Required spacing through air and along the surface, from working voltage, pollution degree, material group and overvoltage category. The two are different failure mechanisms with different rules — which is why altitude derates one and not the other.
Runs entirely in your browser. Nothing is uploaded, stored, or sent anywhere.
Clearance is the shortest distance through air. It protects against a transient flashover, so it is governed by the impulse withstand voltage — which comes from the overvoltage category of the installation, not from the voltage you are running. A 230 V circuit in an OVC III fixed installation needs more clearance than the same 230 V circuit behind a transformer in OVC I.
Creepage is the shortest distance along a surface. It protects against slow tracking: contamination and humidity build a conductive film, carbon paths form, and the insulation degrades over years. It is governed by working voltage, by how dirty the environment gets, and by how well the material resists tracking — the CTI, comparative tracking index.
Because they are different mechanisms, they derate differently. Altitude reduces clearance but not creepage: thinner air flashes over more easily, while surfaces do not care how high up they are. That asymmetry catches people designing for avionics or high-altitude installations.
Standard FR-4 usually falls in material group IIIa — CTI between 175 and 399. That is adequate but not good, and it costs you creepage compared with a high-CTI laminate. If a design is marginal, changing laminate is sometimes cheaper than finding the board area.
The other standard remedy is a slot milled through the board between the two nodes. It removes the surface path entirely, so creepage is then measured around the slot — and if the slot is wide enough, clearance through air becomes the governing figure instead. This is why isolated power supplies have those characteristic slots under the transformer and optocoupler.
PD2 assumes only non-conductive pollution with occasional condensation — ordinary indoor equipment. PD3 assumes conductive pollution and carries a substantial penalty in required spacing. Getting a design classified as PD2 rather than PD3, by sealing the enclosure or applying a qualified conformal coating, frequently costs less than the board area the PD3 spacing would consume.
Spacing is a schematic-and-layout decision that is very expensive to fix late, because it drives component placement, board size and sometimes the enclosure. It belongs in the same early conversation as the safety analysis rather than being discovered at layout review.
Each of these runs entirely in your browser. Nothing is uploaded, stored or sent anywhere, and none of them needs an email address.
See all 49 engineering tools →
We add tools here fairly often and write up the things worth writing up — a stackup that behaved oddly, a standard that turned out to be obsolete, a calculator that was quietly wrong. Join and you get told when something new lands.
One email a month at the very most, and usually less. No drip sequence, no sales cadence, no sharing your address with anyone. Unsubscribe whenever you like.