Sherlock Ohms: Staying Safe from High Voltage

Not many engineers or technicians, as a percentage, work around high voltage with any regularity. The hazards associated with sources capable of being able to "reach out and touch someone," as the old Bell Telephone company commercial used to say. When an arc of voltage is caused to "reach out and just say, 'Hi,'" the greeting can be quite painful, damaging, or even lethal. I have experienced high voltage's awesome ability to bite while operating unawares inside electronic equipment, and I have witnessed other people's reactions to a surprise "Hi."

The day I arrived at my permanent duty station at Robins AFB, in Georgia, following eight months of technical school for ground-based airport surveillance and precision approach radar, the guy who was to be my trainer had visited the base hospital to have his finger treated for a high voltage burn received while performing routine maintenance on a CRT supply chassis. If I remember correctly, he was removing a voltmeter probe from a test point on the front panel and had his finger too close to the metal tip. It was a good introduction for me regarding regarding the dangers of high voltages.

Our radar system had two high power transmitters - one at S-band and one at X-band. Sticking your bare arm inside the some portions of the equipment rack would cause the hairs to stand on end. USAF technical manuals stipulated that removable chassis be removed from the rack and patched in with cables for any alignment procedures with variable capacitors, inductors, and resistors that could not be easily accessed through a hinged front panel door. Everybody, without exception, was taught how to blindly walk his fingers across the tops of components (including very hot vacuum tube shields) to reach adjustments deep in the bowels of the chassis in order to fine tune performance in ways not possible just from the front panel. One example that comes to mind is the ground clutter cancellation circuit, where careful tweaking could "disappear" trees and buildings while clearly painting a little Cessna T-33 jet trainer on final approach.

Engineer Robert Johnson issues a sage caveat on the subject in his recent Sherlock Ohms contribution. Mr. Johnson spent a career designing high power RF amplifiers and developed a healthy (literally) respect for the penchant for differences of potential to attempt to neutralize themselves. He mentions a great little safety tool that is simple to make and use to safeguard your being.

Posted  February 24, 2014