Isolated Conductor Clarification

Technical Questions
#1

I’ve read a few conflicting opinions on this topic so I’m looking for clarification. I have a fixture with a few conductive components that may contact ESD sensitive product.

The conductive material measures less than 1x10^9 ohms point-to-ground. Per ANSI S20.20, an isolated conductor is a conductor that measures greater than or equal to 1x10^9 point-to-ground.

So these components are not “isolated” conductors. From this, I’m concluding that the requirement on the +/-35 volt potential between isolated conductors and ground does not apply.

With all this in mind, are there any charged device model requirements that still need to be considered/followed? Or is this point-to-ground resistance being less than 1x10^9 enough?

Also, there is a statement in ANSI S20.20 that states: “If there is a concern for CDM failures, then a lower limit of 1.0x10^6 ohms for point to point and point to groundable point should be considered.” I don’t have any reason to be concerned with CDM failures, but was looking for clarification on if this statement might apply in situations where there’s contact between product and conductors (non-isolated conductors).

Thanks

#2

Hello Cbor,

This question comes up from time to time and I am glad you asked. There are a few things to consider here. First, you are correct that the conductive items are not considered isolated conductors. Since they measure less that 1x10*9 ohms resistance to ground they will be at the ground potential of 0 volts.

The second point is the low resistance limit of 1x10*6 ohms is meant to apply to worksurfaces. There could be some concern if a device charges then placing it down on a very conductive surface, such as a stainless steel worksurface, could result in a CDM like event no matter what the resistance to ground is. However, for that to happen the device needs to be charged. If the device is not charged, then it is not an issue.

Almost all devices make a low resistance contact at some point. What is important is to ensure at that connection point there is no potential difference.