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National Electrical Code Articles and Information

Based on the 2011 NEC

by Mark Lamendola

National Electrical Code Top Ten Tips: Article 660 -- X-ray Equipment

While they have proven enormously beneficial, medical diagnostic machines that use X-rays also present many hazards. For example, prolonged or repeated exposure even at low levels will result in progressive tissue damage. Risk of cancer is increased with exposure to X-ray.

The manufacturers have addressed this danger in many ways, including equipment design, recommended usage procedure, and user training. And the same is true of nonmedical and nondental equipment, such as industrial X-ray equipment. For this type of equipment, the NEC addresses the issues involved in making this equipment electrically safe. It does that in Article 660.

 

NEC tips for nonmedical and nondental X-ray equipment:

  1. Article 660 addresses safety from an electrical standpoint, only. It does not address direct or stray radiation [660.1].
     
  2. A long-term rating is based on an operating interval of 5 minutes or longer. A momentary rating is based on an operating interval of 5 seconds or less [660.2]. What about the time in between? The NEC is silent about that; if perchance you end up using equipment that's in the 'tween area, consult the manufacturer. And remember to err on the side of caution if you can't get resolution any other way.
     
  3. Portable and transportable are not the same thing. The former means you can carry it by hand [660.2]; the latter means it can be transported in a vehicle (if readily disassembled or not in need of disassembly).
     
  4. For stationary equipment not over 30A, you can supply power via a plug cap and hard service cable or cord [660.4].
     
  5. For portable equipment over 60A, the supply must be an individual branch circuit [660.4].
     
  6. The disconnect must be of adequate capacity for at least 50% of the rating of momentary rated equipment, and 100% of the rating of long-time rated equipment [660.5].
     
  7. The branch circuit conductors must have an ampacity for at least 50% of the rating of momentary rated equipment, and 100% of the rating of long-time rated equipment [660.6].
     
  8. The feeder circuit conductors (supplying more than one branch circuit) must have an ampacity for at least 100% of the rating of momentary rated equipment (based on the two largest X-ray apparatus plus 20% of the momentary ratings of the other X-ray apparatus combined).
     
  9. The transformers and capacitors that are part of an X-ray equipment don't have to comply with Articles 450 and 460 [660.35]. This, however, does not imply anything whatsoever about the transformers and capacitors that are part of your service/feeder/branch power supply.
     
  10. Part IV of Article 660.47 talks about grounding. This is an erroneous use of the word "ground" so do not ground anything.

    See the Article 100 definitions of grounding and bonding. Where Part IV talks about grounding, it means bonding. You can see that from those definitions, for one thing. You can also see this when you follow the instructions of 660.48 to "ground" non-current carrying metal parts in the manner specified in Article 250. Look in Part V of Article 250. It requires that you create a metallic path, which is bonding.

    If you ground where you should bond, you will leave hazardous differences of potential that will probably get somebody killed. The NEC still has not fixed this persistent language deficiency, but now that you know what it really means you can provide a safe installation.

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How the NEC is arranged

  1. The first four Chapters of the NEC apply to all installations.
  2. Article 90 precedes Chapter One, and establishes the authority of the NEC.
  3. Article 80 follows the body of the NEC; it exists as Annex H. It provides the requirements for administration.
  4. Chapters 5, 6, and 7 are the "special" chapters, covering special: occupancies, equipment, and conditions (in that order).
  5. Chapter 8 provides the requirements for communications systems.
  6. Chapter 9 provides tables.
  7. The appendices provide mostly reference information.
  8. Appendix D contains examples that every NEC user should study.

Try your NEC moxy:

  • Do you know the difference between bonding and grounding? Hint: Look in the NEC, Article 100.
  • Does the NEC refer to grounding incorrectly in any of its articles? Yes! So be careful to apply the Article 100 definitions. Don't ground where you should bond.
  • When doing motor load calculations, which Article covers hermetic motors? Answer: While Article 440 covers the application of hermetic motors, it does so only by amending Article 430 because hermetic motors are a special case of motors. For motor load calculations, refer to Article 430.
  • Does the NEC provide a voltage drop requirement? Yes! It does so in a special case, which is Article 648 Sensitive Electronic Equipment. But for general applications, it does not provide a requirement; it merely provides a recommendation in a couple of FPNs.
  • Take our Code Quizzes.

Remember other applicable codes, rules, standards, and references:

  • OSHA's electrical worker safety rules.
  • IEEE standards.
  • NETA standards.
  • NFPA standards.
  • International Codes (if applicable to the installation).
  • State Codes (if the state has them).
  • Local ordinances and permit requirements.
  • Local fire codes.
  • Manufacturer requirements or guidelines.
  • Customer security requirements.
  • Industry standards.
  • Your company's own internal standards, practices, and procedures.
  • Engineering drawing notes.

 

 

 

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