Standard Interpretations - Table of Contents|
| Standard Number:||1910.305(j)(4)(iii); 1910.137; 1910.333|
May 20, 1999
Mr. Ronald R. Cross
Dear Mr. Cross:
Thank you for your July 23, 1996 letter addressed to Mr. Ken Gerecke in the Philadelphia Regional Office of the U.S. Department of Labor, Occupational Safety and Health Administration (OSHA). Your letter was forwarded to this office for response. Please accept our apology for the delay in responding. Your scenario, question, and our reply, follow.
Scenario: The electrical power system in your Casting Department has a high impedance grounded neutral detection system (hereinafter "system") which alarms when a phase-to-ground condition occurs. The referenced electrical switch gear contains a substation which is fed by a 13.8kV, 1,000 kVa transformer which steps the power down to a 480 VAC, 3-phase secondary. The substation is equipped with a high impedance grounded neutral detection system and this system has short-circuit and overload protection, and equipment frame grounding. You state the system remains in compliance with Article 250-27 of the National Electrical Code, 1996 Edition.
The company's "High-Impedance Grounded Neutral Isolation Procedure," (Appendix A) along with system engineering drawings, (Appendices B & C) were provided for review.
Question: Within the context of the provided procedure, at what point in time may qualified electrical personnel restore power to equipment (e.g. a cubicle) once it is determined that a phase-to-ground condition is not present in the equipment powered thereby?
Reply: Although OSHA, for reasons of policy, does not approve company procedures, we made an informal review of the troubleshooting procedure suggested to isolate the phase-to-ground fault condition in the electrical power system shown in your drawings. The high-impedance grounding connection in the electrical power system appears to offer protection to the equipment provided a safe shutdown procedure of equipment is followed as per OSHA 1910.305(j)(4)(iii).
You have indicated that when the system detects a phase-to-ground condition (alarm sounds), your company conducts a planned orderly shutdown of affected equipment in accordance with Article 430-44 of the National Electrical Code. Thus, when the alarm system is activated, an orderly shutdown must proceed without any unnecessary delay. The shutdown process may not be interrupted for any reason, including troubleshooting activities on deenergized portions of the system. This deenergization sequence ensures that when the circuit containing the ground fault is found (indicated by the alarm light going out), it must be isolated as quickly as possible. It must remain deenergized until proper repairs can be made so that when reenergized the alarm light remains off. After the orderly shutdown is completed and after the employer has determined that it is safe to reenergize, the system may then be reenergized, circuit by circuit, while the alarm light is monitored to indicate a continuing safe condition. Any circuit that has been deenergized, but not yet tested for the presence of a phase-to-ground fault condition, must remain deenergized.
The phase-to-ground fault condition for a delta-connected winding motor offers no protection for personnel against electric shock. If a motor frame becomes energized, caused by equipment containing a phase-to-ground fault, an electrical shock hazard could occur to maintenance personnel should they inadvertently make contact while troubleshooting in the area. As protection against this hazard, OSHA recommends that until the fault is isolated and rectified electrical protective equipment, as specified in OSHA Standard 29 CFR 1910.137, be used by operation and maintenance personnel while troubleshooting the electrical power system. This complies with the selection and use of work practices, 29 CFR 1910.333, and permits working with energized parts where safety-related work practices are used to protect employees who may be exposed to the electrical hazards involved.
As equipment is deenergized, the alarm light will go out as soon as the equipment containing the phase-to-ground condition is shut-down. Power must not be restored to equipment previously shut-down while the alarm light is still on.
A general determination cannot be made by OSHA that it is safe to reenergize a portion of the system at your facility since this determination can only be made for a particular instance by examining all the steps actually taken to deenergize the system and to isolate and test a portion of the circuit. Obviously, the employer and the employees involved in designing, deenergizing, isolating, and testing the electrical system are in the best position to make this determination.
We appreciate your interest in employee safety and health and hope you find this information helpful. If we can be of further assistance, please contact the Office of General Industry Compliance Assistance at (202) 693-1850.
Richard Fairfax, Director
Standard Interpretations - Table of Contents|