Clearance is the straight-line air gap between conductors, or between a conductor and accessible metal, measured along the shortest path a spark could take. Required values follow from the peak voltages and transients expected, so they are set by impulse withstand voltage and overvoltage category, with correction factors for altitude above 2000 m. Too little clearance means flashover during a surge even when the surface creepage is generous.
Busbar clearance to the door is 25 mm, fine for 690 V but not with the new 3.3 kV feeder in the same section.
In US utility control rooms a clearance is also the formal permission and tagging that makes equipment safe to work on, so context decides whether someone means millimetres or paperwork.
Also heard as
- air clearance
- electrical clearance
Do not mix it up with
- Creepage distance: Creepage is measured along the insulating surface, clearance straight through air.
Related terms
Creepage distance
Shortest route over an insulating surface from one conductive part to another, the dimension that resists tracking.
Overvoltage category
Class from I to IV ranking equipment by its position in an installation and the surges it must survive.
Flashover
Disruptive discharge over or around insulation that creates a short circuit through the air.
Permit to work
Formal written authority to work on equipment that has been isolated, earthed and declared safe.