I had exactly the same situation with 275Vrms / 750Vp in power supply, with F-type applied part. The mains parts could not meet 2 MOPP so the construction was split for 1 MOPP in the power supply and 1 MOPP in the patient isolation.
Fortunately, the patient isolation could meet this. Also noting that since interpolation is allowed, the increase in creepage/clearance is only small for the additional voltage from 250V to 275Vrms (4.0 to 4.4mm).
However, it is possible that the equipment with F-type insulation is designed exactly for 250V (1.5kVrms, 4.0mm). In this case it may not pass this approach.
The next approach could be to treat the complete insulation path (pri-sec + F-type insulation) as a single item of reinforced insulation. IN that case, the creepage is sure to be OK, but for dielectric strength and insulation thickness it can get messy depending on the construction.
Another approach could be to recognize that if pri-sec fail, the working voltage would fall to mains voltage, so the additional MOPP can meet 4.0mm only. But again a test lab might not accept this.
Finally, general RM/equivalence could be used. The combination of 2 MOOP + 1 MOPP is clearly equivalent to 2 MOPP.
Fortunately, the patient isolation could meet this. Also noting that since interpolation is allowed, the increase in creepage/clearance is only small for the additional voltage from 250V to 275Vrms (4.0 to 4.4mm).
However, it is possible that the equipment with F-type insulation is designed exactly for 250V (1.5kVrms, 4.0mm). In this case it may not pass this approach.
The next approach could be to treat the complete insulation path (pri-sec + F-type insulation) as a single item of reinforced insulation. IN that case, the creepage is sure to be OK, but for dielectric strength and insulation thickness it can get messy depending on the construction.
Another approach could be to recognize that if pri-sec fail, the working voltage would fall to mains voltage, so the additional MOPP can meet 4.0mm only. But again a test lab might not accept this.
Finally, general RM/equivalence could be used. The combination of 2 MOOP + 1 MOPP is clearly equivalent to 2 MOPP.