Underfrequency load shedding (UFLS) is a key system integrity function that will need to be maintained despite potentially more challenging boundary conditions in the future. In this paper, the frequency curve of the Iberian Peninsula grid separation of 2021 has been replicated on a reduced scale physical model with real UFLS protection relays. To assess the prospects for the performance of UFLS schemes in situations with potentially higher stress on the system, two variations have been added to the initial measured scenario: i) a reduced inertia scenario was implemented by replacing synchronous generators by power electronic converters and ii) switching events just before and during the frequency disturbance have been added in a second scenario in order to reproduce a close to reality UFLS relay measurement environment with phase jumps, harmonics etc. In a second step, synthetic frequency events aimed at testing the effects discovered were set up. This includes the effect of longer lines connecting shed loads and the effect of the proximity of rotating generators or power electronic converters. In conclusion, effects on tripping times have been identified experimentally, which will be a valuable information for the design of UFLS schemes in future low inertia grids.