ESD Systems - Questions And Answers

Our ESD policy states that all equipment that is plugged into a wall outlet such as Oscopes and function generators must be insulated from the ESD mat with rubber feet of some kind. It also states that all ESD sensitive items must be in contact with the mat and not insulated. This is a problem when I am troubleshooting a function generator. when it is plugged into the wall I need the rubber feet and when I unplug it I have to remove the insulating rubber feet. Could you shine a little light on this requirement for me so I can go to the ESD police and suggest a change to our procedures. - Anonymous, Orlando, FL

Your ESD policy is designed for overall personnel safety. The reason for isolating the test equipment is to reduce the possibility of an electrical shock to a grounded (ESD Protected) operator. The reason that ESDS devices should be able to make contact with the mat is to ensure they have a way to remove (bring into equilibrium) any charge imbalance to minimize the chance of an ESD event. These are general policies and should have some flexibility. There is no physical way to ground ALL conductors that make up an ESD Sensitive (ESDS) device [as all the traces on the board, pins of a chip, etc., may be guarded by discrete device packages, connectors, etc.]. All grounded operator (if they are using a quality wrist strap or foot grounder) will be grounded with a 1 Megohm resistor in series to ground. This resistor is designed in with only one purpose, the safety of the operator. UL approves quality made wrist straps and foot grounders that employ an 1 Megohm resistor in series with its ground circuit. UL has approved our wrist straps and foot grounds up to 250 VAC because of the 1 Megohm resistor. If you are working on equipment that is under 250 VAC and grounded in series with a 1 Megohm resistor (which will limit the current to an acceptable safe value up to 250 VAC rms) then you should be able to work on this equipment and be ESD protected. THE SAFETY OF THE OPERATOR ALWAYS COMES FIRST IN ANY PROGRAM. Check with your ESD Control program administrator before changing policy. If this is still a concern, table mats too can be grounded with a 1 Megohm resistor to ground via its ground cable.

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