sub panel no bond, why?

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  #1  
Old 01-23-06, 07:41 AM
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sub panel no bond, why?

Why no bond between ground and neutral in sub panel?
And, why a bond between gound and neutral in Main panel.
I've been curious about this.
Can you explaing just exactly why this is.
I'd like to understand the theory.
Thanks in advance.
 
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  #2  
Old 01-23-06, 08:07 AM
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In order to fully describe the theory, the answer would require a book...so if you want to know the detailed answer, I strongly suggest the book 'Soares Book on Grounding and Bonding' http://www.contractor-books.com/IA/2005_Soares.htm

In a brief nutshell: The voltage between two points is only well defined if those points are electrically connected. If your electrical system were not bonded to ground then the 'neutral' could end up at 2400V and the 'hot' at 2520V (example voltages, if your system were not bonded you could get _any_ pair of voltages), rather than the desired 0V and 120V. So for safety you _must_ bond the electrical system to ground.

However you don't want any current to actually flow in the ground system, so you don't want any sort of complete circuit that involves the ground system. A complete circuit requires a loop, so by connecting the electrical system to ground at one, and _only_ one point, you provide the necessary ground reference, but prevent any circuits which involve the ground system.

-Jon
 
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Old 01-23-06, 08:11 AM
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At one place, and only one place, in a residential electrical system the ground and the neutral must be bonded together. This bond allow any fault current on the ground wire to find it's way back to the source via the neutral wire. This will allow current to flow and trip the circuit breaker.

In a normal sub panel setup, you have both a ground wire and a neutral wire connecting the main panel and the sub panel. If you connect these together at the sub panel, then your neutral current will flow back to the main panel on both of the wires, rather than on just the neutral wire. This is wrong because the ground wire is not designed to carry current.
 
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Old 01-23-06, 09:34 AM
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Great explanations. Thanks. I think I'm understanding
this a little better.
So then, ie., the hot wire comes loose in an outlet, touches the metal box, travels back to the main panel via the ground wire, to the ground lug, then over to the neutral lug, via the bonding "strip", then back to the source, thus, allowing the breaker to be overloaded and tripped.
Okay, however, the more I know, I realise there's much more that I don't know. Thus, more questions.
Next question. Okay, the ground lug makes direct contact with the main panel box, and a ground wire, connected to the ground lug, runs to the grounding rod in the Earth. So, is the ground wire from the ground lug to Earth just there to protect the main panel, should there be any current that happens to touch the panel and needs a place to go, before it does major damage to the main panel, or perhaps starts a fire in the box?
 
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Old 01-23-06, 09:57 AM
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What if they dont ground the Main Panel but just use Neutral and two hots? I just saw this at a house... another non professional installation apparently.
 
  #6  
Old 01-23-06, 10:01 AM
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There are two components of the grounding system in a house.

You are correctly describing one part of the system. We ground devices with metal cases (mostly major appliances like refrigerators, washers, etc) to provide a path back to the source for any fault current. If everything goes well, this grounding system will see no current, or in the event of a problem, will see a very high current which will cause the breaker to trip.

The connection between the grounding system in the panel and earth ground (via water pipes or a ground rod , or similar means) is for protection from current from other sources, most notably lightning, and also for reference. As Jon stated, you want your household 120 volts to be centered around 0 volts, not around some other voltage.
 
  #7  
Old 01-23-06, 10:13 AM
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The main ground bond protects your entire electrical system from excessive _voltage_, not current.

Consider a common 1.5V battery. If you connect the two terminals of a volt meter to the two terminals of the battery, you will measure about 1.5 volts. If you connect a suitable lamp to the battery, the lamp will glow. Now lets do a thought experiment: Take your battery (with glowing light) and connect _one_ terminal of the volt meter to one of the battery terminals, and don't connect the other terminal. What voltage is measured?

The answer is not well defined. The voltage measured will _probably_ be near zero, perhaps a few volts. But if there were strong electromagnetic fields nearby, the voltage could be in the 10's of volts, and if it were a dry day and someone had just walked across the carpet and then touched the battery/lamp circuit, the voltage might be _thousands_ of volts. An independent electrical circuit, not connected to anything else, does not have a well stabilized voltage with respect to the rest of the world. The 1.5V circuit remains a 1.5V circuit, but it could be anything with respect to the rest of the world.

Your electrical system is bonded to ground in order to _force_ the neutral voltage to be _0_ with respect to ground, and for all of the other voltages to be well defined relative to ground. This protects things like the insulation on the wires, which is only suitable for a few hundred volts.

-Jon
 
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Old 01-23-06, 10:15 AM
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That was a great explanation
 
  #9  
Old 01-23-06, 10:32 AM
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Wow!! Let me absorb all that. I THINK I understand, but also, now have more questions. What a viscious circle, huh?!? I'll chew on your detailed responses and probably be back for more. THANKS!!
 
  #10  
Old 01-24-06, 09:00 AM
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If they neutral and ground were connected together at the subpanel, then all the current which needs to get back to neutral at the main panel would have 2 parallel wires to travel through: the neutral wire and the ground wire. It would split more or less equally. Therefore, some small voltage would be developed at the subpanel end of that wire with respect to true earth ground, and every tool or whatever which was grounded by its plug would therefore not be at true earth ground.
 
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Old 01-24-06, 10:07 AM
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True Earth ground? Oh sure, throw some more fire wood onto the flames of my confusion! My brain is already on overload. I'm gonna post some more questions, ready or not!
 
  #12  
Old 01-24-06, 12:06 PM
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am I understanding this post correctly

So what you are saying is if you bond it at the subpanel then inadvertantly you will allow you ground to be come hot?

Also is there a way to know if one improperly did this by testing a plug? I would assume if this is the theory then your tester would indicated hot ground or open ground?
 
  #13  
Old 01-25-06, 05:16 PM
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Originally Posted by brianhunter01
So what you are saying is if you bond it at the subpanel then inadvertantly you will allow you ground to be come hot?
Correct. You create a situation in which electrical current is normally flowing on the safety ground; thus making it not-so-safe. This is the same safety ground that is bonded to your plumbing and metal appliances. This is one cause of the hazard where people feel tingling or get shocked while in the shower.

Also is there a way to know if one improperly did this by testing a plug? I would assume if this is the theory then your tester would indicated hot ground or open ground?
It would be rather difficult to test for compared to just opening up the subpanel(s) and looking for isolated neutrals.
 
  #14  
Old 01-25-06, 06:25 PM
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Actually, if the neutral were connected to ground at multiple locations, the ground wires would not become 'hot' in the sense that they would be at high voltage. They would still be at the same sort of low voltage that you expect the neutral to be at.

However there would now be _current_ (movement of electrons) flowing through the equipment grounding system, and the equipment ground wires would be rendered more like the neutral than the safety ground. This can itself present a safety issue (eg. conduit fittings are tested to be suitable as safety grounds, but not tested for suitability as continuous conductors), and reduces the redundancy should something else go wrong (eg. a broken neutral).

There are meters which will tell you the distance from a receptacle to the neutral/ground bond, or meters which will measure the impedance of the neutral to ground path; with these measurements you might be able to detect a 'bootleg' ground or an improperly grounded subpanel.

-Jon
 
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