9.7 Ground-Fault Protection of Equipment and Grounding of Specific Equipment

Ground-Fault Protection of Equipment — the Three Parallel Rules

Ground-fault protection of equipment (GFPE) at high current levels exists to stop arcing ground faults from destroying switchgear. A phase-to-ground arcing fault on a 480Y/277 V system may draw only a few hundred amperes — far below the instantaneous trip of a 2000-ampere breaker — while burning through copper bus at a rate measured in inches per second. GFPE senses the residual current and opens the disconnect.

Three sections impose the same requirement at three points in the system:

All three point to 230.95 for the performance requirements: maximum setting 1200 amperes, maximum time delay one second at 3000 amperes or more, and performance testing when first installed with a written record available to the AHJ.

240.13 Ground-Fault Protection of Equipment. Ground-fault protection of equipment shall be provided in accordance with 230.95 for solidly grounded wye electrical systems of more than 150 volts to ground but not exceeding 1000 volts phase-to-phase for each individual device used as a building or structure main disconnecting means rated 1000 amperes or more. The provisions of this section shall not apply to the disconnecting means for: (1) continuous industrial processes where a non-orderly shutdown will introduce additional or increased hazards; (2) installations where ground-fault protection is provided by other requirements for services or feeders; (3) fire pumps.

Remember what these rules are not: they are not personnel protection. A 1200-ampere GFPE setting will not protect a person. That is the job of Class A GFCIs at 4 to 6 milliamperes under 210.8.

Equipment That Must Be Grounded — 250.110

Exposed non-current-carrying metal parts of fixed equipment likely to become energized shall be connected to an equipment grounding conductor under any of the following conditions:

1. Where within 8 feet (2.5 m) vertically or 5 feet (1.5 m) horizontally of ground or grounded metal objects and subject to contact by persons;
2. Where located in a wet or damp location and not isolated;
3. Where in electrical contact with metal;
4. Where in a hazardous (classified) location as covered by Articles 500 through 517;
5. Where supplied by a wiring method that provides an equipment grounding conductor, with an exception for the metal frames of electrically heated appliances that are permanently and effectively insulated from ground;
6. Where equipment operates with any terminal at over 150 volts to ground.

The 8 feet vertically, 5 feet horizontally dimension pair is a reliable exam item.

Specific Fixed Equipment — 250.112

Exposed non-current-carrying metal parts of the following kinds of equipment, regardless of voltage, shall be connected to an equipment grounding conductor:

• (A) Switchboard, switchgear, and panelboard frames and structures — exception for frames of DC single-polarity switchboards effectively insulated;
• (B) Pipe organs — generator and motor frames in an electrically operated pipe organ;
• (C) Motor frames, as provided by 430.242;
• (D) Enclosures for motor controllers, unless attached to ungrounded portable equipment;
• (E) Elevators and cranes;
• (F) Garages, theaters, and motion picture studios — with an exception for pendant lampholders on circuits not over 150 volts to ground;
• (G) Electric signs, outline lighting, and associated equipment as provided in Article 600;
• (H) Motion picture projection equipment;
• (I) Power-limited remote-control, signaling, and fire alarm circuits where system grounding is required by Part II or Part VIII of Article 250;
• (J) Luminaires as provided in Part V of Article 410;
• (K) Skid-mounted equipment — permanently mounted electrical equipment and skids shall be connected to the equipment grounding conductor sized as required by 250.122;
• (L) Motor-operated water pumps, including the submersible type;
• (M) Metal well casings — where a submersible pump is used in a metal well casing, the casing shall be bonded to the pump circuit equipment grounding conductor.

