I think we must be careful not to confuse two
subjects:
1) Nominal power line voltages and variations.
2) Power line voltages for safety testing.
For the purposes of safety testing, some test
results may be a function of the power line
input voltage. Therefore, it is appropriate
to adjust the power line voltage to that value
that most adversely affects the parameter in
question.
One approach is to simply test at the most
adverse voltage not exceeding the rated
voltage of the equipment.
But, the rated voltage may not include the
extremes of voltage that may actually be
encountered by the equipment. So, IEC 60960
specifies the extremes of voltage for the
purpose of safety testing, notwithstanding
the equipment rating.
For the purposes of safety, equipment need
not operate over the full range of variation
from nominal voltage. But, it must be safe
over that range (i.e., comply with the
standard).
So, IEC 60950 states these requirements in
Sub-clauses 1.4.4 and 1.4.5. For all nominal
supply voltages except 230 V, the supply
variation is taken as +6% and -10%. For 230
V, the supply variation is taken as +10% and
-10%.
Clearly, the 230 V variation was put in the
standard to address the UK voltage situation.
(Interestingly, IEC 60950 Sub-clause 1.6.5
requires the equipment not only to be safe
but to OPERATE over these voltage variations!)
Best regards,
Rich
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