The present invention provides a material for the construction of external coverings accessible by humans or animals. It aims to increase the breakdown voltage between a high voltage VRF, high frequency source of energy and the human or animal, by adaptation of materials and structures. According to an embodiment of the invention, a structure is provided for the cover 18 used in MRI or NMR systems, which carries a greater proportion Vs of the voltage VRF across itself, thereby reducing the voltage Vg across the gap 22 and reducing the likelihood of breakdown.