[Gigs]

It was suggested on another thread that we sticky common information, I suggested that we just develop it into a FAQ, since we definitely have certain frequently asked questions.

I was thinking maybe a rundown of MWBC and their unique failure modes, or a link to a good description of them. That seems to come up a lot.

Subpanels, as was mentioned, and neutral/ground isolation, objectionable currents.

Some Q's to get us going:

Q: Why must the neutral and ground be separated at a subpanel?

A: Because if they are tied together at a subpanel, current will flow on both the neutral and ground wires. This can cause a situation where a broken neutral wire between the subpanel and main panel would float all the chassis of every grounded device up to line voltage! The same danger applies to "ghetto grounding" outlets, where the neutral wire is jumped to the ground screw.

(insert picture of that sort of dangerous fault)

Q: What purpose does a grounding rod serve?
A: Lightning protection only. Very little current conducts through the grounding rod during normal operation, because the earth is a relatively high resistance connection.


Jump in and add more if you want.

 

[hayewe farm]

Actually a grounding rod is to provide a reference level to the common wire for user safety not just lightening protection.

 

[Stubbie]

Actually a grounding rod is to provide a reference level to the common wire for user safety not just lightening protection.

Sorry but that is absolutely incorrect.

 

[Gigs]

Sorry but that is absolutely incorrect.

See that's why we need the FAQ... if we can agree on them at least.

 

[rgsgww]

Why mwbc hots need to be on seperate "legs" of the service panel. To prevent overloading the neutral wire. Ex. 15 amp single poles feed a mwbc and are on the same leg, both hots are loaded to 15 amps. Neutral total current would be 30amps.

Proper way would be to install a double pole breaker (2008 nec, 2005 has some exceptions) this would make sure the mwbc legs are never switched around, and in the case of a fault or during work, both hots would be dead.

 

[joed]

The first answer is wrong. If the ground is still connected the chassis will NOT float up line voltage if the neutral breaks. The ground wire will simply carry all the current for the neutral.
The reason they are separate is because the ground wire is not supposed to carry current except when a fault is present. If the neutral and ground were tied together the ground wire would be carrying current all the time. It would be split between the neutral and the ground.

 

[hayewe farm]

Sorry but that is absolutely incorrect.

Actually it is correct.

Originally the decision was made to use ground as a universal constant reference point for distribution systems so that they could be easily interconnected. The standards in the NEC is for safety not lightening protection.

The following is a quote from EC&M


"The main reason why grounding is used in electrical distribution network is the safety: when all metallic parts in electrical equipments are grounded then if the insulation inside the equipments fails there are no dangerous voltages present in the equipment case. Then the live wire touches the grounded case then the circuit is effectively shorted and fuse will immediatly blow. When the fuse is blown then the dangerous voltages are away. The safety is the primary function of grounding. Grounding systems are designed so that they do provide the necessary safety functions. Grounding also have other functions in some applications but the safety should not be compromised in any case."

 

[rgsgww]

Ground rods and water pipe grounds are used to divert high voltage or lightning to ground. Also are used to equalize ground potential with the system. You are talking about the equipment grounding conductor.

 

[Gigs]

Yep, it's such a common misunderstanding/mixup that even the wikipedia article about it is very confusing. It would be good to have it in a FAQ.

 

[rgsgww]

Yep, I think it would be a good idea to have an faq. Also include information of the hazards of improper installs.

 

[InPhase277]

Actually it is correct.

Originally the decision was made to use ground as a universal constant reference point for distribution systems so that they could be easily interconnected. The standards in the NEC is for safety not lightening protection.

The following is a quote from EC&M


"The main reason why grounding is used in electrical distribution network is the safety: when all metallic parts in electrical equipments are grounded then if the insulation inside the equipments fails there are no dangerous voltages present in the equipment case. Then the live wire touches the grounded case then the circuit is effectively shorted and fuse will immediatly blow. When the fuse is blown then the dangerous voltages are away. The safety is the primary function of grounding. Grounding systems are designed so that they do provide the necessary safety functions. Grounding also have other functions in some applications but the safety should not be compromised in any case."

Put that in the FAQ also: the difference/similarity between grounding and bonding. The quote above describes BONDING not the earth ground. The earth ground has NO relation to the info in the quote.

 

[Stubbie]

Actually it is correct.

As Inphase said you presented a qoute about the equipment grounding system or more appropriately the effective ground fault path and the bonding of metal likely to be energized in the event of a fault. This system is for human safety as EC&M is explaining.

This has nothing to do with the GES (Grounding Electrode System) which are your ground rods, metal water pipes, ground rings and plates etc... This system is for high voltage events like lightning and utility power surges that may damage property.

 

[hayewe farm]

I see so you can just disconnect from the ground rod and water pipe and unbond the neutral ground connection as long as there is no lightening you have nothing to worry about. That's like saying your break pedal stops the car and the brakes have nothing to do with it.

You do know that lightening can come in on a ground. It can even roll across the insulation without entering the conductor.

If the distribution system is not referenced to ground it would be a floating system and reading to ground from the hot or common could be several thousand volts. That's not a theory that's a fact. Even with DC devices with there own power supply if you don't hook the DGs together when you connect two units together you can blow them up that's why you need ground (DG not earth)first connectors on them.

Without a common reference it would be almost impossible to bring a second generating system up and connect it online.

 

[InPhase277]

I see so you can just disconnect from the ground rod and water pipe and unbond the neutral ground connection as long as there is no lightening you have nothing to worry about.

