No definitive answers, just some observations . 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. 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. My test facility has one of these skin-current probes, but it does not operate at 50/60 Hz!!!
The fact that I know of no such device doesn't mean one doesn't exist, but it is interesting to consider the physics of the sensor. The current density to be measured flows as a result of a magnetic field impinging upon a human body. The sensor will be sensing the magnetic field resulting from the current density, but must reject the causative magnetic field impinging upon the same body. Given the vastly larger magnitude of the causative field (due to the high impedance of human skin), this implies a huge amount of selectivity on the part of the sensor. This might be available if the causative and resultant magnetic fields are at right angles to one another, but this is not generally the case. In fact eddy currents flow in a direction so as to oppose the field which caused them (Lenz' Law). That means a skin current probe measuring eddy currents on a surface could not differentiate between cause and effect, it would only measure the net field (cause - effect). There might be some science project of a laboratory set-up in the dungeons of a castle in Transylvania somewhere that is capable of measuring current density on a human body as a result of an impinging magnetic field. If that set-up could be used to determine a correspondence so that the causative magnetic field could be specified instead of the resultant current density, then maybe this physical agents directive could be rewritten to control the causative agent. Then all you need is a shielded magnetic loop and a frequency selective voltmeter (see MIL-STD-461E test method RE101). While there are any number of techniques to measure magnetic fields and RE101 is but one, there is a very important principle to keep in mind. When sampling a field quantity in the quasi-static zone, it is important to use a probe that is representative of the victim protected by the limit, at a distance from the test article that is representative of actual use. You will get different answers that won't correlate through the loop transducer factor using loops of different sizes or at different distances, because the loops are calibrated with a homogenous field penetrating their enclosed area, and this will not be the case making a measurement. So even if a causative magnetic field intensity were specified, a particular sensor would need to be specified as well, in order to get repeatable measurements. From: "Kunde, Brian" <[email protected]> List-Post: [email protected] Date: Tue, 19 Jun 2007 13:55:35 -0400 To: <[email protected]> Subject: Low Freq EMF testing My fellow colleagues, We would like to come up with an internal test to ball-park measure electromagnetic fields at line frequency (50/60Hz) to satisfy our customers when such requests are made regarding compliance to the EU's Physical Agents Directive (2004/40/EC). Any advice regarding test setup and test equipment would be appreciated. Our EMC/Safety lab does not have much expertise in EMF test methods and equipment. We assume that our products would have no problem meeting the directive's requirements, but when customers ask, we would like to give them something. Instead of spending a lot of time and money testing products at qualified labs, we are looking for a simple test setup and test equipment to perform some kind of test ourselves. 1. For IT and Laboratory equipment, what EMF test method standard should we be looking at? Is there a harmonized test standard? 2. What test equipment is required? A Google search popped up a lot of hand held products that claim to measure EMF. Would such products do what we need? 3. Is this approach doable, or have I embarrassed myself by even proposing such a test? Thanks to all in advance. The Other Brian Notice: This communication may contain confidential information intended for the named recipient(s)only. 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