Why must ground & neutral be seperate in subpanel?

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I installed a subpanel when I switched from an electric stove to gas. I used the 40A 220V breaker that formerly served the stove to power the sub panel. The cable is #4 with two conductors and a ground. I have 6- 15 amp breakers in the panel providing branch circuits for my kitchen and other areas of my house. The grounds and neutrals all share the common bus bar in the sub panel. Everything has worked fine for years now. Can someone explain why I read that ground and neutral are to be isolated in the sub panel? Please don't answer because of the NEC since that does not explain why. What is the risk of my current situation?
Thanks, Joe
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snipped-for-privacy@verizon.net wrote:

While I agree that what's there is correct and functional "circuit wise" I think the reason the code requires a separate ground conductor is this:
That common ground/neutral might develop an open between the sub panel and the main panel because of a "loose connection" at one end or the other. If that happened currents returning on the neutrals of those "new" branch circuits would lift the whole "ground" of the sub panel off true ground and would create a dangerous situation by making things like the grouning slots in recepticals rise above ground to dangerous voltage levels.
Jeff
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Jeffry Wisnia
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On 27 Nov 2006 13:25:38 -0800, snipped-for-privacy@verizon.net wrote:

Essentially, because the ground wire is connected to things that people are expected to touch.
When you connect the nuetral and ground together, you create the possibility of the ground becoming hot, and energizing things like the outside of your refridgerator. This is bad. The probability of this happening goes up astronomically if there is a nuetral/ground interconnection anywhere but in the main service entry.
Aside from the chance of something coming loose and putting some random voltage between 105 and 240 volts on the outside of your bathtub, there's also the problem that your ground wire is not zero resistance, which means that if you touch both it and another path to ground, at least some of whatever current is on it will choose to go through you. (This is why, for example, you can get a shock from the casing of a malfunctioning appliance, even if it's grounded.)
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For one thing, it puts current into the ground conductor between the subpanel and main panel.
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The Reason is that there is current goes through a nuteral and no current goes through a ground and if the ground and nutural are tied together they both have current passing through them and the only time that current should go through a ground is when there is a short or something similar...
but to make thing even moree confusing is then why is the nurural and ground tied together at the main pannel and that is to bond the nuural to the earth ground...
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OK, so the ground is bonded to the neutral at the service panel.
What about the transfromer at the pole? Is the neutral center-tap in the North American System bonded to the transformer enclosure? Is this point often connected to a ground wire running down the pole and into the earth?
Also, why in the US systems is the top wire on the pole the hot wire (for the transformer primary) and the neutral is usually several feet below this? Is this arrangement not more prone to lightning damage?
Beachcomber
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Beachcomber wrote:

That was the old way - keeping the hot wire as far from people/animals as possible. The vast majority of new installations will put the ground wire on top.
But what about the rural Canadian systems where there is only a hot wire with no ground at all? Are they any more susceptable to lightning than a US hot top wire system?
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The neutral (white) is a return ... it carries the same current as the hot wire (black). The ground wire is a non current-carrying safety wire (often bare copper). The purpose of the ground wire is to reduce voltages in the case of lightning or an accident (wires falling across other wires outside of your home and raising the voltage with respect to ground to a dangerous level). The ground wire only conducts current in the case of a fault. Ground fault circuit interrupters need the ground wire to detect such faults and open the circuit when they occur.
People are often shocked and even electrocuted with voltages with respect to ground ... one is standing on a wet basement floor ... one is touching a faucet ... one is in the tub or shower. The voltage with respect to ground is the big issue here (for safety reasons).
An ungrounded electrical system in your home would allow voltages to rise to thousands of volts above ground and fry you if you happened to be grounded (in a tub or standing on a wet concrete floor).
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(often
faults
I believe that GFIs are often recommended in cases where there is no ground for safety. They do not need a ground AFAIN.
Bob
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wrote in message

They need the ground wire.
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When a ground is available at an outlet with a GFCI it is preferred (and required) that it be installed,
However, where there is no ground the NEC still recognized that a GFCI can still provide protection by detecting an imbalance in currents between the neutral and hot wire. In this case, the user must be informed (with a small sticker on the outlet) that the GFCI is ungrounded.
Beachcomber
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Charles Schuler wrote:

Actually, they don't.
See section 210-7(d) in the NEC, and section 26-700(9) in the CEC.
GFCIs are a legal substitute for a grounded outlet in an existing installation where there is no ground available in the outlet box.
Chris
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wrote in message

I should have said "Not a good idea."
http://members.tripod.com/~masterslic/FAQ-2/7.html
Problems are they are supposed to labeled as "ungrounded" and external testers won't work.
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wrote:

A GFCI does not need a ground. Then some people get the strange idea that it PROVIDES one. It does not.
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snipped-for-privacy@xmail.com0.invalid says...

No one said GFCIs did. BobF said they are "recommended in cases where there is no ground". They are. What's your beef?
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I said that because people have. Just not in THIS thread, but in this group.

I was saying that only because some people think they provide ground.
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snipped-for-privacy@xmail.com0.invalid says...

Then perhaps you should correct those who are mistaken?

No one in this threadlet.
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A lot of people read these things without making their presence known. Some will make up the wrong thing.
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Will the TEST BUTTON on the GFI work if it does not have a ground?
If so, how does it work?
Mark
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Yes.
Very well [*]
What it does is measure the current leaving the "hot" lead and compares it with the current returning in the "neutral" lead. If those are different by more than a few milliamperes, there is a Ground Fault, that is, current is finding its way to ground through some other path than "neutral". If that is the case, the GFCI Interrupts (the "I" part of "GFCI") the Circuit, very, very quickly, so that no damage is done to whatever caused the Fault (that could be a human body). The test button simulates a fault.
[*] I had it work once for me. I didn't know it had until the lamp I had dropped into water wouldn't come on again.
-Don
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