session six: assessment and selection of arc … safety/arc protective...session six: assessment and...

Session six: Assessment and Selection of Arc Protective Performance of PPE - Go Beyond the Norms?

2013 Electrical Arc Flash Conference – IDC Technologies 1

Session Six:

Assessment and Selection of Arc Protective Performance of PPE – Go Beyond the Norms?

Dr Helmut Eichinger1*

Technical Marketing Manager, Senior Consultant, DuPont International Operations, Geneva, Switzerland * corresponding author: [email protected]

Abstract

The arc protective performance of PPE is usually expressed by its arc rating, typically its ATPV, evaluated according to ASTM, IEC or EN standards. Recent developments, findings and discussions suggest that more considerations should be given to further test result parameters (such as EBT, ELIM and/or Box test Class 1 and 2) and also that the understanding of the meaning of the test results with respect to probability of skin burns needs to be improved. Furthermore, there exist important performance assessment and selection aspects, which are not or only insufficiently dealt with in the existing norms, such as robustness of the arc protective performance as a function of laundering, wear conditions, contaminations, or such as wearer acceptance, aesthetics, comfort and heat stress, or whether and how to include underwear in the selection of appropriate arc protective PPE.

Introduction The standard SANS 724:2010 “Personal protective equipment and clothing against the thermal hazards of an electric arc” is a very comprehensive standard. Some comments and remarks to some statements and requirements in the standard shall help to allow to apply this standard as far as is reasonably practicable. These are the words used by the Occupational Health and Safety Act, 1993 (Act No. 85 of 1993) under section 8 (2) (d), “General Duties of employers to their employees”, when stating: establishing, as far as is reasonably practicable, what hazards to the health or safety of persons are attached to any work which is performed, …, and any plant or machinery which is used in his business, and shall, as far as reasonably practicable, further establish what precautionary measures should be taken with respect to such work,…, plant or machinery in order to protect the health and safety of persons, and he shall provide the necessary means to apply such precautionary measures; This statement as far as is reasonably practicable shall be the guideline for judging to which precision the requirements of SANS 724 need to be fulfilled. One shall also keep in mind that SANS 724 is stating that it provides minimum requirements for personal protective equipment (PPE), and mainly for personal protective clothing.



And the Scope of SANS 724 is stating that this standard does not address electrical shock, arc blast (projectiles, shock waves and hot oil release), the consequences of physical and mental shock and the toxic influences of an electrical arc. One might add, that also the effect of splashes of molten metal (e.g. molten conductors) is dealt with only in the box-test method IEC 61482-1-2, but there also only in a specific, approximate, incomplete way, i.e. because of using a test setup with an aluminium electrode on the top facing a copper electrode at the bottom. Thus in the box test about one third of the emitted energy is caused by the burning aluminium and not generated by the plasma of the arc itself in form of a heat flux spectrum. This is one of the criticisms of the use of IEC 61482-1-2. Does the box test method make sense when there are no aluminium conductors involved in an arc event? In any case, one shall keep an open mind for eventually considering also aspects other than or going beyond the requirements of the SANS 724 standard, when assessing the performance of PPE and when selecting PPE.

Arc Thermal Performance Value (ATPV)

The most commonly used property for characterising the protective performance of materials and of PPE is the arc thermal performance value (ATPV) measured according to ASTM F1959 or IEC 61482-1-1. The ATPV is defined in SANS 724 – like in IEC 61482-1-1:2009 – as in arc testing, the incident energy on a material or a multilayer system of materials that results in a 50 % probability that sufficient heat transfer through the tested specimen is predicted to cause the onset of a second degree skin burn injury based on the Stoll curve, without breakopen In the revision of IEC 61482-1-1, it is intended to change this definition to in arc testing, numerical value of incident energy attributed to a material that describes its thermal properties of attenuating (reducing) a heat flux generated by an electric arc. The ATPV is calculated as a 50% probability that heat transfer through the test specimen reaches the Stoll curve.

The reason for this change is, that the formulation “50 % probability,…, to cause the onset of a second degree skin burn injury“ is an over-simplification, i.e. is strictly spoken not correct!

At first, one needs to better formulate the truth about the meaning of the Stoll curve.



What is the Stoll curve?

The Stoll curve represents a model driven criteria for developing a thermal protection rating for fabrics. It is based on work done in the late 1950’s / early 1960’s by the US Military1 (Naval Air Development Center) that utilised observed data and empirical relationships related to pain and blister effects in human skin.

The actual curve was developed using a derivative of the military research for many thermal protective material ASTM and IEC test method norms. This transformed model essentially evaluates fabric performance relative to an empirical reference following a prescriptive methodology.

