Isolation domains and barriers

A net class has one clearance, held against every other net. Net class voltages cover most boards; write domains and barriers by hand where you need more, such as reinforced isolation the classes cannot express or a group of nets by name. Where nets must keep more distance from one group than from their neighbours (a mains primary from the secondary, a floating rail from the rest of the secondary), put the nets in domains and set a barrier between two domains:

[[domains]]
name = "primary"
classes = ["Mains", "HvBus"]       # every net of these classes
nets = ["PGND", "LLC_*"]           # and these nets, globs allowed

[[domains]]
name = "secondary"
classes = ["Out24V"]
nets = ["GND_SEC", "5VSB"]

[[barriers]]
between = ["primary", "secondary"]
clearance = "4mm"                  # through air, on every copper layer
creepage = "6.4mm"                 # along the board surface, on F.Cu and B.Cu
pollution_degree = 2               # 1, 2 or 3, default 2

A net sits in one domain at most. Inside a domain, and for nets in none, the class clearances apply as before. Between two domains with a barrier, copper on one layer keeps the barrier's clearance (isolation-clearance), pads of one footprint and pours included. Everything that adds copper keeps the barrier from the other domain's copper: pours, agentee route and the layout engine, tie, testpoints, tune, neck, fanouts and stitching vias, from the same rule the check uses. On F.Cu and B.Cu they keep the larger of clearance and creepage, since none of them measures paths around slots.

Creepage is the shortest path between the two coppers along the outer surface they sit on (creepage). It stays on the board and goes around board cutouts and non-plated holes. A slot or hole narrower than the groove width X of the pollution degree (0.25 mm for 1, 1.0 mm for 2, 1.5 mm for 3, as IEC 60664-1 gives it) is bridged and measured straight across. Between copper on F.Cu and copper on B.Cu the path runs down the wall of a board cutout or non-plated hole of any width, or round the board edge, and counts the board's thickness. Solder mask does not count as insulation.

The clearance of a class whose nets sit in a domain is taken as an electrical spacing, so it also holds between pads of one footprint: a TO-220 on a 1.5 mm HvBus class with pads 1.0 mm apart is a clearance error. Pads of one footprint whose nets are in no domain are held to min_clearance alone, since the part's pitch sets their gap.