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Question During surface to ground check of certain Mats and ESD laminate desktops we often fail over 10 e9 due to a fluorescent light near or on the surface being turned on. Also, it seems some energized equipment (even esd approved) seems to induce a voltage in the mat causing it to fail. And just the other day we had a mat fail due to a lamp (black esd approved) sitting on the corner of the mat. The lamp was OFF, but the mat still failed. We had to unplug the lamp from the outlet to clear the fault. Further examination of this phenomenon showed that this mats resitivity is being effected by any equipment line cord being near the mat. Some more that others. My theory is that the magnetic field from the line cord or device is inducing a voltage in the mat. Remove the field and mat passes. My question is, if the magnet field impedes my measurement, would it also impede the the protection offered by the mat. And what guidelines are there for having any powered equipment on the mat.
Answer

We would recommend an electrician evaluate the lighting, and possible replacement of the fluorescent light as its filed might damage ESD sensitive items.

Regarding the mat, we are not aware of any phenomenon where the resistance of an ESD mat would change due to a fluorescent light inducing a charge. It is more likely to interfere with the measuring device, the Megohmmeter. To test this theory, you could shield the Megohmmeter by enclosing in an ESD shielding test, and seeing if an accurate resistance measurement can be taken.

Regarding powered equipment, per ESD Handbook TR20.20 section 5.5.4 Equipment Caused EOS (Electrical Overstress) "Electrical transients generated by large inductors, e.g. solenoids, motors, relays, faulty test equipment, and faulty grounds, can cause EOS. Grounding does not guarantee protection from such transients. Equipment that has large inductors should have protection devices to prevent the electrical 'spikes' (transients) being induced into ESD sensitive hardware. To measure the electrical spikes, use a storage oscilloscope connected to the part of the machine that comes in contact with the hardware."

 
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