OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at http://www.osha.gov.

February 24, 1988

MEMORANDUM FOR: LINDA R. ANKU
  Regional Administrator
 
THRU: LEO CAREY, Director
  Office of Field Programs
 
FROM: THOMAS J. SHEPICH, Director
  Directorate of Compliance Programs
 
SUBJECT: Review of Electrical Utilities Procedures
  for Compliance with Subpart K of Part 1926

29 CFR 1926.404(f)(1)(iv) requires AC systems of 50 to 1000 volts to be grounded if the system can be so grounded that the maximum voltage to ground will not exceed 150 volts. (29 CFR 1910.304(f)(1)(iv) contains a similar requirement for general industry.) A system supplied by a transformer whose secondary voltage is 120 volts between conductors (or is 120 volts between each line conductor and a common neutral and 240 volts between the line conductors) must be grounded. Therefore the installation described in the attachment to Mr. Forbes' letter is in violation of this requirement. Grounding the system described by Mr. Forbes will protect the secondary circuit conductors from having the primary voltage impressed on them (in this case 600 volts) and will limit the voltage above ground on the secondary conductors.