Cord-and-Plug-Connected Equipment — 250.114

Under any of the conditions described in (1) through (4), exposed non-current-carrying metal parts of cord-and-plug-connected equipment shall be connected to an equipment grounding conductor:

1. In hazardous (classified) locations;
2. Where operated at over 150 volts to ground, with exceptions for motors guarded against contact and for metal frames of electrically heated appliances that are permanently and effectively insulated from ground;
3. In residential occupancies: refrigerators, freezers, and air conditioners; clothes-washing, clothes-drying, and dishwashing machines; sump pumps and electrical aquarium equipment; hand-held motor-operated tools, stationary and fixed motor-operated tools, and light industrial motor-operated tools; motor-operated appliances such as hedge clippers, lawn mowers, snow blowers, and wet scrubbers; portable hand lamps;
4. In other than residential occupancies: the same categories, plus cord-and-plug-connected appliances used in damp or wet locations or by persons standing on the ground or on metal floors or working inside of metal tanks or boilers; and tools likely to be used in wet or conductive locations.

Exception to (4): Tools and portable hand lamps likely to be used in wet or conductive locations shall not be required to be connected to an equipment grounding conductor where supplied through an isolating transformer with an ungrounded secondary of not over 50 volts.

250.114 Exception also covers listed tools, listed appliances, and listed equipment covered in (2) through (4) that are protected by a system of double insulation or its equivalent. Such equipment shall be distinctively marked.

Protection of Connections and Clamps

250.10 Protection of Ground Clamps and Fittings. Ground clamps or other fittings shall be approved for general use without protection or shall be protected from physical damage as indicated in (1) or (2): in installations where they are not likely to be damaged; or where enclosed in metal, wood, or equivalent protective covering.

250.12 Clean Surfaces. Non-conductive coatings — such as paint, lacquer, and enamel — on equipment to be grounded shall be removed from threads and other contact surfaces to ensure good electrical continuity, or be connected by means of fittings designed so as to make such removal unnecessary.

250.8 Connection of Grounding and Bonding Equipment. Equipment grounding conductors, grounding electrode conductors, and bonding jumpers shall be connected by one or more of the following means:

1. Listed pressure connectors;
2. Terminal bars;
3. Pressure connectors listed as grounding and bonding equipment;
4. Exothermic welding process;
5. Machine screw-type fasteners that engage not less than two threads or are secured with a nut;
6. Thread-forming machine screws that engage not less than two threads in the enclosure;
7. Connections that are part of a listed assembly;
8. Other listed means.

250.8(B) Methods Not Permitted. Connection devices or fittings that depend solely on solder shall not be used.

The two-thread rule in (5) and (6) is why a 10-32 grounding screw is used in a box rather than a sheet-metal screw: a sheet-metal screw is not a machine screw and typically engages only one thread of the box wall.

Use of Grounded Circuit Conductor for Grounding Equipment — 250.142

250.142(A) Supply-Side Equipment. A grounded circuit conductor shall be permitted to ground non-current-carrying metal parts of equipment, raceways, and other enclosures at any of the following locations:

1. On the supply side or within the enclosure of the AC service-disconnecting means;
2. On the supply side or within the enclosure of the main disconnecting means for separate buildings as provided in 250.32(B) Exception;
3. On the supply side or within the enclosure of the main disconnecting means or overcurrent devices of a separately derived system where permitted by 250.30(A)(1).

250.142(B) Load-Side Equipment. Except as permitted in 250.30(A)(1) and 250.32(B) Exception, a grounded circuit conductor shall not be used for grounding non-current-carrying metal parts of equipment on the load side of the service disconnecting means or on the load side of a separately derived system disconnecting means or the overcurrent devices for a separately derived system not having a main disconnecting means.

There are three exceptions, the practically important one being Exception No. 1: the frames of ranges, wall-mounted ovens, counter-mounted cooking units, and clothes dryers under the conditions permitted for existing branch-circuit installations only, as covered in 250.140.

250.140 permits frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, and clothes dryers to be connected to the grounded circuit conductor for existing branch-circuit installations only, where an equipment grounding conductor is not present, provided the supply circuit is 120/240 V single phase or 208Y/120 V derived from a three-phase four-wire wye system, the grounded conductor is not smaller than 10 AWG copper, the grounded conductor is insulated (or uninsulated and part of a Type SE service-entrance cable originating at the service equipment), and grounding contacts of any receptacles are bonded to the equipment.

That is why old three-wire range and dryer receptacles remain legal where they exist, but every new range and dryer circuit must be four-wire with a separate equipment grounding conductor.