Safety benefit of grounded neutral conductor

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Old 12-29-07, 01:13 AM
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Safety benefit of grounded neutral conductor

What exactly are the safety benefits of the neutral conductor being grounded? What happens to these benefits w/ 240v circuit?
 
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Old 12-29-07, 07:10 AM
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What exactly are you trying to understand? Your question is rather vague.
 
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Old 12-29-07, 07:12 AM
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In theory, Ground has nothing to do with the electrical equipment in your house. If you think, unbond the neutral and touch a live wire to the dirt in your yard......Nothing happens.

Ground is primarily for EXTERNAL activities such as lightning, such that it is directed to the earth, instead of "ZIPPING" and ZAGGING" thru your house wiring uncontrollably.

Thus the BOND. If lighning were to hit an external porch light for example, it would be directed back to the "BONDING POINT" and out to the earth, instead of running aimlessly thru the upstairs hallway and across the bedroom floor..

240 accessories are no different. The neutral is bonded at the panel, and the accessory is "EARTHED" at point of use.
One could also argue the subpanel which is not bonded. Simply put, the external current, such as lightning, is given a direct path out to the earth , touching as little of your stuff as possible.
 
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Old 12-29-07, 07:29 AM
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Unclediezel has touched on half the issue. There is another important part he has left out. What information are you looking for?
 
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Old 12-29-07, 08:23 AM
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Originally Posted by racraft View Post
Unclediezel has touched on half the issue. There is another important part he has left out. What information are you looking for?
Im sure there are several more important parts that I,ve left out---

The who,what ,when where and why of grounding can take up 3 full aisles at the local library. Its one of those topics that is just "DONE" , Because it needs to be done and dont ask questions . I'm a firm believer in understanding why you do something before you attempt it, but, In this case it is simpler to just do it , and not spend the rest of your life trying to figure it out.
 
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Old 12-29-07, 01:53 PM
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Originally Posted by racraft View Post
Unclediezel has touched on half the issue. There is another important part he has left out. What information are you looking for?

I guess the other half


OK.. to be serious... I guess I am asking why is the neutral conductor grounded at the transformer and at the main panel? What benefit does that provide in terms of safety? Or is it just what UncleDiezel said?

I was assuming there was a safety benefit at the receptacle side due to the return path of the 2-wire conductors being grounded. Since the 240v receptacle does not have a grounded conductor, I wondered whether it is giving up some safety benefit or not.

I know that one is not supposed to interrupt the return path via neutral conductor such as at a switch, I assumed this was related to the neutral being grounded.

Also, in the case of 240v circuit what is the wiring protocol for switches? Is there a particular colored wire that one is supposed to not interrupt the path of?

Hope this makes some sense, if not I'll keep trying!

Thanks.
 
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Old 12-29-07, 02:04 PM
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OK.. to be serious... I guess I am asking why is the neutral conductor grounded at the transformer and at the main panel
because that is how they do it.


it provides a path to ground for objectionable currents as close to the service or transformer as possible. By grounding at each end, it would tend to prevent that objectionable current from passing on to the other side of the system.the ground is actually a very poor conductor and realistically, the more grounding there is, the less chance the house will blow up when lightning gets close. (overly simplified but....)

Grounding the neut provides more than a ground source for objectionable currents though, it provides for a 0 reference to ground (I did find it and will find it again if anybody wants to argue this point.. It is in the code)



I was assuming there was a safety benefit at the receptacle side due to the return path of the 2-wire conductors being grounded.
each of them provide a seperate use for the system. Try not to think of the neutral (grounded) conductor to be a safe conductor. The only reason it does not appear to be hot is that it is bonded to the grounding system. If you would remove all ground connections, it too would give you a voltage reading to ground.

Since the 240v receptacle does not have a grounded conductor, I wondered whether it is giving up some safety benefit or not.
Nope. The nuet is not a safety issue. It is a functioning part of the actual cuircuit.



I know that one is not supposed to interrupt the return path via neutral conductor such as at a switch, I assumed this was related to the neutral being grounded.
yes, because if you were to interup the neut, the circuit itself if till hot and the last point of the neut (on the device side) is actually hot as well.



Also, in the case of 240v circuit what is the wiring protocol for switches? Is there a particular colored wire that one is supposed to not interrupt the path of?
Ya, the green one. In most cases, it is preferrable to interrupt both hot wires althoug depending upon the situation, one conducotr interrupted is acceptable. Stick with cutting both sides. it will save some grief.

Hope this makes some sense, if not I'll keep trying!
ditto
 
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Old 12-29-07, 03:14 PM
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Just for giggles..... A true 240 volt circuitdoesnt use a neutral. The 4 wire cord for newer installations is for a "SPLIT" ckt of 120/240. The neutral provides for 120 volts from what was otherwise 240 only. two hots together is 240...and either of those hots to neutral is 120...Ground does its same job as discussed before....Plus prevents the "Case of the appliance" from becoming a current carrying conductor in and of itself....(I told you it could take a Lifetime....).Back in the days of 3 wire appliances, it wasnt uncommon to get a "Tingle" from the side of a clothes dryer.

I'm assuming this is the ":Zero Earth "reference that Nap was referring to?
 
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Old 12-29-07, 11:26 PM
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Many whole books have been written on this subject, so it's going to be tough to cover all the salient points in an unfocused thread. How will we ever know when the discussion is complete?

