Creepage and clearance are the two distances that keep high voltage where it belongs on a PCB. Clearance is the shortest distance through air between two conductors; creepage is the shortest distance along the surface of the board between them. Both must be large enough that voltage cannot arc across or track along the surface, and the design rule check (DRC) in Eagle is where you teach the software to enforce those distances automatically. Setting it up correctly is the difference between a board that is safe by design and one that is safe by luck.
Why both distances matter
Clearance protects against arcing through air: a voltage high enough will jump a gap, and the gap must be wider than the jump distance with margin. Creepage protects against tracking along the surface: contamination, moisture, and the board material itself provide a path that is easier for current to follow than air, so the surface distance must be longer than the air distance for the same voltage. Standards assign required values based on the working voltage, the pollution degree of the environment, and the material group of the PCB laminate.
Low-voltage digital boards can mostly ignore these rules, since the default DRC clearances exceed what a few volts need. The rules become critical the moment mains voltage, motor drivers, relays switching line voltage, or any high-voltage section shares the board with low-voltage circuitry. That is exactly when the DRC setup matters most, because the layout is complex and the eye cannot reliably judge every gap.
Determining the required distances
The required creepage and clearance come from safety standards, not from the CAD tool. Identify which standard applies to the product: the general approach is to find the working voltage (the highest voltage that appears between the conductors in normal operation, including transients), the pollution degree (how dirty the operating environment is), and the material group, then read the required distances from the standard's tables. Mains-powered equipment in a normal indoor environment is the common case, and the tables give specific millimeter values for it.
Be conservative in interpreting the working voltage. Use the peak voltage including transients, not the nominal RMS, and consider fault conditions where a higher voltage might appear. The DRC enforces whatever numbers you enter; entering the wrong numbers produces a board that passes DRC and still fails safety.
Configuring Eagle's DRC
Eagle's DRC dialog sets clearances globally and per net class. The practical approach is to define net classes for the voltage domains on the board: a mains class, a low-voltage class, and so on, then assign each net to its class. In the DRC clearance settings, specify the required conductor-to-conductor distance for each class pair, with the largest required distance governing any pair that includes a high-voltage net.
Eagle checks clearance (through-air distance in the layout) directly. Creepage, the along-surface distance, is harder for the tool to compute exactly, since it must follow the board surface around slots and barriers. The standard workaround is to enforce the creepage distance as a clearance rule as well, which is conservative: anywhere the layout meets the longer creepage requirement, it automatically meets the shorter clearance one. Where board space is tight, physical slots or milled barriers can shorten the required creepage path, but the DRC must then be told about them, usually by keeping copper out of the barrier zone so the tool measures around it.
Layout techniques that satisfy the rules
Design for the DRC rather than fighting it. Keep high-voltage sections compact and grouped, so the large clearances apply to a small region instead of sprawling across the board. Route a clear isolation boundary between voltage domains, a visible moat in the layout that no trace crosses except where intended. Where a signal must cross the boundary, use an isolation component (optocoupler, isolated converter) and give its package the full creepage distance on the board.
Watch the details the DRC might miss: component pads and silkscreen do not conduct, but a copper pour that sneaks under an isolation component can defeat the barrier. Review the finished layout visually around every isolation boundary, because the DRC checks what you told it to check, and a pour or a via you forgot to constrain is still a violation.
Documenting the safety case
Record which standard, which voltage, and which distances the DRC was set to enforce, and keep that record with the project. A board that passes DRC is only as safe as the numbers behind the rules, and the next person to revise the board needs to know what those numbers were. Safety by design is only as durable as its documentation.
Determine the real required distances, encode them as net-class DRC rules, lay out with isolation boundaries, and document the basis. The DRC is a tireless checker; give it the right rules and it will hold the line on every revision.
Related Guides
- DRC Rules for High-Density Signal Escapes: High-density designs, fine-pitch parts, BGAs, dense connectors, live or die at the escape: the short, brutal routing problem of getting dozens or
- Teardrop Pad Reinforcement in Eagle Layouts: A teardrop is a small fillet of copper where a trace meets a pad, flaring the trace out to meet the pad's edge instead of joining it at a sharp neck
- Via Tent Spacing Clearances in Complex Schematics: Via tenting, covering vias with solder mask so the annular ring and hole are sealed, protects against solder bridging, contamination, and accidental
- Net Class Spacing Rules for High Voltage Traces: High voltage on a PCB demands physical distance: traces at different potentials must be spaced far enough apart that the voltage cannot arc across