A practical comparison of surface resistance test electrodes

Journal of Electrostatics 88 (2017) 127e133 Contents lists available at ScienceDirect Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat A practical comparison of surface resistance test electrodes Jeremy Smallwood Electrostatic Solutions Ltd, 13 Redhill Crescent, Bassett, Southampton, SO16 7BQ, UK a r t i c l e i n f o a b s t r a c t Article history: Received 19 September 2016 Received in revised form 22 December 2016 Accepted 27 December 2016 Available online 19 January 2017 Four surface resistance test electrodes are compared using a selection of materials under similar test conditions. The results vary considerably with some materials due to variation in surface resistivity. Using a relatively uniform material two concentric ring electrodes compliant with the same standard differed in results by a factor of 1.8. Silver stripe and copper tape electrodes gave results a factor 0.4 and 0.7 compared to the reference electrode. A 2-pin electrode gave results a factor 4.7 greater. The 2 pin probe cannot be expected to give similar results to the other electrodes for materials that have variable resistivity. © 2017 Elsevier B.V. All rights reserved. Keywords: Surface resistance Standard measurement electrodes 1. Introduction Surface resistance test methods are used for a variety of purposes in industry including evaluation of resistive materials for electrostatic ignition risk, qualification of equipment intended for use in potentially explosive atmospheres under European Directive 2014/34/EU (ATEX) [1] and characterization of packaging materials for use in Electrostatic Discharge (ESD) protection in the electronics industry. Several standard test methods exist for these purposes. The general objective of the measurement in each case is to evaluate the electrostatic charge dissipative ability of the material surface, for avoidance of charge build up and subsequent electrostatic discharges. When the resistance between the electrodes is measured, it is typically reported without compensation for the electrode form, as “surface resistance” rather than surface resistivity. This greatly increases convenience, especially for routine and frequent measurements. Related standards (e.g. IEC 61340-5-3 [9] or IEC 60079-0 [8]) then specify materials in terms of surface resistance for material classification and evaluation purposes. Conductive rubber faced concentric ring electrodes (IEC 613402-3 [5,6] and ESD STM 11.11 [2,3]) were developed for compliance verification evaluation of electrostatic discharge (ESD) control materials used in packaging for the electronics industry. IEC 613402-3 [5,6] is also specified for general measurements of properties of materials with resistance or resistivity in the range 104e1012 U use in electrostatic control. The concentric ring electrodes typically have an outer ring outer diameter of 63 mm and mass of 2.5 kg. E-mail address: jeremys@static-sol.com. http://dx.doi.org/10.1016/j.elstat.2016.12.019 0304-3886/© 2017 Elsevier B.V. All rights reserved. They are suitable for moderate sized flat samples of material, but are unsuited for measurement of small, curved or other non-planar surfaces. With the development of miniature ESD control packaging products there evolved a need for a miniature surface resistance test method that could also be used on non-planar surfaces or within depressions in moulded products. A miniature point-topoint 2-pin probe electrode (IEC 61340-2-3 [5,6] and ESD STM 11.13 [4]) was developed for this purpose. This consists of two sprung loaded and conductive rubber faced pins 3.2 mm in diameter and separated by 3.2 mm. In evaluation of materials for electrostatic hazard control materials in processes and ATEX in Europe, a different electrode system has historically been used for many years, e.g. in IEC 600790 [8]. This consists of two stripes of conductive paint (e.g. silver paint) 1 mm wide and 10 mm long, separated by a gap of 10 mm. This electrode system has recently been published again in IEC 60079-32-2 [7]. This standard also allows the electrodes to be made from conductive rubber or foam strips mounted on insulating supports. The standards that refer to these test methods typically specify classifications or requirements in terms of surface resistance measured according to these methods. It is typically the high resistance limits that are of most interest. For example, in IEC 61340-5-3 [9] a material is classified as insulative if the surface