Good morning
Resistance to ground (and of course, Point to point resistance) is one of the most important measurements in equipment validation.
As you know, UNE-EN61340 5-1 is followed in Europe where it is especified the maximum value of resistance for compliance verification and product qualification.
Less than 10^9 ohms in all the cases (Rpp and Rgp). Until we know, USA standard is similar.
UNE-EN-61340-2-3 defines the specific methods and intruments to carry out these measurments on dissipative worksurfaces. (similar to ANSI/ESD S4.1)
My question is about the use of different resistance test electrodes. In particular, the popular 5 lb cilindric electrodes and the two point probe described in 61340-2-3
Sometime 5 lb electrodes can not be applied because the size or shape of the surface and then two point electrodes are useful.
This procedure is validate on the UNE standard.
After some tests we have checked these electrodes measure different values, even in supposed homogeneus materials, 2 or 3 decades of difference.
But only one limit is accepted without elctrode specification.
How we can qualify a material if two accepted instruments measure different value and there are only one limit?
The IEC specifications and the ANSI/ESD Association specifications are very close and most of the time technically equivalent. For resistance measurements there are really three probes that can be used. The two you mentioned and the concentric ring probe which is used for planer material.
What you are seeing between the 5 lb probes and two point probes are a combination of materials and spacing. It is known that there will be differences between the measurements. Usually, the difference is in the material itself. The two point probe is a much smaller surface area and will detect small material differences even when the material is considered homogenous. The 5 lb probes will not find small differences in the material.
A second difference could be in distance. The two point probe is a fixed distance between the probes. The 5 lb can be spaced far apart. If the material is truly homogenous then there will be differences in the resistance. Think of the material as a series of resistors.
The two point probe as defined can only do point to point resistance. To do resistance to ground the 5 lb probes are needed.
All three probes could result in difference results but usually materials will meet the limits will all three. For a short demonstration of the probes see https://www.youtube.com/watch?v=bADYFOczgJk
Thank you so much for your answer.
If it is know that there will be differences between measures with 5 lb probe and two points (two or three decades in some cases)… why standards do not specify different limits in function of the used probe?
In the field, there are some materials where two points measure above to 10^9 but 5lb below. Can i accept this material as dissipative?
thanks
That is a good question. When using the two point probe you are seeing very small areas of materials. If the materials are injection molding and using carbon fiber or powder, there may be small areas that will not measure 109 or lower. Remember, that if you are measuring packaging materials, then the upper limit is 1011 ohms. If the anomalies are small, maybe you can accept it. Measure around the spot to see if it is an issue.
Ultimatly, the user must make the decision if the material is acceptable or not.