Grounding versus Bonding and Surge Protection Devices (SPDs)
Mike: This is text from IEEE C62.72 that is being published soon. I feel there is some confusion between what is actually bonding and what is actually grounding.
11. Grounding
The purpose of system grounding is to stabilize the voltage rise between any phase conductor and earth or any phase conductor and neutral during normal operations and limit the voltage rise during abnormal conditions. Such abnormal voltage conditions can originate from lightning or line surges, unintentional contact with higher voltage lines, accidental grounding of a system conductor, or arcing ground fault conditions.
The system grounding also provides a low impedance path for a flow of current between system conductors and earth in order to initiate the operation of protective devices.
MH: False, to clear a fault, metal parts must be bonded to an effective ground-fault path in accordance with 250.2 and 250.4(A)(3), (4), and (5).
An effective grounding system can prevent excess voltage rises that can exceed the operating limits and equipment insulation levels.
MH: This statement is true.
An effective grounding system can also prevent adverse and objectionable continuity of service and assure that personnel are not inadvertently exposed to the dangers of electric shock.
MH: False, no amount of grounding (earthing) will reduce voltage on metal parts that will assure personnel will be protected against electric shock. However, the bonding of metal parts together does reduce the voltage “between the metal parts,” but not the earth.
A very low impedance ground path that has been properly installed is of paramount importance for the satisfactory operation of any SPD.
MH: This statement is true as it relates to utility distribution systems, but no premises wiring system. For proper operation of premises SPD, the low impedance path is to the source (bonding), not the earth.
It is extremely important that any SPD specifier or user evaluate the grounding requirements for SPDs. Example of questions to consider when evaluating a grounding system are as follows:
a) How is the equipment, to be protected, grounded or referenced to earth?
b) Is there a common ground grid?
c) Is all equipment effectively bonded via low impedance conductive means.
d) Are the distribution or power class transformers, the building structure, and all equipment (to be protected) bonded and referenced to the same grounding electrode or ground grid?
e) What is the resistance between the grounding terminal of the SPD, the equipment to be protected, and the reference grounding electrode or ground grid or system?
f) If the installation is not new, what is the respective age of the existing grounding system? Grounding systems often deteriorate over time.
g) Has the existing grounding system been regularly inspected, maintained, and tested?
MH: All of the above is irrelevant in relationship to SPDs.
Some articles or paragraphs in national, state, or municipal electrical codes require that the path to ground from circuits, equipment, and metal enclosures for conductors have "sufficiently low impedance" to limit the voltage to ground. However, the limitation of such code requirements is that sufficiently low impedance is not defined. These same electrical codes also state that a single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode. Such wording only specifies the requirement to augment a single electrode by adding one additional electrode if the resistance to ground is 25 ohms or more. The articles do not specify or mandate testing to determine if the initial single electrode provides a resistance, between the single electrode and ground, of 25 ohms or less. In addition, if another electrode is installed, there are no code requirements to verify that the additional electrode had any affect of lowering the resistance below 25 ohms.
MH: True, this is contained in 250.56 of the NEC.
The purposes of the grounding requirements in electrical codes are to require permanent and continuous electrical conductors to safely conduct any fault currents and to limit voltage to ground elevations during abnormal conditions.
MH: False, grounding metal parts to the earth for premises wiring will not limit voltage to ground elevation during abnormal conditions, not does the earth serve any purpose in clearing a ground fault.
Installing a grounding system, which meets the requirements of some electrical codes does may not guarantee a grounding system adequate for successful SPD operations of the selected SPD.
MH: Ground plays no rule in the operation of SPDs, so the ground resistance value is irrevalant.
An ideal grounding system should provide a near zero resistance between bonded components and the grounding electrode of the facility, building, or substation.
MH: True, the voltage between bonded parts and electrode should be close to zero (it doesn’t carry any current and it’s all conductive). But what is the point?
The ground potential rise in any facility increases proportionally to the surge current levels available. High ground potential rises can be created when surge currents impinging on a power distribution system where the facilities grounding system is not effectively bonded and grounded via low impedance means with a relatively high grounding resistance can create significantly high ground potential rises.
MH: True, the premises ground resistance has an impact on ground protection rise from ‘lightning’ but not from a premises ground fault.
These ground potential rises can damage the SPD and cause other hazards and damage that the SPD is intended to prevent.
MH: False, the GPR (voltage of metal parts to remote earth) plays no part in the operation of SPDs. SPDs are not designed to protect electrical equipment against a ground fault (line-to-case fault).
It is therefore recommended that the resistance, between the equipment to be protected and the main grounding electrode, should be 1 ohm or less at the time of installation.
MH: True, bonding of all metal parts is important for proper operation of premises SPD.
In order to verify the grounding resistance, it may be necessary to incorporate soil and grounding resistance testing within the electrical installation specifications for a construction project. Although not normally specified or required, the only way to be assured of the soil and grounding system resistance is through testing. If test results indicate that the ground resistance is high then construction and soil treatment methods exist to lower the soil resistance.
MH: This is true, but the premises ground resistance plays no part in the operation of SPD for premises.
It is not within the scope of this document to specify grounding systems, however, an effective grounding system is critical for the successful operation of any SPD.
MH: False, for the proper operation of premises SPD, a low impedance path is to the source (bonding) is critical, but the earth plays no role.
It is important that the grounding system of any power distribution system receive as much attention in the development phase of construction and during maintenance operations as the energized components and equipment of a power distribution system. An SPD can not be expected to provide adequate protection unless an effective grounding system exists.
MH: False, for the proper operation of premises SPD, a low impedance path is to the source (bonding) is critical, but the earth plays no role.
16.7 Grounding of SPDs
As discussed in Section 11 of this document, the existence and condition of a grounding system, and the connections of SPDs to the ground system, will have a direct effect on the SPD operation and coordination.
MH: False, for the proper operation of premises SPD, a low impedance path is to the source (bonding) is critical, but the earth plays no role.
Deteriorated grounding systems or improper connections of SPDs to the grounding system can eliminate any possibility of coordination and contribute to premature aging of the SPD.
MH: False, for the proper operation of premises SPD, a low impedance path is to the source (bonding) is critical, but the earth plays no role.
