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    Why do we need to bond neutral & ground in breaker box

    Asked Oct 20, 2008, 01:38 AM 5 Answers
    I have noticed that in the Main breaker box we bond the neutral and ground togather.
    The questions are;
    1- Why do we need to do so.
    2- If I plug my drill in to any grounded outlet, why the ground wire is not carring any current as the neutral does and we can touch ground wire where as we can't touch the neutral wire.
    3- Why it's highly recommended to bond the neutral & ground in only main breaker box and not in sub-breaker box .

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    Last edited by Curlyben; Oct 20, 2008 at 02:14 AM.
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    5 Answers
    stanfortyman's Avatar
    stanfortyman Posts: 5,302, Reputation: 264
    Electrical & Lighting Expert
     
    #2

    Oct 20, 2008, 03:27 AM


    1) Because it's code. This bond is where we get the "safety" ground in our branch circuits. This is different from service grounding and ground rods. They are totally different things.

    2) Because the ground is not being used as a circuit conductor. The neutral completes the circuit along with the hot. Just like two hots complete a 240v circuit.

    3) It's not "highly recommended". It's REQUIRED.

    This subject is extremely complex and can take up a lot of time. Time I don't have this morning. Hopefully someone will come along during the day today.
    I'll check back later.
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    tkrussell's Avatar
    tkrussell Posts: 9,638, Reputation: 722
    Senior Electrical & Lighting Expert
     
    #3

    Oct 20, 2008, 05:38 AM


    I like Stan's first answer, because it is Code.

    We both know that is not the answer that will be acceptable to you, so he did the best he could with time available.

    And it is required, not recommended, as Stan mentions.

    This is a complex issue to explain in a nutshell.

    Let me see what I can add, hopefully, meaningful and understandable.


    The "neutral" is technically called the "grounded" conductor. It is grounded by ground rods at the utility transformer by the utility. We must ground it also at the service entrance, which can be at either or both the meter and main switch, technically called the Service Disconnecting Means.

    Most panels, such as you refer to as the Main Breaker Box, has the main switch in it. It is at this point the neutral is grounded by connecting to a grounding electrode.

    Sidebar to confuse you, if the main switch is ahead of the panel, such as outside at the meter, the grounding and bonding of the neutral is done at the main switch, not at the panel.

    It is from this point the neutral must be separate from any other grounding, and must be separate from the equipment grounding conductor (bare or green wire) in any branch circuit. Since the neutral is the return for any 120 volt circuit, it is a current carrying conductor. The bare or green wire of each branch circuit is reserved to only carry current back from a circuit if there is a ground fault (short circuit).

    The equipment grounding conductor is intended to be reserved to be ready to carry any fault current back from a short circuit, in an effort to allow the current to rise quickly to trip the circuit breaker for the circuit with the problem.

    This acts as a safety/redendant return for a circuit. Connection of the neutral/ground at the main switch insures that all neutrals and grounds are now at the same earth potential, and the equipment ground is created, or begins.

    Since a "subpanel" is downstream from the main switch, it is considered as a branch circuit, and is treated the same as a branch circuit, separate and insulated neutral return for any 120 volt circuits, and a separate equipment ground for any metal, such as the panelbox.

    A neutral can be touched with no fear of injury, as it is at zero earth potential, because it has been grounded.

    As we said, this is a complex issue, and I know I bounced around a bit. If this is not clear, get back with any other questions.
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    rtw_travel's Avatar
    rtw_travel Posts: 346, Reputation: 36
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    #4

    Oct 20, 2008, 08:54 AM

    Here's another facet of the answer.

    Appliances with three pronged plugs are designed using the ground to... well... ground the appliance.

    Think of a typical appliance - say dishwasher.

    The motor on the dishwasher uses the hot and neutral wire to provide power to the motor. Other than the motor and controls, everything else in the dishwasher will be bonded to the ground wire. This means that if there is an internal fault in the dishwasher and the hot wire touches anything else, then power will go from the hot to the ground wire, and the breaker will trip on overcurrent. If it was not grounded, then the entire dishwasher will be energized and will shock anyone who touches it.

    You have not asked why many things, such as almost all drills, only have two prong plugs. Do a Google search for 'double insulated', and you'll see how manufacturers have changed the design so they do not rely on household ground wiring for safety.
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    KISS's Avatar
    KISS Posts: 12,505, Reputation: 838
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    #5

    Oct 20, 2008, 09:15 AM

    Good question.

    And yet another reason.

    This is another complex issue. If the ground-neutral bond occurred in multiple places, there could be ground currents created. Since ground is a reference for thinks like your cable TV and everything else in the house, that reference potential will not be zero.

    Ground is NOT supposed to carry current except under fault conditions. It is the reference voltage for computers etc. and should be the same with respect to every other ground, no matter how far away from each other.
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    wclark22's Avatar
    wclark22 Posts: 1, Reputation: 1
    New Member
     
    #6

    Aug 20, 2009, 07:05 AM
    Because if a neutral wire became disconnected (I've seen it happen), the return path for electric current could be along a ground wire. While that itself may not be a hazard, if that ground wire also became disconnected somewhere, parts of the ground system could be energized. That's not EVER supposed to happen.



    The neutral wire is essentially a "low-risk" return path for the electric current in that branch of the system. All of the neutral wires all have the same electrical potential... nothing. At least, no potential compared to ground. There is, of course, 120 volts of potential difference between a neutral wire and any hot wire in the residential system.

    If you touched the metal part of a live neutral wire you should not receive a shock. (But don't try it!) By tying the neutral to ground at one point, half of the conductors (in a typical 120 volt circuit) have no dangerous electrical potential. Of course, the hot wires are still dangerous.
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