Action levels – how to measure, when to act?
The EMF Directive makes a distinction here between thermal and non-thermal effects. Thermal effects are not only dependent on the field strength, but also on the frequency. For this reason, the Directive defines frequency dependent action levels in the range from 100 kHz to 300 GHz. Protective measures are needed if they are exceeded. Any measuring equipment used must therefore evaluate the field strengths correctly for frequency and accurately sum the individual effects, since there are usually lots of frequencies “on air” in the high frequency range – e.g., close to transmitter equipment. Personal field monitors worn on the person can warn staff of excessive field strengths in such situations.
Non-thermal effects are also frequency dependent. The EMF Directive therefore defines frequency dependent action levels for the range from 1 Hz to 10 MHz: Low action levels, above which sensory effects – transient changes in sensory perception – may occur, and high action levels, above which effects detrimental to health are to be expected. Protective measures are already required if the low action levels are exceeded. Preventive measures must be implemented to ensure that the high action levels are not exceeded.
Typical low frequency fields are primarily present in the industrial environment and are often pulsed fields. For this reason, the EMF Directive specifies the use of the weighted peak method, which weights the peak values in the time domain, as the reference method for non sine wave fields. Measuring instruments specifically designed for safety applications already implement this measurement method, also known as Shaped Time Domain (STD).