The "PTB Ex proficiency testing scheme"

Results and findings from the current programmes

Author: Tim Krause; Jia Wu; Lisa Ostermann; Harun Kanbur: Physikalisch-Technische Bundesanstalt (PTB)

1 Introduction

The "PTB Ex proficiency testing scheme" (PTB Ex PTS) was launched back in 2010. It is a project that involves developing interlaboratory comparison programmes to assess the proficiency of different laboratories in the field of explosion protection – the first report on the scheme appeared in this explosion protection journal in 2014. At that time, the pilot phase had just been concluded for the first two programmes; these programmes focused on "Explosion pressure" within the scope of IEC 60079-1 [1] and "Spark ignition" in the context of IEC 60079-11 [2] respectively. The report offered some insights into two new programmes that were in the pipeline, one addressing "Temperature classification" (IEC 60079-0 [3]) and the other addressing "Flame transmission" (IEC 60079-1 [1]). The PTB Ex PTS, which the Physikalisch-Technische Bundesanstalt (PTB) has been developing closely in line with the IECEx system*, was at that stage still very much in its infancy, and the number of participating explosion protection testing laboratories was relatively modest. Seven years on, in 2021, the project has come a long way. This progress merits an up-to-date report that takes a detailed look at some of the results and findings of programmes that are currently under way. The PTB Ex PTS has now established itself globally as an important instrument for verifying that the explosion protection laboratories possess the requisite expertise and are performing to the expected standard. The success of the scheme is also evidenced by the fact that participation is now compulsory for all explosion protection testing laboratories within the IECEx system. Besides the core participant group, which consists of IECEx ExTLs (explosion protection testing laboratories) and IECEx ATFs (additional testing facilities), other participants are getting involved with the project in increasing numbers – namely in-house laboratories attached to manufacturers and operators, as well as research institutes from the field of explosion protection. So while the PTB Ex PTS's main mission is to assess the performance of individual explosion protection testing laboratories, it also serves as a link between various parties in the field of explosion protection, allowing any problems with the test methods and regulatory frameworks to be identified and thereby playing a key role in achieving international harmonisation.

* International Electrotechnical Commission System for Certification to Standards Relating to Equipment for Use in Explosive Atmospheres. The IECEx system is an international conformity assessment system for equipment, services and people and is based on IEC standards.

2 Structure and participants

What started back in 2009 with a pilot phase for two programmes and around 40 participating explosion protection testing laboratories has since evolved into ten completed programmes. We currently have 111 different explosion protection testing laboratories from 34 countries participating in the scheme, as illustrated on the map in Figure 1 below.

As a result of the IECEx Management Committee's decision to make participation in the PTB Ex PTS mandatory, the roughly 82 IECEx testing laboratories have represented the largest group of participants since 2017. However, we are increasingly seeing explosion protection testing laboratories outside of the IECEx system completing one or more programmes, either to assess their own performance or to fulfil the specific requirements of national accreditation bodies.

Figure 1: Map showing distribution of PTB Ex PTS participants across the globe

The PTB Ex PTS consists of individual programmes that each address a particular variable or relevant characteristic of a specific type of protection. Because the types of protection and the associated requirements are described in the corresponding technical standards from the IEC 60079‑0 [3] ff. series, each programme is matched to one or more standards. The individual programmes are in turn divided into two separate phases. In Phase I, participants conduct the test or trial described in the standard and in the programme instructions, and then submit the results to the PTB for evaluation, assessment and analysis. This is followed by an interim report and a workshop to discuss the results, findings and any problems that might arise. Next comes Phase II, in which participants have the opportunity to repeat the tests and submit new results. The final report is then produced on the basis of these two phases. This report analyses and assesses participants' performance in comparison to one another and against a reference value, or "assigned value". This statistical analysis of participating laboratories' performance allows warning signals and action signals to be issued on the basis of the results; for testing laboratories in the IECEx system, these signals may have further consequences. The findings from the programmes are then forwarded to the relevant standards committees so that they can identify any parts of the standards where there is undesirably broad scope for interpretation or pinpoint any aspects of the test methods where there is potential for improvement.