The curve itself does not actually predict the occurrence of a human skin burn injury, but over 40 years of field use suggests it provides adequate protective rating values to fabrics used for thermal protection against burn injury from various hazard levels (such as those estimated by prescriptive analysis and calculation tools like the IEEE 1584, also used in SANS 984).

This methodology is apparently valid – i.e. shown by experience - even though the model is applied incorrectly in the ASTM and IEC arc testing standards and does not meet the validation requirements defined in the original source literature (see below: What are the requirements for the correct use of the Stoll curve).

What is the basis of this model?

In the original literature work, Stoll et al developed a technique using a skin simulant to assess performance from earlier skin burn injury research2 (bare skin). The simulant was placed under the fabric to be evaluated and a rectangular heat exposure was conducted. The energy flux absorbed by the skin simulant underneath the fabric was indicated by a temperature rise. The value, referenced to a specific time interval (typically 3 s) produced a correlation that was extrapolated to fabric performance (mean skin tolerance time to injury from the impressed heat). To provide fabric performance assessments, the test fabric had to be removed immediately at the conclusion of the thermal exposure, “… otherwise, additional heat could flow into the skin from the fabric itself or from the surroundings causing additional damage and upsetting the empirical correlation.” (From Stoll1)

What are the requirements for the correct use of the Stoll curve?

The primary requirement that must be met in order to use the original or a derivative of the model is that the heat exposure used must be rectangular on bare skin (a constant heat is applied nearly instantaneously for a specific duration, and then removed in the same nearly instantaneous fashion). Any deviation from this exposure profile, or shape, invalidates any measured data for evaluating fabrics with the original or any derivatives of the literature model.



Unfortunately, the heat transferred through a fabric (measured on the back face of the fabric), as a result of an arc exposure, is never rectangular. So, strictly speaking, the way the Stoll curve is used in the ASTM or IEC test method standards is not correct.

There are further reasons why the burn injury prediction models based on the Stoll curve do only approximately predict burn injury.

• Which further assumptions used in Burn Injury prediction models are the reasons why these models do not actually predict burn injury?

There are numerous assumptions that are made in order to construct a skin burn injury prediction model, otherwise the problem would be infinitely difficult to address. Essentially, all models establish a closed set of criteria from which to make predictions from. These criteria severely limit direct applicability to predict skin burn injury as they do not represent actual human thermo physical characteristics. Most models incorporate features such as:

• The initial skin temperature is assumed to be 32.5 °C for all cases. Note that the initial skin temperature has a strong effect on all model outcomes – a 1 °C change in initial temperature can produce significantly higher predicted burn injury performance

• Damage increases logarithmically with a linear increase in skin temperature (burns are represented as a first order chemical rate equation)

• Burn injury begins when the skin temperature is > 44°C (instantaneous destruction occurs at 72 °C)

• The amount of damage (depth of tissue injury) is a function of skin temperature and the period of time the temperature is above 44 °C

• Blood profusion is largely ignored (although some models exist for this metabolic function)

• Very little experimental values exist for human burn injury model development. Pig and rat data are the source of most burn injury models

• Current experimental data modeling is based primarily on radiant heat source exposure (the work of Stoll & Greene2)

• The thickness of the individual skin layers is fixed for modeling. The “Stoll” curve was developed based on the skin thickness of the volar surface of the forearm of military volunteers. Note that the thickness of the individual skin layers varies considerably over the surface of the body.

• Skin absorptivity is constant and considered opaque to IR radiation. Hair, skin color, and other skin features are ignored

• The skin composition is held constant (although some models have used water composition at various skin depths to develop performance values)



What is also important to note is there has been no experimental validation of predictive models used for burn injury (to include the Stoll curve). Essentially, all predictive burn injury models provide an artificial reference in order to estimate the relative thermal protective properties of fabrics and systems used in the marketplace.

• Are there better models that could be used?

There are a large number of burn injury predictive models available with various levels of sophistication. There are currently ongoing efforts to transition many of the ASTM test methods that use the Stoll model inappropriately to the base underlying skin burn damage integral model as used in the global thermal manikin test method standards ASTM F1930 and ISO 13506. This transition is being done to resolve at least the “non-rectangular” exposure energy problem to evaluate fabric systems (Stoll curve not valid).But the ASTM and IEC arc test method standards are not yet on the way of being revised to resolve the “non-rectangular” – exposure energy problem, and also the standardization committees have not started work on the above mentioned further assumptions.

As summary and conclusion from the above discussion about the validity of the methodology based on the Stoll curve, one shall retain: The definition of the Stoll curve will be modified in the revisions of the IEC standards, in order to help to correct that the Stoll curve is mainly understood in an oversimplified way as a tool for predicting various degrees of skin burns. However, one can still say, as the experience of the last 30/40 years has shown, that the evaluation of the arc rating values of products based on the Stoll curve criteria as used in the current ASTM and IEC standards have allowed to eliminate products, which would not have provided sufficient protection.

