Machines for EU safety footwear testing – part 7

The tests and equipment used to assess footwear offering different forms of electrical resistance.

by Peter Allen

This article focuses on the SATRA test equipment designed to carry out assessment of electrical properties which are requirements for whole footwear in clause 6.2.2 of the

EN ISO 20345:2011 European safety footwear standard. There are three electrical assessments specified – for ‘conductive’ footwear (clause 6.2.2.1), ‘antistatic’ footwear (clause 6.2.2.2) and ‘electrically insulating’ footwear (clause 6.2.2.3).

Within EN ISO 20345:2011, footwear is classified as either ‘Class I’ or ‘Class II’. Class I is made from leather and other materials, excluding all-rubber or all-polymeric footwear. Class II footwear is all-rubber (that is, entirely vulcanised) or all-polymeric (entirely moulded) footwear. Conductive footwear and anti-static footwear can be either a Class I or Class II product, whereas electrically insulating footwear is applicable only to Class II footwear.

Conductive footwear should be used if it is necessary to minimise electrostatic charges in the shortest possible time – for example, when handling explosives. However, conductive footwear will not provide protection for the user against electrical shock. Anti-static shoes can be chosen if it is necessary to minimise electrostatic build-up by dissipating electrostatic charges. This will help the user to avoid the risk of spark ignition – for instance, of flammable substances and vapours. As with conductive footwear, an anti-static shoe cannot guarantee adequate protection against electrical shock, as it only introduces a limited resistance between foot and floor. The third category – ‘electrically insulated footwear’ – denotes products that protect the wearer against electric shocks by preventing the passage of dangerous current through his or her body via the feet. The intended use is for working on live or close to live parts on installations not exceeding 1,000V ac. Such electrically insulated footwear would often be used in conjunction with other insulated protective garments.

Conductive footwear testing

The requirements for conductive footwear are covered in EN ISO 20345: 2011 clause 6.2.2.1. This specifies that, when measured in accordance with

ISO 20344:2011 clause 5.10 and after conditioning in a dry atmosphere (as set out in ISO 20344:2011 clause 5.10.3.3), the electrical resistance should be not greater than 100kΩ. The conditioning requirement for the dry test is 20±2°C and 30±5 per cent relative humidity (rh) for seven days. The test can be carried out using the SATRA STM 470 conductivity tester. In the conductive test (clause 6.2.2.1), the elements of the equipment required consist of i) an insulating base plate into which a copper plate is set, ii) a 4kg quantity of stainless steel balls, iii) a copper connection plate, and iv) a precision resistance meter.

When conducting this test (see figure 1), the footwear sample is placed onto the copper plate and filled with steel balls. The copper connection plate is pushed into the steel balls, the resistance meter is connected between the copper base plate and the plate which has been set in among the steel balls. A voltage of 100±2V dc is applied via the meter for one minute, at the end of which the measured resistance should be recorded. The test method referenced in clause 6.2.2.1 sets out the requirements for preparation of the sole and the importance of ensuring that good electrical conductivity is maintained.

Testing antistatic footwear

The requirements for antistatic footwear is covered in EN ISO 20345:2011 clause 6.2.2.2. This specifies that tests should be conducted after conditioning in both a dry and a wet atmosphere, in accordance with ISO 20344:2011 clause 5.10. The conditioning requirements are set out in ISO 20344:2011 clause 5.10.3.3. In both conditions (after conditioning in ‘dry’ and ‘wet’ atmospheres), the electrical resistance should be above 100 kiloohms and less than or equal to 1,000 megaohms. Antistatic tests can also be carried out using the

SATRA STM 470 conductivity tester.

The conditioning and procedure for testing the antistatic footwear in the dry condition is the same as the conductive footwear test, although with a different requirement for the measured resistance, as indicated above. For the wet conditions testing, the footwear should be conditioned at 20±2°C and 85±5 per cent rh for seven days.

Before carrying out the wet atmosphere test and after conditioning the sample, a conductive lacquer is applied over the sole in a specified area and allowed to dry. This is done to ensure a good contact between the sole and the copper plate. The use of a lacquer replaces older methods when the sole was placed into a shallow water bath. Another requirement for the use of the lacquer is a specific test to confirm sufficient conductivity is provided within the lacquer layer before carrying out the wet antistatic test. The SATRA STM 470 test device includes an insulated board into which are mounted three supports to the dimensional requirements within

EN ISO 20344:2011 clause 5.10.2.5.

The footwear sample, with its applied and dried lacquer, is placed with the forepart supported by two of the posts 45mm apart. The heel is supported by the third post, which is positioned 180mm from the other posts. The forepart posts are connected under the board to a connection point and a separate connection point is provided for the heel post. By use of the meter and connection points, the resistance can be determined along the length of the footwear, between the single post and the pair of posts. The resistance measured should be less than 1,000 ohms. If this is not achieved, either the type of conductive lacquer or its application should be investigated.

The resistance test for the footwear sample is carried out in a similar manner to the dry test. The footwear sample is placed on the copper base plate and is filled with 4kg of steel balls. The connection plate is pushed in among the steel balls and connection is made using the meter between the base plate and the connection plate in the steel balls. A voltage of 100±2V dc should be applied for one minute before the resistance value is recorded.

Electrically insulating footwear testing

Electrically insulated footwear testing is only applicable to Class II footwear. The testing requirements are set out in EN ISO 20345:2011 clause 6.2.2.3 which, in turn, specifies that the footwear should fulfil the requirements given in

EN 50321. In outline, the test is conducted with the footwear placed into a water bath with either an internal fill of water or steel balls to the same level as the external water. A steadily increasing, variable high voltage is applied through the internal water fill or steel balls to the external water bath (which is earthed). The standard sets out different rating classes for the footwear, depending on the successful outcome of the test against the designated voltage class. The method sets out the particular tests which should be conducted – for example, ‘proof voltage tests’ or ‘withstand voltage tests’. The maximum applied voltage is 10,000V. It should be noted that although SATRA can carry out electrically insulating footwear testing (EN ISO 20345:2011 clause 6.2.2.3) in our testing laboratories, we do not make the equipment required to carry out testing to this requirement.

SATRA test equipment is designed and manufactured in the UK, and is supplied to a global market. Purchasers include brand owners, retailers, manufacturers, component and material suppliers, test houses, government institutions, universities and research institutes. SATRA test equipment draws on the expertise gained over the organisation’s 98 years of history, from fundamental research, development of over 400 SATRA test methods, and the extensive use of SATRA machines in our own laboratories.

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Publishing Data

This article was originally published on page 34 of the July/August 2017 issue of SATRA Bulletin.

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