Rationale for Rule 36-308.
Any non-current-carrying metal object, unless grounded to the station ground electrode, may experience a rise in voltage (potential) to ground or to other nearby objects, either because of induction or by voltage gradients created by the passage of high fault currents to ground. Because these objects are not all of the same nature or size, the means by which each should be connected to the station ground electrode will differ. The object, however, remains the same: to maintain each at a safe voltage (potential) relative to all others and to the earth nearby.
Subrules (2)(a) to (2)(f) address these individual needs. The rationale for Subrule (2)(a)(ii) is to prevent unwanted currents from flowing through single- and multi-bay structures when a voltage gradient exists between the bases of two or more columns.
The base of a single-pole isolating switch or fuse cut-out mounted on a nonmetallic structure is usually elevated well beyond the “touch” distance and does not constitute a hazard. Consequently, the grounding of the base may be optional (Subrule (2)(b)(iii)).
Lightning-arrester grounding paths should be of low impedance because of the high frequency characteristic of lightning discharges. Conductors should therefore be as straight and short as possible to keep the inductance to a minimum (Subrule (2)©(ii)). This is explained further in Item (9) of the Note in Appendix B.
Voltage gradients can exist along the path of a water main, which requires periodic bonding to ground along its length to avoid the presence of unwanted currents.
The non-current-carrying parts of metal equipment such as cable sheaths, potheads, raceways, pipe work, screen guards, meters and instruments, and exposed metal work on buildings shall be grounded to the station ground electrode. The purpose of Subrule (2)(e) is to keep the touch voltage to a minimum.
Subrule (2)(f) is to prevent an elevated ground grid voltage from being propagated along the steel rails of railway spur tracks beyond the boundary of the station ground electrode.
The purpose of any buried loop, be it around a station fence or around a building near a station, is to protect people from lethal shocks by keeping step and touch voltages within acceptable limits (Subrule (3)).
In Subrule (4), connecting the transmission line overhead ground wire to the station ground electrode should decrease the fault current injected into the ground by the station during a fault, which should reduce the GPR.
Subrule (6) is needed to ensure that the conductor will carry the available fault current without annealing.
Subrule (7) allows accurate periodic measurements of the station ground electrode resistance to be made.
Intent for Rule 36-308(e. This is a complex Rule with many provisions, each intended to cover a specific aspect of connecting non-current-carrying parts to the station ground electrode. Our intent is be to ensure that every potential hazard in this area has been addressed.
Intent for Rule 36-308(2)(a). Our intent is that each bay, whether by itself or forming a part of a multi-bay structure, be grounded at each column by a conductor of at least No. 2/0 AWG copper.
Intent for Rule 36-308(2)(b). Since a metallic or nonmetallic structure itself cannot be used as a conductor to ground, our intent is that all metallic non-current-carrying parts mounted thereon be grounded, except as provided for in Subrule (2)(b)(iii).
Intent for Rule 36-308(2)©. Because of the high-frequency nature of lightning discharges, our intent is that the grounding conductor be carried by the most direct route to the station ground electrode.
Intent for Rule 36-308(2)(d). Our intent is that a metal water pipe be grounded at intervals of 12 m or less to prevent unwanted currents from flowing through the pipe.
Intent for Rule 36-308(2)(e). The apparatus in these Subrules are allowed to be grounded with smaller conductors than those in the previous Rules, as their susceptibility to excessive transient voltage is lower.
Intent for Rule 36-308(2)(f). Our intent is to prevent a potential rise of ground grid (voltage) from being transferred beyond the boundary of the station ground electrode area by requiring the use of insulated rail joints at that point on any rail spur entering the station.
Intent for Rule 36-308(3). Where it can be shown that step and touch voltages around a building are within acceptable limits without a loop, then the loop is not necessary.
Intent for Rule 36-308(4). We intend that the overhead ground wire entering the station be connected to the station grounding electrode by at least a 2/0 AWG copper conductor, bare or insulated, to protect against dangerous voltages during transient circuit conditions.
Intent for Rule 36-308(5). The grounding conductor on a grounded neutral system should be capable of carrying normal full-load current continuously in case of a discontinuity in the neutral conductor. We intend that the grounding conductor have an ampacity not less than the neutral conductor.
Intent for Rule 36-308(6). We intend that the transformer neutral on a grounded system be connected to the station ground electrode by a grounding conductor not less than 2/0 AWG copper. The grounding conductor shall also have sufficient ampacity to carry the maximum ground fault current of the transformer as specified in Table 51, and be in addition to the requirement of Subparagraph (2)(b)(i).
Intent for Rule 36-308(7). We intend that the conductors in this Subrule be disconnectable from the station ground electrode, to allow the resistance of the station ground electrode to be readily measured when required.
Tony Moscioni
Electrical Inspector
Electrical Safety Authority
