Testing for protection against heat

SATRA test machines used to assess safety footwear materials designed to safeguard against heat.

by Peter Allen

The SATRA Bulletin article ‘Heat-resistant safety footwear’ described two SATRA machines used for testing against the effects of exposure to contact heat and radiant heat (STM 531 and STM 518). This month, the test machines under consideration are the STM 471 – which allows assessment of insulation against heat provided by footwear soling construction – and the STM 524 test machine, which is used to assess materials for resistance to molten metal splash.

In some instances, heat sources can be quite specific – for instance, molten metal splashes from welding – while, in other cases, heat can emanate from a number of diverse sources, (such as high under-sole temperatures, which can arise from a number of different hazardous conditions). Safety standards which specify a test for the determination of insulation against under sole heat include ‘Wildfire firefighting personal protective equipment – Requirements and test methods’ (ISO 16073:2011), ‘Footwear for firefighters’ (EN 15090:2006) and ‘Personal protective equipment – Footwear protecting against thermal risks and molten metal splashes as found in foundries and welding – Requirements and test methods’ (EN ISO 20349:2010). All of these reference a test defined within ‘Personal protective equipment – Test methods for footwear’ (EN ISO 20344:2004-5.12 – ‘Determination of insulation from heat’). The SATRA STM 471 Heated Sandbath (figure 1) can be used to conduct this test.

Heated Sandbath

The SATRA STM 471 test equipment comprises a sand-containing tray fitted with a thermostatically controlled hot plate at its base. A layer of sand, as defined in EN ISO 20344, covers the hotplate to a depth of 30mm. In conducting the test, the sandbath is typically preheated for two hours to achieve an equilibrium temperature. The footwear sample (which is tested whole) is fitted with a temperature probe positioned against the insole in the footwear forepart – above the contact point the sole makes against a flat plane. The footwear is filled with 5mm diameter stainless steel balls, and the footwear is worked through the sand until the sole is in contact with the hotplate, after which the sand is levelled off around the footwear.

The specifying test method (such as EN ISO 20349:2010) will typically define a temperature of the hotplate, as well as specifying requirements regarding the effects of the exposure of the soles to heat. In addition, it will define a time within which the internal temperature rise measured by the temperature probe must not exceed a specified value from the initial temperature of the conditioned footwear sample – for instance, a hotplate temperature of 150°C and a temperature rise <42°C after 30 minutes’ exposure within the sandbath. A fitted mesh guard protects the operator from the heat source during the test. The test equipment incorporates an illuminated power on/off switch, a temperature controller for the hotplate and a digital display of the temperature sensing probe output. The machine is supplied with the temperature probe along with a quantity of steel balls and sand conforming to the requirements of the standard.

Molten metal splashes

With the SATRA STM 524 Molten Metal Splash Test machine (figure 2), tests can be carried out as detailed in the requirements of EN 348:1992 – ‘Protective clothing – Determination of behaviour of materials on impact of small splashes of molten metal’. This standard allows an assessment to be made of the garments worn by workers employed in foundries and in welding operations. It is referenced in ‘Personal protective equipment – Footwear protecting against thermal risks and molten metal splashes as found in foundries and welding – Requirements and test methods’ (EN ISO 20349:2010).

The test machine incorporates an oxyacetylene torch used to melt a steel rod, molten drops from the rod being directed onto a vertically-oriented test specimen. The steel rod is automatically fed to the torch for continuous melting, and the machine permits adjustments to be made to the position of the torch which, along with adjustments to the flame, allows the production of molten droplets at the specified mass and at the specified rate of droplet delivery defined within the standard.

The material sample taken from the footwear upper (which should constitute all the footwear layers of construction) is tensioned across a support block and incorporates a temperature sensor. This allows the temperature to be measured on the reverse side of the specimen, as molten droplets are directed onto the external material surface. To meet the requirements for small molten metal splash – one of the tests within EN ISO 20349:2010 – the material under test should be subjected to at least 25 droplet impacts before a temperature rise of 40°C is recorded. In addition, the test methods call for some additional visual assessments to be made of the condition of the materials on completion of the test.

The test equipment is supplied with glazed safety doors and a means of directing welding fumes into the laboratory’s existing fume extraction system. The machine also incorporates a metal droplet catching tray.

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Publishing Data

This article was originally published on page 30 of the February 2012 issue of SATRA Bulletin.

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