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Clearance
and Creepage Distance Measurement
Moe
Lamothe, P.Eng, Lamothe Approvals Inc.
There
are a number of guidelines that must be used when measuring clearances and
creepage distances. The following illustrations are extracted from Annex F
of IEC 60950-1 First Edition
to illustrate the major points.
Value
of X used in illustrations below:
|
Pollution
Degree
|
X
(mm)
|
|
|
0.25
|
|
2
|
1.0
|
|
3
|
1.5
|
Groove
width < X – ignore groove
Groove
width > X – ignore groove for clearances, consider for creepage
distance.
V-shaped
groove – ignore groove for clearance, consider as shown for creepage
distance.
Rib
– follow upper line for clearance, follow contour for creepage distance.
Remember
- Clearance is always measured as the shortest straight-line distance.
Creepage distance is measured over the surface.
Uncemented
joint
– ignored for both creepage distance and clearance measurement. Groove
widths are considered for creepage if wider than X but ignored if they are
less than X. Tape layers used to wrap transformer windings are considered
uncemented.
For
PC boards, the measurement is taken from the edge of the trace or land
and all intermediate traces are subtracted even if they are not used.
Recessed
screw head – the gap is < X and is ignored. The upper line is
used for clearance; the lower line is the creepage distance.
Recessed
screw head – the gap is > X and is considered. Creepage distance
and clearance are the same distance.
Moe
Lamothe is the president of Lamothe Approvals Inc.
(Georgetown, ON, Canada). He can be reached at
info@lamotheapprovals.com.
The
copyright for all of the text, tables and illustrations remains with Lamothe Approvals Inc. Permission is granted to print or reproduce
this document provided that it properly attributed to Lamothe Approvals Inc.
Prepared
May 2003
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