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art IX. Instruments, Meters, and Relays 250.170 Instrument Transformer Circuits. Secondary circuits of current and potential instrument transformers shall be grounded if the primary windings are connected to circuits of 300 volts or more to ground and, if installed on or in switchgear and on switchb...

art IX. Instruments, Meters, and Relays 250.170 Instrument Transformer Circuits. Secondary circuits of current and potential instrument transformers shall be grounded if the primary windings are connected to circuits of 300 volts or more to ground and, if installed on or in switchgear and on switchboards, shall be grounded irrespective of voltage. Exception No. 1: Circuits where the primary windings are connected to circuits of 1000 volts or less with no live parts or wiring exposed or accessible to other than qualified persons. Exception No. 2: Current transformer secondaries connected in a three-phase delta configuration shall not be required to be grounded. 250.172 Instrument Transformer Cases. Cases or frames of instrument transformers shall be connected to the equipment grounding conductor if accessible to other than qualified persons. Exception: Cases or frames of current transformers, the primaries of which are not over 150 volts to ground and that are used exclusively to supply current to meters. 250.174 Cases of Instruments, Meters, and Relays Operating at 1000 Volts or Less. Instruments, meters, and relays operating with windings or working parts at 1000 volts or less shall be connected to the equipment grounding conductor as specified in 250.174(A), (B), or (C). (A) Not on Switchgear or Switchboards. Instruments, meters, and relays not located on switchgear or switchboards operating with windings or working parts at 300 volts or more to ground, and accessible to other than qualified persons, shall have the cases and other exposed metal parts connected to the equipment grounding conductor. (B) On Switchgear or Dead-Front Switchboards. Instruments, meters, and relays (whether operated from current and potential transformers or connected directly in the circuit) on switchgear or switchboards having no live parts on the front of the panels shall have the cases connected to the equipment grounding conductor. (C) On Live-Front Switchboards. Instruments, meters, and relays (whether operated from current and potential transformers or connected directly in the circuit) on switchboards having exposed live parts on the front of panels shall not have their cases connected to the equipment grounding conductor. Mats of insulating rubber or other approved means of floor insulation shall be provided for the operator where the voltage to ground exceeds 150 volts. 250.176 Cases of Instruments, Meters, and Relays - Operating at Over 1000 Volts. If instruments, meters, and relays have current-carrying parts of over 1000 volts to ground, they shall be isolated by elevation or protected by a barrier(s), grounded metal, or insulating covers or guards. Their cases shall not be connected to the equipment grounding conductor. Exception: Cases of electrostatic ground detectors shall be permitted to be connected to an equipment grounding conductor if the internal ground segments of the instrument are connected to the instrument case and grounded and the ground detector is isolated by elevation. 250.178 Instrument Equipment Grounding Conductor. The equipment grounding conductor for secondary circuits of instrument transformers and for instrument cases shall not be smaller than 12 AWG copper or 10 AWG aluminum or copper-clad aluminum. Cases of instrument transformers, instruments, meters, and relays that are mounted directly on grounded metal surfaces of enclosures or grounded metal of switchgear or switchboard panels shall not be required to be connected to an additional equipment grounding conductor. Part X. Grounding of Systems and Circuits of over 1000 Volts 250.180 General. If systems over 1000 volts are grounded, they shall comply with all applicable requirements of 250.1 through 250.178 and with 250.182 through 250.194, which supplement and modify the preceding sections. ENHANCED CONTENT Collapse As a general rule, Table 250.66, for sizing GECs, and Table 250.122, for sizing EGCs, apply to systems operating over 1 kilovolt. There are, however, special requirements in Part X for EGCs for electrical systems utilizing shielded solid dielectric insulated cables rated 2001 to 35,000 volts. Those electrical systems are commonly referred to as medium- and high-voltage systems. 