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Old September 19th, 2018, 10:13 AM   #1
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Nearbyatom is offline
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UL508A minimum spacing

In the UL book table 10.2 it states that a minimum spacing of 1" needs to be maintained for voltages 251-600V if "through air", and a min spacing of 2" if "over surface" at the same voltages.

What does "through air" and "over surface" mean?
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Old September 19th, 2018, 01:34 PM   #2
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jraef is offline
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Through air means if you look at the two lugs on a device and look for the smallest air space between them, it must measure 1" or more.

Over surface means the distance is measured as if you were an ant crawling on the surface in as short of a path as possible along a common (non-conductive) surface between terminals, often referred to as "down, over and up". So if for example you have a circuit breaker where the lugs are less than 1" air gap apart, you can add a barrier between them that has a total surface distance from one to the other of 2 inches or more.

Case in point: If you removed those black barriers, the distance between lugs would be less than 1" but with them in place, it is more than 2" across the surfaces.
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