Electrical Grounding by dandanhuanghuang


									       Electrical Grounding

                        Chris Burgess
                       James Stowers
                       Rene Thomson

“Packing up to 100 million volts of electricity, a bolt of lightning
has the power to tear through buildings, explode walls made of
brick and concrete, disturb steel and ignite deadly fires. The cost
of this damage can be enormous.”
          Purposes for Grounding
Personnel Safety: Proper grounding of power sources should prevent system
   source voltages from permanently appearing on metallic frames that
   personnel physically come in contact with.

Equipment Circuit Protection: To prevent electrical fires and limit damage to
   equipment and associated circuit conductors, the Building Safety Protection
   System shall provide a low resistance/impedance path for lightning currents
   to flow to earth when lightning strikes.

Electrostatic Discharges (ESD): ESD should be reduced by maintaining low
   resistance/impedance paths between grounded points throughout any
   ground plane.
Where Lightning Strikes
                   Getting Started
• System Description –
   – The entire lightning protection system shall be designed and
     installed in accordance with:
       • A) National Fire Protection Assoc. (NFPA) Document # 780
       • B) Underwriters’ Laboratories, Inc. (UL) Standard # 96A
       • C) Lightning Protection Institute (LPI) Standard # 175

• Submittals –
   – A complete shop drawing shall be submitted to the architect and
     engineer for approval prior to commencement of the installation.
     The shop drawing will show the extent of the system layout
     designed for the structure along with details of the products to be
     used in the installation.
A grounding system neither attracts, repels or prevents a lightning strike. The
lightning protection system simply provides a safe path for the lightning current to
follow - allowing the harmful current to be guided safely into the ground and away
from your home.
•   A true earth ground, is a rod driven into the earth to a minimum depth of 8
                           Earth Electrodes
         Type                     Advantages                             Disadvantages
Vertical rods           Simple design. Easy to install in good       High impedance. Hard to install
                        soils. Hardware readily available. Can be    in rocky soil. Step voltage on
                        extended to reach the water table.           earth surface can be high under
                                                                     large fault currents or during a
                                                                     direct lightning strike.
Plates                  Can achieve low resistance contact in        Most difficult to install. Should
                        limited area.                                be installed vertically.

Horizontal bare wires   Low impulse impedance. Good RF               Subject to resistance fluctuations
    (radials)           counterpoise when laid in star pattern.      with soils drying. Not
                                                                     recommended with unstable
Incidental electrodes   Can achieve very low resistance in certain   Little or no control over future
     (water pipes,      applications.                                alternations. Must be employed
     buried tanks)                                                   with other electrodes, not as sole

Ring ground             Straightforward design. Easy to install      Problems with asphalt and
                        around existing facility. Hardware readily   concrete around the facility? Not
                        available. Very efficient due to volume.     desirable where large rocks are
                                                                     near surface.
    How to Ground a Building Effectively
•   Gather Accurate Soil Resistivity Data
    Soil resistivity is a physical property unique to every soil. Prior to designing a
    grounding system, soil resistivity testing must be completed to determine the
    electrical properties of the soil at and around the site.
•   Inside Grounding Should Be Configured As Single Point
    Inside grounding involves the electrical bonding of all internal metallic's in a building
    to the outside grounding system. Each facility should have a master ground bar
    (MGB) that serves as a hub with all grounding for the building being either directly or
    indirectly bonded to it.
•   Outside Grounding Should Address Lightning
    Lightning storms can produce peak currents up to 400,000 amps. It is important that
    the grounding system is configured properly and is sufficiently robust to handle the
    intense energy contained in a lightning strike. Outside grounding must provide a low
    impedance path to allow lightning energy to dissipate into the earth without causing
    any damage to the protected structure and its contents.
•   Surge Protection Is Crucial
    Surge protection should be installed at the main AC service entrance and on any
    service panel feeding critical equipment. Surge protection should also be installed on
    incoming twisted pair telephone lines, data lines and co-axial cables entering your
    facility. The majority of surges are not caused by direct lightning strikes to the facility
    but are either carried in on utility lines, or generated internally.
Components of a Grounding System
Minimum Equipment Ground Conductor

Under table 250.122 In the NEC2005 Book, you can find minimum size
requirement for up to 6000 Amp. (Pg#70-112)
     A "GFCI" is a ground fault circuit
  The GFCI is designed to protect people from severe or fatal electric
  shocks. Because a GFCI detects ground faults, it can also prevent
  some electrical fires and reduce the severity of others by interrupting
  the flow of electric current.
  In the home's wiring system, the GFCI constantly monitors electricity
  flowing in a circuit, to sense any loss of current. If the current flowing
  through the circuit differs by a small amount from that returning, the
  GFCI quickly switches off power to that circuit. The GFCI interrupts
  power faster than a blink of an eye to prevent a lethal dose of
  electricity. You may receive a painful shock, but you should not be
  electrocuted or receive a serious shock injury.

Three common types: Receptacle Type, Circuit Breaker Type,
   Portable Type

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