Electric arc protection in focus

Published:  19 April, 2010

Electric arc protection is a minefield according to many in the industry. And they’re right. Just like the risk itself, it is an explosive subject that inspires high intensity debate where a lot of people are making a lot of noise as they argue their points of view. 

Why? Well, like other areas of PPE where standards are in debate, there is much ongoing discussion and disagreement at all levels about the ‘right way’ and the ‘best way’ to test and select garments. For those at the sharp end of this issue, the companies who need to provide protection to their staff, the technical ins and outs of these arguments might not seem important, but the inevitable consequence of this debate is confusion, and while a little knowledge can be dangerous, in this case no knowledge is arguably more so. So, if you are confused about open and closed testing, ratings and curves, well, so is everyone else, and this article isn’t going to provide a helpful solution. But what it will try to do is to explain some of what is getting people so hot under the collar in an unbiased manner.

What is an electric arc?

An electric arc is an ongoing plasma discharge resulting from a current flowing through normally non-conductive media such as air. That is to say, a continuous high current electrical discharge ‘jumping’ across a gap between conductors, or a conductor and the ground.

The effects of an electric arc vary depending on the individual circumstances but are all terrifying: extreme temperatures that can reach up to 12,000 °C – or four times the surface temperature of the sun – explosive forces caused by the rapid expansion of gases and elements such as copper within the arc, intense light, high noise levels and toxic fumes.

‘In a low level arc it tends to be the heat and light that does the damage, but the ballistic effects at high intensity faults can be very severe,’ explains Mike Frain from Electrical Safety UK Ltd. ‘Most deaths caused by electricity will be through electrocution, but burn injuries are more often the results of an arc, and a lot of people who have experienced this phenomena first hand will never work again. Severe burns to the hands face and torso are one consequence, but an arc flash can also cause internal injuries through in the inhalation of highly toxic and superheated gases, permanent damage to the eyes because of UV radiation and damage to the ear drums.’

An electric arc is a hazard that deserves respect. It most often affects electrical workers and ‘age and experience mean nothing’ says Mike. It could be caused by a technical fault, or a mistake made by a person such as dropping tools across  energised parts or using sub standard electrical test equipment. It could be something as simple as moisture or dust on an insulating surface or even the unwelcome presence of birds & rodents inside switchgear but the most common cause is human interaction with energised equipment.

Who is at risk?

The easy answer is anyone working on or around electricity, but in reality the risk is broader than that. As well as electricians workers in a range of industries such as utilities, telecommunications, petrochemical and gas, wind farms, mining, foundries and chemical plants may be at risk from electric arc. And because there are so many different situations in which an arc flash can occur there is no one blanket approach that can applied to protect against it.

What should you do?

Risk assessment, risk assessment, risk assessment. This is the cornerstone of any electric arc policy and the results of it should form the foundation of whatever you do next.

‘You can’t determine how bad an arc might be by looking at the equipment,’ cautions Mike Frain, who explains that employers have a clear responsibility under EU laws to carry out risk assessments on any work activities where there is a potential danger to workers. ‘Time and time again I have come across companies who provide PPE without doing a risk assessment, which goes against not only European law, but also the UK’s Health & Safety at Work Regulations and the PPE regulations. No one should provide PPE except on the basis on a prediction of what could happen based on a thorough risk assessment.’

Much of the pioneering work in the prediction of electric arcs is carried out the in the US, and to an extent to UK is playing catch up, says Mike. But, he says, generally speaking we are very good at electrical safety, with a year on year reduction in personal injury. This is the result of years of improvements to electrical equipment, techniques and good working practices. However, there are still 15-20 fatalities a year and hundreds of injuries. ‘The approach to electrical safety in the UK is always to work de-energised, but sometimes people do have to work on energised circuits, and this is where we lag behind. Everyone knows there is a risk, but often they turn to PPE because it is available, but they are not doing so based on scientific data.’

