Doug, I accept the reasoning for a clearance when there are mounted components, but can we now consider areas on PCB's where there are NO components. See PCB cross sectional drawings below -
Scenario A
This is a representation of the 2nd paragraph of 2.9.5 lines 1 and 2.
The coating will provide the necessary insulation so the separation
distances can be reduced from those of tables 3, 4, 5 and 6.
Scenario B
This is a representation of the 2nd paragraph of 2.9.5 lines 2 and 3.
Assume the following:
Secondary Circuit
Basic Insulation
Working Voltage - 150 V r.m.s
Transient rating - 1500V
Pollution degree - 2
Material group - IIIa
The clearance distance given by Table 5 for the above is - 1.0mm
The creepage distance given in Table 6 for the above is - 1.6mm
Is it not possible that if the distance between the conductors was
specificed from Table 5 that the creepage distance would be
insufficient, given that a charge is more likely to travel across a
surface (creepage) than through air (clearance)?
Note 2 to Table 6 states "If the creepage distance derived from table 6
is less than the applicable clearance from tables 3 and 4 or from 5, as
appropriate, then the value for that clearance shall be applied as the
value for the minimum creepage distance.
This only happens when the CTI of the material is above 600 and the
voltage is below 200V (for a 1500V transient).
The CTI is not specified in 2.9.5, is it assumed that the CTI will be
above 600 otherwise it won't pass the electric strength tests?
I suspect that line 3 of the second paragraph should read "the distances
in tables 3,4,5 and/or 6 apply".
Regards
Richard Steele
Equipment Engineering Group
Fujitsu Telecommunications Europe Limited
Douglas Mckean wrote:
>
> When analyzing printed circuit boards, BOTH clearance AND creepage
> are issues. The flat pcb fab (no parts installed or soldered) would
> most likely have nothing but creepage issues - distances along
> the surface of the board ("as the crow walks").
>
> An assembled pcb with parts that could compromise creepage
> (parts laid down on the board for example or wires added
> as a mod after assembly or maybe even a piggyback board)
> would have clearance issues as well ("as the crow flies").
>
> Thus, the reason why both clearance and creepage are included
> in all the IEC-950 based standards that I've seen: UL-1950,
> EN-60950, ...
>
> Regards, Doug
>
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