resistance measured according to IEC 61340-2-3 [2,3] (2 pin or CR probe) is  1011 U. An insulative material would, where possible, be excluded from use in an ESD Protected Area. In contrast in IEC 60079-0 [8] requires the surface resistance of enclosures for equipment used within a flammable atmosphere area to be  109 U 128 J. Smallwood / Journal of Electrostatics 88 (2017) 127e133 calculated for surface resistivity given the same surface resistance measurement result. In order to avoid this problem in calculating KCR, the formula given in IEC 61340-2-3:2000 [5] is used to compare electrode systems in Table 1. Calculation of KCR according to IEC 61340-2-3:2016 [6] gives only a small difference compared to the IEC 61340-2-3:2000 [5] formula (0.100 compared to 0.096). As the surface resistance values measured with each electrode are compared directly by experiment, differences in results due to different formulae for conversion to surface resistivity are avoided. For a parallel stripe electrode such as IEC 60079-0 [8] (which is the same as IEC 60079-32-2 [7]) the electrode constant is Kst ¼ g l Fig. 1. Concentric ring (CR) electrode. (@ 50 ± 5% r.h.) or 1011 U (@ 30 ± 5%r.h.) measured according to the conductive paint stripe electrode method given in the standard. So, it is particularly important that the measurement results are repeatable and reproducible in these resistance ranges. In theory the surface resistance Rs measured by an electrode system across a surface of uniform surface resistivity rs is of the form Rs ¼ K rs where K is a constant dependent on the electrode surface contact geometry. Any electrode systems having the same K might be expected to give the same results when measuring a material of uniform surface resistivity. For the concentric ring electrode system the value of K is defined differently in different standards. In STM 11.11e2015 [3] surface resistivity is simply quoted as a factor of 10 times the surface resistance, making KCR ¼ 0.1. In IEC 61340-2-3:2000 [5], the relationship between surface resistivity and surface resistance was given as rs ¼ Rs ðd1 þ gÞp g which leads to KCR ¼ g pðd1 þ gÞ where d1 is the diameter of the inner contact electrode and g is the gap between the inner electrode and inside of the outer electrode (IEC 61340-2-3 [5,6]). In practice it is the inner diameter of the outer electrode that is specified instead of the electrode gap. All dimensions are specified in the standards with a tolerance (see Fig. 1). In STM 11.11e2006 [2] and 61340-2-3:2016 [6] the formula relating Rs and rs is defined as rs ¼ 2pRs loge dd2 1 This leads to KCR ¼   loge d2=d 1 2p As the objective of this paper is to directly compare surface resistance measured using different electrodes, conversion to surface resistivity is not required. Clearly, different conversion factors defined in different standards could lead to different values where l is the length of the stripe and g is the gap between the electrodes, and any fringe effects at the electrode ends are neglected. The dimensions may be in meters or mm (see Fig. 2). When the electrode tolerances are taken into account, each electrode system gives a range for K that would be expected for electrodes built according to the standards. For the CR electrodes, the minimum K is given for minimum gap, and vice versa. For the stripe electrode the minimum K is given for minimum gap and maximum length, and vice versa. The calculated minimum, nominal and maximum values are given in Table 1. It can be seen that the range of constants for the various standard cells is very similar, ranging from about 0.091 to about 0.106. The approximate variation in measurement result due to this, for a uniform resistivity sample, should be about ±5% for the CR electrode and ±6% for the stripe electrode system. The electrode configuration for the 2 pin electrode system is shown in Fig. 3. So far the author has not found an equation deriving a constant K for this configuration. Typical materials and products under test can have variable resistance characteristics across the surface that can lead to differences in results according to the type and scale of the electrode and direction or location of measurement. The concentric ring electrode has infinite rotational symmetry and can be expected to give the same result with all orientations even on an anisotropic material. In contrast a parallel stripe electrode can be expected to give results variable with orientation on an anisotropic material. The concentric ring electrodes