Answer to Question #5931 Submitted to "Ask the Experts"

I am an electrician who field tests high-voltage switchgear that uses vacuum bottles to minimize contact erosion. Periodically, these bottles must undergo vacuum integrity testing using high-voltage alternating current (AC) "hypots," applying the voltage across the open contacts and looking for current leakage with a microammeter. What are the x-ray hazards associated with this hypot testing? Is shielding necessary? Would the sheet metal of an outdoor unit substation be adequate shielding? As long as there is no current leakage, will no x rays be produced? We are using a hypot capable of producing 60 kilovolts AC.

As long as there is no current leakage, no x-rays will be produced. Additionally, to generate x rays, an x-ray machine uses direct current (DC) in the glass-vacuum tube to accelerate and then impact the high-energy electrons on a target, usually tungsten. Only after the electrons are accelerated through a DC voltage/potential difference of tens to hundreds of kilovolts and then suddenly stopped on the positively charged anode can x rays be generated. I don't think that the AC voltage would cause the generation of x rays, even with current leakage.

Also, for your information, the x-ray intensity, generated by a 60-kilovolt DC current, can be reduced to one-half its original intensity by about 8 mm (0.3 inches) of glass or about 0.6 mm (0.024 inches) of steel.
 
John P. Hageman, MS, CHP