The next question then is: • How precise can the arc rating values, such as the most commonly used ATPV,

be determined by the ASTM and IEC test method standards?

The principle of the ASTM and IEC test method is to determine the incident energy up to which the heat energy transmitted through the material(s) does not exceed the Stoll curve criteria when test specimens of the material(s) are exposed to heat energy from an electric arc.



Three two-sensor panels, each covered by a test specimen, are spaced at 120° around a pair of electrodes, between which an electric arc will be generated (see Figure 1). One monitoring sensor is positioned on each side of each two-sensor panel.

During each arc event, the amounts of heat energy transferred through the test specimens are measured during and after exposure to an electric arc.

Figure 1: The schematic view of the test set-up showing the three test panels around the arc electrodes. Not shown are the monitor sensors, which have to be positioned one to the left and one to the right of each of the three panels

The heat flux of the exposure are measured with the 3 times 2 monitor sensors and the heat flux transferred through the test specimen(s) are measured with the 2 sensors of each of the 3 panels covered by the test specimens. The change in temperature versus time is used, along with the known thermo-physical properties of copper, to determine the respective heat energies delivered to and through the test specimens.

For each test specimen, the average response curve of the 2 sensors of a panel sensor, i.e. the curve of transmitted heat through the test specimen on a panel versus time is plotted and compared with the Stoll curve. The plot in Figure 2 shows a panel sensor response curve, which exceeds the Stoll curve.



Figure 2: Plot of a panel sensor response curve (EPanel) exceeding the Stoll curve (EStoll)

It has been determined, that because of the stochastic nature of an electric arc, at least 20 test specimens need to be tested, i.e. a series of at least 7 arc event tests need to be carried out, in order to obtain statistical meaningful arc rating values. These 7 arc events need to have different arc energy levels, in order to obtain various levels of incident energies, so that one obtains a statistically sufficient number of response curves which are above and a sufficient number of response curves which are below the Stoll curve.

Figure 3 shows - as an example - the resulting sensor response curves from one arc test event, i.e. on the top the three average response curves obtained from each pair of monitors sensors next to each panel and below the three average response curves obtained from each pair of panel sensors.



Figure 3: Example of 3 average monitor and 3 average panel response curves

For each test specimen, a data point with the coordinate x = “average of incident energy measured by the 2 sensors of a panel” and with the coordinate y = 1, if the average panel response curve is exceeding the Stoll curve, or y= 0, if the average panel response curve is not exceeding the Stoll curve, is then entered into a plot showing “Probability of exceeding Stoll curve” versus “Incident Energy” (see upper diagram in Figure 4).

By performing a nominal logistic regression on the resulting set of data points, one then determines the 50 % probability value of exceeding the Stoll curve or not. The resulting value is the ATPV, i.e. the incident energy value that would just intersect the Stoll curve.

Figure 4: Determination of the arc rating values ATPV and EBT

• Upper diagram: Example of a plot used for the determination of the ATPV of 1997 kJ/m2, i.e. by help of the plot of data points showing “Probability of exceeding Stoll curve“ versus “Incident Energy”,

and

• Lower diagram: Example of a plot used for the determination of the EBT of about 2250 kJ/m2, i.e. by help of the plot of data points showing “Probability of breakopen” versus “Incident Energy”.

• .



Figure 4 shows also, how in many cases also the breakopen threshold energy (EBT) can be determined or at least estimated by help of the same set of data points which have been measured for the determination of the ATPV. The EBT is defined in arc testing as the numerical value of incident energy attributed to a material that describes its breakopen properties when exposed to heat flux of electric arc. The EBT is calculated as a 50% probability that breakopen occurs.

For each test specimen, a data point with the coordinate x = “average of incident energy measured by the 2 sensors of a panel” and with the coordinate y = 1, if the test specimen on the panel shows breakopen, or y= 0, if the test specimen shows no breakopen, is then entered into a plot showing “probability of breakopen” versus “Incident Energy” (see lower diagram in Figure 4).

By performing a nominal logistic regression on the resulting set of data points, one then determines the 50 % probability value of occurrence of breakopen. The resulting value is the EBT (In the current version of IEC 61482-1-1, this value is called the EBT50; but in the previous version of this test method standard, and also in the next revision of the IEC standard, it has been and shall be called again EBT without the subscript 50, like in the current ASTM F1959). The plot for the determination of the EBT shown in Figure 4 shows the example of a set of data points, which is not sufficient for a statistical valid determination of the EBT according to the criteria of the test method standards ASTM F1959 or IEC 61482-1-1. One can at best give an estimate, that the EBT of the tested three layer sample is about 2250 kJ/m2. Further data points at higher incident energy exposure would be needed in order to obtain a balanced distribution of data points corresponding to test specimens which show or do not show breakopen. Only from such a more balanced distribution could then a statistically meaningful EBT be calculated by help of logistic regression. The test method standard ASTM F1959 reports the results of some intra-laboratory testing conducted in 2005 at Kinectrics, Inc., Toronto, Ontario, Canada. The repeatability r of this test method has been established by help of repeated determination of the arc rating of three samples. For the sample A, the arc rating has been measured 6 times; the average arc rating of sample A has been determined to be 5.3 cal/cm2, with a repeatability r of 0.5 cal/cm2. For sample B, the average arc rating has been 7.0 cal/cm2, with repeatability of 0.99 cal/cm2. For sample C, the arc rating has been 17.0 cal/cm2, with repeatability of 1.22 cal/cm2.



