Clearance
Clearance turns up in 3 fields: electrotechnology, nuclear, PCBs. Here is what it means to the people in each one.
Clearance in electrotechnology
Straight-line air gap between two conductive parts, the dimension that decides whether a surge can flash across.
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."
Clearance in nuclear
The release of materials with very low radioactivity from regulatory control, so they can be reused or disposed of normally.
Removal of regulatory control from material whose activity concentration is below set clearance levels, for example around 0.1 Bq per gram for cobalt-60 and caesium-137 in international guidance. Cleared material can be recycled, reused or sent to ordinary landfill. Demonstrating clearance requires careful measurement and documentation, and it is central to decommissioning, where most of the plant's mass is metal and concrete that can be cleared.
"Scaffold poles passed clearance on the bulk monitor, they can go back to the contractor yard."
Clearance in PCBs
Minimum spacing between copper features of different nets, or between copper and the board edge or holes.
Clearance in PCB design is the gap enforced between conductors of different nets, and between copper and drills, board edges or keepouts. Fabricators set a manufacturing minimum, commonly 0.1 to 0.15 mm (4 to 6 mils) on standard work and finer for HDI, while safety rules for mains-connected boards demand millimetres of creepage and clearance rather than mils. Clearance is checked by the design rule check and again by the fabricator's DFM review.
"Default clearance is 6 mil, but make it 8 mm across the isolation barrier between the mains side and the low-voltage side."