August 14, 2017 – With more volts than ever before in electric vehicles (EVs) and on solar-paneled rooftops, first responder and electrical worker safety is a growing concern. Researchers at the Department of Energy's Oak Ridge National Laboratory (ORNL) are addressing the challenge with the development of a probe to accurately detect direct-current (DC) energy.
The DC “Hot Stick” probe designed under the sponsorship of the U.S. Fire Administration, a division of the U.S. Department of Homeland Security, not only detects DC voltage, but also ensures that the probe is properly tapped into the electrical lines being tested. The latter is essential because false negatives may be obtained if a meter is not making proper contact, noted ORNL lead scientist Bruce Warmack of the Electrical and Electronics Systems Research Division.
Exposure to these high voltages is increasingly commonplace as technologies such as hybrid cars, EVs and solar panel arrays, all of which contain large amounts of DC power, are adopted by a growing number of consumers. EVs, for instance, may carry as much as 400-600 volts, Warmack said. Battery packs are typically situated in the rear of EVs, with electrical cables running to the controller up front and to other devices.
“If you can imagine in an accident, a vehicle may be splayed open, and first responders need to know whether the car is carrying a charge before they can even touch it, much less cut through cables and extract a victim,” Warmack said. “That much voltage is potentially lethal and can cause arc flash hazards or trigger explosions.”
Similarly, installers and other technicians working on solar panel arrays need protection as well. Those installations can carry several hundred volts, Warmack noted.
The patent-pending probe that Warmack and his colleagues at ORNL created is designed to penetrate cable insulation and make proven contact with a conductor. The probe then indicates whether a cable is fully discharged of energy.
Making that contact is important. In preliminary work on the project, ORNL researcher Roger Kisner suggested a probe tip verification system that would ensure direct, galvanic contact was made.
One of the challenges for the project was dealing with DC voltage, which is more difficult than AC to work with, Warmack said. “In certain cases you can get electrical fields from DC, but it is easily interfered with by plastics and other kinds of materials. It’s more or less a science experiment to find DC unless you make direct contact with the conductor using a voltmeter,” he added.
The scientists created a device similar in appearance to most existing AC probes and familiar to first responders. In fact, the DC probe can be designed to slip onto the end of an AC probe, and its functionality is quite simple. A green light indicates proper contact, and if voltage is detected, a red light is illuminated.
The prototype is already being tested on a first responder vehicle in Frederick County, Maryland, Warmack noted. The Fire Administration has provided positive feedback so far, he said.
“The probe would give a firefighter or other first responder assurance very quickly that the conductor is either not charged or needs to be handled with care,” Warmack said. “Our probe makes sure that an emergency worker or electrician has made contact and isn’t just hitting air or a piece of plastic. There’s really no tool that does this as effectively. This technology can save lives.”
ORNL engineer Nance Ericson, who designed the circuits for the probe, noted that while the system was designed for electric vehicles and solar panels, it could also be used to make measurements in a variety of settings. The circuit he developed can detect the presence of both AC and DC energy—or can ensure that there is no stored energy present—a feature not available in existing electrical instruments.
“We see this technology extending well beyond the first responder community and improving the safety of electrical measurements in a wide variety of applications ranging from industrial power to home appliance repair,” Ericson said.
Going forward, Warmack and his colleagues envision the development of more probes to test across the country, as well as versions to check for DC voltage on smaller gauge wires found in other electrical systems. Future work could also see the addition of a microcontroller to the probe to give a digital readout, and to capture and process data.
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