|
|
|
A free electronic newsletter covering news and other topics for those interested in RF safety issues.
LIVE, Web-Based RF Safety Training
|
Overview There are two major IEEE (Institute of Electrical and Electronics Engineers) standards relating to RF radiation.
The IEEE standards both have both have dual designations as ANSI (American National Standards Institute) standards, i.e. ANSI C95.1-1999. The IEEE C95.1 standard has exposure limits for electric fields and magnetic fields that are whole-body and time averaged. It has relaxed limits for an appendage and also has exposure limits for induced and contact currents. History The first ANSI standard that covered RF radiation exposure was issued in 1966. It was four pages in length and recommended that exposure be limited to field levels no higher than 10 mW/cm². Exposure limits were expressed as Radio Frequency Protection Guides (RFPG). Under ANSI rules, committees must meet every five years to reevaluate a standard and either reaffirm or change it as required based on the latest available information. The original standard was updated without substantial change once in the 1970's. The first major change came in the 1982 version. ANSI C95.1-1982 became the first SAR-based human exposure standard in the world. All the major standards in the world today are similar and have the characteristic frequency-dependent exposure limits. There are some important differences but the exposure limits for electric and magnetic fields are all based on the same concepts of body heating and electrostimulation (shocks and burns). Five years after this major update the now much larger group began the process of reviewing the standard. Amid an atmosphere of dissention and concern over liability issues that ANSI was concerned about, the sponsorship of the standard was switched from ANSI to IEEE. IEEE is a larger, stronger organization that is international in scope. Thus the next update of the standard, which incorporated several major changes, was IEEE C95.1-1991. A year later it was ratified by ANSI. The latest version represents only minor changes and clarifications from that landmark standard. Exposure Limits The IEEE standard has exposure limits for electric fields and magnetic fields that are whole-body and time averaged. Language is much stronger—limits are expressed in terms of Maximum Permissible Exposure (MPE). MPE limits for the magnetic (H) field are relaxed below 100 MHz since the exposure limits at lower frequencies are based more on electrostimulation than body heating and both induced and contact currents are related to the strength of the electric field. There are also limits for induced currents and contact currents. This is a two-tier standard. MPE limits are specified for Controlled Environments and Uncontrolled Environments.
Table 1 Maximum Permissible Exposure Limits for Controlled Environments
RF Electromagnetic Fields
RF Induced and Contact Currents
Table 2 Maximum Permissible Exposure Limits for Uncontrolled Environments
RF Electromagnetic Fields
RF Induced and Contact Currents
Controlled versus Uncontrolled Environments
Nothing in the IEEE standard has caused more confusion and discussion than the definitions and applications for the two environments. The IEEE standard does define both terms but the definitions have drawn a great deal of criticism. The FCC regulations use definitions that are similar but with several changes made in a attempt to improve them. And now the FCC is poised to further clarify its two-tier limits.
In all other areas of industrial hygiene the term "controlled" environment normally means that there is a safety program in place pertinent to the particular physical or chemical hazard, in this case RF radiation. This is the often stated position of OSHA and increasingly, of the FCC. Quite simply, you have to have an RF safety program in place in order to qualify to use the Controlled Environment Maximum Permissible Exposure limits.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Send mail to
web@rfsafetysolutions.com with
questions or comments about this web site.
|
