Indoor substations supply the electricity on which wastewater treatment plants, pumping stations, chemical facilities and the cooling systems of power plants depend, so their failure in an earthquake can cascade into pollutant releases and loss of containment far beyond the substation itself. The weak links are the transformers and gas-insulated switchgear (GIS), which stand on the floors of stiff reinforced concrete (RC) frames that amplify the ground motion. Lead rubber bearing (LRB) isolation is an obvious remedy, but its benefit is usually reported as a single reduction obtained under far-field records, and little is known about how much of it survives near-fault, pulse-type shaking. This study evaluates LRB isolation of a three-storey indoor substation frame carrying a transformer and GIS. A three-dimensional nonlinear time-history model and ABAQUS continuum models of a published 1/4-scale shaking-table specimen were validated against the measured frequencies and peak responses and subjected to the 1940 El Centro (far-field) and 1999 Chi-Chi TCU052 (near-fault) records at peak ground accelerations of 0.07–0.62 g. At the rare-earthquake level the bearings reduced the base shear by 79 % and 34 %, the maximum inter-storey drift by 81 % and 65 % and the transformer acceleration by 65 % and 53 % under the far-field and near-fault records, respectively. The equipment amplification factor fell from 2.7 to below 1.0 and the frame remained elastic, but the near-fault bearing displacement (69 mm) exceeded the 55 mm limit. A spectral filtering index (SFI), the ratio of the spectral acceleration at the effective period and damping of the isolated system to that at the fixed-base period, explained the difference: across 28 isolated analyses the base-shear ratio followed 0.17 + 1.03 SFI (R² = 0.87), and isolation stopped reducing the base shear once the SFI exceeded about 0.8. The SFI therefore indicates, at the early design stage, whether an isolation system will protect the equipment whose failure could trigger a secondary environmental disaster, and it flags the case in which bearings stiffer than specified make the substation respond worse than a conventional one.