You might think that providing PPE without a risk assessment, or specifying the top level of protection available just to be on the safe side, is not so bad, or at least better than nothing, but it is not as simple as that. Without an assessment of the possible scale of the risk, companies have no idea the level of protection they need to provide, and over-protecting can be just as dangerous as under-protecting if workers are provided with bulky, heavy and uncomfortable garments that then don’t get worn at all. Furthermore, inappropriate PPE may dumb down the vital safety defence senses such as sound, sight and smell.

Mike explains that risk assessments should be based on sound engineering practice, and in his view the best way to assess the severity of electric arc risk is to use the US-based Institute of Electrical and Electronics Engineers (IEEE) 1584 guide for arc flash calculations. ‘The IEEE is regarded as the largest learned body on electrical engineering in the world, and the guide provides predictive techniques built on empirical data for what happens in high-power situations. Using this data companies should be able to predict the amount of incident energy in the workplace and therefore the severity of an electric arc.’

Once a risk assessment has been carried out, and a risk has been evaluated, the first step should not be providing PPE, points out Elaina Harvey from DuPont Personal Protection. ‘Before companies even consider protective clothing they should have engineered out the risk as much as possible. The PPE at Work regulations state that employers need to provide workers with suitable PPE only if the risk cannot be dealt with in another way, and these regulations introduce the concept of PPE as a last resort measure. If, after every effort has been made to control the risk, companies decide they still need PPE, they have two options – and this is where it gets complicated.’

The International Electrotechnical Commission (IEC) has developed the IEC 61482 series of standards for clothing to protect against the thermal hazards of an electric arc – they do not also address electric shock hazards, the effects of UV or noise or any of the other associated effects of an arc. This IEC series is in various stages of becoming European standards. IEC 61482-2 (to become EN 61482-2) describes the performance requirements for garments, while IEC 61482-1-1 and IEC 61482-1-2 (already the equivalent EN norms) describe the two different test methods. 

‘These might not be the best way of doing things, but they are the only test methods we have in the current legislation,’ says Elaina. ‘At testing standards level there is still a huge amount of debate about electric arc, how it occurs and how it behaves, and it is not an easy subject. Once end users have determined the level of protection they require, they need to decide which of these two test methods is most appropriate.’

Arc in a box, EN 61482-1-2

By far the most prevalent test method for electric arc clothing in Europe is ‘arc in a box’ or ‘closed arc’, commonly referred to as the box test, and under the old ENV 50354 standard this was the only test method specified. 

The box test was developed around the utility market to assess the risk with enclosed low voltage equipment, and can be used to test both fabrics and finished garments. It uses a fixed set of parameters such as distance of specimen from the arc (30cm) and its duration (0.5 seconds), and is divided into Class 1 and Class 2, with a pass or fail result. The test takes place within a three-sided plaster box sealed by insulating plates at the top and bottom. The arc is directed towards the opening to simulate an arc hitting the front of a worker at chest height.

The test is carried out in an electrical set up with an open supply voltage of 400V and a short circuit current of 4kA (Class 1 set up) or 7kA (Class 2 set up).

When tested at either Class 1 or Class 2 conditions, a visual inspection of garments determines whether they pass or fail, with key criteria including after flame of less of five seconds, no melting on the inner side of the fabric and no holes of more than five mm on the inner layer. In a change to the old ENV 50354 standard, fabrics undergoing the box test now also have to take into account the heat transferred through the fabric which is compared to the Stoll Curve, a calculation of the amount of energy it takes to cause second degree burns to the skin. Previously this was a voluntary requirement. The heat transfer is measured by sensors, and if the values are below the curve, ie the energy transferred is insufficient to cause second degree burns, the fabric will pass the test.

According to Andrew Braimbridge from Sioen, different countries in Europe tend to go for different protection levels. In the UK, he says, we tend to specify level 2 as a matter of course, whereas in France, for example, level 1 is more commonly requested.

