CISPR 11 / EN 55011: DC power power ports: What frequency to measure at?

EN IEC 55011:2025 also known as CISPR-11 specifies that the frequency range for DC power ports depends on the maximum specified length of the attached cable. The following information is for Group 1 equipment (see What is ‘Group 1’ and ‘Group 2’ according to EN55011 / CISPR11)

• For a specified length of <3m, no measurement is neccessary at all!
• For a specified length of >= 3m, <30m from f[MHz] = 60/L to 30 MHz
  • For example, for 5m specified length: from 12MHz to 30 MHz
  • For example, for 10m specified length: from 6MHz to 30 MHz
• If the specified length is >=30m (or no maximum length is specified): From 150kHz to 30MHz.
  • For Class A equipment only, there is a special exception: No limits apply if the equipment is installed using good engineering practice regarding EMC (sic!). Good engineering practice is, for example use of shielded cables, grounded metallic cable trays, >= 30m separation distance from residential environments, symmetrical DC port configurations, or installations internal to the building.

Source: EN IEC 55011:2025, Table 6 (rules), Table 5 (Class A limits), Table 3 (Class B limits)