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November 2, 2004

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Heat Resistant Safety Gloves

Heat Resistant Safety Gloves

Heat Resistant Safety Gloves

When it comes to heat resistant safety gloves the range to choose from is huge, so specifiers need to be sure to select the right gloves for the job to prevent injuries to the wearer. One criterion on which decisions can be based is compliance of products to British and European standards. A new standard governing one aspect of glove safety – heat resistance – has just been released, so Brian Bennett explains its main provisions and how it differs from the previous standard.

Heat-resistant gloves have been used for many years and pre-date more recent types of safety gloves, such as those designed for chemical and cut resistance. They are used in a wide range of applications, from handling hot plates and food containers in kitchens to more severe tasks, such as fire-fighting and handling molten metal. As a result, there are many types of heat-resistant glove available, ranging from relatively lightweight gloves to very heavy mitts for very high temperatures. As some might melt or burn if used in the wrong context it is very important to use the correct glove for the job.

The most important standard for heat-resistant gloves is the European standard known as EN407. It can be used in its own right but it is also used as the basis for other heat-resistant glove standards. The latest version of this standard for heat-resistant gloves has just been released and will be referred to as BS EN407:2004 in the UK. There are a couple of significant differences between this and the original standard, which was released in 1994, but before looking at those specifically, it is useful to examine generally the hazards associated with heat using the new standard as a background.

There are three modes of heat transmission – conductive, convective and radiant – but once the outside surface of the glove becomes hot the major mode of heat transmission is likely to be conductive, as most heat-resistant gloves are made from fairly dense materials, so the contribution from the other modes is likely to be minimal. The tests in EN407, with the exception of ‘burning behaviour’ (see page 55), measure short-term resistance to heat, i.e. measured in seconds rather than minutes or hours, and allocate a performance level to the product. So, if protection is required for long periods at high (or low) temperatures tests other than those used in EN407 would be required.

Contact heat test
To understand how the heat flows through the glove the contact heat test can be applied. As the name suggests this simulates what happens when a hot object is placed on the glove and how the heat is conducted through it. There are four performance levels in this test but taking, for example, the second performance level – 250ºC – at the very instant of contact it is only the very top of the surface of the glove that is at this temperature; the rest of the glove is still at room temperature. As time passes the entire glove heats up, with those parts that are nearest the hot surface being the hotter. Eventually, the inside surface of the glove reaches a temperature that is too hot to bear and the glove must be removed.

The contact heat test effectively measures this time and to pass the test the temperature inside the glove must not reach the prescribed limit until after 15 seconds. It must be borne in mind that this time depends on a number of factors, such as the weight of the object being handled, and how frequently it is handled. As much of the thermal resistance is due to the air in the glove a heavy object, which squashes the fabric, will reduce its thermal resistance.

Note also that times for handling at different temperatures with the same glove cannot be extrapolated from the results of one test – for example, if the time at 250ºC is 20 seconds the result will not be 10 seconds at 500ºC. It may, in fact, be only one or two seconds and indeed in practice the times do fall off quite quickly, so a glove designed for 250ºC should therefore not be used at a higher temperature.

This time of 15 seconds also depends on the temperature of the glove before it comes into contact with the hot object. If it is being used in hot conditions, or in a hot climate, or it has been used to handle something hot a few seconds or minutes previously, it will be at an elevated temperature to start with. Repetitive handling at high temperatures can therefore be a problem as the glove gradually heats up, thus reducing the handling time – sometimes quite considerably. There are various ways to overcome this. The easiest way is to use a glove with a higher performance level, or to rotate gloves, but in no circumstances should the gloves be put in water to cool (unless they are actually on fire!), as the thermal conductivity of water is about 20 times greater than for dry air, meaning the heat will be transmitted much faster.

This fact is taken into account in the latest version of BS EN 659, which is the standard for fire-fighters’ gloves. This measures thermal resistance in both wet and dry conditions and sets minimum values for both. So, while there are gloves that can be used in both wet and dry conditions, most gloves cannot. The assumption must therefore be that gloves are not to be used in this way unless the manufacturer has tested and designed the product for such use.

Convective heat test
It is not often that the convective heat test needs to be considered for handling hot objects, although it is naturally included in the aforementioned BS EN 659 standard for fire-fighters’ gloves. This is partly because, as mentioned above, convective heat becomes predominantly conductive after the surface has become hot. However, the test differs from the contact heat test in that it measures the performance of both the back and front of the glove. In practice there are also difficulties in relating the meaning of a performance level in this test to the actual situation, so, if in doubt, it is probably better to rely on the ‘contact heat’ performance level but discuss this with your supplier if you have such a requirement.