250.182 Derived Neutral Systems. A system neutral point derived from a grounding transformer shall be permitted to be used for grounding systems over 1 kV. 250.184 Solidly Grounded Neutral Systems. Solidly grounded neutral systems shall be permitted to be either single point grounded or multigrounded neutral. (A) Neutral Conductor. (1) Insulation Level. The minimum insulation level for neutral conductors of solidly grounded systems shall be 600 volts. Exception No. 1: For multigrounded neutral systems as permitted in 250.184(C), bare copper conductors shall be permitted to be used for the neutral conductor of the following: * (1) Service-entrance conductors * (2) Service laterals or underground service conductors * (3) Direct-buried portions of feeders Exception No. 2: Bare conductors shall be permitted for the neutral conductor of overhead portions installed outdoors. Exception No. 3: The grounded neutral conductor shall be permitted to be a bare conductor if isolated from phase conductors and protected from physical damage. Informational Note: See 225.4 for conductor covering where within 3.0 m (10 ft) of any building or other structure. (2) Ampacity. The neutral conductor shall have an ampacity that is not less than the load imposed and be not less than 331/3 percent of the ampacity of the phase conductors. Exception: In industrial and commercial premises under engineering supervision, it shall be permissible to size the ampacity of the neutral conductor to not less than 20 percent of the ampacity of the phase conductor. (B) Single-Point Grounded Neutral System. If a single-point grounded neutral system is used, the following shall apply: * (1) A single-point grounded neutral system shall be permitted to be supplied from one of the following: o a. A separately derived system o b. A multigrounded neutral system with an equipment grounding conductor connected to the multigrounded neutral conductor at the source of the single-point grounded neutral system * (2) A grounding electrode shall be provided for the system. * (3) A grounding electrode conductor shall connect the grounding electrode to the system neutral conductor. * (4) A bonding jumper shall connect the equipment grounding conductor to the grounding electrode conductor. * (5) An equipment grounding conductor shall be provided to each building, structure, and equipment enclosure. * (6) A neutral conductor shall only be required if phase-to-neutral loads are supplied. * (7) The neutral conductor, if provided, shall be insulated and isolated from earth except at one location. * (8) An equipment grounding conductor shall be run with the phase conductors and shall comply with all of the following: o a. Shall not carry continuous load o b. Shall be bare, covered, or insulated o c. Shall have ampacity for fault current duty (C) Multigrounded Neutral Systems. If a multigrounded neutral system is used, the following shall apply: * (1) The neutral conductor of a solidly grounded neutral system shall be permitted to be grounded at more than one point. Grounding shall be permitted at one or more of the following locations: o a. Transformers supplying conductors to a building or other structure o b. Underground circuits if the neutral conductor is exposed o c. Overhead circuits installed outdoors * (2) The multigrounded neutral conductor shall be grounded at each transformer and at other additional locations by connection to a grounding electrode. * (3) At least one grounding electrode shall be installed and connected to the multigrounded neutral conductor every 400 m (1300 ft). * (4) The maximum distance between any two adjacent electrodes shall not be more than 400 m (1300 ft). * (5) In a multigrounded shielded cable system, the shielding shall be grounded at each cable joint that is exposed to personnel contact. Exception: In a multipoint grounded system, a grounding electrode shall not be required to bond the neutral conductor in an uninterrupted conductor exceeding 400 m (1300 ft) if the only purpose for removing the cable jacket is for bonding the neutral conductor to a grounding electrode. ENHANCED CONTENT Collapse Large-scale photovoltaic electric supply systems (solar farms) and wind electric systems (wind farms) of the types covered in Articles 691 and 694 encompass large tracts of land, and long conductor runs are the norm. Connecting neutral conductors to a grounding electrode at specific intervals is impractical, but, more importantly, it introduces potential operational problems. Recent changes to ANSI/IEEE C2, National Electrical Safety Code(r) (NESC(r)), permit intervals greater than 1300 feet between grounding electrode connections to minimize the number of locations where the cable jacket is removed. Multiple points of grounding necessitate removing the cable jacket to make the connection, which compromises the overall cable integrity and increases the possibility of premature cable failure. Because the cables involved in these installations are typically shielded, the shield is also included in the grounding connection, which has the potential of exposing the shield to undesirable EMI. 250.186 Grounding Service-Supplied Alternating-Current Systems. (A) Systems with a Grounded Conductor