A deviation is not automatically forbidden, but it weakens the shortcut that a clean, relevant, OJEU-cited standard normally provides. The EMC guide states that the secure route is to apply the relevant cited standards without deviation while making the EMC assessment; if the manufacturer deviates, the technical documentation should give detailed information on those deviations.
Common deviation cases include skipping tests, changing the standard test method, relying on design precautions or comparison with similar apparatus, applying only emission or only immunity coverage, using an older withdrawn reference, or using non-harmonised EMC specifications because no harmonised standard covers the technology or environment.
For each deviation, document the specific clause or test not followed, the reason, the alternative evidence, the EMC phenomenon affected, the intended environment, the conclusion against Annex I, and the person approving the residual risk. Do not describe a product as covered by a harmonised standard without making clear which parts were applied and which parts were justified separately.