One of the primary uses of electricity
is for general lighting and the local DNO must ensure that its supply
is suitable for this purpose. Repeated sudden changes in voltage of a
few per cent are noticeable and are likely to cause annoyance.
The local DNO must ensure that these
sudden variations are kept within acceptable levels and this means
placing limits on consumers’ apparatus which demands surges of
current large enough to cause lighting to flicker.
In order to evaluate flicker in
measurable terms, two levels have been selected: the threshold of
visibility and the threshold of annoyance.
Both are functions of
frequency of occurrence as well as voltage change.
Since both these thresholds are
subjective it has been necessary to carry out experiments with
various forms of lighting and panels of observers to ascertain
consensus relationships between frequency of occurrence and
percentage voltage change for the two thresholds.
The DNOs have used this information in
setting the planning levels for flicker contained in Engineering
Recommendation P28, which govern motor starting currents, etc.
The network impedance from the source
to the point of common coupling between the lighting and the
offending load is of paramount importance and thus the local office
of the DNO should be consulted in cases where the possibility of
creating an annoyance arises.
Intermittently loaded or frequently
started motors, such as those on lifts, car crushers, etc., together
with instantaneous water heaters, arc welders and furnaces, are all
potential sources of disturbance.
Large electric furnaces present a
particular problem and it is frequently necessary to connect them to
a higher voltage system than is necessary to meet their load in order
to achieve a lower source impedance.
Fluctuations occurring about ten times
a second exhibit the maximum annoyance to most people, but even those
as intermittent as one or two an hour will annoy if the step change
is of sufficient magnitude.
CENELEC Standard EN61000-3-3, limits
voltage fluctuation emissions from equipment rated less than or equal
to 16 A and EN61000-3-11 limits emissions from equipment rated from
16 A to 75A.
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