at the Service Point. If an ac system is grounded at any point and is provided with a grounded conductor at the service point, a grounded conductor(s) shall be installed and routed with the ungrounded conductors to each service disconnecting means and shall be connected to each disconnecting means grounded conductor(s) terminal or bus. A main bonding jumper shall connect the grounded conductor(s) to each service disconnecting means enclosure. The grounded conductor(s) shall be installed in accordance with 250.186(A)(1) through (A)(4). The size of the solidly grounded circuit conductor(s) shall be the larger of that determined by 250.184 or 250.186(A)(1) or (A)(2). Exception: If two or more service disconnecting means are located in a single assembly listed for use as service equipment, it shall be permitted to connect the grounded conductor(s) to the assembly common grounded conductor(s) terminal or bus. The assembly shall include a main bonding jumper for connecting the grounded conductor(s) to the assembly enclosure. (1) Sizing for a Single Raceway or Overhead Conductor. The grounded conductor shall not be smaller than the required grounding electrode conductor specified in Table 250.102(C)(1) but shall not be required to be larger than the largest ungrounded service-entrance conductor(s). (2) Parallel Conductors in Two or More Raceways or Overhead Conductors. If the ungrounded service-entrance conductors are installed in parallel in two or more raceways or as overhead parallel conductors, the grounded conductors shall also be installed in parallel. The size of the grounded conductor in each raceway or overhead shall be based on the total circular mil area of the parallel ungrounded conductors in the raceway or overhead, as indicated in 250.186(A)(1), but not smaller than 1/0 AWG. Informational Note: See 310.10(G) for grounded conductors connected in parallel. (3) Delta-Connected Service. The grounded conductor of a 3-phase, 3-wire delta service shall have an ampacity not less than that of the ungrounded conductors. (4) Impedance Grounded Systems. Impedance grounded systems shall be installed in accordance with 250.187. (B) Systems Without a Grounded Conductor at the Service Point. If an ac system is grounded at any point and is not provided with a grounded conductor at the service point, a supply-side bonding jumper shall be installed and routed with the ungrounded conductors to each service disconnecting means and shall be connected to each disconnecting means equipment grounding conductor terminal or bus. The supply-side bonding jumper shall be installed in accordance with 250.186(B)(1) through (B)(3). Exception: If two or more service disconnecting means are located in a single assembly listed for use as service equipment, it shall be permitted to connect the supply- side bonding jumper to the assembly common equipment grounding terminal or bus. (1) Sizing for a Single Raceway or Overhead Conductor. The supply-side bonding jumper shall not be smaller than the required grounding electrode conductor specified in Table 250.102(C)(1) but shall not be required to be larger than the largest ungrounded service-entrance conductor(s). (2) Parallel Conductors in Two or More Raceways or Overhead Conductors. If the ungrounded service-entrance conductors are installed in parallel in two or more raceways or overhead conductors, the supply-side bonding jumper shall also be installed in parallel. The size of the supply-side bonding jumper in each raceway or overhead shall be based on the total circular mil area of the parallel ungrounded conductors in the raceway or overhead, as indicated in 250.186(A)(1), but not smaller than 1/0 AWG. (3) Impedance Grounded Systems. Impedance grounded systems shall be installed in accordance with 250.187. 250.187 Impedance Grounded Systems. Impedance grounded systems in which a grounding impedance device, typically a resistor, limits the ground-fault current shall be permitted if all of the following conditions are met: * (1) The conditions of maintenance and supervision ensure that only qualified persons service the installation. * (2) Ground detectors are installed on the system. * (3) Line-to-neutral loads are not served. Impedance grounded systems shall comply with 250.187(A) through (D). (A) Location. The grounding impedance device shall be installed between the grounding electrode conductor and the impedance grounding conductor connected to the system neutral point. (B) Insulated. The impedance grounding conductor shall be insulated for the maximum neutral voltage. Exception: A bare impedance grounding conductor shall be permitted if the bare portion of the grounding impedance device and conductor are not in a readily accessible location and securely separated from the ungrounded conductors. Informational Note: The maximum neutral voltage in a 3-phase wye system is 57.7 percent of the phase-to-phase voltage. (C) System Neutral Point Connection. The system neutral point shall not be connected to ground, except through the grounding impedance device. (D) Equipment Grounding Conductors. Equipment grounding conductors shall be permitted to be bare and shall be electrically connected to the ground bus and grounding electrode conductor. 