was rated for 250V, whereas the meter itself had an advertised rating of 750V. The incident occurred while measuring 480VAC.
Since a case relating to a product's integrity requires finding deficient design, poor construction, or lack of warning, the plaintiff had to prove the defendant was liable for the design of this meter.
UL standards require that the half-ampere fuse (the rating of the meter's fuse) carry half an ampere indefinitely. However, should the current double to 1A, the fuse should open within 120 sec. Obviously, the fuse will open even faster with more current.
The proper placement and rating of components, especially the fuse, in a multimeter is crucial to the safe operation of any unit. Even if equipped with special fuses, the maximum current reaching the fuse is critical. We define the interrupting rating of a fuse as the maximum current the unit will safely interrupt. For special fuses used in meters, this is often 10,000A. In conjunction with this, the meter manufacturer must control the size of the wire used in the meter leads as well as their lead length to limit the maximum current that may flow to the fuse.
The 480V, 3-phase power at the circuit breaker panel was capable of supplying a fault current in the neighborhood of 40,000A. However, the defense attorney's engineer noted the resistance of the meter's leads and the branch circuit wiring to the contactor would have limited the maximum current to a range of 1200A to 2500A. But since test results indicated meter leads explode at about 2000A, and the electrician's leads had not exploded, the current was likely in the range of 1200A to 2000A.
Engineers on both sides of the case recognized the literature supplied with the meter failed to mention or warn of the hazards noted above. The seller was also unaware of this information. Since both parties had evidence to support a case against the other, they settled the lawsuit out of court. The injured electrician received compensation, and the seller added warnings to his meters.
What's the lesson here? Had the electrician used a meter properly designed for the task, the size of the arc produced would have been half or even less than that actually experienced. Also, the time duration of the arc would have been substantially less; resulting in less energy available to cause burns.
To avoid a similar situation, electrical professionals should always use proper test equipment and safety gear when working hot. In this case, appropriate gloves and safety glasses could have easily prevented or minimized the electrician's injuries.
Crawford, P.E., is a Forensic Electrical Engineer and Electrical Contractor; Buske, P.E., is an engineer with Buske Engineering.