Yes, you can do everything except un-bond the neutral and ground in the branch circuit, because that bond actuates the overcurrent devices in case of a short.

That's like saying your break pedal stops the car and the brakes have nothing to do with it.

It's absolutely not a similar situation. The analogy is way off. A better analogy would be that you can disconnect the emergency brake and the car will still stop just fine with the regular brakes

If the distribution system is not referenced to ground it would be a floating system and reading to ground from the hot or common could be several thousand volts.

So? That is the value of the electric field due to capacitance. It is phantom, no real current will flow. The air above your head is about 200 V positive compared to the air near your feet on any given day, but you are not being electrocuted continuosly.

That's not a theory that's a fact. Even with DC devices with there own power supply if you don't hook the DGs together when you connect two units together you can blow them up that's why you need ground (DG not earth)first connectors on them.

This has nothing to do with earth grounding or the reasons why we do it.

Without a common reference it would be almost impossible to bring a second generating system up and connect it online.

Right, a common reference is not the earth. A common reference is the CIRCUIT ground. Consider it this way: wire two 12 V batteries in series. The wire between the two batteries is called "ground" and is the point where we reference the voltages of the series system, namely, +12 V and -12 V. There is no earth connection, yet it is still called ground.

The "ground" in our electrical system is just a connection to the system neutral so that fault current can flow in a complete path to its source. The earth ground is a different beast altogether, even though the two are connected. Our electrical system will function just fine without the earth ground. Your car still starts without an earth ground, and it's voltage is not several thousand "real" volts above ground. The space shuttle's electrical system works great, and it is nowhere near the ground.

 

[Stubbie]

I see so you can just disconnect from the ground rod and water pipe and unbond the neutral ground connection as long as there is no lightening you have nothing to worry about. That's like saying your break pedal stops the car and the brakes have nothing to do with it.

Remove all the grounding electrodes you want and even remove the neutral to ground bond (main bonding jumper) at the service equipment and your electrical system will be just fine. You will have safety protection everywhere but the metal case of the service equipment. You will not have high voltage protection from lightning.



I think what you need to do in this discussion is explain why you think connection to ground rods and water pipes offer human safety and why the neutral to ground (earth) connection at the main panel provides safety to humans.

 

[goose134]

Stubbie and InPhase are right on. I can never explain it as well as these two but the information is dead on.

 

[goose134]

In regard to the OP, I think a few diagrams of three way wiring, receptacles, and switches might help. I believe someone on this site had a good visual of how a MWBC works as well. That might be helpful.
I know it has been mentioned, but sub panels MUST be brought into the sticky. I think that grounding and bonding would be a Herculian chore to explain and enumerate all the different possible scenarios for most DIY participants ( no offense intended). Those issues will always come up and will always be addressed. Let's not hijack the thread people!

 

[jamiedolan]

I see so you can just disconnect from the ground rod and water pipe and unbond the neutral ground connection as long as there is no lightening you have nothing to worry about. That's like saying your break pedal stops the car and the brakes have nothing to do with it.

You do know that lightening can come in on a ground. It can even roll across the insulation without entering the conductor.

If the distribution system is not referenced to ground it would be a floating system and reading to ground from the hot or common could be several thousand volts. That's not a theory that's a fact. Even with DC devices with there own power supply if you don't hook the DGs together when you connect two units together you can blow them up that's why you need ground (DG not earth)first connectors on them.

Without a common reference it would be almost impossible to bring a second generating system up and connect it online.

Quoting Mike Holt;

"What is the Purpose of the Ground Rod?
The ground rods serve no purpose in clearing line to line or line to case faults. An electrical system will operate “ungrounded” and some industrial systems are designed to operate ungrounded. The purpose of the Ground Rod, or more accurately Grounding Electrode System is given in Section 250.4(A) (1) and (2):
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
(2) Grounding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.

Authors Note: A ground fault at a metal street lighting pole, with a ground rod but no low impedance (green wire) path back to the source, does not allow sufficient current to flow to open the over current protective device when using the earth as the sole return path. Even at 25 ohms ground resistance, a ground rod will not assist in clearing the fault for a 120 V supply source. At 25 ohms resistance the maximum current will be 4.8 amps at 120V. The metal lighting pole will remain energized and a serious shock hazard. A supplemental grounding electrode can be added at street lighting and traffic signal poles, but Section 250.54 states the earth shall not be used as the sole equipment grounding conductor."

Check out Grounding Versus Bonding Book - 2005 by Mike Holt:

http://www.mikeholt.com/bookitem.ph...rchtext=&product_type=Book&category_from=Book


Jamie

 

[Gigs]

In regard to the OP, I think a few diagrams of three way wiring, receptacles, and switches might help. I believe someone on this site had a good visual of how a MWBC works as well. That might be helpful.
I know it has been mentioned, but sub panels MUST be brought into the sticky. I think that grounding and bonding would be a Herculian chore to explain and enumerate all the different possible scenarios for most DIY participants ( no offense intended). Those issues will always come up and will always be addressed. Let's not hijack the thread people!

Well, we can't be a complete how-to book, but I do agree the most important points of subpanels should be included.

 

[hayewe farm]

Gentalemen, thank you for the dicussion and enlightenment, but I do have one question.
scenerio: The only earth ground connection is a high impedance connection at the the transformer and the house bonding connection where in place, the neutral and ground fault are connected together at the panel but not to a water line or a ground electrode. This in effect connects the "neutral" to the frame of a stove through the ground fault connection. If you touch the stove and a water pipe would you get shocked? If not why wouldn't you?