NEC Article 250 Sections 250.1 through 250.4
The purpose of the National Electrical 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)]. Slide 94 and 95
The purpose and objective of Article 250 - Grounding is to insure that the electrical system is safe against electric shock and fires by limiting the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and as well as a ground-fault (line-to-case fault) . The rules contained in Article 250 identify the installation methods that must be followed to insure a safe electrical installation. Slide 96
Authors Comment: The grounding rules covered in this book are in reference to solidly grounded alternating current systems, such as 60/120, 120, 120/240, 208Y/120, 480Y/277 V. Other system configurations, such as 3-wire corner-grounded delta system, ungrounded system, or high resistance grounding system are permitted by the National Electrical Code, but they are rarely installed, except in industrial applications.
Because of the size and complexity of Grounding and Bonding,
Article 250 is subdivided into the following Parts: Slide 97
Part A - General Requirements, Slide 98
Authors Comment: An effective ground-fault current path is created when all electrically conductive materials that are likely to be energized are bonded together and to the grounded (neutral) at the electrical supply. Effective bonding is accomplished through the use of equipment grounding conductors, bonding jumpers, metallic raceways, connectors and couplings, metallic sheathed cable and cable fittings, and other approved devices recognized for the purpose. A ground-fault path is effective when it is properly sized so that it will safely carry the maximum ground-fault current likely to be imposed on it. For the purpose of this book, the effective ground-fault current path will be called the fault current path.
Ground-Fault. A ground-fault is an unintentional electrical connection between an ungrounded (hot) conductor and metal enclosures, raceways, equipment, or earth. Figure 250-2
Ground-Fault Current Path. An electrically conductive path from the point of a ground-fault (line-to-case fault) on a wiring system through conductors, equipment, or the earth extending to the grounded (neutral) terminal at the electrical supply source.
FPN: The ground-fault current paths could consist of equipment grounding conductors, metallic raceways, metallic cable sheaths, electrical equipment, and other electrically conductive material such as metallic water and gas piping, steel framing members, stucco mesh, metal ducting, reinforcing steel, shields of communications cables, or the earth itself.
Authors Comment: These definitions originated from the Usability Task Group recommendations for clearly describing what grounding and bonding is intended to accomplish.
250.3 Other Code Sections
250.4 General Requirements for Grounding and Bonding.
(A) Grounded Systems.
Authors Comment: An electrical system as used in this subsection refers to the power source such as a transformer, generator or photovoltaic system.
(1) Electrical System Grounding. Grounded electrical systems (transformer, generators, etc) must be connected to the earth for the purpose of limiting the voltage imposed by lightning, line surges, or unintentional contact with higher voltage lines, by shunting the energy to the earth.
Authors Comment: For systems over 600 V, the earth helps the utility to clear a line-to-ground fault, but for systems operating at less than 600, the earth will not remove dangerous voltages on the metal parts of the electrical system from a line-to-ground fault.
In addition, electrical systems (power supplies) are grounded to the earth for the purpose of stabilizing the voltage to earth during normal operation. Figure 250-3
DANGER: A separately derived system must have the system grounded
to the earth to stabilize the phase-to-ground voltage under normal operation. According
to the IEEE Std. 242 (Buff Book), if a ground fault is intermittent or allowed to
continue, the ungrounded system could be subjected to possible severe overvoltage to ground,
which can be as high as six or eight times phase voltage. This can puncture insulation
and result in additional ground faults. These overvoltage are caused by repetitive charging
of the system capacitance, or by resonance between the system capacitance and the inductances
of equipment in the system.
The IEEE Green Book also states that field experience and theoretical studies have shown that arcing, restriking, or vibrating ground faults on ungrounded systems can, under certain conditions, produce surge voltages as high as six times normal. Neutral grounding is effective in reducing transient voltage buildup from such intermittent ground faults by reducing neutral displacement from ground potential and reducing destructive effectiveness of any high-frequency voltage oscillations following each arc initiation or restrike,
Ungrounded systems are also difficult to trouble shoot because the line-to-ground voltage will not be consistent.
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