250.188 Grounding of Systems Supplying Portable or Mobile Equipment. Systems supplying portable or mobile equipment over 1000 volts, other than substations installed on a temporary basis, shall comply with 250.188(A) through (F). (A) Portable or Mobile Equipment. Portable or mobile equipment over 1000 volts shall be supplied from a system having its neutral conductor grounded through an impedance. If a delta- connected system over 1000 volts is used to supply portable or mobile equipment, a system neutral point and associated neutral conductor shall be derived. ENHANCED CONTENT Collapse The term portable describes equipment that is easily carried from one location to another. The term mobile describes equipment that is easily moved on wheels, treads, skids, or similar means. (B) Exposed Non-Current-Carrying Metal Parts. Exposed non-current-carrying metal parts of portable or mobile equipment shall be connected by an equipment grounding conductor to the point at which the system neutral impedance is grounded. (C) Ground-Fault Current. The voltage developed between the portable or mobile equipment frame and ground by the flow of maximum ground-fault current shall not exceed 100 volts. (D) Ground-Fault Detection and Relaying. Ground-fault detection and relaying shall be provided to automatically de- energize any component of a system over 1000 volts that has developed a ground fault. The continuity of the equipment grounding conductor shall be continuously monitored so as to automatically de-energize the circuit of the system over 1000 volts to the portable or mobile equipment upon loss of continuity of the equipment grounding conductor. (E) Isolation. The grounding electrode to which the portable or mobile equipment system neutral impedance is connected shall be isolated from and separated in the ground by at least 6.0 m (20 ft) from any other system or equipment grounding electrode, and there shall be no direct connection between the grounding electrodes, such as buried pipe and fence, and so forth. (F) Trailing Cable and Couplers. Trailing cable and couplers of systems over 1000 volts for interconnection of portable or mobile equipment shall meet the requirements of Part III of Article 400 for cables and 495.65 for couplers. 250.190 Grounding of Equipment. (A) Equipment Grounding. All non-current-carrying metal parts of fixed, portable, and mobile equipment and associated fences, housings, enclosures, and supporting structures shall be grounded. Exception: If isolated from ground and located such that any person in contact with ground cannot contact such metal parts when the equipment is energized, the metal parts shall not be required to be grounded. Informational Note: See 250.110, Exception No. 2, for pole-mounted distribution apparatus. (B) Grounding Electrode Conductor. If a grounding electrode conductor connects non-current-carrying metal parts to ground, the grounding electrode conductor shall be sized in accordance with Table 250.66, based on the size of the largest ungrounded service, feeder, or branch-circuit conductors supplying the equipment. The grounding electrode conductor shall not be smaller than 6 AWG copper or 4 AWG aluminum or copper-clad aluminum. (C) Equipment Grounding Conductor. Equipment grounding conductors shall comply with 250.190(C)(1) through (C)(3). (1) General. Equipment grounding conductors that are not an integral part of a cable assembly shall not be smaller than 6 AWG copper or 4 AWG aluminum or copper-clad aluminum. (2) Shielded Cables. The metallic insulation shield encircling the current-carrying conductors shall be permitted to be used as an equipment grounding conductor, if it is rated for clearing time of ground-fault current protective device operation without damaging the metallic shield. The metallic tape insulation shield and drain wire insulation shield shall not be used as an equipment grounding conductor for solidly grounded systems. ENHANCED CONTENT Collapse Shields comprised of copper tape and drain wires cannot be used as the EGC in solidly grounded systems. The use of shielded cables is specified in 315.44. Those requirements are based on the system voltage, installation conditions, and cable construction. As explained in the informational note, the primary purposes of the shielding are to confine voltage stresses on the insulation, dissipate insulation leakage current, and drain off the capacitive charging current. Shown below are three different types of single-conductor shielded cable construction. Shielded cables are also available in