The structure of the PTB Ex PTS and the design of the individual programmes are based on the general requirements pertaining to proficiency testing set out in ISO/IEC 17043 [5]. The statistical evaluation, analysis and assessment methods are defined in ISO 13528 [6]. Neither of these documents is a standard dedicated specifically to proficiency testing in the field of explosion protection; instead, these documents describe general requirements that can be adapted to suit the circumstances in question.

Table 1: List of concluded (grey) and new (green) programmes in the PTB Ex PTS

3 Programmes

Table 1 lists the programmes offered as part of the PTB Ex PTS, along with their launch year and the standard(s) to which they correspond. The "Flameproof joints" and "Small component temperature" programmes will be run for the 2021/2022 programme cycle.

The programmes that were run for the 2019/2020 programme cycle and have been recently concluded will be described in detail below.

3.1 "Tests of enclosures" programme

The general test procedure for the "Tests of enclosures" programme is described in IEC 60079-0 [3] and IEC 60529 [8]. Enclosures used in hazardous areas must fulfil certain criteria and must undergo corresponding testing. These tests are defined in the aforementioned standards. The enclosures' impact strength and their protection against the ingress of solid foreign objects (dust) and water are two key parameters for testing and assessing explosion protection.

Figure 2: Test sample enclosure TE undergoing IP 5X testing

The characteristic of interest in this programme was therefore enclosures' compliance with their specified IP rating. Other aspects besides this characteristic were analysed, such as how the heat and cold resistance tests were conducted and how the test conditions during testing compared. The test samples TE on which the tests were conducted comprised 12 previously prepared glass fibre reinforced polyester resin empty enclosures with external dimensions measuring 170 mm x 170 mm x 91 mm, as pictured in Figure 2.

In line with the programme description, participating explosion protection testing laboratories were required to conduct the following tests:

Thermal resistance test pursuant to section 26.8/26.9 of IEC 60079‑0, Edition 7 [3],
Impact strength test pursuant to section 26.4.2 of IEC 60079-0, Edition 7 [3],
IP test pursuant to section 13/14 of IEC 60529, Edition 2.2 [8],
- Protection against the ingress of water (IP X4),
- Protection against the ingress of dust (IP 5X).

3.2 "Battery testing" programme

The general test procedure for the "Battery testing" programme is described in IEC 60079-11 [2]. The ongoing digital modernisation and networking of processes and applications in the field of explosion protection brings with it new challenges with regard to mobility. These include a significant increase in the number of batteries and cells needed to power mobile equipment.

Figure 3: Test sample primary cell BT undergoing short-circuit testing

These batteries and cells are often used in equipment with the "intrinsic safety" type of protection and must be tested in accordance with the aforementioned standard. The maximum surface temperature and the internal resistance of batteries and cells are key parameters for testing and assessing explosion protection. For this reason, these two parameters were chosen as variables/characteristics of interest for the programme. The test samples BT pictured in Figure 3 consist of 26 IEC LR6 primary cells.

In line with the programme description, participating explosion protection testing laboratories were required to conduct the following tests:

Short-circuit test to determine
- The thermal hotspot,
- The maximum surface temperature pursuant to section 10.5.3 b) of IEC 60079‑11 [2],
- The internal resistance pursuant to section 10.5.3 a) of IEC 60079-11 [2]

4 Results & findings

For each of the programmes, a selection of the results will be shown below and the findings from the programme analysis will be discussed. Due to the differences between the variables/characteristics in the two programmes, the assigned value and assessment criteria are defined differently for each programme.

Figure 4: Graph showing the results of Phase I and Phase II of the IP 5X dust test

4.1 "Tests of enclosures" programme

Figure 4 shows the results for both phases of the IP 5X dust test for determining the IP rating. The Y-axis shows the number of enclosures that were penetrated by dust and accordingly rated as "failed" by the participating laboratory. The X-axis shows the participants in anonymised form. The assigned value against which the individual results are compared is shown in red and is based on the totality of the participants' results (median). The assessment limits, which take the form of a warning signal and an action signal, appear as a yellow dotted line and a red dotted line respectively; they have been determined by the PTB based on the programme design and the form of the test samples. Participating laboratories LC0085 to LC0089 took part in Phase II only.

