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Thursday, April 13, 2006

X-Ray Detector Shock - Lessons Learned About Electrical Grounding

This x-ray detector probe, virtually identical to the one that caused an electrical shock to a researcher who was just adjusting its position, is a commercial product, about the size of cigar.   Its aluminum sleeve housing was to have been grounded by a copper foil from the internal printed circuit board which, in turn, was grounded through the probe cable wiring. In this case, the ground path was interrupted (assembly failure) and the case charged up 480 volts. The human hazard level is considered as 5 milliamperes (5 mA, or 0.005 amperes).   Fortunately, the design of this equipment limited the current to less than 0.0005 amperes, and the researcher only felt a jab to the hand - estimated at far less than that of a static shock from a carpet-and-doorknob lightning generator. A proper ground would have prevented this. A similar fault in more powerful equipment could have been fatal.

The Purpose of Grounds

Grounded metal covers or enclosures are the last physical and electrical barrier between you and the source of electrical energy contained inside. The enclosure is intended to contain the electrical hazard: the users should only be able touch a 0-volt enclosure. When a metal enclosure is not grounded, the enclosure is often able to "float" up to the voltage inside the equipment. A reliable ground path will divert electrical current to a safe earth ground, prevent the enclosure from floating up to high voltage, and protect you from a shock or worse. This is true of x-ray detectors, electric drill motors, computers, monitors, microwave ovens, and most electrical equipment fitted with a three-prong power cord from the manufacturer.

ADVICE: When you plug in equipment, make a quick check of the power plug and cord. Is the third pin missing?   Get it fixed before use. Don't use those two-to-three pin adapters, unless an electrician verifies it is installed properly and the ground is effective (the "flying wire" needs to be connected to an earth ground via a grounded screw to a grounded box in the wall: if your receptacle is in a plastic outlet box, you may lose the bet). Best of all, don't use these adapters. Instead, have an electrician install a grounding outlet. Or, use a three-wire extension cord connected to a properly grounded outlet.

In the rush to return faulty equipment to service, it is easy to overlook a single, obscure or hidden ground connection. It might be a missing screw, a bent foil, or just a missing hairpin spring. It is especially important that all the grounds be restored after repairs. As you disassemble faulty equipment for troubleshooting and repair, always pay close attention to cover screws and grounding wiring or bonding. Make notes or sketches, if necessary. Restore it to original condition. A final voltmeter check of the "case voltage" to a known good ground may detect a defective or missing ground.

Check your office, lab, and shop equipment, and at home, your microwave oven, and kitchen and bathroom appliances, for improperly used adapters or missing ground pins.


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