multiconductor configurations such as Type MV (Article 315) and Type MC (Article 330) cables. Where the ground-fault current is relatively low (as in impedance grounded neutral systems), the metallic shield of any of the cable types pictured here is permitted to serve as the EGC if it is rated for clearing time of ground-fault current without being damaged. Cable manufacturers can provide permissible short-circuit currents for a metallic shield based on the fault clearing time of the OCPD. (Photo courtesy of Chuck Mello) Where the system is solidly grounded, neither the metallic tape insulation shield (top cable shown) nor the drain wire insulation shield (middle cable) can be used as the EGC, because they do not have sufficient circular mil area to provide the effective ground-fault return path required by 250.4(A)(5). A metallic insulation shield encircling the conductor (bottom cable), commonly called concentric neutral cable, has larger conductor strands in the concentric wrap. Because of its larger overall circular mil area, this concentric neutral cable is permitted to be used as an EGC in a solidly grounded system if the metallic shield will not be damaged during the time it takes to open the circuit OCPD. The copper metallic tape is typically 5 mils thick and is helically applied with a 12.5 percent or larger overlap over the insulation shield. Drain wires are typically 24 AWG bare copper wires. Where the cable shield cannot carry ground-fault current without damage, a separate EGC must be installed. A separate EGC would also be required where tape or drain-wire-type shields are not permitted to carry fault current. The EGC can be integral to a cable assembly, can be run as a separate conductor in a raceway or cable tray, or can be one of the types of EGCs specified in 250.118, such as rigid metal conduit or intermediate metal conduit. Wire- type EGCs are sized in accordance with 250.122. See also the Informational Note to 250.190(C)(3), for more information on how the rating of OCPDs used in systems operating over 1000 volts is determined. (3) Sizing. Equipment grounding conductors shall be sized in accordance with Table 250.122 based on the current rating of the fuse or the overcurrent setting of the protective relay. Informational Note: The overcurrent rating for a circuit breaker is the combination of the current transformer ratio and the current pickup setting of the protective relay. ENHANCED CONTENT Collapse This requirement applies to EGCs that are separately installed as specified in 250.190(C)(1). It also applies to a conductor installed in a cable assembly, other than the cable shield, that is used as an EGC. An EGC contained within a cable assembly can be a single conductor, or it can be sectioned (comprising multiple conductors within the cable jacket or sheath to form a single EGC) as permitted by 310.10(G)(5). 250.191 Grounding System at Alternating-Current Substations. For ac substations, the grounding system shall be in accordance with Part III of this article. Informational Note: See IEEE 80, IEEE Guide for Safety in AC Substation Grounding, for further information on outdoor ac substation grounding. 250.194 Grounding and Bonding of Fences and Other Metal Structures. Metal fences enclosing, and other metal structures in or surrounding, a substation with exposed electrical conductors and equipment shall be grounded and bonded to limit step, touch, and transfer voltages. (A) Metal Fences. If metal fences are located within 5 m (16 ft) of the exposed electrical conductors or equipment, the fence shall be bonded to the grounding electrode system with wire-type bonding jumpers as follows: * (1) Bonding jumpers shall be installed at each fence corner and at maximum 50 m (160 ft) intervals along the fence. * (2) If bare overhead conductors cross the fence, bonding jumpers shall be installed on each side of the crossing. * (3) Gates shall be bonded to the gate support post, and each gate support post shall be bonded to the grounding electrode system. * (4) Any gate or other opening in the fence shall be bonded across the opening by a buried bonding jumper. * (5) The grounding grid or grounding electrode systems shall be extended to cover the swing of all gates. * (6) The barbed wire strands above the fence shall be bonded to the grounding electrode system. Alternate designs performed under engineering supervision shall be permitted for grounding or bonding of metal fences. Informational Note No. 1: A nonconducting fence or section may provide isolation for transfer of voltage to other areas. Informational Note No. 2: See IEEE 80, IEEE Guide for Safety In AC Substation Grounding, for design and installation of fence grounding. (B) Metal Structures. All exposed conductive metal structures, including guy wires within 2.5 m (8 ft) vertically or 5 m (16 ft) horizontally of exposed conductors or equipment and subject to cont