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
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11/11/2016 16:38