EU Harmonised Standards are crucial tools for ensuring product safety, facilitating market access, and supporting regulatory compliance within the European Union. Essentially, harmonised standards are like any other standard published by an organisation like CEN or ISO. What makes a normal standard harmonised is that it is published inside the official journal of the European Union and it becomes a legally binding document that bestows legal rights.
In this regard they are a manufacturers best friend. They ensure a predictable playing field which, if you comply with them, give you the presumption of conformity.
The presumption of conformity provides legal assurance that the product meets the essential health, safety, and environmental protection requirements set out in EU legislation on the area that the harmonised standard covers. Essentially it means that there can be no question by authorities that your product does not comply.
Take as an example the FPR requirements introducing a limit for heavy metals in products. This must be tested, and the test is done using a harmonised standard. If the test, using the harmonised standard, shows you comply with the requirements, market surveillance authorities may be allowed to re-do the test at their own cost but unless they conclude a different result cannot question the compliance of the product.
If you use a non-harmonised standard, the authorities would be able to question whether the standard provides an equivalent degree of assurance as the harmonised standard. Because this is in practice difficult to prove it is recommended not to use non-harmonised standards due to the risks to legal liability.
If there are no harmonised standards this concept of equivalence and presumption of conformity does exist.
The link of standards and the EU legal acts ('The EU Acquis') and the impact of this is a much debated topic. It was established by the European courts. Initially in C-613-14 (James Elliot case) which confirmed that a harmonised standard is a legal document and a harmonised standard has a legal impact. This was further refined in C-588/21 confirming that because standards are ' documents they should be public.
At the moment however standards are not freely available, and so they must be purchased. All EU countries have standards bodies, but some offer cheaper rates and so it is worth looking around to see where you can get the standards the cheapest (i.e. https://www.evs.ee/en/).