are designed for flat surfaces, and typical products measured may have curved or textured surfaces. Some products may have features that are too small to apply large electrode systems. The 2 pin electrode structure allows measurements on small areas or curved surfaces. Conductive rubber faced electrodes are easy to apply, do not affect the surface material and are easily removed leaving no trace after measurement. The IEC 60079-0 [8] painted stripe electrode can conform to a surface curvature or profile but may affect the surface material and could be difficult or impossible to remove after measurement. For this reason, IEC 60079-32-2 [7] allows alternative electrodes of the same geometry. The standard electrode systems therefore have advantages and disadvantages according to the form of the sample under test. This paper gives a comparison of results and experience of using standard test electrodes on a variety of materials with resistance in the range GU to TU. In addition, the use of self-adhesive conductive (Cu) tape electrodes is explored. These electrodes have been used in practice by the author for many years. These can conform to moderate surface contours or textures and are often conveniently applied. They normally do not physically affect the surface and can be removed, leaving only a residue of adhesive that can be easily cleaned off if necessary (see Fig. 4). The objective is to directly compare the electrodes and surface J. Smallwood / Journal of Electrostatics 88 (2017) 127e133 129 Table 1 Standard electrode parameters (CR electrodes calculated according to IEC 61340-2-3:2000) [5]. Electrode dimensions IEC 61340-2-3 [5,6] STM 11.11 [2,3] IEC 60079-32-2 [7] IEC 60079-0 [8] Type CR CR paint stripe Electrode parameters d1 d2 g 30.5 ± 1 30.5 ± 0.64 57 ± 1 57.15 ± 0.64 12.5e14.0 12.7e14.0 10 ± 0.5 Fig. 2. Paint stripe electrode. Fig. 3. 2 pin electrode system (IEC 61340-2-3 [5,6] and ESD STM 11.13 [4]). l K min K nominal K max 100 ± 1 0.091 0.092 0.094 0.096 0.097 0.100 0.101 0.101 0.106 dimensions of these electrodes are compared in Table 2. The electrodes are shown in Fig. 5. The Trek 152P-CR-1 probe (TCR) is an active probe that was used with a compatible Trek 152-1 resistance meter. The Warmbier 880 CR probe (WCR) was used with a Metriso 3000 resistance meter. Both probes had mass of 2.5 kg. Resistance measurements above 1 MU were made at 100 V test voltage. The Trek 152-1 m had a lower resistance measurement limit of 1 kU and upper limit of 10 TU. The Metriso had an upper limit of 1 TU. Before measurements started, the contact resistance of the commercial CR and 2 pin electrodes and Cu strip electrodes was measured using a stainless steel test plate. In all cases the resistance was found to be ≪ 1 kU at 10 V test voltage. The measurements made with CR electrodes were used first to establish a comparison of the CR electrodes and afterwards as a basis for comparison with the 2 point probe electrode system. After this, comparison was made with silver (Ag) paint stripe electrodes and copper (Cu) tape electrodes. The Ag paint electrodes were fabricated by drawing lines with a silver paint pen (ITW Circuitworks® Flex Conductive Pen CW2900) by hand using a ruler (Fig. 6). In practice it was found to be difficult to maintain a uniform electrode line width and inter electrode gap and avoid smudging of the paint. Copper (Cu) tape electrodes were fabricated from conductive Table 2 Commercial electrode parameters. Fig. 4. Copper (Cu) strip electrodes. resistance results obtained with them, and so the electrodes are compared under the same humidity and test voltage conditions rather than those specified in the standard test methods. For all of the electrodes except the 2 pin electrodes, the value of K is calculated to be 0.1. For a homogenous test material and identical test conditions, the measured surface resistance result should, in the absence of other influential factors, be the same. The 2 pin electrodes are intended by the standards to be used in situations where the CR electrodes are unsuitable due to curvature or size of the specimen under test, and so also would be expected to give similar results. This paper provides experimental evidence of variations that occur in practical measurements and discusses the influence of material non-homogeneity on the results. Electrode Type Electrode parameters d1 d2 g K Warmbier 880 (WCR) Trek 152P-CR-1 (TCR) CR CR 30.2 30.7 57.6 57.7 13.68 13.52 0.099 0.097 2. Experimental Two types of commercial CR electrodes available for the experiments. These were measured with a vernier gauge to determine their constant K and establish compliance with the standards before for use in experiments. As the electrode contacts were flexible polymeric materials and could easily be deformed, it was difficult to do this precisely. The electrodes were within the requirements of IEC 61340-2-3 [5,6] and ESD STM 11.11 [2,3]. The Fig. 5. Commercial electrodes. 