IEEE Std 1653.6-2018 pdf download – IEEE Recommended Practice for Grounding of DC Equipment Enclosures in Traction Power Distribution Facilities.
5.2 NEC requirements In accordance with NEC Article 90.2(B)(1), installation for railways for generation, transformation, transmission, and distribution of power used exclusively for the operation of rolling stock are not covered by the NEC® [B4]. 5 5.3 Solid grounding Solidly grounded dc enclosures are connected to earth ground through the bonding of neighboring equipment, building structure, and grounded electrodes with no intentional impedances installed between the enclosure and ground. The operation of solidly grounded equipment relies on the presence of very low-impedance connections between equipment enclosures and ground in order to minimize voltage potentials during normal operation and during fault conditions. Due to the low operating voltage and correspondingly high operating current, particular attention must be paid to providing an adequate earthing system and bonding conductor size. Insufficiently sized conductors may simply fuse (open circuit) and allow the affected equipment enclosure to elevate to the system voltage. An earthing system that exhibits excessive resistance may raise an entire building to the system voltage. If the fault is remote from the supply, as is often the case in transit systems, conductor resistance may limit currents below feeder breaker trip settings and the fault may continue indefinitely. An enclosure fault to railway negative may go almost totally unnoticed. Over time, the fault may cause earthing electrodes to be rendered ineffective by stray current corrosion. A negative fault may also cause enough current to flow to ground such that equipment bonds melt unnoticed and inadvertently isolate the equipment from safety grounds. As a result, it is generally not recommended to solidly ground positive polarity dc equipment enclosures due to the high-energy nature of dc faults and the fact that some ground faults can be low in magnitude and not detectable by overcurrent protective devices.