RF Protective Garments
Personal monitors can tell you where you cannot go or should not remain, but what if you need to work in an area with significant RF fields? RF protective garments are often a good solution to this problem. RF protective garments or suits are comprised of the following major components:
The suit components are comprised of a combination of Nomex, the same flameproof fabric used by racecar drivers, and stainless steel. The conductive stainless steel wires provide the attenuation. Manufacturers use from 10 to 25 percent stainless steel in the construction. While more stainless steel may seem desirable, it can lead to arcing and there are limits on the level of the RF field that you should enter due to other factors.
These garments can be considered Personal Protective Equipment (PPE). RF protective garments are often used by personnel who climb towers where FM radio and/or television broadcast antennas are located. They are also used by the military for certain applications. As there is with RF personal monitors, there is often confusion about RF protective garments.
These garments provide a substantial amount of protection—10 dB (10:1) minimum and often more for protection against emitters that operate above 50 MHz. But there are limitations and cautions, especially at lower frequencies, such as in the AM radio band.
Protection is limited. Putting on an RF protective garment should not be equated with Clark Kent changing into his Superman outfit in a phone booth. The manufacturers of these garments stress that you should know the intensity of the field that you are entering. Yet, most survey instruments are not usable at these high field levels, which leaves computer modeling as the most common method. The conservative, recommended approach is to assume 10 dB of protection and restrict your work to areas that are no higher than 1.000 percent of the MPE limits.
Suits must be worn properly. Suits should never be used without the hood above 800 to 900 MHz due to the potential for eye and brain damage at those frequencies. Hoods are generally not required below 400 to 500 MHz. At frequencies in between, the suits can be used at levels of 300 to 500 percent of the MPE. It is critical that the body of the suit makes intimate contact with the special conductive socks. The suits need to have a path to ground to work properly. If you leave the socks off, you not only do not have a good path to ground, but the currents in the suit pass through your ankles. The current density in the ankles can be very high due to the small cross-sectional are and can produce very high Specific Absorption Rates. If your ankles feel warm, something is wrong!
RF personal monitors do not work underneath RF protective garments. Wearing an RF personal monitor under an RF protective suit appears to make sense to those who are not familiar with the personal monitors and the suits. It would seem that the monitor would simply detect what is getting through to the wearer. Unfortunately, it is far more complicated than that. There are two basic reasons why this does not work.
Distance. The minimum measurement distance for RF sensors is either 20 cm or 5 cm, depending on the standard. In practice, many probes do not work with only a 5-cm (about two inches) spacing. It is one of the reasons that the IEEE increased the distance to 20 cm. If someone wears a personal monitor under an RF protective garment, the conductive fabric is normally touching the monitor and is less than half a centimeter from the sensors. All types of abnormal reactions can occur with the sensor this close to a potential radiator (the conductive fabric can become a radiator). Capacitive coupling can occur. which usually means that the levels are overestimated. but they can also short out the field.
Currents in Fabric. The strong currents that flow through the conductive fabric and the fields that occur on the surface can interfere with the control electronics. Under some conditions, this will cause false alarms. Often the monitor’s ability to function properly will be disrupted in terms of electromagnetic compatibility (EMC), and the monitor will not alarm at all, regardless of the magnitude of the RF field.
Wearing an RF personal monitor under an RF protective suit does not work. It is potentially unsafe for the wearer and would never be recommended by the manufacturer of the personal monitor.
The coverall, gloves, socks, and hood can impede mobility and add to heat stress. These are the negatives to what is often the only solution to working near active antennas. The hood can also limit visibility. If you expect to be exposed to an RF field with an intensity greater than 300 percent of the occupational MPE limit or from a microwave source, the need for protecting your head greatly outweighs the negative impact on your visibility.
A high-power RF personal monitor is available for use on the outside of RF protective garments. This combination of suit and monitor is the ideal solution to working in high-level RF fields. Just remember, if the monitor sounds an alarm at 1,000 percent, it is time to back off and get the power reduced before you proceed! Contact RF Safety Solutions for details and pricing.