IEC 60071-1-2011 pdf free download – Insulation co-ordination – Part 1: Definitions, principles and rules.
When the adoption of an assumed maximum is considered adequate, the representativeovervoltage of the various classes shall be;
– For the continuous power-frequency voltage: a power-frequency voltage with r.m.s. value
equal to the highest voltage of the system, and with duration corresponding to the lifetimeof the equipment.
For the temporary overvoltage: a standard power-frequency short-duration voltage with anr.m.s. value equal to the assumed maximum of the temporary overvoltages divided by v2 .For the slow-front overvoltage: a standard switching impulse with peak value equal to thepeak value of the assumed maximum of the slow-front overvoltages.
For the fast-front overvoltage: a standard lightning impulse with peak value equal to thepeak value of the assumed maximum of the fast-front overvoltages phase to earth.
NOTE For GlS or GlL with three-phase enclosure and insulation levels chosen among the lowest ones for agiven U, the phase-to-phase overvoltages may need consideration.
For the very-fast-front overvoltage: the characteristics for this class of overvoltage arespecified by the relevant apparatus committees.
For the slow-front phase-to-phase overvoltage: a standard combined switching impulsewith peak value equal to the peak value of the assumed maximum of the slow-front phase-to-phase overvoltages.
For the slow-front [or fast-front] longitudinal overvoltage: a combined voltage consisting ofa standard switching [or lightning] impulse and of a power-frequency voltage,each withpeak value equal to the two relevant assumed maximum peak values,and with the instantof impulse peak coinciding with the peak of the power-frequency of opposite polarity.
5.3Determination of the co-ordination withstand voltages (Uew)
The determination of the co-ordination withstand voltages consists of determining the lowestvalues of the withstand voltages of the insulation meeting the performance criterion whensubjected to the representative overvoltages under service conditions.
The co-ordination withstand voltages of the insulation have the shape of the representativeovervoltages of the relevant class and their values are obtained by multiplying the values ofthe representative overvoltages by a co-ordination factor. The value of the co-ordinationfactor depends on the accuracy of the evaluation of the representative overvoltages and onan empirical,or on a statistical appraisal of the distribution of the overvoltages and of theinsulation characteristics.
The co-ordination withstand voltages can be determined as either conventional assumedwithstand voltages or statistical withstand voltages.This affects the determination procedureand the values of the co-ordination factor.
Simulations of overvoltage events combined with the simultaneous evaluation of the risk offailure，using the relevant insulation characteristics，permit the direct determination of thestatistical co-ordination withstand voltages without the intermediate step of determining therepresentative overvoltages.
5.4Determination of the required withstand voltage (urw)
The determination of the required withstand voltages of the insulation consists of convertingthe co-ordination withstand voltages to appropriate standard test conditions. This isaccomplished by multiplying the co-ordination withstand voltages by factors whichcompensate the differences between the actual in-service conditions of the insulation andthose in the standard withstand voltage tests.
The factors to be applied shall compensate atmospheric conditions by the atmosphericcorrection factor K and the effects listed below by a safety factor K,.
Effects combined in a safety factor K s :
– the differences in the equipment assembly;
– the dispersion in the product quality;
– the quality of installation;
– the ageing of the insulation during the expected lifetime;
– other unknown influences.IEC 60071-1 pdf download.