130 J. Smallwood / Journal of Electrostatics 88 (2017) 127e133 Fig. 7. Results of CR and 2 pin electrode measurements on insulative material. Fig. 6. Fabricated electrodes. self adhesive copper “EMC” tape (Advance AD528) 10 mm wide and 100 mm in length applied by hand to the surface. A 10 mm wide tape is used as is easily handled, was readily available in the author's toolkit and from suppliers. The resistance result was not expected to vary greatly with the electrode width. The interelectrode gap was 10 mm. The calculated constant for this arrangement was 0.10 (see Fig. 6). In order to compare measurements as nearly as possible on the same position, a 100 mm  100 mm grid was marked on the material. Electrodes could then be set by eye on the center of each grid square and measurement of the marked material was avoided. The material was supported on a highly insulating (>1013 U) PVC slab. All samples were conditioned for at least 24 h and tested at 50% ± 3% humidity rather than the humidity levels specified by the standards. This is because the objective was to compare the different electrodes under identical conditions, and not to evaluate the materials under standard conditions. In practice, varying humidity can have significant effect on resistance measurement results, typically giving lower resistance results at higher humidity. For the same reason, the fabricated electrodes were also used at the same applied voltage (10 V and 100 V) as the standard electrodes. Typical materials can often give lower resistance results at higher applied voltage. 3. Results Initial measurements were made on a flexible insulative material measured using the CR probes to have resistance of the order 1011e1012 U. A flexible material was used to avoid problems with obtaining good electrode contact over the large CR electrode surface. Measurements were also made on a flexible “conductive” mat material used in ESD control. Measurements were then made using the 2 pin probe. In each case the resistance result was plotted on a graph against the measurement location, so that measurements at each position could be directly compared (Fig. 7). The CR electrode measurements on the insulative material gave reasonably consistent results. In order reduce the effect of material resistance on the comparison of the electrodes, the results at each position were “normalised” arbitrarily taking the WCR electrode as the reference at each measurement location. The ratio of the result for each electrode to the WCR electrode result was calculated for each location. The average and standard deviation of these ratios could then be calculated (Table 3). Using this method it was calculated that the TCR electrode gave on average a factor of 2.7 times higher resistance result than the WCR electrode. When 2 pin electrode measurements were added, these were found to be considerably higher than the CR electrodes and highly variable. In many cases 2 pin probe results were an order of magnitude or more higher than the CR probe results. 75% of these results exceeded the 1013 U upper measureable limit of the equipment although none of the CR probe results exceeded 1012 U. Those that did so were recorded as 1013 U to allow some analysis to be made. The 2 pin electrode gave on average a factor of nearly 85 times higher resistance than the Warmbier CR electrode (see Fig. 7). Measurements on the “conductive” ESD control material using the CR electrodes showed the material to be highly variable in resistance in different measurement positions. Measurements with the 2 pin electrodes showed extraordinarily variable results, on average a factor of about 1.5  104 times the WCR electrode (See Fig. 8). It was suspected from these results that the materials used in initial tests had highly variable surface resistance that would make comparison of electrodes difficult. Other materials were then tested to try to find a more uniform material for electrode comparison purposes that would have a resistance approximately in the desired range. The most promising material found was an art paper. After conditioning