Precision of test method ASTM F1959

Test number Sample A ATPV

cal/cm2

Sample B ATPV

cal/cm2

Sample C EBT

cal/cm2

1 5.1 7.3 17.4

2 5.1 6.4 17.3

3 7.4 7.4 16.8

4 6.9 6.9 17.5

5 7.0 7.0 16.8

6 7.3 7.3 16.4

average 5.3 7.0 17.0

repeatability r 0.5 0.99 1.22

Two single test results, obtained in the same laboratory under normal test method procedures that differ by more than the above indicated repeatability values r, must be considered as derived from different or non-identical sample populations. The reproducibility of the test method ASTM F1959, i.e. the difference between results from various test laboratories was not established, as in 2005 there was only one testing facility in North America capable of performing the test. It is envisaged that during the current revision of IEC 61482-1-1, an inter-lab testing program between at least two, if not three or four laboratories shall be carried out. The costs for such testing will be high, estimated to more than 100’000 Euro. As summary and conclusion from the above discussion about the determination of the arc rating values ATPV and/or EBT and the precision of these arc rating values, we shall retain: Roughly speaking the repeatability of the determination of the arc rating of a material is currently known to be of about 10%. ATPV or EBT values, which differ by less than 10% can be considered as being the same.



• What does this knowledge about the precision of the arc rating values mean when considering the requirements of Table 1 of SANS 724?

Table 1 - SANS 724 – Recommended clothing type to be worn per hazard/risk category

1 2 3 4

Hazard / risk

category

Required minimum arc rating of PPE

Clothing Description

cal/cm2 J/cm2

0 n/a n/a

Non-melting, flammable materials (i.e. untreated cotton, rayon, wool, silk or blends of these materials) with a minimum fabric weight of 150 g/m2

1 4 16.74 Arc rated FR shirt, FR trousers or FR coverall

2 8 33.47

Arc rated FR shirt, FR trousers or FR coverall

3 25 104.6

Arc rated FR shirt and FR trousers or FR coverall, and arc flash suit selected so that the system arc rating complies with the required minimum

4 40 167.36


The values in column 3 of Table 1 of SANS 724 may be misinterpreted as suggesting that arc rating of PPE, when expressed in J/cm2, can be measured with a precision up to the second digit after the decimal point. From the above discussion we know, that this is not the case. The up or down round values in column three of the below modified Table 1a put – within the limits of the precision of the test method, and “as far as reasonably practicable” – equally severe minimum performance requirements, than the values in Table 1 of SANS 724. Table 1a lists also the minimum performance requirement not only in J/cm2, but in the “real” SI units “kJ/m2”.



Table 1a - (modified Table 1 / SANS 724) – Recommended clothing type to be worn per hazard/risk category

1 2 3 4 5

Hazard / risk

category

Required minimum arc

rating of PPE


cal/cm2 J/cm2 kJ/m2

0 n/a n/a n/a


1 4

15 16 16.74 17

150 160 167.4


2 8

30 32 33.47 34

300 320 334.7


3 25

100 104.6 105

1000 1046


4 40

160 167.36 168

1600 1673.6


But if up or down rounding of the arc rating values given in J/cm2 or kJ/m2 is reasonably practicable because of the limits of the precision of the test method, then also the values expressed in cal/cm2 do not need to remain strict values anymore. Table 1b translates the up or down rounded values in J/cm2 or kJ/m2 into reasonably permissible minimum requirement values in cal/cm2.



Table 1b - (modified Tabl1 /SANS 724) – Recommended clothing type to be worn per hazard/risk category

1 2 3 4 5

Hazard / risk category

Required minimum arc rating of PPE


cal/cm2 J/cm2 kJ/m2

0 n/a n/a n/a


1

3.6 3.8 4 4.1

15 16 16.74 17

150 160 167.4


2

7.2 7.6 8 8.1

30 32 33.47 34

300 320 334.7


3

23.9 25 25.1

100 104.6 105

1000 1046


4 38.2 40 40.2

160 167.36 168

1600 1673.6


A key requirement for any material or material assembly that shall be part of arc protective clothing is that it has to be Flame Resistant (FR). FR is defined in IEC 61482-2 by passing the surface ignition test according to ISO 15025/Procedure A and optionally also the edge ignition Procedure B, or in ASTM F1506 by passing the vertical flame test ASTM D6413.

