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. One big advance in radiation safety is the emergence of digital X-rays. The amount of energy required is far less. Digital dental X-rays, for example, require only a fraction of the X-ray energy formerly required to make an image on film. Veterninarians are even moving to digital, for this and other reasons.
X-ray safety isn't the issue it used to be. But the machines still have high-voltage supplies so they are still electrically dangerous. 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:
- Article 660 addresses safety from an electrical standpoint, only. It
does not address direct or stray radiation [660.1].
- 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.
- 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).
- For stationary equipment not over 30A, you can supply power via a plug
cap and hard service cable or cord [660.4].
- For portable equipment over 60A, the supply must be an individual branch
- 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].
- 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(A)].
- 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) [6606.(B)].
- 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.
- 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.