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A free electronic newsletter covering news and other topics for those interested in RF safety issues.
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Why and where do shocks and burns occur? A shock or an RF burn occurs when you come into contact with either an RF radiator or a re-radiator. RF radiators are usually some type of antenna. Many antenna designs cause RF current to flow in their metallic components which in turn is radiated into space. Touch one of these surfaces and the energy flows through your body to ground. Similarly, the same thing can happen if you touch a re-radiator. Any ungrounded, conductive (usually metal) object that is in the field of a strong RF source can be illuminated by the RF field and re-radiate the energy back into space. It is often very difficult to make a good RF ground so objects that appear to be well grounded are often "floating." For example, the U.S. Navy goes to great lengths to add copper straps to the railings that are welded to the hull of the ship. Yet, when the straps are broken due to damage or corrosion from salt, it is not uncommon for sailors to receive burns when they touch the railing. When you touch a re-radiator, you provide a path to ground through you. A surge of energy occurs at the point of contact. This results in a shock and, in many cases, an RF burn. Factors that impact shocks and burns The primary factors that determine if you will receive a shock or burn should you contact a conductive object are:
Although the relationship between field strength and shock and burn potential is very complicated and difficult to calculate, it is clear that the shock potential increases with the strength of the electric field. The potential for shocks and burns are related to the frequency and how well grounded you are in a way similar to the concept of SAR. If you are very well grounded, you have a much higher chance of receiving a shock or burn. If the frequency is close to where you make an ideal, quarter-wave antenna, then the potential is much higher. This occurs at about 40 MHz for well grounded adults. Many engineers that have received shocks and burns approach a conductive object with trepidation and reach out to touch it much like one would lightly touch a wall to see if the paint was still wet. This is absolutely the worse thing that you can do! When you touch an object with the tip of your finger, all the current flows through a very small area. If you grasp the object or touch it with the palm of your hand, the area that makes contact is about 100 times larger. And the current density is only about one percent as much. This is the reason that the IEEE standard for contact current is based on a grasp. It was felt by the committee members that limiting the field levels to a point where a point contact would not produce a shock or burn would result in very restrictive exposure limits. The standard would be based on concern for shock rather than body heating. |
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