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Question I have been looking into using drag chains on our conductive wire carts. Our floors read 10 E6, but when taking a reading between the wire cart and the floor, with the drag chain properly installed I get a reading of 10 E11. Is there a standard for the reading between the wire cart and the floor as far as the resistance is concerned? - Gabriel , St. Charles, IL
Answer The ESDA is revising the MIL-STD-1686 with the current draft standard ESD DS20.20-1998. In this document, they are proposing that mobile equipment should be kept below 1E9 ohms. The large difference (5 magnitudes) you are measuring between the cart and floor is indicative of an open path to ground. You should check the construction of the cart first. If the uprights connect the shelving with a plastic or other insulative bushing, then a hole needs to be drilled and a metal screw or bolt attached (for each upright) to electrically connect it to each shelf. Also, it the cart is painted or anodized, this could cause a higher resistance reading too. Check both the contact point of the drag chain to the metal cart (bottom shelf) and the contact surface to the floor. You can measure the shelf to chain with an Ohmmeter, but need a megohmeter to measure the chain to floor resistance reading. There should be very little resistance between the bottom shelf and the drag chain (measured above the serial resistor) and the resistance between the floor and chain should be close to the floors resistance. It has also been observed that some carts, even while grounded with drag chains, can tribocharge at substantial potentials. You will want to check this out with a grounded field meter as well.
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