Creepage & Clearance: How IEC 60664-1 Sets Insulation Distances

What IEC 60664-1 is

IEC 60664-1 is the base standard for insulation coordination for equipment within low-voltage systems. In practice, it's the standard engineers reach for to answer a very concrete question: how far apart do two conductors at different potentials need to be so the insulation between them doesn't break down? It answers that with two separate distances — clearance and creepage — and a small set of environmental inputs that scale them.

Creepage vs clearance — two different failure modes

These get used interchangeably in conversation, but they're physically distinct:

• Clearance is the shortest distance through the air between two conductors. It guards against the air itself ionising and arcing over — a fast, voltage-driven breakdown.
• Creepage is the shortest distance along the surface of the solid insulation between them. It guards against a conductive track slowly forming across a contaminated surface (tracking) — a slow, contamination-driven failure.

Because they fail for different reasons, they're driven by different inputs. Clearance cares about voltage and air density; creepage cares about voltage, how dirty the surface gets, and how tracking-resistant the insulating material is.

The inputs that set the numbers

• Working voltage — the RMS or DC voltage actually across the insulation. Both clearance and creepage scale with it.
• Pollution degree (PD1–PD4) — how much conductive contamination the surface sees. A sealed module is PD1–PD2; an open industrial environment is PD3–PD4. Higher pollution demands more creepage, and the standard imposes minimum floors (PD2 ≥ 0.2 mm, PD3 ≥ 0.8 mm, PD4 ≥ 1.6 mm) because a tiny gap can be bridged entirely by a particle or a droplet regardless of voltage.
• Material group (CTI) — the Comparative Tracking Index of the insulator, in four groups (I, II, IIIa, IIIb). A higher CTI material resists surface tracking, so it needs less creepage for the same conditions.
• Altitude — thinner air at altitude breaks down more easily, so clearance is corrected upward with height (creepage, a surface phenomenon, is not).

Where the common simplifications bite

Most calculators (including this one, by design) drive clearance directly from the working voltage, altitude-corrected. What that skips is stepping the working voltage up through an overvoltage category to a rated impulse withstand voltage (IEC 60664-1 Table F.1) first. That step matters for circuits exposed to significant transient overvoltages — most obviously anything connected directly to the mains — and skipping it will understate the required clearance for those cases. If your circuit sees real transients, add your own margin.

Two more things worth knowing: the standard's distance tables approximate a power law, not a straight line or a step, so interpolating between tabulated voltage points (rather than jumping to the next-higher band) is both legitimate and less over-conservative. And the base numbers assume functional insulation — basic, supplementary, and reinforced insulation are handled separately and need their own treatment.