Service Equipment Grounding (3-14-2K)
The National Electrical Code specifies that the proper method of grounding the electrical service of a building or structure shall be in accordance with Section 250-24(a). Section 250-24(a) directs that alternating current systems which are required to be grounded by Section 250-20(b), have a grounding electrode conductor bonded from the service grounded (neutral) conductor to an acceptable grounding electrode as listed in Sections 250-50 or 250-521 [250-24(a)(1)], Figure 1.
Note: Illustrations not available on the Internet.
1 An acceptable grounding electrode system would include [250-50]:
- Underground metal piping electrically continuous for a minimum of 10 feet supplemented with ground rods.
- Effectively grounded metal frame of a building.
- Bare, galvanized, or other electrically conductive coating, reinforcing steel rods not smaller than 1/2 inch in diameter if the total length of the steel is not less than 20 feet.
- Ground ring encircling the building or structure in direct contact with the earth at a depth below the earth's surface of not less than 2 1/2 feet consisting of at least 20 feet of bare coper conductor not smaller than No. 2.
Neutral-to-Ground Connection - Section 250-24(a)(1)
The grounding electrode conductor, which is used to bond the service grounded (neutral) conductor to the grounding electrode, can be installed at any accessible location from the load end of the service-drop or service-lateral up to and including the service disconnecting means, Figure 1.
CAUTION: Some inspectors insist the connection to the grounded (neutral) conductor originate from the meter socket enclosure. Other inspectors prefer that the connection originates from the service disconnect enclosure. The Code states that either location is acceptable.
Objectionable Neutral Current - Section 250-24(a)(5).
A neutral-to-ground connection shall not be made on the load side of the service disconnecting means except as permitted for separately derived systems [250-30(a)(1)], separate buildings or structures [250-32(b)(2)], or meter enclosures [250-142 Ex. 2]. A neutral-to-ground connection at any other location will cause objectionable neutral current to flow on the equipment grounding conductor (conductive metal parts of the electrical equipment). This violates Section 250-6(a) and can create an electric shock or fires, as well as power quality problems, Figure 2.
Author's Comment:The dangers and power quality problems of improper neutral-to-ground connections were covered in the previous issue of Power Quality magazine.
Grounded (neutral) Conductor Required - Section 250-24(b).
For the metal parts of the electrical equipment to be safe from electric shock, the electrical equipment must be electrically connected (bonded) to the power suply grounded (neutral) conductor, typically at the transformer. The low impedance path necessary to clear phase-to-ground faults is accomplished by bonding the equipment grounding conductors (metal parts of the electrical system) such as metal enclosures, rigid metal conduit, intermediate metal conduit, electrical metallic tubing, armored cable, etc., to the grounded (neutral) service conductor [Sections 250-2(b) and 250-118].
At service equipment (the building disconnect), the low impedance fault return path is established by bonding the metal service disconnect enclosure to the grounded (neutral) service conductor within the service disconnecting means. This neutral-to-ground connection is accomplished by the installation of a main bonding jumper (screw or strap) [250-28], which is suplied by the equipment manufacturer [384-3(c)], Figure 3.
DANGER If the conductors from the electric utility do not include a grounded (neutral) conductor [violation of Section 250-24(b)], then the earth must serve as the fault return path [violation of Section 250-2(d)]. Under this condition, if there is a phase-to-ground fault, the high impedance path of the earth will not allow sufficient fault current to flow to open the circuit overcurrent protection, and the metal parts of the electrical system would remain energized practically at line voltage, Figure 4.