Understanding Electrical Clearances

If you’re looking for a more practical use of electrical clearances read the Laying Tracks article. It should clear that air on how to apply the proper clearances on the majority of traces used during a printed circuit board design. However, if you’re looking for some tips on how to deal with some high voltage traces then read on.

Electrical Clearance:
The electrical clearance should be calculated as the shortest distance between two conductive tracks or pads measured through air. Clearance is needed between high voltage traces to prevent a dielectric breakdown between traces caused by the air ionizing. If humidity, altitude, and temperature are environments your board design may be subjected to pay close attention. The dielectric breakdown level is heavily influenced by these three factors.

In the event your designing a power supply and you’ve got 240V mains you’ll want to consult the relevant standards to ensure you meet the regulator guidelines. As a general rule you’ll want to keep a minimum of 315mil between the tracks. This includes the high voltage mains as well as any isolated signal tracks.

Creepage Distance:
Creepage is similar to electrical clearance however is calculated as the shortest path between two conductive tracks/pads measures along the insulation as opposed to through the air. Creepage distance is important to protect against tracking which produces a partially conducting path of of localized deterioration on the surface of an insulating material as a result of the electric discharges close to an insulating surface. No general rules of thumb are provided however you may reference IEC112 for a fuller explanation of tracking.