Electromagnetic Interference

With the proliferation of electronic equipment that is a part of our homes & businesses today, has come a cacophony of electronic noise.  This noise, can exist in nearly any part of the electromagnetic spectrum from DC through microwave and beyond.  We’ve all heard the hum of electromagnetic noise Perhaps the most problematic is noise in the 2.5GHz range which has the annoying side effect of interfering with our wifi networks.  While this is certainly a real problem, and I have identified it as the source of legitimate interference in the past when diagnosing wifi network problems, I happen to also believe it is a convenient scapegoat used by IT-folk when at their wits end about why your wifi connection keeps getting dropped.

But either way, to illustrate a simpler case, I’ll explain a scenario I encountered with a client in the healthcare industry.  A hospital contacted me saying they were experiencing intermittent noise on an ECG (electrocardiogram) machine so severe that for some patients the ECG output was completely unreadable by the doctor.  As a result the patient had to be moved to a different part of the hospital or asked to return at a different time when hopefully the machine would work.  They had already beat up the vendor of the machine, and replaced an expensive set of skin electrodes and associated wiring.  They provided me with scrolls of logs indicating what times of day and for whom the machine would not work properly.  I examined these, looking for periodicity that could point us to a cycling load, a certain operator, etc.  Nothing.  It was random.  I had an assistant lie down and get a “free” ECG from the nurse so we could see whether the offending electrical noise was currently present.  Of course not.  But we returned another day armed with a wide range spectrum analyzer and a couple different antennas, some directional.  The spectrum was relatively quiet throughout, except the 60Hz background electromagnet radiation that you expect to find in any building with 60Hz electrical service.  The intensity of the 60Hz energy varied a lot as we moved around, and there was definitely some present at the point of use.  The ECG’s I had been provided had a time scale that didn’t permit inspection of the waveform of the trace on the printout.  But it turned out that we could zoom in on the trace while it was on the screen.  The noise was periodic, and counting the divisions on the screen revealed it was in large part the same 60Hz spectral content that I was seeing on the spectrum analyzer.  From there it was a simple matter of systematically shutting off equipment one load at a time to see where the noise was originating from.  In this case it turned out to be a simple floor standing lamp near the examining table.   As supplementary lighting, it was on sometimes, not others, the staff reporting that they used it as needed.  It was also moved around the room to some extent.  With my assistant again reluctantly on the ECG machine, we flipped the lamp on & off and observed the noise on the trace appear and disappear accordingly.  There is nothing better than finding a very simple solution to a tricky problem.

I would have like to do a leakage current test on the lamp, but the hospital opted to discard it.  There are specific guidelines for maximum leakage currents in patient care areas, which vary by proximity, equipment, and procedure.  Electromagnetic noise is among the less important symptoms of leakage current, patient electrocution being the foremost.  Healthcare providers should have periodic leakage current measurements as part of their electrical safety program.  There are specific ANSI, UL, NFPA and IEC standards governing allowable leakage values depending on many factors.  If your business is experiencing EMI problems, or your healthcare facility needs an updated electrical safety program before the next JCAHO inspection, we can help.

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