I dont think that an EMI scenario where the RF field is sufficient to engergise or de-energise a 28 volt relay is likely. I have never seen any direct effect to an electromagnetic relay from significant RF fields from 2 - 2000 MHz.
Problems only start when the relay has a semiconductor drive circuit (either integrated in the relay or remote from it). With insufficient or defective decoupling and filtering unwanted operation can be caused down to a few Volts/metre at VHF/UHF frequencies. ie 3 or 4 orders of magnitude less than my experience above.