7.6.1 Box Test versus Open Arc
There has been considerable argument over the relative advantages and disadvantages between the box test and open out test methods. This has even manifested itself in comments made in International standards and official guidance on the subject of protection against the thermal effects of electrical arcs. The following are examples of such comments.
The following comment is a quotation from the National Foreword to the box test standard – EN 61482- 1-2:2007 from the UK technical committee PEL/78. “It is the opinion of UK technical committee PEL/78 that the “box test” does not provide the user with a realistic and reliable test. A premise of this test is that the fault currents will not exceed 4000A or 7000A and the worker will not be closer than the specified distance from the arc (in reality this cannot be guaranteed)”. “The UK technical committee PEL/78 believes that the open arc test IEC 61482-1-1 will provide the best way to determine whether a particular material will provide the best protection for the worker for any given job”.
The following comment came from the *DGUV 5188 Thermal Hazards from Electric Fault Arc -Guide to The Selection of Personal Protective Equipment for Electrical Work. “On the other hand, Arc-Man test results lead to the so- called Arc Thermal Performance Value, or ATPV. In this context, the incident energy is determined according to a statistical methodology, by which a 50% probability exists of suffering second-degree skin burns behind the PPE. Even if an electric fault accident is relatively improbable, the EU directive regarding PPE allows no interpretation of PPE that would tolerate such injury. For this reason, as a matter of principle, such test methods should not be used within the EU”. This is very clear but as I have said previously, to be wearing PPE which had an ATPV at the exact level of the actual arcing incident energy in a real-life situation would be improbable. It is also the case that there are safety factors built into how the incident energy calculations are applied. This is where the new test parameter the Incident Energy Limit (ELIM) has been introduced. ELIM differs from the ATPV in that there is 0% chance of a second-degree burn rather than a 50% probability.
*Note - DGUV is the German Social Accident Insurance which is the national, compulsory program that insures workers for injuries or illness incurred through their employment.
Both methods are not perfect because how could they be? Electrical arcs are violent and often unpredictable events, so the research has served well to provide two methods of meeting international standards. I have therefore provided information on both standards and also calculator tools for determining the ATPV/ELIM associated with the open arc method as well as the (APC) box test. In this case you have the tools to do some comparisons. The following are my thoughts on the relative merits and drawbacks to each method.
The setup of the box test closely resembles low voltage utility termination equipment or cut out, the similarities being copper and aluminium electrodes, the voltage and the prospective current. The closeness of the sample to the arc is also plausible for this type of equipment which is often installed in enclosed areas. Lots of low voltage cut outs are still mounted in cupboards under stairs for instance and service cupboards are notoriously tight on space. Indeed, “the (box test) conditions are intended to represent the practical situation in low-voltage installations and low-voltage networks. The qualitative results (re-burning, melting, hole formation etc), derived at either 4 kA/500ms or at 7 kA/500ms, may be used to assess whether any given test system (fabric or protective clothing) would be appropriate under exactly the same real working conditions”. (ISSA Guideline for the selection of personal protective clothing when exposed to the thermal effects of an electric arc) However, these conditions are dissimilar to conditions outside electrical utilities. For instance, very few modern industrial installations have a mixture of copper and aluminium and in most situations the working distances are greater. In addition, the fault currents within industrial facilities are much greater so the box test is unlike “the same real working conditions.”
The box test is much harder to correlate to quantitative risk evaluation via arc flash calculations such as IEEE 1584. In addition, the arcing current and time to clear are two variables that are essential to obtain an accurate estimate of arc power/incident energy. Perhaps one could use IEEE 1584 and then apply that to obtain a disconnection time to determine the box test APC.
The electrical source for the open arc test is a large generator setup which has to be decoupled from the mains supply. The cost of these facilities is high which explains the reason why there are so few of them in the world. The electrical source for the box test is a directly coupled mains transformer making it much easier and cheaper to perform the test.
The open arc test is carried out on the garment whereas the open arc test only needs the fabric. In addition, there are only 4 exposures required for the box test making it cheaper. The drawback is that the box test does not subject the whole garment to the thermal effects of the arc and therefore, the open arc test could be seen as giving a truer prediction of the garment performance in an actual arc event.
Advocates of the box test would point out that the open arc method can only capture the radiated thermal effects of an arc whereas the box test specimens will be more effected by splashes of vaporised and molten materials.
The box test has, as previously explained, got just two exposure levels which are a prospective short circuit current of 4 kA for 500ms (APC1) and 7 kA for 500ms (APC2) The material must pass either to be given an arc protection class (APC) as well as fulfil the qualitative criteria in the standard. Having just two levels means exposures higher than APC2 cannot be evaluated which could mean that exposures above 12 to 20 cal/cm2 cannot be protected. APC1 can return an approximate ATPV of 4-8 cal/cm2 and Class 2 is from 12-20 cal/cm2. However, it can be argued that pass/fail makes interpretation more straightforward as no statistical analysis is required to determine classification levels. The downside of a pass/fail result is that repeatability for fabrics that only just pass is not as easy to identify compared with specific values given for ATPV or ELIM.
I have seen evidence of where a major utility company who were using box test garments in the knowledge that the supplied PPE may be underrated for certain site conditions. This was based upon what the perceived level of acceptance to protection would be and the phrases used were “comfort/risk balance” and “some protection was better than none at all”. I am sure that a better strategy would be to do the field studies first and then determine the PPE requirements in terms of ATPV based upon the task, distance, fault levels, protection arrangement as well as the prevention techniques detailed in Chapter 5: Prevention & Minimisation.
My recommendation would be for the box test rated PPE to be used for utility type low voltage systems only for instance to replicate potential hazards in; service entrance boxes, cable distribution, street lighting cut-outs, cabinets, distribution substations or comparable installations and/or where the arc is directed to the front of a worker at the height of the breastbone. For all HV and LV industrial and commercial situations, utilities, power stations and transport infrastructure I would strongly recommend the use of open arc rated PPE.