There has always been a discussion between Europe and North America about which test is the more severe and/or more appropriate one.

The surface ignition test is more looking after integrity of the surface in case of exposure to flames, i.e. whether the flame causes hole formation or not. The critical parameter in the edge ignition test is rather the char length.



But good materials fulfill both, ASTM and IEC FR requirements.

Also SANS 724 contains the key requirement, that only FR material layers shall be considered as being part of the arc protective clothing or of a multilayer or multi-garment systems, where eventually one garment is worn on top of another (e.g. a FR flash suit over a FR coverall). For example, ordinary cotton underwear, i.e. underwear which does not meet the FR requirements of the ASTM or IEC standards, shall not be considered as part of the arc protective clothing system. And the arc rating of the system has to be determined and has to meet the minimum arc rating performance requirements.

But in principle, if the system consists of garments worn on top of another, the inner garments do not need to be tested for their own arc ratings. And it is also not absolutely, necessarily needed that the arc rating of each of the inner garments has to be measured or known as long as the garments will be worn together, when there is the risk of occurrence of an electric arc.

In this sense, the use of long-sleeved FR underwear shall be permissible too as part of the arc protective clothing system, and permissible without that an arc rating needs to be determined for the FR underwear. But the arc rating of the whole system consisting for example of long-sleeved FR underwear, FR coverall and arc flash suit needs to be measured (see suggested Table 1c).



Table 1c - (modified Table 1 / SANS 724) – Recommended clothing type to be worn per hazard/risk category

1 2 3 4 5

Hazard / risk category

Required minimum arc rating of PPE Clothing Description

cal/cm2 J/cm2 kJ/m2

0 n/a n/a n/a


1

3.6 3.8 4 4.1

15 16 16.74 17

150 160 167.4

Arc rated FR shirt, arc rated FR trousers or arc rated FR coverall

2

7.2 7.6 8 8.1

30 32 33.47 34

300 320 334.7

Arc rated FR shirt and arc rated FR trousers, or arc rated FR coverall or long sleeve FR underwear and FR shirt, FR trousers and FR coverall so that the system arc rating complies with the required minimum

3

23.9 25 25.1

100 104.6 105

1000 1046

Arc rated FR shirt and FR trousers or FR coverall, and arc flash suit selected so that the system arc rating complies with the required minimum or long sleeve FR underwear and FR shirt, FR trousers and FR coverall, and arc flash suit selected so that the system arc rating complies with the required minimum

4 38.2 40 40.2

160 167.36 168

1600 1673.6

Arc rated FR shirt and FR trousers or FR coverall, and arc flash suit selected so that the system arc rating complies with the required minimum or long sleeve FR underwear and FR shirt, FR trousers and FR coverall, and arc flash suit selected so that the system arc rating complies with the required minimum



• Criticism of CENELEC consultant with respect to use of ATPV; proposal of an additional arc rating value: the incident energy limit (ELIM)

The CEN consultant is criticizing the use of the ATPV, based on the observation that the various materials have a more or less sharp logistic regression S-curve. Thus the value, which is “really” meaningful for the assessment of the protective performance of a material or material system should not be the ATPV but the incident energy value, which corresponds to the start of the mix zone.

The mix zone is defined as the range of incident energies, inside which in case of the determination of the ATPV the difference between the measured curve of energy transmitted through a test specimen and the Stoll curve is either positive or negative, or inside which in case of the determination of the EBT there occurs a positive or negative outcome for breakopen or underlayer ignition (i.e. inside which there either occurs or not breakopen of the test specimen or underlayer ignition inside the test specimen).

One shall note that the low value of the mix-zone range is the lowest incident energy indicating a positive result, and the high value of the mix-zone range is the highest incident energy indicating a negative result. And one shall also note that a mix zone is established when the highest incident energy with a negative result is greater than the lowest incident energy with a positive result.

It is evident, that when comparing two materials with same ATPV value, the one which has a sharper logistic regression S-curve (i.e. a higher ELIM) provides better protection than the material with a rather flat logistic regression S-curve: If the ELIM is higher, the risk is lower that at an incident energy of some per cent below the ATPV one may measure a heat transfer through test specimens which is exceeding the Stoll curve.

Experience seems to indicate that a sharper S-curve is a sign for more homogenous, isotropic fabric constructions, whereas a flatter S-curve may be an indication of higher variations in fabric construction or fabric properties between different test specimens of the same material sample. One shall note, that not only the composition of a clothing material is important, but also the quality of how the various components are made into a woven fabric.

