A thermal well (10, 10', 10") for sensing temperature conditions in transformer structures or other environments involving high voltage potentials and/or explosive ambient conditions while reducing the risk of arcing or corona-based sensor failure or degradation, wherein the thermal well is adapted to be positioned between high potential transformer coils or windings, the thermal well comprising: a continuous low relative dielectric constant structure with an internal cavity (26, 26') extending along a longitudinal direction from a first end to an opening at a second end, the internal cavity including a first region proximate the first end, and a second region (28, 28') extending longitudinally outward from the first region to the opening, wherein the low relative dielectric constant structure comprises Teflon; a thermal sensor (14, 14') at least partially located within the internal cavity (26, 26') and having one or more wires extending out of the opening at the second end of the internal cavity (26, 26'); a first low relative dielectric constant potting material encapsulating at least a portion of the thermal sensor (14, 14') within the at least a portion of the first region of the internal cavity (26, 26'); and a second low relative dielectric constant potting material filling at least a portion of the second region (28, 28') of the internal cavity (26, 26'); characterized in that the low relative dielectric constant structure comprises a substantially T-shaped configuration to provide an abutment surface allowing the thermal well to be installed with a first portion of the low relative dielectric constant structure being disposed between adjacent windings of a transformer coil, when installed in the transformer coil, and a second portion of the low relative dielectric constant structure being above the transformer coil with the abutment surface resting on an upper end of the transformer coil, when installed in the transformer coil, wherein the low relative dielectric constant structure includes: the first portion having a first outer dimension; the second portion proximate the first portion, the second portion including the at least one abutment surface extending outwardly therefrom and having a second outer dimension that is greater than the first outer dimension, wherein the internal cavity extends through the second portion into the first portion to a suitable depth to facilitate repeatable location of the thermal sensor relative to the transformer coils. a continuous low relative dielectric constant structure with an internal cavity (26, 26') extending along a longitudinal direction from a first end to an opening at a second end, the internal cavity including a first region proximate the first end, and a second region (28, 28') extending longitudinally outward from the first region to the opening, wherein the low relative dielectric constant structure comprises Teflon; a thermal sensor (14, 14') at least partially located within the internal cavity (26, 26') and having one or more wires extending out of the opening at the second end of the internal cavity (26, 26'); a first low relative dielectric constant potting material encapsulating at least a portion of the thermal sensor (14, 14') within the at least a portion of the first region of the internal cavity (26, 26'); and a second low relative dielectric constant potting material filling at least a portion of the second region (28, 28') of the internal cavity (26, 26'); characterized in that the low relative dielectric constant structure comprises a substantially T-shaped configuration to provide an abutment surface allowing the thermal well to be installed with a first portion of the low relative dielectric constant structure being disposed between adjacent windings of a transformer coil, when installed in the transformer coil, and a second portion of the low relative dielectric constant structure being above the transformer coil with the abutment surface resting on an upper end of the transformer coil, when installed in the transformer coil, wherein the low relative dielectric constant structure includes: the first portion having a first outer dimension; the second portion proximate the first portion, the second portion including the at least one abutment surface extending outwardly therefrom and having a second outer dimension that is greater than the first outer dimension, wherein the internal cavity extends through the second portion into the first portion to a suitable depth to facilitate repeatable location of the thermal sensor relative to the transformer coils. the first portion having a first outer dimension; the second portion proximate the first portion, the second portion including the at least one abutment surface extending outwardly therefrom and having a second outer dimension that is greater than the first outer dimension, wherein the internal cavity extends through the second portion into the first portion to a suitable depth to facilitate repeatable location of the thermal sensor relative to the transformer coils.