KANBrief 2/12

Test finger: tested and found to be too short

A test finger can be used to test whether the enclosures of machines and installations are designed such that persons are not able to come into contact with dangerous parts. An assessment (pdf) commissioned by KAN has found however that test fingers to EN 60529 do not always guarantee this protection.

The design of enclosures must prevent persons from touching any dangerous electrical or mechanical parts. In order to test this, a jointed test finger is employed which is intended to simulate a human finger. The design of the test finger is set out in EN 60529:2000, “Degrees of protection provided by enclosures (IP code)”, which specifies a length of 80 mm and a diameter of 12 mm.

In the course of KAN’s study of anthropometric data in standards (KAN Report 44, “Anthropometric data in standards”; 2009), it was found that the length of the test finger, which was defined over 30 years ago, no longer corresponds to the anthropometric reality within the population. For this reason, the ASER institute was commissioned in June 2011 with the task of reviewing whether the underlying data are still up to date. In addition to the length and width of the finger, factors such as a realistic series of joint angles and the influence of fingernails were to be examined. In the first step, current distributions of index finger length and width were compared with the dimensions of the test finger. In addition to the German values, data based upon ISO/TR 7250-2:2010 "Basic human body measurements for technological design – Part 2: Statistical summaries of body measurements from individual ISO populations" from other countries were also included in the evaluation.

The test finger must be lengthened

The assessment found that the diameter of the test finger presented a high level of safety: with values of between 14 and 18 mm, the finger width of almost all adults both in Germany and in other ISO countries is substantially greater than the test finger diameter of 12 mm. Consequently, enclosure openings which the test finger is unable to penetrate are also inaccessible to the human finger. The situation is different with regard to the length of the test finger.

At the current length of 80 mm, protection is not fully assured for a not inconsiderable percentage of the population in Germany. If the distributions of the index finger length recorded in other countries are also taken into account, the deviation from the test finger length is even greater. From an anthropometric perspective, the length of the test finger should therefore be extended.

The assessment concludes that a test finger length of over 90 mm is required in order to allow for the actual index finger lengths of the population in the countries under consideration. To make allowance for the length distribution of all countries if possible and for the potential penetration depth of the index finger, which owing to the skin fold at the base of the finger is greater than the length measured in accordance with the standard, extension by 15 mm is proposed. In order for fingernails of different length to be included in the design of the test finger, a further 5 mm must be added to the length.

EN ISO 13857 governing safety distances on machines (EN ISO 13857:2008: Safety of machinery – safety distances to prevent hazard zones being reached by upper and lower limbs) stipulates a safety distance of at least 120 mm for square openings which can be penetrated by a finger (i.e. 12 to 20 mm). In order for the test finger also to cover this standard, it must have a total length of 120 mm.

The length of the individual test finger phalanxes to EN 60529 does not reflect those of actual human index fingers. Whereas on the test finger, the shortest phalanx is the lowest, i.e. that nearest the body, on human beings it is generally the top phalanx. A worst-case analysis (long, thin fingers) is however considered sufficient. It is not necessary for different types of test finger to be specified in the standard.

In order for the results of the assessment to be implemented, the ASER institute proposes the use of a plug-on sleeve for the testing of larger enclosure openings. KAN will discuss with experts in an ergonomics workshop whether this solution is suitable for use in practice and how the results of the assessment can best be implemented.

Dr. Beate Schlutter