Figure 5 shows the logistic regression curve of a fabric with a very narrow mixed-zone. The mix zone in the upper plot of Figure 4 shows the logistic regression curve of a three layer fabric system with a considerably larger mixed zone. But one has to note, that it is quite logic that two or multi-layer systems show a wider mix zone, thus a flatter logistic regression S-curve, because of all the higher degree and probability of possible variations between test specimens.

However, due to the currently proposed definition of the ELIM, the ELIM is not an arc rating value, which is determined in a statistically robust way. One can easily



see that one single erratic measurement could determine the value of the ELIM (for example, in case that for some unknown or perhaps erroneous reason one would measure the occurrence of the exceeding of the Stoll curve or observe a breakopen of one single test specimens at very low incident energy). There will be further discussions in the project team working on the revision of IEC 61482-1-1, how to avoid the possible excessive influence of one outlier on the value of the ELIM.

Nevertheless, from the above discussion of the shape of the logistic regression curve one may retain, that when comparing two materials with same ATPV rating, the material with the sharper logistic regression S-curve (i.e. with the higher ELIM) is probably the better protecting one.

Figure 5: The logistic regression curve of a fabric with a very narrow mix zone.

• Why is it perhaps advisable to also know the EBT of a material in addition to its ATPV, and to use the EBT in addition to the ATPV for the selection of protective clothing PPE?

Usually the EBT of a clothing material or of an assembly of several layers of material is higher than the ATPV. However, it may happen for some materials or material assemblies that breakopen of test specimens occur at incident energy values, at which the heat transfer through the material(s) is not exceeding the Stoll curve. It is therefore possible that for some materials the EBT is lower than the ATPV, or that even no ATPV can be determined. From such materials the arc rating according to ASTM F1959 and IEC 61482-1-1 and which shall be used for the selection of PPE, is given by the EBT.



Please note that Table 1 of SANS 724 is not setting a required minimum value for the ATPV, but for the “arc rating”, as the arc rating for some materials is not given by the ATPV, but by the EBT, i.e. in cases where the EBT is lower than the ATPV.

But also in the more usual cases that the EBT is higher than the ATPV, one should consider to include the EBT in the process of selection of PPE. When comparing clothing with the same ATPV, it is safer for the user to select the clothing with the higher EBT. Why?

The selection of PPE is based on the criterion that the arc rating of the selected PPE shall be larger than the incident energy to which the wearer of the PPE may be exposed, determined by help of the hazard and risk assessment. But what happens if the actual arc event is somewhat more severe than assessed?

In case of the example shown in Figure 5, the ATPV of the clothing is slightly above 47.7 cal/cm2 and the EBT estimated to be about 53.8 cal/cm2, i.e. about 13 % above the ATPV. If this clothing has been selected for protection against an assessed incident energy of about 47 cal/cm2, but if the incident energy caused by the actual arc event is 10% higher than assessed, the wearer would probably suffer some small amount of 2nd degree skin burns. But if the actual incident energy would be increasingly higher than 13%, the clothing is increasingly more likely to break entirely open and expose the non-FR underwear (e.g. cotton underwear) or the skin of the wearer directly to the effects of the electric arc event. This may cause ignition of non-FR cotton underwear; the exothermically burning underwear will then become a very severe additional hazard on its own.

If however the EBT is much higher than the ATPV, the probability of an eventual breakopen of the clothing, i.e. the probability of eventually exposing the skin or the underwear of the wearer directly to the effects of the electric arc is much lower. The Industrial Standard of China Power Utility for arc protective clothing – for example – specifies that for its category 1, i.e. for clothing with an ATPV between 6 and 8 cal/cm2, the EBT has to be about 2.2 times higher than the ATPV, i.e. the EBT has to be between 13 and 18 cal/cm2. And for its category 2 clothing, i.e. with an ATPV between 8 and 25 cal/cm2, the EBT has to be between 18 and 25 cal/cm2 (see Table 2).

From the above discussion one may conclude that it is advisable, safer that in case of 2 products with the same ATPV, one selects the product with the higher EBT.



Table 2 - Industrial Standard of China Power Utility for arc protective PPE

Protection

ATPV

cal/cm2

ATPV

J/cm2

EBT50

cal/cm2

EBT50

J/cm2

BW

g/m2

Cat.1 6 - 8 25.74~33.47 13 - 18 54.41~75.33 150~200

Cat.2 8 - 25 33.47~104.60 18 - 25 75.33~104.60 200~290

Cat.3 25 - 40 104.60~167.36 25 - 40 104.60~167.36 290~600

Cat.4 > 40 >167.36 > 40 >167.36 290~600

• How to use Class 1 or Class 2 rating of protective clothing according to box test IEC 61482-1-2?

