This question was answered by the Lux technical team.
It’s not easy to find light fixtures suitable for conditions where there is constant vibration, especially when standard IP-rated luminaires constantly fail in such conditions.
Having a light fixture attached to a source of vibration requires a performance specification that can handle such demanding conditions. The introduction of the LED has made the situation somewhat easier; having electronic components mounted onto PCB material is very different to having fragile filaments or discharge tubes inside a glass envelope. But the problem remains.
Vibration can come in a number of guises:
Arhythmic forced vibration is what happens when intermittent shocks are applied to equipment, such as the effect of driving over uneven ground. The vibration can be severe and can cause damage within the equipment.
Rhythmic forced vibration is what happens when equipment undergoes continuous vibration, such as when equipment is fixed directly to, or close by, vibrating plant. The force may not be severe, but its continuous nature can damage components due to the length of time of exposure.
Harmonic vibration can also occur. It’s possible for external vibrations to set up and ‘feed’ a harmonic vibration in the equipment.
Very few lighting fixtures are designated vibration-proof, and many of those advertised as such do not have test data to support the claim. In practice, fixtures are often mounted onto vibration dampers. These dampers provide a break between the source of the vibration and the fixture, protecting the components within the luminaire.
Equipment designated as vibration-proof may have been designed specifically for the purpose. There are three features to look for:
- The overall mass of the fixture should be as low as possible
- The equipment should be designed to be as rigid as possible
- The equipment should be designed so that its ‘natural vibration frequency’ lies outside the intended operating/testing range of the installation.
For lighting equipment, the standard that applies is BS EN 60068-2-6:2008, which refers to section 4.20 of BS EN 60598-1:2015 – rough service luminaires. It states: ‘Rough service luminaires shall have adequate resistance to vibrations. Compliance is checked by the following vibration test (as specified in IEC 60068-2-6). The luminaire is fastened in its most onerous but normal position of installation to a vibration generator. The direction of vibration is in the most onerous direction and the severity is: duration: 30 min, amplitude: 0,35 mm, frequency range: 10 Hz, 55 Hz, 10 Hz, sweep rate: approximately one octave per minute. After the test, the luminaire shall have no loosened parts which could impair the safety.’
The tests are designed to expose any mechanical weakness in the equipment. A fixture’s vibration certification only applies to the specific conditions that were used in the test. It shouldn’t be assumed that the fixture can be mounted in any other way.
For one transport authority, very specific vibration tests were required. Luminaires were subjected to one hour of vibration in each of three orthogonal axes, with vibrational stress being applied from 3Hz to 50Hz at 0.125mm displacement. Luminaires were then checked to ensure that no fixings had come loose and that all electronic components were still working. The rationale behind these tests was the proximity of the luminaires to trains entering and leaving platforms, setting up constant vibration. In other conditions, luminaires were mounted very close to plant and machinery.
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