It can be difficult to find luminaires suitable for conditions where substantial vibration is present. The introduction of LED sources has made the situation somewhat easier, as having solid state components mounted on a substrate is very different to having tungsten filaments or discharge sources in a vacuum. But the problem remains as luminaires become more complex and require more components.
The need for luminaires to deal with vibration from transportation is obvious. But many permanent installations are also subjected to at least some form of resonance or vibration. Understanding the vibrational force likely to occur, especially for equipment mounted in hard to reach areas, is crucial to judging product suitability, and thus longevity. Vibration analyses are routinely carried out in new construction or structural renovations, especially in applications near roads. Suspended structures like bridges, and areas near motors and other MEP/HVAC equipment need even more attention.
When a full analysis is not possible, portable vibration monitors/data loggers are available to rent which can help define the forces. But what if no form of vibration analysis is available? Different types of vibration can be difficult to foresee in some scenarios without an analysis, so the cumulative effect of resonance and vibration can be difficult to judge. Rules of thumb and good practice are therefore key. A few are:
- Locating lighting at a distance from motors and roadways.
- Building in more mass to mounting supports to dampen acceleration forces.
- Mounting motors and MEP/HVAC on motor mounts.
- Protecting luminaires from high winds.
With product ratings, there are numerous variations of tests to choose from to suit a given application. Some are country specific, some are meant for defense testing, and there are many other industry specific vibration testing standards. Misconceptions and misuse of standards are common, so what is the best way to understand if a product will be suitable to deal with vibration? For lighting specifically, two standards are typically used to judge suitability:
- IEC 60068-2-6:2008 Sinusoidal Vibration.
- ANSI C136.31-2018 Minimum vibration withstand capability and vibration test methods for roadway and area luminaires.
IEC 60068-2-6 testing subjects a specimen to sinusoidal vibration over a given frequency range for a given period. IEC 60068-2-6 also requires the luminaire to be functional during testing. During the tests, vibration response data are to be monitored and examined to determine critical frequencies. The two main procedures are results of both sweeping and fixed frequencies. Frequencies, intensity, and the behavior of the luminaire must be stated in the test report. But again, the test is administered using sinusoidal vibration only. Sinusoidal vibration is not very common in nature, but it provides an excellent engineering tool that enables us to understand complex vibrations by breaking them down into simple, one-tone vibrations.
ANSI C136.31 testing is primarily meant for roadway and area lighting. The ANSI standard contains the now common 3G test acceleration rating. If the luminaire is to be placed on a bridge or overpass, the luminaire system is to be tested at 3.0 G. All other applications of luminaire systems are meant to be tested at 1.5 G. With ANSI C136.31, the luminaire is fastened in its most onerous but normal position of installation to a vibration generator or shaker. Increasingly severe tests are administered. After the test, the luminaire shall have no loosened parts which could impair safety and be fully operational. It is important to note that the 3G rating of a luminaire means that it can withstand fundamental resonant frequency vibrations, for 100,000 cycles in each plane (x, y, z). This is intended to provide a worst-case scenario as the product’s natural resonance is taken into account. Meaning, at other less severe frequencies for a given product, it could perform better and could take more than 3G of accelerative vibrational force.