SANS 724 is also referring to possible arc rating of PPE according to the box test method IEC 61482-1-2. Figure 6 is giving a schematic view of the box test set-up. The box test does not allow to assign a property value to a material or clothing, like this is done by the open arc test method ASTM F1959 and IEC 61482-1-1.

The property value according to IEC 61482-1-1 is in first approximation independent of the testing parameters such as arc current, arc gap, distance from the arc, etc. What counts is the incident energy measured by the monitor sensors and the simultaneously measured heat transfer through the material. The box test however evaluates whether a material or clothing offers sufficient protection – based on the Stoll curve criterion - when exposed to Class 1 or Class 2 testing conditions, but without that - because of the spatial directionality of the emitted incident energy - the incident energy onto the test specimens can be measured at the same time as the heat transfer through the material.

The testing conditions are characterised by the generation of an electric arc between an aluminium electrode at the top and a copper electrode at the bottom, with a gap of 3 cm between the electrodes facing each other, and the arc confined into a small 2 litre box made of plaster, with the box open only one side and the test specimen positioned in front of the open side of the box at a distance of 30 cm from the electric arc. The open circuit voltage of the test circuit is 400 V, the short circuit current of the test circuit is either 4 kA (Class 1) or 7 kA (Class 2), and the arc duration is 500 msec. The key question then is: Will a Box-test Class 1 or Class 2 rated clothing offer sufficient protection for “my” exposure conditions, i.e. in case of “my” equipment? How can I extrapolate from the box test conditions to “my” equipment situation?



For the similar problem of how to correlate the arc rating determined according to ASTM F1959 or IEC 61482-1-1 to “my” equipment situation, there exists the initial empirical solution of NFPA 70E and the later incident energy calculation Guideline IEEE 1584. In 2012 the German Guideline BGI GUV-I 5188 has been launched as a tool for judging whether either a box test Class 1 or Class 2 rated clothing will offer sufficient protection in “my” equipment situation. Experience will show how effective this new Guideline can be used. However, some – many? – people continue to express their doubts whether this second approach of arc rating of materials and clothing by help of box text Classes and this new tool for making the link to “my” equipment situation is really needed.

Figure 6: Schematic view of box test set-up according to IEC 61482-1-2

• What is the difference between clothing rated box test Class 2 and clothing rated appropriate for HCR 2 according to NFPA 70E, i.e. clothing with ATPV > 8 cal/cm2 ?

Box test Class 1 and Class 2 are completely different ratings than the HRC ratings according to NFPA 70E. For example, a material usually needs to have an ATPV of about 20 to 25 cal/cm2, in order to pass box test Class 2. However for clothing, which shall be appropriate for HCR 2 it is sufficient that such clothing has an ATPV greater than 8 cal/cm2.

The discussion about the arc rating of materials and clothing needs to be completed with some considerations concerning the durability of the arc rating. SANS 724 and also the ASTM and IEC product standards require that protective clothing shall keep its arc rating during its whole service life when cleaned in accordance with the manufacturer’s instructions.



• Does it make sense to determine the arc rating after 25 or 50 or 100 cleaning cycles according to manufacturer’s instructions?

Yes and no.

Yes, as such determination of the arc rating after numerous cleaning cycles gives an indication about whether the FR properties of a material are permanent and cannot be washed out.

No, as numerous cleaning cycles alone cannot simulate actual service life. Fabrics rather tend to shrink during repeated washing and drying cycles, which may lead to the effect that the arc rating after the numerous cleaning cycles can be higher than initially or after a few cleaning cycles only.

Furthermore, pre-treatment before arc testing only by cleaning cycles will not simulate the actual stretching and wear which materials will suffer during actual service life. It makes good sense that SANS 724 in clause 6.5.3 is not setting the obligation that a manufacturer has to indicate the service life of a garment in terms of cleaning cycles, but that the manufacturer and the user have the freedom to define and determine the service life also by other means which are acceptable to both.

It might also be advisable to test a material not only after cleaning strictly according to manufacturer’s instructions, but to test a material also for how robust the arc rating is with respect to not strict observance of manufacturer’s cleaning instructions. This may be advisable in particular in the case that a user has no assured control how clothing will be cleaned by a laundrying company or when the clothing is cleaned by employees at home. One should consider to evaluate or ask the manufacturer to evaluate to which extent the material is sensitive – for example - to chlorine containing bleaches, different soaps and softeners.

Further considerations for the selection of arc protective clothing

Besides the above extensive discussion about the arc rating of materials and clothing, there are other properties, which are important for the selection of appropriate PPE.



The manufacturer’s user information should indicate

Mechanical durability: Table 3 gives an indication of typical tensile strength and trap tear properties of typical materials and how these properties may change as a function of repeated laundering.

Table 3 -

Chemical durability: To which extent is a material compatible with chemicals, which may be present in the work environment?

