Assessing the fitness for purpose of devices designed to allow operatives to descend safely from working at height.
A descender is a device which includes a line typically made from wire rope, textile rope or webbing, and which allows users to move themselves or others from a higher to a lower position under a controlled rate of descent. Descenders may be used in a variety of applications, one of the most important of which is for rescue.
In Europe, descenders for rescue are tested to EN 341:2011. The standard includes test procedures that require a series of high-level descent tests to assess the product’s ability to perform satisfactorily after repeated use.
The standard currently categorises descenders into two types: ‘automatic’, which incorporates a braking system that requires no intervention by the user once the descent has commenced, and ‘manually-operated’ products with a braking system that requires the user to take action. EN 341 refers to these as ‘Type 1’ and ‘Type 2’ respectively. It also includes a classification system (‘A’ to ‘D’), based on descent energy the device is capable of withstanding (see box 1).
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Box 1: EN 341 classification of descender devices |
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Class of device |
Descent energy (J) |
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A |
Up to 7.5 x 106 |
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B |
Up to 1.5 x 106 |
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C |
Up to 0.5 x 106 |
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D |
For only one descent – descent energy depends on the maximum descent height and the maximum rated load. |
Most of the testing in EN 341:2011 is required to be carried out on the same device without any changes being made between tests. Following multiple descents, the device should still retain an acceptable safety factor.
EN 341:2011 includes general requirements which call for manufacturers to specify the minimum and maximum rated load, the latter being at least 100kg. It also includes requirements for design, materials and construction – such as a line integrity test, so that appropriate materials are used in the production of the device.
Dynamic strength test
Manually operated descender devices are tested for both dynamic and static strength in each locked position intended by the manufacturer. Dynamic strength (an assessment of the device’s ability to withstand shock loading) is tested with 4m of the line extracted, after which it is subjected to a 60cm freefall with a test mass in accordance with the maximum rated load. The descender device should not release the test mass, and no part of the descender device should show any signs of breaking or tearing. For Class D devices, the impact force is recorded, as this measurement is required in the static strength test.
Descent function test
For Classes A, B and C, assessment of function is carried out using the same device previously tested for dynamic strength. For the dry condition assessment prior to testing, the device is conditioned at a temperature of 20 ± 2ºC and relative humidity (RH) of 65 ± 5 per cent for at least 72 hours. Two descents for each test condition and device set-up are then carried out; one under the minimum rated load and one under the maximum plus 25 per cent.
Manually operated devices should be tested in the hands-off position or, if applicable, with any panic locking element engaged. The descent speed should be 0.5m-2m per second and measured within 30 seconds of the completion of the maximum descent. The temperature of any parts of the device that will be touched to control the descent according to the user instructions should be a maximum of 48ºC.
The same test must then be performed on the same device after submerging the descender in fresh water for 60 minutes and allowing it to drain for 15 minutes before the test commences. A new device must then be used to carry out the same test after submerging the descender in fresh water for 60 minutes and allowed to drain for 15 minutes, before it is placed in a conditioned atmosphere of -4°C for a minimum of four hours. For a Class D unit designed for a single descent, the same tests are conducted. However, a new device may be used for each condition. The manufacturer may also claim the descender can be used in very cold conditions, in which case the same process as above is used and then the product is placed into the coldest atmosphere for which it is claimed to be suitable.
Descent energy test
To assess a Class A, B or C device’s ability to perform in line with the manufacturer’s stated rating, a test is conducted using the same descender after the wet conditioned test, with the number of descents required calculated according to the class of the descender (see box 1). Note that single-use Class D products are not tested in this way. The descents are carried out at regular intervals with a mass equivalent to the maximum rated load. During the last descent, the descent speed is measured and should be 0.5m-2m per second. Once again, the temperatures are measured and assessed as before. Clearly for this type of test, a facility to carry out large drop heights is necessary. Some test facilities use a powered capstan which is an accepted method, but it is no substitute for carrying out a test over the actual height claimed. SATRA works with the National Lift Tower in Northampton, UK, which has a long drop testing facility. This is situated within the main structure of the building, where one of the lift shafts has been adapted to carry out descent tests on equipment, including fall arresters and descenders, at heights of up to 100m. Tests can be repeated in rapid succession.
Static strength test
To assess if the overall breaking strength of the device is sufficient, a test referred to as a ‘static strength test’ is conducted. Once again, the same descender sample as used for the previous tests is subject to this test force. Class A, B and C descender devices are subjected to static strength tests of ten times the maximum rated load, but at least 12kN which is applied for three minutes, during which time the descender must withstand the force. For Class D devices, the static strength requirement is twice the impact force recorded in the dynamic strength test.
Other required tests
The standard includes two requirements specifically for manually operated descender devices. One requirement, applying to all manually operated devices, relates to the force to release and operate the control element of the device – referred to as ‘operating force’. The other requirement relates to what is termed the ‘holding force’ and applies to those devices where the user controls the descent manually by holding the line.
The operating force test is carried out before the function tests. It involves attaching a mass or force equivalent to the device’s maximum rated load in a specified manner, and then measuring the force required to activate the manual control to allow the descent to start. The force should not exceed 450N. Devices that are designed both to move with the user, and to be operated from a fixed position, are tested in both configurations.
The holding force test is carried once before the function tests and once after the descent energy test. As in the procedure for measuring operating force, the test involves attaching a mass or force equivalent to the device’s maximum rated load in a specified manner. However, in this case, the force applied to the line going in the descender device necessary to hold the mass is measured. This should not exceed 200N.
Corrosion resistance is assessed by subjecting the device to a warm salt water mist. Descenders are required to pass a 48-hour corrosion test in accordance with EN ISO 9227. A check is made to ensure that the function of the device is not impaired and that there is no visual evidence of corrosion – either externally or internally.