Pollution Degree 1, 2 and 3: The Number Behind Every Clearance

Pollution degree is one of the two inputs, along with overvoltage category, that determine the clearance required between two conductors. It is frequently misread as a description of the working environment as a whole, which leads designers to claim pollution degree 2 for equipment used in a dusty workshop.

The three degrees that matter

Degree 1 assumes no pollution, or only dry non-conductive pollution, and is normally achieved only inside a sealed enclosure or a potted assembly. Degree 2 assumes only non-conductive pollution, with occasional temporary conductivity caused by condensation. Degree 3 assumes conductive pollution, or dry non-conductive pollution that becomes conductive because condensation is expected. Degree 4, for continuous conductive pollution, is outside the usual scope of the standard.

Where the micro-environment is

The relevant location is the surface between two conductive parts, not the room. An instrument with a sealed case can have pollution degree 2 inside and sit in a degree 3 environment. A PCB with conformal coating can claim a lower degree at coated areas than at bare areas, and the two must be assessed separately.

How it is decided in practice

The manufacturer declares the pollution degree, and the declaration must be defensible from the construction and from the instructions. Declaring degree 2 for an instrument with ventilation openings that admit dust, and no filtration, is the kind of claim that fails when a test house examines the construction rather than the data sheet.

Related technical notes

• Overvoltage Category I to IV: Where the Instrument Sits in the Supply Chain
• Walking the Clearance Table Without Getting Lost
• Creepage Distance and CTI: When the Material Decides the Gap