Recommended Practice for the Measurement of Radio Frequency Emission from Industrial, Scientific, and Medical (ISM) Equipment Installed on User's Premises
|Publication Date:||9 June 1988|
This document describes equipment inspection and radio frequency (rf) electromagnetic field measurement procedures for evaluation of rf industrial, scientific, and medical (ISM) equipment installed in the user's facility. The term, "ISM equipment," as used here, includes equipment that generates rf energy for purposes other than radio communications, to cause physical, chemical, or biological changes; for example, industrial heaters (dielectric and induction), medical diathermy, ultrasonic equipment, rf plasma devices, and rf stabilized welders. These procedures are designed to help ensure that the equipment does not interfere with radio communications, navigation, and other essential radio services. The engineer responsible for the measurements should take all reasonable precautions to ensure that the maximum emission from the ISM equipment under test (EUT) has been measured.
Radio frequency field-strength measurements of installed ISM equipment may be required if any of the following conditions exist:
1) The emission from the EUT was not measured by the manufacturer.
2) Because of its size or special operating conditions, the EUT could not be tested before installation.
3) Installed ISM equipment is suspected of causing interference.
4) ISM equipment has been modified in a way that could affect its rf emissions.
5) As the equipment ages, there is a question about its continued compliance.
6) There is a question about the safety to nearby personnel because of the emissions from the equipment.
Measurements should be made under the direction of an engineer skilled in making and interpreting rf field-strength measurements. These measurements are made after the equipment is installed and ready for use at its place of use, and after it has been inspected as described in 2.2. The measurement report should generally be kept on file for at least three years after new measurements are made or after the equipment is no longer in use.
There are significant differences between the "open field" or anechoic chamber measurements common in electromagnetic compatibility (EMC) and electromagnetic interference (EMI) work and the on-site measurements of installed ISM equipment. Some of these differences are described below.
1) The measurement conditions are usually more difficult because of crowded measurement locations, reflections from surrounding walls and equipment, and signals from other sources.
2) It is reasonable to take advantage of the shielding provided by walls and other equipment located between the ISM Equipment Under Test (EUT) and the location at which field-strength information is required. While the effect of a single wall may be small, the total effect of other equipment and building structures may be significant. In any case, it is not necessary to make measurements in a way which eliminates the benefits of this shielding. Some equipment is designed to be operated in a shielded enclosure, either because of its own sensitivity to outside interference, or because of its emission characteristics. When measuring the emission from this equipment, take advantage of the attenuation of the enclosure by making the measurements outside the enclosure, with the equipment in its normal operating position.
3) Since the measurement equipment is disconnected, moved, and reconnected many times during a set of measurements, its calibration should be checked frequently.
4) ISM equipment may often be grouped for field-strength measurements, taking one set of measurements for the entire group.
6) On-site measurements of installed ISM equipment are usually unique to the particular site because of effects of local shielding. However, they may apply to the same piece of equipment or to identical equipment installed at other sites that provide equivalent shielding and grounding.
7) The emission measured from the ISM equipment is a function of the environment surrounding the equipment. In most industrial locations, the environment is likely to change. Therefore, when measuring emission, 1) make sure the environment is typical of its usual operating condition, and, 2) determine, to the extent possible, if probable environment changes will increase emission. If so, the user should be warned. When measuring emission because of suspected electromagnetic interference, the conditions existing at the time of the suspected interference should be duplicated as closely as possible.