Clearing up confusion on bonding and grounding solidly grounded transformers
After a national arc-flash hazard analysis project was performed at eight recently constructed parts distribution warehouse sites for a Global 100 company as part of an OSHA Voluntary Protection Program (VPP), management found the results to be somewhat shocking.
During the data gathering process, Electrical Service Solutions, Inc., discovered more than 35 violations of the NEC involving improper bonding and grounding of transformers. Violations ranged from system bonding jumpers that were missing, undersized, improperly terminated, and installed in two locations to grounding electrode conductors that were either missing, undersized, improperly terminated to the electrode, and/or connected to the separately derived system in a location other than where the system bonding jumper was connected. These findings reiterate the fact that a significant amount of confusion still remains in the industry on the topic of bonding and grounding of transformers. Let’s take a closer look at the areas where most of the misconceptions arise.
To understand the concept of bonding and grounding for safety, the installer must know that for normal load current, short circuit current, or ground-fault current to flow, there must be a continuous circuit or path — and a difference of potential. The 2011 NEC defines the effective ground-fault current path as “an intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors on high-impedance grounded systems.” An effective ground-fault current path is an essential part of the overcurrent protection system.
Normal load current, short circuit current, or ground-fault current will use any and all completed paths, dividing in opposite proportion to the impedance in each path, to return to its source and then back to the origin of the fault. The unintentional ground-fault current flow in these completed paths facilitates the sure instantaneous operation of the overcurrent device, rapidly interrupting the energy source supplying the ground fault. The ground-fault current path must be complete and meet three important criteria:
A ground-fault current path for a grounded separately derived system/transformer that doesn’t meet these criteria becomes a silent and often lethal source of electrical shock when a ground fault occurs. If an effective ground-fault current path isn’t established and a ground-fault occurs on the derived ungrounded circuit conductors of a transformer, ground-fault current will not flow; therefore, the operation of the overcurrent protection device in the ground-fault current path won’t be initiated. Electrical raceways, enclosures, and equipment will become energized with dangerous energy, continually searching for a path back to its source. When a human body completes the ground-fault current path, it results in electrical shock or electrocution. Unlike obvious indications of faulty wiring of branch or feeder circuits, defective high-impedance ground-fault current paths are difficult to detect, because these circuits are predominantly called upon when a ground fault occurs.
Following is an overview of essential areas related to bonding and grounding single, solidly grounded, 480V – 208Y/120V, delta-to-wye, 3-phase transformers.
This path allows unintentional ground-fault current to flow from the point of a ground fault on the derived ungrounded circuit conductors, to the derived source, then back to the origin of the ground fault. This unintentional ground-fault current flow elevates the current in the transformer primary winding for ground faults between the derived source of the transformer and the first overcurrent protection device — or facilitates the operation of the transformer secondary overcurrent protection devices if the ground fault is on the load side of these devices. The system bonding jumper is one of the key elements that forms the effective ground-fault current path from the furthermost downstream point in the electrical system back to the derived source, the secondary winding of the transformer. If the system bonding jumper isn’t properly installed (Photo 1 and Photo 2), an effective ground-fault current path will not be established.
Table 250.66 of the 2011 NEC is used to size the system bonding jumper based on the size of the derived ungrounded circuit conductors supplied by the secondary of the transformer. Because the system bonding jumper is part of the ground-fault current path, it’s necessary to maintain a proportional size relationship between the derived ungrounded circuit conductors and the system bonding jumper. Where the derived ungrounded circuit conductors are larger than the maximum sizes given in this table, 250.28(D)(1) requires the system bonding jumper be not less than 12.5% of the area of the largest derived ungrounded circuit conductor. For purposes of this article, this requirement will be designated the “12.5% rule.”
The grounding electrode makes the earth connection for the transformer secondary circuit. It must be an effective connection, and all grounding paths must be connected to it. To prevent objectionable current flow, the grounding electrode conductor connection to the grounded conductor must be made at the same point on the separately derived system where the system bonding jumper and supply-side bonding jumper are connected, as specified in Sec. 250.30(A)(5).
Section 250.66 and Table 250.66 are used to size the grounding electrode conductor based on the size of the derived ungrounded circuit conductors supplied by the secondary of the transformer; however, because the maximum current in a grounding electrode conductor is limited by the impedance path through the grounding electrode and earth — and is not intended to be part of the effective ground-fault current path — the 12.5% rule does not apply.
Table 250.66 is used to size these bonding jumper conductors based on the size of the derived ungrounded circuit conductors supplied by the secondary of the transformer. Because the metal water piping system(s) or exposed building frame structural metal in the area served by the transformer will predominantly be used as a grounding electrode as identified in 250.30(A)(4), the rules for grounding electrode conductors apply. Therefore, the 12.5% rule does not apply. To prevent objectionable current flow, this bonding jumper conductor connection shall be made at the same point on the separately derived system where the grounding electrode conductor is connected, as stated in 250.104(D).
A separate bonding jumper from the grounded conductor to the metal water piping system(s) and exposed building frame structural metal is not required when either is used as a grounding electrode, as identified in 250.30(A)(4), and if a bonding jumper is installed between the exposed building frame structural metal and metal water piping system in the area served by the transformer.
Table 250.66 is used to size the supply-side bonding jumper based on the size of the derived ungrounded circuit conductors supplied by the secondary of the transformer. The supply-side bonding jumper is part of the ground-fault current path. Therefore, the 12.5% rule does apply.
When the system bonding jumper is not located at the derived source of the separately derived system, the grounded conductor (neutral) serves as part of the supply-side bonding jumper during a ground-fault condition. Thus, in addition to the existing requirements for grounded conductor sizing, the grounded conductor must comply with the same minimum sizing requirements as the supply-side bonding jumper per 250.30(A)(3).
Section 250.122 and Table 250.122 are used to size the equipment grounding conductor based on the rating or setting of automatic overcurrent devices in the circuit ahead of electrical raceways, enclosures, and equipment.
The Table (click here to see Table) summarizes the components described in this article, applicable 2011 NEC sections, and sizing table headings. It provides component functions and outlines minimum requirements as identified in the NEC related to bonding/grounding for safety of single, solidly grounded, 480V – 208Y/120V, delta-to-wye, 3-phase transformers.
Schamel is the president of Electrical Service Solutions, Inc., Huntington Beach, CA. He can be reached at email@example.com.