Why can't you connect Ground/Neutral bus bars on a sub

sky jumper

the one other comment i'd add is that tying ground to neutral is better than leaving ground open. with an open ground you do have a potential for a "hot" chassis. with gnd tied to neutral at least it will trip the ckt breaker.

also consider conduit states like Illinois where the ground is distributed through the conduit - not a separate ground wire. for devices such as lighting fixtures this system relies on a ground connection being made through the fixture's mounting screws (unless you go out of your way to actually connect the fixture's ground wire to the outlet box - which nobody does). I installed a ceiling fan this week that had a painted mounting bracket with a ground wire attached to the painted surface (presumably the metallic body of the bracket would receive ground through the mount screws from the outlet box/conduit) but then the painted surface would partially insulate the rest of the fixture! this is not a good ground, and in my opinion is more dangerous than tying neutral to ground.

kbsparky

WE had a situation where the main bonding jumper screw was loose, so the panel box enclosure, and HVAC ductwork, and metal ceiling grid ended up being "hot" ... it was quite a mess!

This is what can happen when current-carrying circuits are connected to your equipment ground: undesirable currents begin flowing across all kinds of things, that normally should not be carrying juice.

The other thing to consider is electricity does not take the path of least resistance. It takes all paths, with the majority of current flowing over the path of least resistance. Some of the current will be flowing over other paths of greater resistance.

IF you happen to come in contact with one of those paths of current flow, some of it may end up flowing through you, which can range from a mild shock to electrocution, depending on the amount of current flowing.

Keeping your grounds (non-current-carrying) separated from the neutrals (current-carrying) can minimize these hazards.

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-KB

Life is uncertain -- eat dessert first!!
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Leah Frances

This girl does.

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If I could only remember to THINK about what I was doing before I did it.

Jim Port

Skyjumper, I hope you are not advocating that tieing the neutral to ground at a receptacle. Doing so could cause someone to receive a seroius shock or worse. This is called a bootleg ground and could have deadly consequences.

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Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.

a7ecorsair

Orange you a good girl

sky jumper

i'm certainly not trying to get people killed -- I am merely questioning the conventional wisdom of the dangers of tying ground to neutral. with due respect to the licensed electricians here, I have not yet heard a convincing explanation that addresses the questions and scenarios i've outlined. I like to base my opinions on science and an understanding of the physical laws that govern the flow of electricity as opposed to what is spelled out in a code book.

as you may have guessed I am not a licensed electrician - I am an electrical engineer with extensive training in physics, and this code requirement does not make sense to me - to be clear, I do undestand the need to ground the conducting chassis/surface of a device - but I question the need to provide two separate conducting paths back to panel ground/neutral -- and I do understand the argument to separate current carrying circuits from non-current carrying -- but I question the rationale behind that argument (for the reasons stated in my previous posts). I will readily admit that I do not have much experience "in the field" where untrained handymen and homeowners do things that cause fires and death -- i'm just wondering what those mistakes are because to me it seems the risk here is almost non-existent. even with kbsparky's annecdotal story - how did the hot get shorted to the panel/ducting to beging with and why didn't the main breaker trip as a result? how would a ground connection have prevented it?

Daniel Holzman

I believe skyjumper has made a very good point, one which seems to have been missed completely throughout this discussion. Current flow through a wire in and of itself is not dangerous even if you touch a bare wire. As he pointed out, current will only flow through YOUR body if you are at a different potential (voltage) than the wire. If the wire is the neutral or the equipment ground, it is by definition connected to the ground at the main panel, so the wire should be very close to ground potential, which by the way is taken to be zero BY CONVENTION, not because there is some magic about the ground truly being at zero voltage.

So let's discuss real world dangerous situations that can arise, and how an independent ground wire actually functions.

Case 1, very dangerous: The hot wire becomes disconnected from the device. In the case of AC current, the potential of the hot wire varies between about +170V and -170V 60 times per second. Assuming you are at ground potential, and have a resistance of 1000 ohms (could be higher, could be lower), and you touch the hot wire, you are going to get a shock due to the RMS voltage potential difference of 120V, and in fact you will get I = V/R = approximately 1/8 amp of current through you, which can be quite dangerous. If you are wet, and your resistance is lower, you will get more amperage, and you could die. Bad news. Note that the presence of an independent equipment ground in this case does not help you out a bit, and note that the breaker will not trip either.