and test with the CR and 2 pin electrodes this appeared to give reasonably uniform results (Fig. 9). The 2 pin electrode was used to check for anisotropy by comparing measurements in 12 positions with the electrodes oriented in x and y directions. No significant anisotropy was found. Once again the TCR electrodes gave higher resistance than the WCR electrodes (Table 4). The 2 pin electrodes gave higher resistance results than the CR electrodes. It can be seen that there remain differences in the surface resistance results at different positions on the material although it appears much more uniform than the materials initially used. Two types of fabricated electrodes were then tested on a selection of positions on the art paper. A smaller number of measurements of this type were made partly because once applied, there would be no possibility of removing them without damage to the material. Further measurements on those locations with different electrodes would not be possible. The silver paint electrodes were found to be most difficult and time consuming to fabricate accurately. This analysis showed that for art paper TCR electrode gave results an average of 1.8 times the WCR electrode (Table 4). The 2 pin electrode gave results an average of 4.7 times higher. In contrast, the Ag paint and Cu tape electrodes gave results an average of 0.4 J. Smallwood / Journal of Electrostatics 88 (2017) 127e133 131 Table 3 Normalised ratios of measurements with each electrode at each location on insulative and conductive materials. Electrode WCR TCR 2 pin Insulative material Conductive material No. of measurements Average Standard deviation No. of measurements Average Standard deviation 38 38 38 1.0 2.7 84.9 0.0 0.53 67.7 16 13 15 1.0 1.6 14800 0.0 1.0 36400 and 0.7 times the WCR electrode respectively. The standard deviations of results (TCR 0.38, 2 pin 0.79, Cu 0.19 and Ag 0.11) in each case showed reasonable consistency. A similar analysis was done on measurements made on a more variable paper (Tyvek®). The basic results are given in Fig. 10. In this case TCR electrode gave results an average of 1.2 times the WCR electrode whereas the 2 pin electrode gave results an average of 5.5 times higher. The Cu tape electrodes gave results an average of 1.8 times the WCR electrode. The Ag stripe electrode was not tested. The variability of the 2 pin electrode results for Tyvek® can be seen in the higher standard deviation of 3.51 compared to 0.79 for the art paper. In contrast the standard deviation of results for the TCR electrode was 0.38 for art paper and 0.56 for Tyvek®. The standard deviation for the Cu tape electrode for art and Tyvek papers were 0.19 and 0.92 respectively. 4. Discussion Fig. 8. Results of measurements on „conductive“ ESD control material. It is clear from these experiments that practical measurement results can be much more variable that would be expected from theoretical calculations assuming materials of uniform surface resistivity. Part of this variability is undoubtedly due to inherent variability in the surface resistivity of the material under test. There are, however, other unidentified factors. Based on electrode Fig. 9. Measurement results on art paper. Fig. 10. Measurement results on Tyvek® paper. Table 4 Normalised ratios of measurements with each electrode at each location for art and Tyvek® paper. Tyvek® paper Electrode Art paper No. of measurements Avg. Std. Dev. No. of measurements Avg. Std. Dev. WCR (reference) Trek CR 2 pin Cu tape Ag stripe 24 24 24 8 4 1.0 1.8 4.7 0.7 0.4 0.0 0.38 0.79 0.19 0.11 20 20 20 4 0 1.0 1.2 5.5 1.8 0.0 0.56 3.51 0.92 132 J. Smallwood / Journal of Electrostatics 88 (2017) 127e133 dimensions the ratio of the electrode constants of the Warmbier CR and Trek CR electrodes would predict that the Trek CR would give results a factor of 0.98 times the Warmbier CR electrode. In practice a factor of 1.8 was found. Similarly the Ag stripe and Cu tape electrodes would be expected to give 1.1 times the resistance of the Warmbier CR electrode but in practice 0.4 and 0.7 were found respectively. Whether these differences are due to resistance of the contact between the electrode material with the test sample, or other additional factors, is unclear. It seems clear that the 2 pin electrode responds to variation in the surface resistance of the material under test in a very different way to the other electrodes. Examination of the geometry of the electrodes suggests an explanation for this. The CR electrodes measure the material surface