In this context, radioeléctrica refers to the fact that the electromagnetic radiation (see Phil's answer) is at frequencies falling within the range defined by the ITU as being 'radio frequencies' (as distinct from electromagnetic radiation at audio frequencies, for example). The risks associated with radio-frequency electromagnetic radiation depend on the frequency. Most frequencies are harmless to humans – some could cook you alive if the source is powerful enough and you got close enough (think ‘microwave oven’).

All three occurrences of radioeléctrica in the ST extract can be translated as ‘radio-frequency’:

Fijación de límites de exposición o emisión radioeléctrica distintos a los previstos en norma sectorial.
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Establishment of limits for exposure to, or emission of, radio-frequency electromagnetic* radiation that differ from the industry standards.
* electromagnetic can be omitted here, as the context is already well-defined

instalaciones radioeléctricas pertenecientes a redes de comunicaciones electrónicas
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radio-frequency installations forming part of electronic communications networks

instalación de estaciones o infraestructuras radioeléctricas
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the installation of stations or radio-frequency infrastructures