Grounding the neutral limits the voltage on any wire in your house to 120 volts to ground (independent of whether the circuit is a 120-volt circuit or a 240-volt circuit). This in turn limits the voltage that hits my body to 120 volts for any one thing I touch. If the neutral wasn't grounded, the voltage could be higher on some wires and lower on others. The grounded center-tap neutral is what makes North American residential wiring safer than European residential wiring.

There's a lot more to the story of course, but to me that's the most important part.
 
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Old 12-30-07, 12:36 PM
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Confining the discussion to a single-phase residential Service ,the essential reason for Grounding a Service Entrance Conductor is to "short-circuit" the Primary winding of the distribution transformer in the event that a hi-tension conductor that supplies the Primary makes accidental contact with one of the Secondary Conductors. Should this occur without a Grounded Service Conductor, there would be (say) 7000 volts-to-Ground on each Service Conductor.

One of the Primary conductors is Ground-connected. If the other Primary conductor ( 7000 volts-to-Ground) makes contact with the Secondary "Neutral" which is Ground-connected at it's Service termination , the Primary winding is effectively "short-circuited", both Primary conductors Ground-connected.

If the Primary conductor makes contact with an Un-Grounded Service conductor, the section of the Secondary winding between the Grounded / Un-Grounded Secondary conductors , which is a very low-impedance path, effectively "short-circuits" the Primary winding, causing a large fault-current which will "blow" the fuse that limits the Primary current.
 
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Old 01-02-08, 09:09 AM
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How about the safety UNbenefits of a grounded neutral?

To carry this talk one step forward; A safer solution is to isolate the branch circuits from ground altogether. Using a 1:1 isolation transformer on each branch circuit with an UNgrounded secondary, you can achieve a 120volt (or 240volt) circuit that will not result in a serious shock during a touch of one conductor and ground. Its also about $$, and isolation transformers don't come cheap.
 
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Old 01-30-08, 08:25 AM
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This is an interesting thread. I have a few engineering thougts/questions so please work with me.

So in engineering terms the 2nd hot wire is actually functioning as a neutral because of the sine wave differents between the 2 hots? One hot wire is sine wave and the 2nd hot is co-sine wave? and never the "twane shall meet?"

So the incoming SEU bare is actually a ground wire not a neutral. In 120V circuits the neutral directs all returns to ground of the same wave as the hot. It is not a positive charge (as the incoming) that is why there is no tingle. But there is a slight arc if a neutral is removed. Why?

In 240V the hots carry each other's return in co-sine wave back out to the transformer. Right? Does this go back to the power source or is it terminated at the transformer?

Thanks for your patience.
 
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Old 01-30-08, 09:22 AM
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Originally Posted by Thonati View Post
So in engineering terms the 2nd hot wire is actually functioning as a neutral because of the sine wave differents between the 2 hots?
This is why the code only uses the term "neutral" in very specific context. What people usually call the neutral is actually called the "grounded conductor". This is really to reinforce the point that the "neutral" is a current-carrying conductor just like any hot wire, it just happens to be bonded to ground whereas hots are not bonded to ground.

So the incoming SEU bare is actually a ground wire not a neutral.
It's a grounded conductor. It carries the difference in current between the two hot legs which in common parlance is the "neutral". The service entrance is a special case because the bare neutral is grounded at both ends by your home's ground rods and by the power company ground rods at the pole.

In 120V circuits the neutral directs all returns to ground of the same wave as the hot.
It completes the circuit back to the center-tap of the source transformer, not the ground (earth).

It is not a positive charge (as the incoming) that is why there is no tingle.
In alternating current (AC) power systems, the positive and negative charge reverses 60 times per second. The hot is positive and the neutral is negative, then the neutral is positive and the hot is negative.

There is usually no tingle when you touch a grounded conductor (neutral) because it is bonded to all of the exposed conductive surface and is therefore at the same potential with respect to anything else you could be touching at the same time. Don't let this fool you though, there are many situations where a neutral can deliver a lethal shock just like a hot.

But there is a slight arc if a neutral is removed. Why?
Because in a 120V circuit, the neutral carries exactly the same current as the hot. All of the electrons that flow "out" on the hot must flow "back" on the neutral. In a 240V/120V such as a multiwire circuit or main service, the neutral carries the difference in current between the hots.

In 240V the hots carry each other's return in co-sine wave back out to the transformer. Right?
In 240V, there is only one sine wave, thus the term "single phase power".

Does this go back to the power source or is it terminated at the transformer?
For the purpose of this discussion, you can think of the transformer as the power source.
 
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Old 01-30-08, 10:18 AM
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Originally Posted by Thonati View Post
This is an interesting thread. I have a few engineering thougts/questions so please work with me.

So in engineering terms the 2nd hot wire is actually functioning as a neutral because of the sine wave differents between the 2 hots? One hot wire is sine wave and the 2nd hot is co-sine wave? and never the "twane shall meet?"
A rather difficult use of some terms here.
For the purposes of this industry:

HOT: means significant voltage on a conductor relative local ground.
Note that a current carrying neutral may not be "HOT" when carrying current, but may become "HOT" if disconnected from the panel.
Sine/Cosine: In engineering terms, these currents (or voltages) will be 90 degrees different. In a split 240/120, the hot legs are 180 degrees offset, therefore, Sine/Cosine are NOT used to describe them.
 
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