resistance over a distance of at least the perimeter of the inner electrode, which is around 96 mm. The Ag stripe and Cu tape electrodes measure the material over a distance of 100 mm. Ignoring the possibility of conduction from the rear of the electrode the 2 pin probe measures the material over a much smaller distance that is around half the periphery of each electrode, or about 5 mm. It is instructive to consider the response of the electrodes to small regions of more conductive or more insulative material that may occur in the measurement location (Fig. 11 and Fig. 12). For the CR and stripe or tape electrodes, a small region of high resistance material of the order of the dimension of the electrode gap and crossing the gap is likely to have only a small effect on the measured resistance because most of the material between the electrodes remains unaffected. It acts like a high resistance in parallel with the average resistance of the material measured. In contrast a small region of low resistance material of similar dimensions between the electrodes will have a relatively large effect in reducing the measured result. The result tends to be highly influenced by the minimum resistance region found between the electrodes. A small region of high or low resistance that does not bridge the electrode gap has relatively little effect. For the 2 pin electrode system, a similar size region of high or low resistance bridging the gap may have a significant effect on the measured result as the majority of the conduction path between the electrodes is affected (Fig. 12). If one of the electrodes happens to sit on a region of high resistance, a high resistance result will be obtained. Thus, small regions of high or low resistance can be expected to significantly affect the result. This can easily be demonstrated in practice by placing material of another resistance under the electrodes. In a simple experiment, 10 mm strips of office paper and polymer foil were used to give more conducting and more insulating regions on the art paper surface. The art paper substrate was measured first with each electrode in one location. The surface resistance of the strips was measured with the strip laid on the art paper and both pins of the 2 pin electrode on the strip. The resistance was then measured with Fig. 11. The effect of regions of more or less conductive material on the CR and stripe or tape electrodes. Fig. 12. The effect of regions of more or less conductive material on the 2 pin electrodes. each strip laid on the art paper across the diameter of the TCR electrode, and under one pin of the 2 pin electrode (Table 5). It can be seen that the presence of an insulating polymer strip laid under the TCR electrode raised the result by a factor of approximately 2 from 7.4  109 U to 1.6  1010 U. The same strip under one pin of the 2 pin electrode raised the result from 2.2  1010 U to 1.2  1013 U, an increase of 550 times. With the strip under both of the 2 pin electrodes the resistance was >2  1013 U. A more conducting paper strip under the CR electrode reduced the result by approximately a factor of 0.2 from 7.4  109 U to 1.3  109 U. The same strip under one pin of the 2 pin electrode reduced the result from 2.2  1010 U to 1.1  1010 U, a factor of 0.5. With the strip bridging both 2 pin electrodes the resistance was 1.0  109 U. Similar effects can be expected to occur with high and low resistance regions of dimensions down to the inter electrode gaps and diameter of the electrode pins. A statistical number of measurements made using the 2 pin electrode arguably would better represent the range of resistivity of the material than similar measurements made using the other electrodes. For evaluation of materials for electrostatic control and risk avoidance all these methods may be equally useful. The CR, Cu tape and Ag stripe electrodes give comparable results even with variation in resistance of the material. The 2 pin electrode cannot, however, be expected to give similar results to the other electrodes when there is appreciable variation in material resistance. 