Radiant heat test
Radiant heat tests are carried out when a safety product is subjected to the radiant heat from a very hot object, such as a red-hot piece of steel, or a furnace. One of the main changes in the new standard relates to this test, which is described in detail in the international standard ISO 6942:2002 and which is considered to give more representative values than the earlier version of BS EN407, which used EN366 as a basis. As a result, when products are tested to the new standard there may be situations where the results are different from those quoted from earlier tests. The earlier version tended to lump gloves without reflective coatings into the lower performance levels and those with reflective coatings into the highest performance level.

The latest standard will give a more even distribution between the various performance levels, although it remains the case that if high levels of reflectance are required – for example, working in front of a kiln – then a glove with a shiny reflective surface will usually be more suitable. A common misconception is that gloves with reflective surfaces are the best to use for all purposes that require heat resistance. This is certainly not the case, as such surfaces are easily damaged both by contact with very hot objects, and as a result of mechanical action.

Molten metal tests
The other two tests in BS EN407 are for gloves for use in conjunction with molten metal. The ‘small splashes of molten metal’ test is intended to replicate welding and was previously used for this purpose but this test has been superseded by the standard BS EN12477, which was specifically written for welders’ gloves. This latter standard is currently being amended and the amendment is likely to be available next year. The other test is the one for ‘large quantities of molten metal’, which is self-explanatory. It is worth noting that this test uses molten iron, so if the user is using a metal other than molten iron the test should be repeated with that metal as molten metals stick to different surfaces differently, and this can have a significant bearing on the protection offered by the glove.

Heat-resistant leather gloves generally tend to perform well in molten splash tests but you will need to verify that the glove has been tested and can be used for the job for which it is intended. Also bear in mind that cheap leather, which has not been treated for heat resistance, can shrink significantly when it becomes very hot.

Burning behaviour
Apart from the radiant heat test, the other significant change in the new standard is that any glove that the manufacturer wishes to record as having a performance level 3 or 4 – in, for example the contact heat test – should also record a performance level 3 or 4 in the burning behaviour test. This is the very first test in BS EN407:2004 and is carried out by placing a burner at the tip of the glove or mitt. Generally speaking, most gloves either achieve the top level 4, or are not tested because they are made from materials that have been treated for flame resistance, or are inherently flame-resistant (or not, as the case may be), although occasionally a few will give other performance levels.

Always bear in mind that even gloves that achieve the top level 4 may catch fire in some circumstances, although should not do so if used in accordance with the manufacturer’s instructions.

In fact there were various gloves on the market that achieved level 3 in the contact heat test but these were manufactured from cotton and therefore failed the criterion that they should be non-flammable. While it is possible to hold something at 350ºC or even 500ºC with a sufficiently thick cotton glove, the glove is likely to catch fire in the process. And while the heat generated by the flames may be low in comparison with the heat source and may not be a problem in terms of heat conduction it is clearly a hazard to be wearing burning gloves.

Although a few asbestos products are still made most of the gloves now used for high-temperature work are made from high-performance materials, such as the aramids, and more exotic fibres, such as PBI and PBO. These materials are inherently flame-resistant and usually achieve the top performance level 4 in the burning behaviour test.

Wear it well
Generally speaking it is a good idea to wear gloves that are loose-fitting, if the job allows, so that they can be thrown off in a hurry if they get too hot, and to wear a gauntlet that at least covers the lower part of the arm. If there is frequent contact between the arm (as opposed to the hand) and a hot surface this may need to be discussed with the supplier, as the cuffs of some gloves are not made of the same materials as the glove and there is no requirement in the standard for the cuffs themselves. Occasionally it may also be necessary to ensure the cuff of the glove is tight-fitting so that nothing hot can fall inside.

Gloves should always be kept clean, especially of substances such as grease, which can alter both thermal conductivity and flame-resistance. The reflective surface of gloves intended for radiant heat protection should also be regularly cleaned according to the manufacturer’s instructions, as this maintains both the reflectivity and the reflective surface itself. Bear in mind that the tests were carried out on a new glove so any glove that is damaged, worn or dirty will not give the same results in the test if it is carried out again.

There are still various gloves on the market that have not been tested or certified in accordance with current legislation. To avoid any problems I would suggest that the buyer, or safety officer ask for an EC Type Examination Certificate, which is a document issued by a government-appointed body, for any heat-resistant product they wish to buy. Any reputable manufacturer will be able to supply such a document, which will confirm that the glove has indeed been tested to BS EN407. I would also suggest that anyone using heat-resistant gloves to any significant extent obtain a copy of the latest standard BS EN407:2004 (available from the British Standards Institution), which gives more details of the various performance levels.

 

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