Does this 12” rule indicate what should be the static voltage (not field) at a distance of 12 inches from the ESD source? S20.20 states that the field should be less than 2000V/inch. So if we take the distance as 12 inches, we get a voltage of 167V (2000volts per inch/12 inches). So does that mean, that at a distance of 12 inches, the static voltage should be < 167V?
Welcome to the EOS/ESD Association forum.
Just to be clear, this is a question about insulators not isolated conductors. So to answer your questions on insulators, if an insulator is more then 12 in away from an ESD sensitive device, there is no need to measure it. Second, the field (which is volt/in) drops off by the square of the distance not linearly. So if you double the distance, the field is reduced by 2 squared or 4 times.
Also be careful of using the meter. The field meters that measure volt/in are calibrated at a specific distance, typically 1 in. The measurement is only valid 1 in away from the insulator. The proper procedure is the measure the field emanating from the insulator, then determine if it is a threat depending on the distance from the ESD sensitive item. So for example, if you measured an insulator and the results were 1000 volts/in, you need to keep it at least 1 in away from the ESD sensitive device. If it measured 2500 volts/in you would need to keep it at least 12 in away from the device. If you cannot do that, you must do something to reduce the field from the insulator.
I hope that helps.
Thank you for your question. The 12" rule applies to charged insulators so it is electric field only since an insulator does not have a voltage (potential). The charge cannot flow from an insulator so technically there is no “Potential”. An electric field is not linear since the field strength drops off with the square of the distance away from the source. While the result of the arithmetic using a linear calculation will be close to the result of a measured electric field 12 inches from the source, the instruments used to measure the field integrate what they “see” at distance. So, the observed field will likely be less than 167 volts/inch at 12 inches from a 2000 volts/inch source.
Our close proximity rule at 125 volts/inch at 1 inch is derived from the IEC61340-5-1 field strength value of 5,000 volts/meter which equates to 127 volts/inch. The concern with the electric fields is due to the threat of induction (grounding a sensitive item in the presence of an electric field). Over the years, there have been studies using actual components to determine their susceptibility to electric fields and our current values are quite conservative and offer a lot of protection. In the future if devices are in use that are more sensitive than those currently in use, the values may have to be adjusted. End-users should adjust their specifications accordingly today.
Our 35 volts for isolated conductors limit is set based on the off-set voltage allowed for ionizers. An imbalanced ionizer can charge isolated conductors and the potential can be transferred by contact to another conductor at a different potential (or perhaps ground). Isolated conductors with a charge are considered a direct ESD threat although the 35 volts limit is also considered very conservative.