Durable appearance and colour fastness: Aesthetics are a key element for acceptance of clothing by workers.

Figure 7 – Appearance before and after 100 launderings

Comparison of FR cotton nylon (Left) and Protera™ (right)

before and after 100 launderings.



Comfort: Light weight and air-permeability of the fabric and a good garment design are parameters, which determine the comfort of protective clothing. Comfort is not only another key aspect for acceptance of the clothing by workers, but high discomfort will be hindering for the worker and may even increase the risk on an arc incident.

• Summary of conclusions

1. According to the South African law and the SANS 724 standard, hazard and risk assessment shall be carried out “as far as is reasonably practicable”.

2. SANS 724 is setting minimum performance requirements for protective

performance of PPE against the thermal hazards of an electric arc. There is room for use of further arc protective property results obtained from testing.

3. But at first, the understanding of the meaning of the ATPV needs to be

improved: The ATPV is NOT the value of the incident energy, at which the wearer of the PPE, when exposed to such an incident energy, will suffer with 50% probability 2nd degree skin burns. Correct is, that the ATPV is calculated as a 50% probability that the heat transfer curve measured behind the test specimen reaches the Stoll curve.

4. And the truth about the Stoll curve is, that the curve itself does only

approximately predict the occurrence of a human skin burn injury.

5. And the primary requirement for the correct use of the Stoll curve is that the heat exposure used must be rectangular. Unfortunately, the heat transferred through a fabric (measured on the back face of the fabric), as a result of an arc exposure, is never rectangular. So, strictly speaking, the way the Stoll curve is used in the ASTM or IEC test method standards is not correct.

6. However, over 40 years of field use suggests that the Stoll curve provides

adequate protective rating values to fabrics used for thermal protection against burn injury from various hazard levels, including the electric arc hazards estimated by prescriptive analysis and calculation tools like the IEEE 1584, also used in SANS 984. And one can claim that the experience of the last 30 / 40 years has shown, that the evaluation of the arc rating values of products based on the Stoll curve criteria as used in the current ASTM and IEC standards have allowed to eliminate products, which would not have provided sufficient protection.

7. When selecting protective clothing in accordance with the minimum arc rating

requirements of SANS 724, one shall not only be aware of the fact that the Stoll curve and the measured arc rating values ATPV have only an approximate meaning as predictors of skin burn injuries. One should also take into account that the repeatability of the determination of the arc rating of a material is currently known to be of about 10%. Thus, ATPV or EBT values, which differ by less than 10%, can be considered as being the same.



8. Therefore it is suggested that the arc rating requirement values in Table 1 of SANS 724 can reasonably, practicably be read as suggested in the modified Table 1b. For example, HCR2 does not need to be defined by the strict value of 8 cal/cm2, but one could consider for example that also the value of 7.2 cal/cm2 = 300 kJ/m2 (i.e. the down rounded value of 33.4.7 kJ/m2 = 8 cal/cm2) is acceptable for HCR2..

9. It is further suggested that also the use of long-sleeved FR underwear shall be permissible as part of the arc protective clothing system, and permissible without that an arc rating needs to be determined for the FR underwear. But the arc rating of the whole system consisting for example of long-sleeved FR underwear, FR coverall and arc flash suit needs to be measured (see suggested Table 1c).

10. Another additional PPE selection criterion could be the new proposed arc rating value ELIM. When comparing two materials with same ATPV rating, the material with the sharper logistic regression S-curve (i.e. with the higher ELIM) is probably the better protecting one.

11. And it is advisable and safer that in case of 2 products with the same ATPV, one selects the product with the higher EBT.

12. SANS 724 is setting minimum performance requirements not only for the arc rating, There is room for applying further considerations for selecting the better PPE, i.e. the PPE which is offering the better, best combination of protection, durability, comfort and life cycle costs The user shall be interested to obtain information from his supplier about how sensitive a material/ clothing is with respect to non-observance of the manufacturer’s laundering instructions, and to obtain information about mechanical durability, chemical durability, durable appearance and colour fastness

13. Finally, in addition to the arc rating values and other property values of PPE, which are important for the selection of PPE, the user shall also value consistent product quality and long term reliability of the supplier and manufacturer of PPE.

References

1 Stoll, A.M., and Chianta, M.A., “Method and rating system for evaluation of thermal protection”. Aerospace Med. 40 (11); 1232-1237. 1969

2 Stoll, A.M., and Greene, L.C., “Relationship between Pain and Tissue Damage Due to Thermal Radiation,” J. Appl. Physiol. 14(3) (1958), 373-382

3 Weaver, J.A., and Stoll, A.M., “Mathematical Model of Skin Exposed to Thermal Radiation,” Aerospace Med. 40(1) (1969), 24-30

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