Open arc, EN 61482-1-1

The ‘open arc’ test method was developed in the US and instead of testing fabric and clothing within a closed arc scenario, where the energy of the arc is magnified outwards towards the person, the open arc test simulates arc flash in an open environment. Unlike the box test, it does not produce a pass or fail result, but instead the arrangement of the test is intended to serve as the source of the thermal incident energy of the arc so that the ATPV rating – the Arc Thermal Protection Value of a fabric of garment – can be determined. This is the amount of thermal energy, expressed in calories per cm squared, that a fabric/garment can support  - according to the Stoll Curve - before the wearer suffers second degree burns.

The open arc test uses sensors on both sides of the test specimens to measure the incident energy passing onto and through the fabric or garment. Like the box test it has certain fixed criteria – an 8kA arc current and 30cm distance between the specimens (three in this case) and the arc – however, the ATPV rating is determined by varying the amount of incident energy through arcs of different durations (between 0.2 and 2 seconds).

The advantage of the ATPV rating is that it can be compared directly with the incident energy level determined by a risk assessment, enabling the end user to select a protection level that is as high or higher.

Which test method is best?

There is no easy answer to this question, and indeed it is not so much a question of which test is best – there is no right or wrong test – but which is more appropriate to any given scenario.

So, where’s the controversy? Well, both test methods have their detractors and supporters. Some argue that the conditions of the box test are too restrictive, for instance, that the distance from the arc cannot be guaranteed, or that the box test takes place within a plaster box, whereas in a real-life scenario this would probably be metal, creating an arc that would behave differently. Others point to the fact that the box test does not provide an ATPV rating, and that the open arc test therefore provides more reliable and accurate results which allow the end user to match items of PPE according to their assessed risk. Certainlu, in the view of Mike Frain and DuPont, the open arc is the preferred test method, but, on the other hand, open arc testing is relatively new to Europe – in fact, the only certified test house for open arc is currently in Canada, although DuPont has an open arc test rig at its technical research centre in Geneva – and it was developed in line with US working practices.

Ivan Deceuninck from Sioen, which is a major supplier of arc protective clothing in Europe, takes a very balanced view of the debate. ‘Both test methods can be linked to different working conditions, for example a linesman would be exposed to an open arc, and an electrician working in a confined space would be exposed to a closed arc. While it is true that the arc in a box test is not exactly the same as it would be in reality, the same can be said for the open arc test, and in fact for all test methods. Lab conditions are never the same as reality. For instance, the certifications here apply to a single phase arc, whereas electricians are often working in tri-phase conditions.’

Generally speaking, European manufacturers test to the box test because this is the European test, however for many end users trying to find out about electric arc protection, the majority of information that can be found on the Internet comes from the US and refers to the calorific or ATPV rating. Hence, there is a lot of confusion at end user level – and indeed even among suppliers and test houses. This is not a simple subject. Where problems occur is when confusion prevents end users taking action, and no one wants that. So the message is clear. Do your risk assessment, do everything you can to mitigate that risk and if you decide you still need to put PPE in place and don’t know which method is best, talk to your supplier.

What else?

PPE for electric arc is not just the clothing. The most serious burns are usually to the hands, which are likely to be closer to the arc where the heat is most intense, and the face is another area of the body that will be exposed. You need to consider the other accessories – gloves, footwear, eyewear, and even voltage rated tools. And you need to balance the protection provided with the fact that the wearer still needs to be able to carry out their work.

‘The problem with an arc blast is that it is so intense and so quick that you need quite heavy clothing to protect against it,’ explains Andrew Braimbridge from Sioen. ‘In the UK people tend to want the highest protection as a matter of course, but once they see how heavy these garments are, they usually have a rethink. As with all areas of PPE, what everyone is looking for is protection and wearability. The best insulation against an arc blast is the air gaps between layers, so the more layers – and the right sort of layers – the better the protection. When people come to us and ask for a certain level of protection, for example, 25 cal/cm2, we ask them what they will be wearing underneath our jacket, and the only way to really tell is to have the whole ensemble tested. For example, you could have a jacket that provides 20 cal/cm2, a shirt that provides 8 and a polo that provides 4. The total level of protection provided will then by much higher than simply the sum of these numbers.’

  • Operation Florian

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