Case 2: The hot wire frays a little, and a small hair of wire touches the frame of the appliance. In this case, the frame of the appliance is going to be at an elevated voltage, likely somewhere close to 120V RMS, because the hot wire is touching it. If there is no independent equipment ground, there will likely be only a very small current flow through the frame, because the neutral is typically not connected to the frame of the appliance, and the frame is probably not connected to ground well or perhaps not at all. However, the frame is still at a relatively high voltage, so if you touch the frame, you will get a shock, due to the voltage difference between the frame and you. Current will flow through the frame, through you, and to ground through you, and if the wire hair is large enough, and you are wet, you could get seriously injured or killed. If there is an independent equipment ground, it will provide a low resistance path back to ground, and by Kirchoffs law, there will be very little (not zero, but very little) flow through you, because your resistance is higher than the equipment ground wire.

Note that if the neutral were connected to the frame of the appliance, the neutral would carry the stray current back to ground with lower resistance than you, so again you would sustain only a small amount of current flow through your body. In fact, you might not notice at all that the frame was energized. And if the hair of the wire is small enough, you will not trip the breaker.

So I believe the main reason for the independent equipment ground is to ground the frame of the appliance, since we typically do not use the neutral to ground the appliance frame any longer. However, with dryers we often ground the frame through the neutral, if we have a three wire plug.

Saturday Cowboy

With all due respect, you have to have a really good understanding of advanced electrical theory(and i have seen far too many sparkies that don't) to understand why it is not a good idea.(ohm's law, AC theory etc.) Yes we will all admit that in a perfectly wired and working system there is little risk. But that is not a good way to design a system(only safe under ideal conditions). Plus it is code required(for most of us that settles the matter weather we like it or not)

The short of it is you will always have current(in the whole house) where it is not soposed to be. It will find a time and place for it to devolop into a problem.

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Ragged Trousered Philanthropist


Please follow the code - its there for your safety no matter how inconvenient.

jlmran

Simple answer to OP: it violates the code.

What you really meant to ask: why is it a code violation?

Sorry...I get hung up on semantics.

Saturday Cowboy

Those things are always connected. The problem came when the ground was lost and because there was something using the ground as a return path or neutral if you will, the ground was hot and instead of "draining" the bootleg current to ground, which it couldn't, it sent the current to everything that the ground was connected to.

I'm guessing that the duct and ceiling where connected to building steel which is required to be bonded.

This link from another post shows how some weird stuff can become energized when monkeying with grounds
http://www.youtube.com/watch?v=EQiMr...layer_embedded

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Ragged Trousered Philanthropist


Please follow the code - its there for your safety no matter how inconvenient.

sky jumper

thanks D Holzman. we speak the same language, and your explanations are spot on I believe.

the only caveat to our premise that no (or very little) current will flow through one's body when touching a bare grounded/neutral wire is in situations where the wavelength of the energy is comparable to the length of the conductor in question. in such cases you can actually realize a votage gradient along the length of a single conductor. however, the wavelength of 60Hz is something like 3k miles, so that is not at all a concern in household. it becomes a problem with radio frequency (which is where i do all my work).

jlmran

Bonding neutral and ground bars in a subpanel would result in equipment chassis (serviced by main panel) becoming energized, without a conventional 'fault' occurring.

Being energized doesn't necessarily mean it will shock.

1488rob

Iknew you werent electrical skyjumper like all us skydivers we allways want to question things ..why we would never make good lawyers..having to refer to someone elses work. Who are you on dropzone .com?

I have a question towards this.. On my mainpanel of the halfbuilt house I bought both the neutral and ground wires are wired to same buss and it passed. All breakers are on that side. when I add breakers to the left side wil I be able to wire both neutral and ground to the ground buss?, if not.it will be alot of rewiring as theres no space to put wires left on nuetral buss

Stubbie

I'm not sure why this is so hard for you to understand. All metal likely to be energized in a ground fault must be bonded. All that metal is readily accessible to touch by humans. You are advocating that all metal should be allowed to have current flowing on it by bonding it to the neutral return path and forgo a non current carrying path constructed separate from the neutral.

