From: "Lance Hewitt" 
Newsgroups: sci.engr.electrical.compliance
Subject: PCB Clearances
Date: Tue, 21 Dec 1999 17:11:22 +1030
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I am hoping someone can ease my confusion.

I am laying out the circuit boards for a new piece of domestic audio
equipment (power amplifier with SMPS). Through out the design we have been
using EN60065 to guide us. In this standard is Table II which specifies
minimum creepage and clearances verses voltage. For example 200V needs a bit
over 2mm and 400V needs about 3.5mm (for basic or supplementary insulation).
However Clause 4.3.1 states that these distances may be reduced by a
specified formula if the conductors are part of a PCB which complies with
IEC 249-2 and references figure 13. This now produces distances of: 200V =
0.73mm and 400 = 1.25mm. Clause 4.3.1 then goes on to say " This reduction
in creepage distances is permitted only as far as overheating  is concerned
(see Subclause 11.2)" What does this actually mean?

Does it just mean the modified distances must not be reduced below these
levels when heated (just the same as for those specified in table II), or is
there more to it than this?

I have been reading the book "Self Assessment for Electrical Safety" (don't
worry we will not be self assessing!). In there it talks about creepage and
clearances in EN60950 (IT equipment I think). In this standard they appear
to take into account PCB material and pollution levels. If I use these guide
lines I end up with 200V = 2mm and 400V = 4mm. This is a long way from
0.73mm and 1.25mm calculated above.

EN60065 (1993) does not seem to care about PCB material or pollution in this
context. Will I be safe and legal if I use the reduced distances calculated
from EN60065?

If the answer is yes, does the next part of 4.3.1: "the above reduced values
apply to the conductors themselves, but not to mounted components or
associated soldered connection." mean that the distances specified in table
II, not the calculated reduced distances, apply to the solder pads on a PCB?

If the answer to this is yes does this mean a diode with a reverse voltage
of 1kV in a SOD-87A package can never be used at its rated voltage as the
distance between pads is less than 2.3mm, it is only 3.5mm long and table II
says you need 6.5mm distance for 1kV?

I assume all this is for the top and bottom layers as to my understanding
clearance and creepage do not apply to internal layers. Is it fair to say
the distances between conductors on internal layers is governed by the
insulation resistance and breakdown voltage of the PCB material and if
passes EN60065 Clause 10.3 its OK?

Have I over complicated something simple?
Thanks in advance for anyone's help.
Regards, Lance.