All exposed, noncurrent-carrying metal parts of a unit, when connected to the lot service equipment, shall be grounded by means of a grounding conductor run with the circuit conductors or in a listed power supply cord provided with an approved polarized multi-prong plug. One prong of the plug shall be for the sole purpose of connecting that grounding conductor, by means of a listed and approved grounding receptacle, to the grounded terminal at the lot service. The conductor shall be insulated and identified by a green color.