For Phase I, it is evident that the results of seven of the participants significantly differ from the assigned value and have therefore triggered a warning signal. One of the explosion protection testing laboratories has crossed the action signal threshold. Phase II shows an overall improvement in results, in particular for participants who crossed the threshold for a warning or action signal in Phase I. None of the results have triggered a warning or action signal in Phase II.

An analysis of the data identified the following main reasons for the differing results of the IP 5X dust test:

Some of the previously prepared weak points in the impact strength test had not been correctly identified,
A striking piece with a collar/shoulder had been used for some of the participants' impact strength tests, and this did not comply with the geometric requirements of the current edition, Edition 7, of IEC 60079-0 [3],
The striking piece was not always able to fall vertically and freely in some of the participants' impact strength tests.

When the programme data was analysed, another issue came to light that, while highly unlikely to have affected the results in this case, may still be relevant to other test samples and test specifications: The temperature requirements during the impact strength test were not always met.

Figure 5: Graph showing the results for Phase II of the process for determining the internal resistance

4.2 "Battery testing" programme

Figure 5 shows the results for Phase II of the process for determining the internal resistance of the primary cells. The Y-axis shows the internal resistance of the cell. The X-axis shows the participants in anonymised form. For each participant, there is a blue box with a blue mark (line) inside it; the blue mark inside the box is the participant's result, which is the arithmetic mean of the individual measurements (shown as square blue dots), and the blue box itself represents the standard deviation of these measurements. The assigned value is the robust mean of the participants' results in accordance with ISO 13528 [6]. The warning signal and action signal thresholds are set on the basis of the robust standard deviation multiplied by two and three respectively.

The results show that the majority of the participants' results are within the acceptable range. Five of the participants have crossed the threshold for a warning signal. Another participant is outside of the acceptable range, triggering an action signal. In Phase I (not shown here), three warning signals and four action signals were merited. For Phase II, the robust standard deviation as a measure of the variation is s_II* = 12.9 mΩ. For Phase I, the value was s_I* = 16.1 mΩ. This shows that the Phase II results improved on the Phase I results, both in terms of the warning and action signals and in terms of the overall variation in the participants' results.

An analysis of the programme data identified the following three main reasons for the differing results of the process for determining the internal resistance:

Mechanical switches had been used in some of the participants' test set-ups, which may have resulted in elevated internal resistances along the short-circuit section,
A low sampling frequency (< 1 kHz) had been used in some cases when collecting measurement data, which may have resulted in reduced short-circuit currents,

The cells' internal resistance had in some cases been calculated using different maximum open-circuit voltages (measured value as opposed to the figure specified in Table 13 of IEC 60079‑0 [3]).

5 Summary and long-term plans

The results discussed in this report are a selected extract from the analysis of participants' data from the "Tests of enclosures" and "Battery testing" programmes. It is apparent that, despite the use of the same set of technical requirements in the form of standards, participants' results may nevertheless differ from one another. Some of these differences can be attributed to differences in the way measuring equipment is used and to errors in the test set-up and in the preparation of test samples. These kinds of influences can be significantly reduced by clarifying the issues and providing appropriate training. However, the programme evaluations also reveal that the variations in the results can sometimes also be explained by the scope that the standards allow for interpretation.

The PTB Ex PTS's objective is to examine the test methods used on a daily basis in the explosion protection sector and to identify any weak points so that the comparability of the results of explosion protection testing laboratories across the world can ultimately be improved. The programmes that have been developed have the capacity to bring us gradually closer to achieving this objective. We are already witnessing this in the way that programme participants are achieving increasingly consistent results across Phases I and II of the programmes. There has been a direct reduction in discrepancies between results, and outliers have been minimised. In the long term, suggestions for improvements to the test methods based on the findings from the programmes will be submitted to the relevant standards committee with the aim of bringing about lasting improvements to the regulatory frameworks that will benefit all parties and make the requirements of these frameworks clearer.