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January 16, 2005

Power Supply Safety

"...where I lay stunned with my ears ringing, my lungs hurting, my head aching, and my nostrils filled with the stench of my burnt eyebrows."

I know of no source of statistics that collects the number of power supply designers, technicians, or assembly workers killed in designing power supplies or the number of users killed by design faults -- but I suspect it is non-trivial. This is because I know of one statistic. Over 80 sailors electrocuted aboard U.S. Navy ships over a period of several years. Often these were experienced electricians who were familiar with electrical practice on grounded commercial systems, but not familiar with the safety requirements of the ungrounded shipboard power systems. Safety needs to be taken seriously.

There are two important aspects of power supply safety. First is to keep the power supply user and their maintenance people safe. The second is to keep yourself, your technicians, and manufacturing personnel safe during the design and manufacturing of your power supply.

Safety Considerations in Power Supply Design in the Texas Instruments 2004/05 Power Supply Design Seminar is concerned with the first. The second is outside the scope of the reviewed paper, but I have added a few personal anecdotes on this important subject.

Bob Mammano of Texas Instruments and Lal Bahra of Underwriters Laboratories pretty much followed the problem solving approach used in this website. First they make you aware of safety problems, cover the relevance of design categories and specifications, present a method of solving the problems, and then write about solution details. Safety problems to be aware of include:

  • electrical shock
  • energy hazards
  • fire
  • heat related hazards
  • and mechanical

Relevance is covered in a discussion of the various safety standards and the equipment to which they apply. The solvability method is to partition the power supply into elements and then analyze each element for safety issues. Finally details are given such as required insulation thickness and layering, clearance and creepage distances, etc.

The structure of the paper is:

  • Introduction
  • Principles of Safety
  • Safety Standards for Power Supplies
  • Elements of a Power Supply
  • Protection with Insulation
  • Working Voltage
  • Insulating Materials
  • Clearance and Creepage
  • Designing for Safety
  • Power Supply Certification
  • Acknowledgement

All in all, the paper is excellent in making the designer aware of safety issues related to the power supply operator and maintainer.

An important subject not discussed is how you keep yourself and fellow workers from harm during the design and manufacturer of the power supply. Here are some personal anectodes that illustrate some of these pitfalls.

The first occurred when I was eight years old. My brother was trying to learn about electrical circuits by wiring bells and lights in various circuits and then energizing them with 115 V ac. The circuit was on a large round oak table in our dining room and power came through two long wires from the chandelier above the table and ending in non-insulated alligator clips.

The thing I did not notice was that my brother always put on rubber gloves before picking up the alligator clips. After showing me what fun it was, he cautioned me not to touch anything when he left the room. I immediately picked up the clips to try my hand with the lights and bells. The first shock involuntarily caused a spasm in my hands that would not allow me to let go of the electrodes. 115Vac went from one hand through my arm, upper torso, arm, and out the other hand. Unable to let go and shouting at the top of my lungs, I made two quick involuntary circles of the table and was then thrown deep into the fire place where I was pressed between the back wall and grate -- still unable to let go or even move in my tight fire-place-cage. It was probably less than 15 seconds before my brother responded to my yelling and pulled the wires out of the chandelier, but it seemed to me more like a lifetime.

According to the National Institute for Occupational Safety and Health (NIOSH), a total of 5,348 workers were electrocuted from 1980 through 1992, an average of 411/year. NIOSH considers this figure is under reported. Also it does not include non-worker electrocutions in households, etc. I consdider myself lucky to have lived beyond my eighth year. (Ray Ridley in his seminars always warns to touch a circuit with the back of your hand, so the involuntary spasm due to shock takes you away from the circuit instead of forcing you to grab it. I recognize the worth of this advice from personal experience.)

Another incidence occurred when I was older and supposedly wiser. I was taking a rectifier and filter circuit to its input voltage limits to determine its mode of failure. I had the circuit in an oven and my head in the oven as I slowly turned up a variac while I monitored the voltage and current meters and the circuit. All at once the hermetically sealed electrolytic tantalum foil capacitor on the rectifier output blew its end plug, sounding like a 45 caliber pistol being fired -- right in my ear in a confined space. I involuntarily jerked my head up away from the circuit and hit my head hard on the top of the oven and brought it down over the circuit just in time to take a deep breath of carcinogenic electrolyte fumes being released by the capacitor. The reflex action also pushed my face into the burning circuit. A deep stabbing pain in my lungs and burnt eyebrows sent me back away from the oven onto my back on the laboratory floor -- where I lay stunned with my ears ringing, my lungs hurting, my head aching, and my nostrils filled with the stench of my burnt eyebrows. Alone in the lab during late evening hours, I sat quietly for a half hour before packing up and driving home.

Another story concerns being hit by aluminum spalling when a hermetically sealed nickel-cadmium D cell was inserted backwards into the circuit. It held up for about 6 minutes before it exploded, bending and spalling the 1/16 inch aluminum frame in its circuit drawer enclosure. I just happened to be in the lab doing other things, but the spalling hit me from 12 feet away.

One interesting accident I observed was when an engineer placed a cup of hot coffee on his bench while testing a high current circuit. The banana plug wiring was neatly dressed in place but not tied. The force induced by the current caused the wires to jump apart when current was applied, knocking the coffee off into the engineer's lap. This was before Stella's time, so their was no million dollar settlement. (For more about Stella, see StellaAwards.com .)

I personally know of only one death caused by unsafe power supply design practice, but one is one too many.

There is a point to be made from these anecdotes. The circuit design engineer usually knows the most about the dangers associated with the circuits he works with. Often times from personal experience. It is his responsibility to communicate this information through all the sources available to him. There is often a safety boiler plate in functional tests, manuals, and other documents associated with a power supply. Make sure that it is more than the standard boiler plate -- make sure it contains all your knowledge of what can be unsafe about your circuit.

Here is the detailed bibliography information and abstract. After the 2004/05 seminar is complete it will probably appear on the Texas Instrument website. Until then you might be able to request a copy from the authors or a TI field engineer.

Reference: Mammano, Bob, and Lal Bahra, Safety Considerations in Power Supply Design, Texas Instruments 2004/05 Power Supply Design Seminar, SEM1600, pp. 1-1 to 1-13. 13 pages, 9 figures, 3 tables, 0 references (several specifications are mentioned in the paper), 0 appendix.

Author Abstract: Increasingly the responsibilities of a power supply designer extend beyond merely meeting of functional specification, with designing to meet safety standards an important collateral task. Since all commercial and home-use supplies must eventually be certified as to safety, knowledge of the requirements should be a part of every designers repertoire. This simplified overview has been prepared with the collaboration of Underwriters Laboratory Inc. to provide a basic introduction to the issues and design solutions implicit in assuring the safety for both the user and service personnel of your power supply products, as well as easing the certification process.

Posted by Jerrold Foutz at January 16, 2005 02:57 PM