Why is a ground screw needed?

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  #1  
Old 02-03-06, 11:21 PM
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Why is a ground screw needed?

I've always wondered about this: What's the point of a ground screw in a metal box? Doesn't the ground wire go back to the panel? Why would you ever need to screw a ground wire to the box?

Thank you for enlightening me.
 
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Old 02-03-06, 11:35 PM
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Grounding the box prevents it from becoming hot if a hot wire gets loose in the box.
If someone opens the box with the power on you don't want the box hot.
 
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Old 02-04-06, 12:26 AM
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Thanks very much, GWIZ. It's amazing how little I know about just about everything

Much appreciated.
 
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Old 02-04-06, 01:04 AM
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grounding is good

Also, electricity does not "like" to have unbalanced current inside a ferrous (iron-containing) object.

Current flows makes a magnetic field.
Balanced current flows (all circuit conductors inside the same cable/conduit) makes magnetic fields that are equal and opposite, so they pretty much cancel each other.

Unbalanced current flows make a strong magnetic field since there is no opposite current flow to make a cancelling magnetic field.

That field extends through space to induce an opposing magnetic field and an opposing induced current flow in the steel surrounding it, and this opposing field chokes the current flow.

(This also imparts energy to the steel causing it to heat up.)

The magnetic field can be so strong that the wires can actually move with great force.

Tying in the metal components allows electricity to flow over the outside of the ferrous parts and ultimately reach the service panel.

This is critically important with steel conduit types.
Unbalanced fault current travels primarily over the outer steel layer.
Therefore, the NEC requires that these steel components be bonded to insure that the fault path is solid.

There are other reasons including minimizing potentials between parts and avoiding antenna effects (ungrounded metal can resonate in response to radio frequencies). Grounding the box help to shield against RF noise both into and out of the box.


The principal reason is (as G stated above) so that the box cannot be energized. This is a good enough reason by itself and probably the only one that you have to remember.
 
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Old 02-04-06, 07:55 AM
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Bolide; You were doing so good until these comments directed at "shielding".


Unbalanced fault current travels primarily over the outer steel layer.
Interesting, but debatable comment. Are you refering to "skin effect"? or some other effect" Skin effect at 60Hz is about .3" in copper, more in steel, not a factor for thin steel boxes or EMT... Lightning has very high frequency components, so you will get partial credit for this...
Therefore, the NEC requires that these steel components be bonded to insure that the fault path is solid.
I'll buy that.

There are other reasons including minimizing potentials between parts and avoiding antenna effects (ungrounded metal can resonate in response to radio frequencies). Grounding the box help to shield against RF noise both into and out of the box.
Definately not true. Grounding (bonding) and shielding are not the same and are unrelated in regards RF interference. Examples, You can put a person in a metal wire globe, suspend it in space and not ground it. You can then shoot a lightning bolt thru (actually around) it and the person will be unhurt. What is important here is that the person is SHIELDED, and that shield does not have to be grounded. Another example is the cell phone you are wearing right now. It has many shields in it and none are attached to local (ground). They still perform very well as shields. Grounding does not make a shield work any better. Shields work because they won't allow electric fields to penetrate, and/or they set up a counter magnetic field.
Also, grounded metal can resonate with RF fields as well as ungrounded; as examples of this, look at buried (and grounded) wire radials on a HF vertical antenna.
Sorry for the thread hijack. But, very occasionally, skin effect and shielding may affect our 60Hz work.
 
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Old 02-04-06, 08:42 AM
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This Mike Holt article discusses the topic also.
 
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Old 02-04-06, 08:48 AM
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Are you refering to "skin effect"?
No, to magnetic exclusion.

Grounding (bonding) and shielding are not the same and are unrelated in regards RF interference.
See Mike Holt's article and his note about magnetic fields.

Grounding does not make a shield work any better.
Because the ground drains away charge and reduces re-radiation.

grounded metal can resonate with RF fields as well as ungrounded;
I agree.



very occasionally, skin effect and shielding may affect our 60Hz work.
Not what I had in mind anyway. Next discussion: the 200% neutral: necessity or hype?
 
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Old 02-04-06, 10:42 AM
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more neutral Cu

You might start a new thread, use a scary title to keep out the non-electric pros.
This thread is done. It was successfully answered on the first response.
 
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Old 02-04-06, 10:43 AM
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Thanks for the additional comments. Very interesting. It's a pretty big world out there and it's nice to venture off your front door step and take a look around.
 
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Old 02-04-06, 11:05 AM
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Any & all metallic surfaces in a premises that may become "energized" are required to be at "Ground-potenial", i.e., connected to Ground, via a variety of Ground-connections & fittings.

"Any & all" includes the metallic surfaces of fixed and portable appliances and tools.That is the purpose of 3-slot "Grounding" receptacles and 3-prong cord-caps.
 
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