The first characteristic numeral (1 through 4) always implies the satisfaction of two conditions simultaneously: protection of persons against access to hazardous parts, and protection of the equipment against the ingress of solid foreign objects. This dual role ensures both operational safety and physical integrity against external contamination.

The definitions for protection against access rely on access probes:

• 1 (Back of hand): Requires a 50 mm sphere probe to maintain adequate clearance from hazardous parts and not fully penetrate the opening.
• 2 (Finger): Requires a jointed test finger (12 mm dia., 80 mm length) to maintain adequate clearance. The finger may penetrate up to its 80 mm length, but the stop face (50 mm x 20 mm) shall not pass through the opening.
• 3 (Tool): Requires a 2.5 mm probe to maintain adequate clearance and shall not penetrate.
• 4 (Wire): Requires a 1.0 mm probe to maintain adequate clearance and shall not penetrate.

The protection against solid foreign objects uses similar size metrics:

• 1 (≥ 50 mm diameter): The 50 mm sphere probe shall not fully penetrate.
• 2 (≥ 12.5 mm diameter): The 12.5 mm sphere probe shall not fully penetrate.
• 3 (≥ 2.5 mm diameter): The 2.5 mm probe shall not penetrate at all.
• 4 (≥ 1.0 mm diameter): The 1.0 mm probe shall not penetrate at all.

For numerals 1 and 2, “not fully penetrate” means the full diameter of the sphere shall not pass through an opening. The protection requirement for numerals 3 and 4, which involves small rods, shifts to absolute exclusion of the probe. Achieving a stated degree of protection implies compliance with all lower degrees of protection, although lower tests need not be performed if compliance is obvious.