If you do this .. neutral current has the potential to flow on all sorts of metal devices and appliances via the connection of a power cord or bonding. If a branch circuit neutral would open (very common) then bonded metal would be a viable path for large amounts of current to use to return to the source. If you do not bond the metal to a separate effective fault path then all that metal comes to line voltage if any of it comes in contact with a ungrounded conductor. Now you definitely have a potential voltage ready to push that current thru you so it can return to the source.

So the point is you do not want current flowing on metal that is easily in contact with human touch or where it is unsuspected by the typical unskilled homeowner or person.

You are not correct to think that because a grounded conductor is at zero potential nothing will happen if you come in contact with metal that is carrying neutral current. Fact is nothing is really at zero potentiall just some floating potential that hopefully is close to zero...earth is a good example of this floating reference.

Also remember that AC current is reversing potential rapidly during a complete cycle of the sine wave. Point being if I disconnected a neutral from a panel neutral bar and told you to take the end of that neutral wire (grounded conductor) in one hand and touch the neutral bar in the panel with the other what do you suspect is going to happen ...? My point being you got to go way out of your way to get in contact with neutral current in a system that is grounded. In an system where there are only 2 wires (hot and neutral) in the branch circuits all metal that is easily touched by humans would become energized at line voltage in the event of a fault because you would not bond it to the neutral. You cannot let neutral current use bonded metal as a return path. To do so puts you at a much higher risk of injury or death.
The exception being dryer and range circuits prior to the 1996 code cycle. These are no longer allowed. These 3 wire circuits are actually pretty safe which supports your argument but in the even of an open in the neutral of the branch circuit you just energized the metal of the appliance. Nothing really happens other than think about touching the dryer chassis then touching the washer chassis , I believe you just gave that neutral current a return path to the source and it is going to use you to get there. It only takes milliamps to kill you not amps and milliamps will flow thru you in that event or any event where you put your body in series with two low impedance paths to the source. A 4 wire branch circuit prevents this event as the bond to the dryer metal is removed from the neutral.

So the idea is to keep neutral current off bonded metal and to bond that metal to a separate effective ground fault path that will let a breaker open the circuit in the event of a hot wire (ungrounded conductor) touching that bonded metal.

It would be true if I connected bonded metal to the neutral that a breaker will still open on fault to a hot wire ... after all it could care less what wire (ground or neutal) allows enough current to flow thru it to trip open.

Problem is Ive given the neutral current hundreds of more ways to seek the source that are easily touched by humans that may provide that link.

Given a normally operating system it really is foolish to bond neutral to bonded metal IMO and the NEC.

Neutral current will kill you it is nothing to allow out of the walls to places it isn't expected or easily touched.

There are always events that can throw all this out the window such as an open service neutral ... regardless I'd rather have neutral current flowing on insulated conductors than over bare metal that I can easily touch in places I do not expect it. Electricity is dangerous we cannot make it completely safe but we have to limit the possibilities of it being less safe.

Remember to that circuit breakers do not monitor what is going on with the neutral. Just ask a plumber or two that has had a near death experience with unwanted or objectionable neutral current on metal water pipes and drains..

So to summarize you have these choices

Bond neutral to ground and energize all your bonded metal that humans readily come in contact.

Do not bond metal to neutral and risk line voltage on your bonded metal in the event of a phase fault.

Bond metal to a separate grounding path that is not part of the neutral current carrying system. Doing so keeps neutral current off bonded metal in places you do not expect it to be and the unsuspecting unskilled person cannot easily come in contact with it... part of the reason you only bond neutral to ground in the service equipment.

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" One nice thing about the NEC articles ... you have lots of choices"

Stubbie

kbsparky

It was not the hot that was shorted, it was the neutral that was not properly bonded, thus creating the parallel paths of current flowing through the building steel components. The same thing can happen with any sub-panel whose neutral and ground wires are not kept separated.

Anyone standing on a concrete floor, and happened to touch something as innocent looking as the metal ceiling grid was getting zapped, since the neutral current was flowing through that, as well as the metal ductwork of the heating system, etc. Touching the frame of a fridge and an adjacent stainless steel sink would have the same result.

This effect is more pronounced when a rather large load of current is flowing through the neutral, creating a potential difference between the neutral and ground.

Keeping those non-current carrying metal parts separated from the current-carrying neutral conductor is critical in preventing such hazards.

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-KB

Life is uncertain -- eat dessert first!!
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