I tried your requirement. Just changed the option as required & created the equipment in IE01.
Even though I don't open that tab, while saving the equipment, I get the error message as Fill the Required field.
If you want to make for specific equipment category, click on Influencing push button in the top. In the next screen, double click on Equipment category. Then mention your required equipment category, press enter & then make the settings for plant section as "Required".
So that, only for that equipment category, plant section field will be made as mandatory.