Optional additional letters (A-D) specify enhanced human access protection, while supplementary letters (H, M, S, W) define critical testing conditions and intended application.

The IP Code allows for the inclusion of two sets of optional letters that modify or add context to the core protection rating.

Additional Letters (A, B, C, D): Protection against access to hazardous parts. These letters are used if the actual protection against access is higher than that indicated by the first characteristic numeral (IPXX). Alternatively, if only access protection is indicated, the first characteristic numeral is replaced by ‘X’.

• A: Protected against access with the back of the hand (50 mm sphere probe).
• B: Protected against access with a finger (jointed test finger).
• C: Protected against access with a tool (2.5 mm rod, 100 mm length).
• D: Protected against access with a wire (1.0 mm wire, 100 mm length). For letters C and D, the probe may penetrate its full length, but the stop face must not pass through the opening. For example, IP23CS shows access protection C (tool) which is higher than the protection offered by numeral 2 (finger).

Supplementary Letters (H, M, S, W): Additional information. These letters clarify the operational state or environment under which the protection level was verified.

• H: High-voltage apparatus.
• M: Tested for harmful effects due to water ingress when movable parts (e.g., rotor) are in motion.
• S: Tested for harmful effects due to water ingress when movable parts are stationary.
• W: Suitable for use under specified weather conditions and provided with additional protective features.

The absence of letters S and M implies that the degree of protection should hold regardless of whether parts are in motion or not, potentially requiring testing under both conditions. If multiple supplementary letters are used, they must be placed in alphabetical sequence.