5. Conclusions Standard concentric ring electrodes according to IEC 61340-2-3 [5,6] and ESD STM 11.11 [2,3] have been compared with a standard 2 pin electrode according to IEC 61340-2-3 [5,6] and ESD STM 11.13 [4] and a silver paint stripe electrode according to IEC 60079-0 [8] and IEC 60079-32-2 [7]. These standard electrodes have been compared with an experimental conductive self adhesive copper tape electrode system. All except the 2 pin electrode configuration had a theoretical K of around 0.1. All these methods were found to be susceptible to considerable variation in results due to variability of local resistivity of the material under test and other unknown factors. When compared using the most uniform material available (art paper), the TCR electrodes gave results on average a factor of 1.8 higher, the 2 pin electrode system 4.7 higher, copper tape 0.7 and silver paint stripe 0.4 times the WCR electrode arbitrarily used as reference. When test materials had significant variability, the WCR and TCR electrodes remained reasonably comparable although the 2 pin electrodes gave highly variable results. A small region of high resistance under one electrode can lead to a high resistance result whereas a small region of low resistance can bridge the electrodes and give a low resistance result. The WCR, TCR, Cu tape and Ag stripe electrodes are only moderately affected by small regions of high resistance. All the electrodes are highly influenced by regions J. Smallwood / Journal of Electrostatics 88 (2017) 127e133 133 Table 5 Effect of high and low resistance strips placed under electrodes on the measurement result. Material strips used Measurement configuration Under both pins of 2 pin electrode No strip polymer strip office paper strip Under one pin of 2 pin electrode Across diameter of TCR electrode 1.2  1013 1.1  1010 7.4  109 1.6  1010 1.3  109 10 2.2  10 >2  1013 1.0  109 of lower resistance bridging the electrodes. It seems likely that with common ESD control materials poor reproducibility and variation of results is likely due to material local resistance variations. In particular, the 2 pin electrode is likely to give a wide variation of results reflecting high and low resistance regions in the material. The other electrodes are likely to give results representing mainly the lower resistance regions in the material. A statistical number of measurements made using the 2 pin electrode could better represent the range of resistivity of the material than similar measurements made using the other electrodes. With a relatively uniform test material (art paper) there remained differences between the resistance measured with each electrode that are unexplained by differences in electrode geometry. These indicate that contact resistance or other as yet unidentified factors have considerable effect on the measurement results. The concentric ring, Cu tape and Ag stripe electrodes can be considered to give reasonably similar surface resistance results within an order of magnitude. The 2 pin probe cannot be expected to give similar results to the other electrodes for materials that may have be non-uniform resistivity. References [1] Directive 2014/34/EU of the European Parliament and of the Council of 26 February 2014 on the Harmonisation of the Laws of the Member States Relating to Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres (Recast), 2014, p. 309. OJEU L 96, 29.3.2014. [2] ESD Association, Surface Resistance Measurement of Static Dissipative Planar Materials, ANSI/ESD STM 11.11e2006, 2006, ISBN 1-58537-124-6. [3] ESD Association, Surface Resistance Measurement of Static Dissipative Planar Materials, ANSI/ESD STM 11.11e2015, 2015, ISBN 1-58537-280-3. [4] ESD Association, Two-Point Resistance Measurement, ANSI/ESD STM 11.13e2015, 2015. ISBN: 1-58537-279-x. [5] International Electrotechnical Commission, Electrostatics e Part 2e3: Methods of Test for Determining the Resistance and Resistivity of Solid Planar Materials Used to Avoid Electrostatic Charge Accumulation, IEC 61340-2-3:2000, 2000, ISBN 978-2-8322-3475-4. [6] International Electrotechnical Commission, Electrostatics e Part 2e3: Methods of Test for Determining the Resistance and Resistivity of Solid Planar Materials Used to Avoid Electrostatic Charge Accumulation, IEC 61340-2-3:2016, 2016, ISBN 978-2-8322-3475-4. [7] International Electrotechnical Commission, Explosive Atmospheres Part 32-2. Electrostatic Hazards Tests. IEC 60079-32-2, ISBN 978 2 8322 2276 8. [8] International Electrotechnical Commission, Explosive Atmospheres - Part 0: Equipment - General Requirements, IEC 60079e0:2011, 2011, ISBN 978-288912-519-7. [9] International Electrotechnical Commission, Electrostatics e Part 5-3: Protection of Electronic Devices from Electrostatic Phenomena - Properties and Requirements Classifications for Packaging Intended for Electrostatic Discharge Sensitive Devices, IEC 61340-5-3:2015, 2015, ISBN 978-2-8322-2787-9.