Brian, Some exposures can be measured on the person, some cannot.
If a person is standing under a power line in a homogenous field, a field can be set up in the person's body, and current will flow through the body out of the feet if not well insulated, or through the hands if grasping something earthed. There are methods suggested in some standards for measuring this type of induced current flow, and limits given for this. Another exposure would be from an inductive loop device that would couple into body tissue, often times non-homogenous. In this case there is no probe made to stick into one's organs to measure the induced current (thank goodness). To get around these measurement difficulties, standards have what they call "reference levels". Reference levels are related to the "basic restriction" which is the actual limit established by standards groups. The basic restriction is the limit of the induced current that should not be exceeded in the body, but usually cannot be measured. So methods were established by the standards groups to relate field strength of the external field to induced current, and this is called "reference levels". The reference levels are conservative, so if one passes the reference level, compliance with the basic restriction is guaranteed. If one fails the reference level, it does not necessarily mean that the basic restriction has been exceeded; rather, it means that a different analysis based on the basic restriction must be performed, such as a numerical analysis. Usually the numerical method is more complex, so it is always wiser to try the measurement of the reference level first. This reduces the issue to how do you measure an external field that a body would be occupying. This is much simpler than measuring organs... :-) ICNIRP established the limits that are referenced/incorporated in the EU Recommendation and Directive. They provided the limit, but not the method. The method is developed by standards groups such as CENELEC and the IEC. CENELEC and the IEC made product specific standards defining measurement parameters, giving distances and methods. I know which ones apply to EAS and RFID, but I'll need to follow up with another email for 50/60 Hz. Are your products "appliances" or "power industry" devices? Don Umbdenstock Manager Compliance Engineering Tyco Safety Products / Sensormatic 6600 Congress Avenue Boca Raton, FL 33487 USA Phone: 561.912.6440 [email protected] From: [email protected] [mailto:[email protected]] On Behalf Of Kunde, Brian Sent: Thursday, June 21, 2007 10:15 AM To: [email protected] Subject: RE: Low Freq EMF testing Thanks to everyone of the information. It has been helpful. Can anyone recommend test equipment for measuring EMF/ELF? I received some information but would like more. Any and all recommendations are appreciated. Thanks, The Other Brian From: [email protected] [mailto:[email protected]] On Behalf Of John Woodgate Sent: Tuesday, June 19, 2007 4:54 PM To: [email protected] Subject: Re: Low Freq EMF testing In message <c29d95f3.57cc7%[email protected]>, dated Tue, 19 Jun 2007, Ken Javor <[email protected]> writes: >I had no clue what this latest directive was, but recalled seeing it on >the cover of Conformity magazine. From there to the url, where the >limit at power-line frequencies is given not as an electric or magnetic >field, but as a current density, presumably a current density flowing >on the victim - a human body or simulation thereof. This is an example of the confusion caused by having many standards. The current density is the BASIS for the ICNIRP recommendations, by they are expressed (at power frequency) as field strength in amps per metre. The standard that deals with current density (IEC/EN 62226-2-1, I suppose) is about calculation methods; it does not deal with limits or the ICNIRP recommendations. > So it appears that what is needed is a current probe that measures >current density flowing on a surface, as opposed to a clamp-on probe. No, you measure the field strength. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk There are benefits from being irrational - just ask the square root of 2. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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