The detailed
procedure for insulation co-ordination set out in IEC 60071-1 (European
standard EN60071 1 is identical) consists of the selection of a set of standard
withstand voltages which characterize the insulation of the equipment of the
system. This set of withstands correspond to each of the different stresses to
which the system may be subject:
• Continuous
power frequency voltage (the highest voltage of the system for the life of the
system).
• Slow-front
overvoltage (a standard switching impulse).
• Fast-front
overvoltage (a standard lightning impulse).
•
Very-fast-front overvoltage (depends on the characteristics of the connected apparatus).
•
Longitudinal overvoltage (a voltage between terminals combining a power frequency
voltage at one end with a switching (or lightning) impulse at the other).
These
voltages and overvoltages need to be determined in amplitude, shape and
duration by system study. For each class of overvoltage, the analysis then determines
a ‘representative overvoltage’, taking account of the characteristics of the
insulation.
The
representative overvoltage may be characterized by one of:
• an assumed
maximum,
• a set of
peak values,
• a complete
statistical distribution of peak values.
The next
step is the determination of ‘co-ordination withstand’ voltages – the lowest
values of the withstand voltages of the insulation in use which meet the system
or equipment performance criteria when subjected to the ‘representative overvoltages’
under service conditions. Factors are then applied to compensate for:
• the
differences in equipment assembly,
• the
dispersion of the quality of the products within the system,
• the
quality of installation,
• the ageing
of installation during its lifetime,
•
atmospheric conditions,
•
contingency for other factors.
This results
in so-called ‘required withstand voltages’ – test voltages that must be
withstood in a standard withstand test.
In specifying equipment the next step
is then to specify a standard test withstand voltage (a set of specific test voltages
is provided in IEC 60071-1) which is the next above the required withstand
voltage, assuming the same shape of test voltage.
A test conversion factor must be applied to
the required withstand voltage if the test voltage is of a different shape to
the class of overvoltage in question.
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