Risk prediction of lamp loosening in road luminaires

Now and in the future, retrofit LED lamps will replace existing lamps in road and street luminaires. Given the increased mass of the LED lamp compared with its conventional design, potential risk of lamp loosening due to vibrations of the luminaire is feared by a manufacturer.

Philips Engineering Solutions supported the development of a retrofit LED lamp by predicting the risk of lamp loosening. By analyzing the loosening mechanism, the lighting pole dynamics and the wind load, this risk is investigated and reduced on beforehand.

Introduction

Street and road lighting is subjected to wind excitation, causing lighting poles and, consequently the luminaires to vibrate. In turn, this results in dynamic acceleration forces acting on the lamps within the luminaires. The retro-fit LED lamps used to replace the conventional light sources typically have more mass, due to the required drivers and cooling measures. As a result, the acceleration forces on the lamp and its holder will be larger. This increases the risk of loosening of the lamp and thereby a malfunction of the entire lighting system.

Lamp loosening mechanism

The mechanism for lamp loosening due to vibrations first needs to be understood in order to assess the risk of loosening. The lamps are screwed into lamp holders that are standardized and have limited height. Consequently, it will be mainly the friction force in the electrical contacts that prevents the lamp from rotating in case the lamp is subjected to alternating (acceleration) forces. If this friction force is not sufficient, relative movement of the lamp with respect to its holder will result in loosening. In turn, this reduces the friction force, thereby reinforcing the loosening.

Lighting pole dynamics

To determine the acceleration level of the lamp due to wind excitation, consider the dynamics of the lighting pole, which is excited by the wind. Typically, mainly the first bending mode (1 – 7 Hz) of a pole is excited. Excitation of higher order vibration modes of the lighting pole would result in larger acceleration levels. However, specific wind profiles would be required to effectively excite these modes. Consequently, the acceleration level at the lamp is dominated by the low-frequent (1 – 7 Hz) swaying of the lighting pole. The magnitude depends on the pole characteristics and therefor a large number of different lighting poles are taken into account for the risk analysis.

Wind loads

For a specific region or country, the expected maximal wind load in a certain period of time is determined in accordance with the EN 40-3-1 norm, which is used by e.g. pole manufactures to compute maximal deflection. A method is developed by Philips Engineering Solutions to extract dynamic loads from the norm. The norm uses wind maps, issued for each country and specifying the 10 minute mean basic wind velocity, exceeded only once per 50 years, as well as e.g. terrain characteristics. For the risk analysis, a large number of countries is included.

Conclusions / Results

A customer developing a retrofit LED lamp for road luminaires needed to know if their new lamps would have a potential risk of loosening. With white-box modeling of the lamp loosing mechanism, in combination with lighting pole dynamics and wind profiles, the risk of lamp loosing is predicted. The assessment enabled successful introduction into the market.