Grounding questions

Discussion in 'Electrical Forum discussion & Blog' started by t20sl, Aug 29, 2010.

  1. t20sl

    t20sl New Member

    Messages:
    3
    Location:
    Ohio
    I have a 3 gang box with qty 2 single pole switches and qty 1 three way switch. Box is fiberglass and each switch has a grounding screw. The 3 way switch is on a completely different circuit breaker. Other 2 switches are served bay another breaker. Question is should all 3 grounding wires be connected to each other? Also do I need to ground the switches themselves also? Thanks and if it not too much trouble explain the reason(s) so it helps me understand.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,055
    Location:
    New England
    Twist all of the grounds together and run a pigtail to each switch.
  3. t20sl

    t20sl New Member

    Messages:
    3
    Location:
    Ohio
    Reason's why putting grounds together from different circuits???? Can't this cause a ground eddy current??? Any pro's out there? Why do you ground a switch in a plastic box? Thanks
  4. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    No, if the circuit conductors are terminated or spliced in a box then all grounds associated with those conductors must be spliced. 250.148


    Because the yoke is metallic and code requires it. 404.9(B)
  5. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,534
    Location:
    North Carolina
    What is grounding?
    It is connecting to earth.

    Why do we ground? There are only four reasons to connect to earth as outlined below;
    250.4 (A) Grounded Systems.
    (1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation

    What is bonding?
    (4) Bonding of Electrically Conductive Materials and Other Equipment. Normally non–current-carrying electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.

    Why should we bond?
    (5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.

    Above are quotes from the 2008 NEC that explains why we connect to earth or ground if you please and why we bond.
    The bare or green conductor that we call the EGC (equipment grounding conductor) is also the conductor we use to bond all noncurrent carrying metal to the service grounded (neutral) conductor at the service disconnecting means. To ensure that this takes place the NEC requires that all EGC be bonded together .

    Notice the last sentence of #(5) above and we will do a little study in law, Ohm’s Law that is.
    250.56 states that if we don’t have 25 ohms of resistance we must install another electrode.
    Using this 25 ohms let’s do a little math.
    Using 120 volts with these 25 ohms of resistance we can divide the ohms into the voltage and get the current of 4.8 amps. This will not be enough current to open the breaker of blow the fuse. Even on a 408/277 the current from any one leg and ground would be 11 amps and still too low to open the overcurrent device (fuse or breaker)

    We do not ground but for the reasons given in 250.4(A)(1) and this plays no role in the opening of the overcurrent device. When we bond we insure that the overcurrent device opens and the exposed 6/32 screw that secures the plat cover to the device can not hurt us when we come in contact with them operating the switch.



    T
  6. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    The ground wire in the system will normally have zero current. I think your term "eddy currents" is an electronic concept, not related to power. In reality, IF a fault causes current to ground, multiple paths from the fault location back to the earth connection at the panel will reduce the potential developed on the metal grounded item where the fault occurred.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,055
    Location:
    New England
    Ground is there for safety, and normally serves no purpose except when there is a fault. Millions of older houses have no grounds at the boxes, being wired with x/2 without ground wires. No circuit relies on ground except maybe a surge suppressor. Even a GFCI will work fine without a ground.

    Internally, within a complex electronic circuit, the design of the ground plane and routing can be as complex as the rest of the operational circuitry, but in a house...tie them together! Ran into numerous issues with grounds internal to things like radars, and sometimes with grounds when powering a large system with multiple generators, but within your house, you have one power source...it's not an issue.
  8. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    That is the point though. The grounding conductor is intended to carry fault current. Nothing else. This is why codes have changed and evolved over the years to require equipment grounds on pretty much everything.
  9. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    This is why I can't wait 'till they finally clarify the NEC on this.
    Not all "grounding" is simply connecting to the earth. Equipment grounding is really equipment bonding. This is once change I am looking forward to....if it ever comes.
  10. t20sl

    t20sl New Member

    Messages:
    3
    Location:
    Ohio
    Thank you all for helping me understand this issue.
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,055
    Location:
    New England
    The vast majority of things you plug in do not have a 3-prong plug. So, in those cases, the ground does little. It could save you if the hot ended up on the frame and energized the cover screw, but little else. Having a ground IS a good thing, not saying it isn't, it's just that missing it is not necessarily going to create magically an unsafe situation. It IS there for fault protection, like insurance policies. And, like many things in life, mandated in new or remodeled situations, and a good idea regardless.
  12. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
    Cave Creek, Arizona
    You ground the switch because it is to protect YOU, not the box.
  13. CapstanRec

    CapstanRec New Member

    Messages:
    23
    Location:
    New Orleans
    The comment on eddy currents (i.e. ground loop) is correct. In general, each branch circuit should have one path to ground. It's extremely critical to avoid ground loops when designing audio equipment. In the home, it's not as critical but I still think it's best practive to have only one path to ground.
  14. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    Audio systems are NOT construction wiring. Not even close.
    There is NO probelm having multiple bonding paths back to the source.
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