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Questions And Answers

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Question What is the purpose of Surface-to-Surface resistance measurements across a dissipative surface vs. RTG measurements on the same surface taken over several points on the surface?  Does one need to do both?
Answer

Normally, for worksurface compliance verification testing, resistance to ground (RTG) using one 5 pound electrode is sufficient. The recommendation is to conduct resistance point to point (RTT) using two five pound electrodes when doing troubleshooting.

Per ANSI/ESD S20.20-2007 Table 3, the noted test method for the product qualification of worksurfaces is ANSI/ESD S4.1 and both RTG and RTT are performed in an environmental chamber. The end user can rely upon the tests having been done by the worksurface or mat supplier.

Per ANSI/ESD S20.20-2007 Table 3, the noted test method for the compliance verification of worksurfaces is ESD TR53-2006 and only RTG is required tested at ambient environment. The test procedure includes: “Place the electrode in the center of the ESD worksurface and apply 10 volts. Wait for the integrated checker or meter to stabilize. If the indicated resistance is less than 1.0 x 10E6 ohms, continue testing. If the indicated resistance is equal to or greater than 1.0 x 10E6 ohms, switch the integrated checker or meter to 100 volts and retest the ESD worksurface. Tests should include those ESD worksurface areas that are subject to wear or are visibly dirty.”

The Troubleshooting section includes a RTT test: “Ensure the environmental ambient conditions have not dried out the worksurface material. To determine this condition, the resistance between two locations on the top of the worksurface ([RTT] point-to-point) can be performed per ANSl/ESD S4.1 using an integrated checker or meter and two electrodes with test leads.”

The end user should have a written ESD control plan including a compliance verification plan that details the frequency and sampling (if used) of the compliance frequency testing.

However, as a practical matter, one should test each type or color of worksurfaces material if one is not sure what its RTT resistance is. This is particularly impotant if one uses ground cords that include a one megohm current limiting resistor. Just for the worst case example, say you have a stainless steel mat but the ground cord has a one megohm current limiting resistor. The RTG would measure 1 x 10E6 with the RTT being only one ohm.

 
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