DSH 727 Creepage and clearance
http://decisions.iecee.org/icons/ecblank.gif DSH 727
Creepage and clearance
29.1.3
60335-1(ed.4);am1;am2
Question:
Clause 29.1.3, reads: "29.1.3 Clearances of reinforced insulation shall be not less than those specified for basic insulation in table 16, but using the next higher step for rated impulse voltage as a reference."
The standard is not very precise about how to apply this clause in the situation for appliances having higher working voltages than rated voltage, for which the clearance are determined by interpolation, as described in clause in clause 29.1.5.
Examples:
1) Appliances with working voltage = rated voltage = 120V, overvoltage category II. For basic insulation, the rated Impulse voltage for 120V appliance = 1500 V. For reinforced insulation, the rated impulse voltage to take into account is 2500V (category above 1500V).
2) Appliance with working voltage 300V AC, rated voltage 120V AC, overvoltage category II. For basic insulation, Impulse voltage determined following 29.1.5 =1754 V.
For reinforced insulation, what is the correct impulse voltage ?:
a) 2500 V ?
b) 4000 V ?
c) Value given by application of IEC60664-1:2007, clause 5.1.6: 160% * 1754 = 2800V ?
d) Others ?
Extract of clause 5.1.6 of IEC60664-1:2007:
“With respect to impulse voltages, clearances of reinforced insulation shall be dimensioned as specified in Table F.2 corresponding to the rated impulse voltage but one step higher in the preferred series of values in 4.2.3 than that specified for basic insulation. If the impulse withstand voltage required for basic insulation according to 4.3.3.4.2(parts or circuits within equipment specifically protected against transient over voltages) , is other than a value taken from the preferred series, reinforced insulation shall be dimensioned to withstand 160% of the impulse withstand voltage required for basic insulation.
Decision:
(1) If the clearances applied for basic insulation are selected from table F.7 of IEC 60664-1, the clearances of reinforced insulation shall be dimensioned as specified in Table F.7 to withstand 160 % of the withstand voltage required for basic insulation.
(2) If the clearances applied for basic insulation are selected from clause 4 of IEC 60664-4, the clearances of reinforced insulation shall be twice the value required for basic insulation.
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