Grounding - Performance (7-6-99)
The purpose of the National Electrical this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity [90-1(a). In addition the NEC contains provisions that are considered necessary for safety and compliance with the NEC with proper maintenance should result in an installation that is essentially free from hazard [90-1(b)].
The purpose and objective of Article 250 - Grounding and Bonding is to insure that the electrical system is safe. The requirements contained in Article 250 are intended to protect against electric shock and fires from high voltage surges from lightning as well as ground-faults. The rules contained in Article 250 of the NEC identify the installation methods that must be followed so as to insure a safe electrical installation.
Author's Comment: The grounding rules covered in this column are in reference to solidly grounded 60/120, 120, 120/240, 208Y/120, 277, or 480Y/277 volt alternating current systems. Other system configurations are recognized and permitted by the NEC, such as 3-phase, 3-wire corner-grounded delta system, ungrounded system, or high resistance grounding system. However, these systems are not solidly grounded, are rarely installed except in industrial applications, and are beyond the scope of this column.
The following subjects are important to understand before you attempt to apply any of grounding and bonding requirements contained in Article 250 [250-2]. These subjects will be covered in great detail over the next few months.
System Earth Grounding [250-2(a)]
The intentional grounding of one conductor of an alternating-current system to the earth so as to limit elevated voltage on conductors from high voltage surges imposed by lightning, line surges, or unintentional contact with higher voltage lines and to stabilize the phase-to-ground voltage.
Equipment Earth Grounding and Safety Grounding [250-2(b)]
The intentional grounding of metal parts of the electrical system to the earth so as to limit voltage on the metal parts from lightning, line surges, or unintentional contact with higher voltage systems and to remove dangerous voltage (touch potential) from the metal parts of the electrical system from ground-faults.
Bonding insures that dangerous voltage on interior metal water piping systems, metal gas piping systems and exposed structural steel framing members resulting from ground-faults can be quickly cleared by opening the circuit overcurrent protection device.
Low Impedance Ground-Fault Path [250-2(d)]
The low impedance ground-fault return path must be mechanically and electrically continuous, have the capacity to safely carry fault-current, and have sufficiently low impedance (less than 1 ohm) to clear ground-faults by the opening of the circuit overcurrent protection device.
Earth Cannot be Used for Equipment Safety Grounding [250-2(d)]
The earth, such as a grounding electrode consisting of a ground rod or concrete foundation steel, cannot be used to provide the low impedance path necessary to remove dangerous voltage resulting from ground-faults.