Working it Hot (11-18-99)

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Mike, Our plant has a 12,300-volt dry transformer. I asked our electrical contractor to check the voltage the other day and he insisted on doing it with the power energized. He opened the transformer to check the taps and some metal object apparently fell into the transformer and caused an explosion.

Is there any safety standard that exists regarding under what circumstances electrical equipment should be serviced, tested, maintained, etc. while energized?

From: George R. Corron <

I hate to bring this point up, but is there any way to check the voltage with the transformer DE-energized?


How else could the output voltage of a transformer be determined without it being energized? Sometimes "you gotta do what you gotta do", after taking the appropriate safety precautions, such as wearing the proper safety apparel, checking the area for loose objects that may fall into the energized area, blocking the area from unauthorized access while live parts are exposed, etc.

From: Slye, Carl A (Homestead ARB) <

At Homestead Air Reserve base we work on similar transformers. Normally they are energized until we can get the covers off. We are required to work a two-man concept, and wear protective equipment. Rubber gloves and hot sticks if needed. We make sure the equipment is clear of any hazards before opening it or working around it. The only safety standard that I know of is good common sense and be aware of your surroundings. At Homestead we practice what is called Operational Risk Management. We try to assess possible causes of accidents before they occur. This management plan is used throughout the Federal Government

From: Kohlhepp, Michael C. <

What in the world was this guy thinking? Checking the taps on the transformer could have been done with the transformer de-energized. The winding ratios/tap settings should be on the nameplate. I would say common sense should have prevailed in this circumstance. Maybe he was trying to check the secondary voltage. The voltage definitely could have been checked at this point safely by a qualified electrician. OSHA 1910.269 contains standards for hi-voltage safety. It can be found on the web at

From: Earl Dean <

Check this link out

From: Scheidecker, Brad <

As I am sure you know, OSHA has specific rules regarding working on or near energized exposed electrical sources. I believe it is 1910, Subpart S. In the contractor's can he check the voltage on a system unless it is energized?

From: Jody Wages <

Anyone working with transformers operating at these voltages should be qualified and licensed for this work. Accidents can always happen and electricians should always be aware of the environment and surrounding materials within the area. Proper workspace clearances are established in the NEC for switchgear and panelboard locations.

From: <

Yes, OSHA covers the topic fairly thoroughly. The tricky part of the OSHA regulations is the use of terms such as "qualified personnel", "qualified persons", and so on. OSHA does not necessarily mean a person has to be a journeyman electrician or anything like that, to be qualified. Generally, they mean any person who is aware of the hazards involved.

There is one other very important thing I'd like to point out. He referred to "our plant" and he referred to the electrician as an electrical contractor. Plant personnel should be aware that any time a plant person is on site the plant management is responsible for everyone's safety, including the contractor and the plant management is responsible for the contractor's actions. This is true even if there is a construction manager on site.

From: Saporita, Vince <

NFPA 70E should be utilized. It is the Standard for Electrical Safety Requirements for Employee Workplaces (1995 Edition).

From: Dela,Sonny, GLENDALE, Engineering <

The following note is found in IEEE Std 1100-1992 (Chapter 5, page 108)
"CAUTION: Workers involved in opening energized power panels are required to abide by the prescription of ANSI/NFPA 70E - Standard for Electrical Safety Requirements for Employee Workplaces OSHA CFR 1910 (Subpart S-Electrical; 1910.331 - Safety Related Work Practices ANSI C@-1990 and ANSI/NFPA 70E concerning appropriate protective equipment as well as government regulations codified in OSHA CFR 1910[B3], and in ANSI C2-1990 [B1]