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What eye protection is required for CNC plasma cutting?

May 26, 2013

Don’t stare at the bright light!

A plasma cutting arc, like any electric arc, gives off a broad spectrum of electromagnetic radiation, which extends all the way from Infrared light (IR), through the visible spectrum, and into the Ultra Violet (UV) range. Plasma cutting arcs can also be very intense, because the arc current is typically anywhere from 100 to 800 Amperes. Needless to say, looking at an arc that intense can easily cause eye damage, including permanent damage leading to blindness.

Recommended eye protection for plasma cutting

There are a few sources of recommendations that everyone should be aware of. These include OSHA and ANSI. The following information comes from the OSHA Fact Sheet “Eye Protection against Radiant Energy during Welding and Cutting in Shipyard Employment”, and also references the ANSI numbers. The protective shade numbers refer to the darkness shade of safety glasses or welding shields, the higher the number the darker the glass.

OSHA Shade Recommendations For Plasma Cutting - Click for larger view

ANSI and the American Welding Society (AWS) publish a slightly more detailed chart, in ANSI Z49.1:2005 Safety in Welding, Cutting, and Allied Processes.

ANSI and AWS eye shade recommendations for plasma cutting - Click for larger view

In both cases, the recommendations offer the rule of thumb to “start with a shade that is too dark to see the weld zone. Then, go to a lighter shade which gives a sufficient view of the weld zone without going below the minimum”.

By comparison, the ANSI recommendations for SMAW, GMAW, and FCAW welding are all in the shade 10 – 14 range also. So how do you apply that to operation of a? When you are welding, you pretty much stand still and look at the weld zone through a helmet, so you can manually operate the welding torch that is just inches away from your face.

Operating a CNC machine is a little different:

  • the operator is often much farther away from the arc.
  • the operator needs to move around, so he must be able to clearly see his surroundings (it would be rather unsafe to walk around the shop under a welding helmet).
  • the operator needs to be able to see the machine control as well as the overall machine while the arc is on.
  • the arc is moving, and the operator is not always looking in that direction.

With these considerations in mind, how can a CNC plasma operator wear glasses as dark as a welder’s shield and still see enough to get anything done?

Differing Opinions

Obviously, if you were to operate an 800 Amp plasma torch at the same distance as a typical welding torch, you would need a welding helmet for protection. But nobody stares at the arc while operating a CNC machine, unless trying to diagnose something while cutting, and then a welder’s shield is usually held between the face and the torch to allow safe viewing. For normal operation of a CNC machine, the above recommendations are too dark.

There is also an opinion held by some in the industry that the only protection your eyes really need is protection against UV and IR light. Those people believe that visible light can’t hurt your eyes, and since almost any safety glasses (even clear ones) filter out UV and IR light, the dark shades are not required!

I tend to disagree with that notion, here’s why:

Evidence

While the evidence is not definitive, an increasing body of research points to intense visible light as a cause of eye damage leading to problems such as macular degeneration, retinal detachment, and permanent blindness. The source of the problem is “blue light”. Visible light includes wavelengths in the range from 380 to 780 nm. Shorter wavelengths within this spectrum create light in the “blue” range. These are the more energetic wavelengths, making them more hazardous for long-term retinal exposures.

Plasma arc cutting light spectrum - Click for larger view

This spectral analysis shows what the relative intensity of light across the spectrum might look like coming from a typical plasma cutting arc. The most intense visible light is in the “blue light” range.

The Danger

Since a plasma arc generates light across the entire spectrum, it definitely includes “blue light” in the light that you see. And because the visible light from a plasma arc is very intense, the damaging “blue light” from a plasma arc is also very intense, and therefore potentially dangerous to the human eye.

But here is one of the biggest problems – you can’t feel retinal damage until it’s too late. If you damaged your retina, there's no pain, and the only way you know is when your eyesight starts to degrade. By the time you see an eye doctor about the problem, the damage is permanent.

Conclusion

Somewhere there is a happy medium, where the CNC plasma operator’s eyes are safe from damage, but he can still see well enough to operate the controls and walk around the shop without tripping over stuff. The recommended shades may be too dark to actually work in, but wearing clear safety glasses is not the answer. Safety glasses with yellow lenses are recommended to block potentially damaging blue light, and a secondary shield between the operator and the cutting torch, perhaps a welding curtain or dark plastic shield, could shade the operator from the most damaging radiation.

References:

OSHA Fact Sheet - Eye Protection against Radiant Energy during Welding and Cutting in Shipyard Employment

ANSI Z49.1:2005 Safety in Welding, Cutting, and Allied Processes

Blue Light Eye Damage

The Risk of Eye Damage from Bright and Blue Light Therapy

The Effects of Blue Light on Ocular Health

The Scientific Journal of The Royal College of Opthalmologists - Retinal light toxicity

Posted in Cutting Systems , Tagged with Plasma, Safety

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