Shore Power isolation transformers originally featured E-I laminated cores with straight coils. ABYC recommends a metallic shield separating the primary and secondary windings and connected to earth ground via the green wire in the shore cable, and furthermore that the transformer casing must be metallic and bonded to boat ground. – Unfortunately, in a transformer constructed along those lines the risk remains of internal wires working loose due to engine vibration and either inadvertently re-connecting boat ground and earth ground or arcing against the case, creating a fire and chock hazard.