A string trimmer will save a lot of time with yard work. They are inexpensive, and simple to use. There are a few things that you should keep in mind when using one regarding safety.
1. Trimmer Line
All trimmers are made to accommodate a certain size of line. If you use a line that is too heavy, it can stress the motor and cause the trimmer to not operate properly. If you no longer have the owner's manual when it's time to replace the line, it's best to remove the old line to take with you when you purchase the new line.
2. Protective Clothing
When you use a string trimmer you should always wear protective clothing. It may be uncomfortable in the summer to wear long pants, but it's advisable. The trimmer can kick up unseen debris such as rocks and small twigs, and cause injury to exposed skin. Try to wear boots that will protect your feet and ankles as well. It's also important to protect your eyes with a good pair of goggles or safety glasses for the same reason. Wearing protective ear covering is also a good idea since some trimmers can be loud.
3. Wet Weather
Using in a string trimmer in wet weather is not a good idea. Not only do you run the risk of slipping and falling, the trimmer won't operate at full capacity if it's being used on wet grass or brush. This can eventually wear down the motor, forcing you to replace it before its typical lifespan.
It's best to allow the trimmer to cool down before refueling it. If you add fuel to a warm engine, spilled gasoline can ignite as it hits hot engine parts. To avoid this, make sure you are operating with a full tank before you begin your yard work. If it becomes necessary to refuel while you are working, put the trimmer down and move on to another project to allow the trimmer time to completely cool off.
Many trimmers are compact in size, and would easily fit in a storage closet or pantry. Because most trimmers operate with fuel, it's important to not store them in enclosed areas. An electric trimmer can be safely stored indoors in a closet, but if the trimmer uses gas, it should be stored outdoors in a ventilated area.
6. Installing Blades
Unless your trimmer model is interchangeable (and very few are) you should not try to replace the string with a blade. String trimmers are made specifically to use with a line, and adding a blade is not only dangerous, it will cause the trimmer to not work correctly.
A string trimmer should not be used to cut thick shrubbery or brush. Since the line is made of nylon, it won't be thick enough to cut through this. Also never walk backwards or sideways as you work. Keep a clear line of sight on what you are working on.