Understanding the safety hazards that go with fiber optic cable is critical for those who install or maintain fiber optic systems.

As electrical professionals, most of us take fiber optic (FO) safety for granted. Since fiber optic cable carries no electricity, we don't worry about electrocution. Similarly, we don't think about personal or property damage due to fire because it isn't a source of heat or combustion. In fact, since its light source is invisible to the naked eye, we aren't even sure when it's transmitting data signals. It's this lack of understanding that poses the greatest threat to those of us installing or maintaining fiber optic systems.

Invisible light. Take a look at the wavelengths shown in the Table, left (not available online). Almost all of them are 800 nm (nanometers or billionths of meters) or higher. These all classify as infrared light, which our eyes do not see (very similar in color to the light used by television remote controls). Visible light has a wavelength between 380 nm and 750 nm. Light beyond this range is invisible to us. However, even though we cannot see this light, it can cause severe damage to our eyes.

The power levels and wavelengths found in common fiber systems range from 50 nanowatts (nW) to 10 milliwatts (mW), as shown in the table, below. While this may not sound like a lot of wattage, keep in mind this light can potentially pump through a fiber that's only 9 millionths of a meter (microns) in diameter. Even at these low levels of power, that's a fairly high level of watts per square centimeter.

Dangerous situations arise when untrained people pick up a live fiber, and look directly into it. They see no light. Therefore, they assume there's no danger. However, such unsuspecting people can end up with a burned retina in a very short time.

Do not confuse looking into a live fiber with performing continuity checks. We call one of the better testing tools available today a visual tracer. Essentially, a visual tracer is a visible light you shine down the fiber. Then, you use your eye to trace the fiber through its course to the end. The tracer itself can be a flashlight (difficult to use), a modified flashlight, or even a microscope that holds the fiber in place while you direct a source of light into it. The better tracers use special test sources, which use a bright red LED source. The big benefit is the power levels of all visual tracers are too low to cause eye damage.

For single-mode cables, we sometimes use a more powerful tool, called a visual fault locator (VFL). These testers use red lasers that have enough power to actually show breaks in the fiber through the cable jacket. They are stronger than simple fiber tracers, but still not powerful enough to do bodily damage.

You can also purchase a special film card to identify live fibers by eye. This small card, which costs about $10), converts the infrared light to visible light. By using it, you can tell quickly and easily whether a fiber is live or not.

The odds of going blind by looking into the broken end of an optical fiber are virtually nil, since the broken surface tends to scatter the light coming through it. However, it is possible for you to suffer injury by mishandling polished optical fibers, but only under certain circumstances, including: 1. The light source must be high-powered. Only the more powerful lasers are strong enough to cause injury. For example, some Cable TV lasers are powerful enough to do damage. 2. The beam of light exiting the fiber must be narrow. Do you remember the old trick of starting a fire with sunlight and a magnifying glass? Just as in that case, the light from a fiber must be very tightly focused to cause harm.

Microscopic glass needles. A more serious hazard of optical fiber work is the fibers themselves. Fibers are pieces of glass. And like all glass, they can cause injury.

Because of this, you need to handle fiber with care. First of all, you must be very careful when handling open fibers; that is fibers not contained in a cable. (Modern optical fiber cables are very safe, and pose no danger to you. It is when the cables open that hazards arise.) If you were to accidentally jab yourself with one of these open fibers, you could easily end up with a painful sliver. What's worse is this sliver may not be visible! Remember: These slivers are made of transparent glass and can be very difficult to see.

You'll be surprised to know that jabbing yourself with a fiber is not the most hazardous situation. The real danger is when fibers are stripped, trimmed, and cut. These operations result in short, nearly microscopic pieces of glass lying around a work area. These are short, thin, invisible needles. If they're left lying around, someone will inevitably end up touching or handling them. As sharp and thin as these glass shards are, they can easily penetrate your skin. And unlike a wood sliver, these glass slivers will not degrade inside your skin.

These cut pieces of fiber are very dangerous. If they were to end up in your lunch, they could cause internal bleeding and conceivably death.

To avoid this problem, you should make generous use of masking tape (or any other type of tape) to catch the waste fiber pieces. Some technicians wrap the tape around a few fingers, sticky side out. This catches the fibers as soon as they are cut. You should also frequently blot the entire work area with tape to pick up stray pieces. Once the pickup operation is complete, you need to fold the tape upon itself and carefully dispose it. Never leave this tape lying around. Remember fibers are insidious, since they are very difficult to see; especially when you're not expecting them. If you happen to sit on some of these cut pieces, you won't soon forget it.

Associated hazards. On a final note, you should take care when installing optical fiber in a hazardous or difficult environment. Since runs of fiber frequently share facilities with power conductors (whether on poles or in underground raceways), you will often find yourself working in confined spaces (such as manholes) or on a pole. These locations pose hazards such as induced voltages, explosive gases, or energized equipment or cables. When working in these areas, you must follow appropriate safety rules and procedures.