Electrical Contracting News (ECN) December 2016 - Page 26

FEATURE OUTDOOR LIGHTING STREET WISE Energy efficient LED light sources and the associated electronic control gear offer numerous benefits, such as long life, reduced maintenance and adjustable light. But for street lighting the problem of possible overvoltages has to be addressed. Martin Thompson of Tridonic looks at how to provide reliable protection against overvoltages. I n contrast to the situation with indoor lighting, the problem of possible overvoltages, also known as surge voltages, has to be addressed when using LED light sources and the associated control gear for street lighting applications. A surge protection concept tailored to combat 10kV between L/N and earth and 6kV between L and N is, therefore, absolutely essential. Operating and maintenance costs for public lighting account for a large proportion of the energy costs of municipal and local authorities but there is enormous potential for savings to be made in lighting applications by replacing conventional light sources with LED light sources. This can only succeed, however, if sufficient attention is given to protecting LED street lights against overvoltages. Unlike conventional light sources, LEDs are operated at low voltages and are therefore sensitive to overvoltages, along with the electronic control gear. Overvoltages can be caused by switching operations in the power supply system or in nearby industrial facilities, electrostatic discharges (during maintenance work for example), or lightning strikes either on the LED street light or the power supply cable or in the vicinity, leading to galvanic or inductive coupling. Small overvoltages have little effect even on unprotected LED modules but frequent such overvoltages may have an adverse effect on the life of the LED light sources. Large overvoltages, such as those produced by a lightning strike, may well instantly destroy the LED modules or electronic control gear of several LED street lights. Overvoltages due to switching operations in power supply systems often occur between phase and neutral, in other words between L and N (differential mode interference). They reach peak values of 6kV and affect only the control gear. Standard control gear therefore has built-in surge protection of 4kV to 6kV so these overvoltages are absorbed. Overvoltages due to lightning strikes are much more difficult to calculate. They primarily occur between the power lines (L/N) and earth (PE) – differential mode interference – and can quickly reach several tens of kilovolts. The result is an induced voltage that can destroy the lights in entire street runs. The risk of lightning strikes is not the same everywhere, however, as there are strong geographical differences. The lightning ground flash density Ng, which defines the number of lightning strikes per square kilometre and year, is one in Belgium and 150 in South Africa, for example. Such regional differences therefore have to be taken into consideration. An LED lighting system consists of the luminaire housing, the LED module with an optical system for directing the light (lenses, reflectors) and the electronic control gear for supplying the LED module with appropriate power. Luminaires are ‘Overvoltages can be caused by switching operations in the power supply system.’ designed so that all the conductive parts have a defined connection to protective earth. A good surge protection concept combats 10kV between L/N and earth and 6kV between L and N. This level of protection is tested to IEC61000-4-5 and will withstand even multiple overvoltage events. It is therefore recommended that, wherever possible, luminaires in protection class I should be used because in accordance with relevant standards high voltages should be compensated only with a protective earth. For historical reasons, 26 | December 2016 26-27 Outdoor Lighting – Tridonic.indd 26 11/11/2016 16:38