neutral vs ground

[kimbri]

Why is the neutral and the ground connected at the same point at the service? Why don't you get a shock from the neutral when it is "hot"?

 

[LarryFine]

Re: neutral vs ground

Your first question answers the second. A proper neutral is never "hot", voltage drops aside.

 

[iwire]

Re: neutral vs ground

Originally posted by kimbri:
Why is the neutral and the ground connected at the same point at the service?

To facilitate the operation of the overcurrent devices.

Originally posted by kimbri:
Why don't you get a shock from the neutral when it is "hot"?

Because it is 'grounded' but keep in mind an open neutral can shock you.

 

[bthielen]

Re: neutral vs ground

You can find many references on this site to the term "neutral" and how it may be poor terminology. The more accurate term for a neutral wire is "grounded conductor". An open neutral is no longer grounded and therefore not neutral anymore, but is in reality an extension of the "hot" therefore, you could get a shock as easily as if you contacted the "hot" conductor.

Bob

 

[charlie b]

Re: neutral vs ground

I have written a "canned" answer to this question. It's too long for the FAQ section. So I'll just post it again here.

Why grounds and neutrals are tied only at the main service, and not at a subpanel.

Reference: NEC article 250.42.

The reasons that there is a difference are (1) That current is always seeking a path back to its source, and (2) That current will take every available path.

The function of the equipment grounding conductors (EGC), that ones that connect to the ground bar in the panel, is to carry fault current. If a fault occurs with a piece of equipment, such that a hot conductor comes into contact with the case or other external metal part, any person who touches that equipment is going to get a shock. The shock can be enough to kill, but the current will not be high enough to cause the breaker to trip.

However, with the EGC creating a path from the case back to the ground bar, then via the ground screw or bonding jumper to the neutral bar, the current in this path will be high enough to trip the breaker. This will terminate the event before the person can receive a fatal shock. That is why the ground and neutral buses are connected at the main service disconnecting means ? to complete the current path from the fault point back to the source. In this context, I am treating the main panel as the "source." Once the current gets to that point, it has nowhere else go.

If you also connect the ground and neutral at a subpanel, then there will be two paths for current to flow back to the source during normal operation. Current will be flowing in the neutral most of the time (unless the loads running at the moment are perfectly balanced among the phases). But with the ground and neutral tied together both at the main panel and at the subpanel, the EGC will be in parallel with the neutral wire. Therefore, the EGC will carry current. This will cause the external metal parts of each and every component that has an EGC its to become energized. You could not safely touch anything in the facility.

 

[roger]

Re: neutral vs ground

A Neutral (Grounded Conductor) properly connected will carry current back to source but will not have voltage on it due to the fact that voltage has dropped to zero across the load(s) it is connected to.

For instance, a 120 watt lamp (120 watts for simplicity) fed with 120 volts will have 1 amp flowing through the complete circuit trying to get home (source), but the voltage only flows to the load on the hot conductor and is used up going across the fillament dropping to zero.

If you open the neutral on the downstream side of the load, the 1 amp of current can not flow back to it's home (source) leaving it with 120 volts at this point since it has not dropped to zero. Now if any part of your body gets between the break in the conductor, bridging it together, the voltage will drop across you and the 120 watt lamp in series providing a shock.

Roger

 

[LarryFine]

Re: neutral vs ground

Originally posted by roger:
...but the voltage only flows to the load on the hot conductor and is used up going across the fillament dropping to zero.

This gives the impression that there is not an equal amount of current on the neutral conductor, which I know is not wnat you mean. You're referring to voltage-to-earth.

If you open the neutral on the downstream side of the load, the 1 amp of current can not flow back to it's home (source) leaving it with 120 volts at this point since it has not dropped to zero.

Meaning that, with zero current, there is zero voltage drop across the intended load, leaving full voltage across the break in the circuit.

Now if any part of your body gets between the break in the conductor, bridging it together...

(or bridging from the load (hot) side of the open in the neutral to another grounded surface)

...the voltage will drop across you and the 120 watt lamp in series providing a shock.

Absolutely! The voltage drop across each load section of a series circuit is proportionate to that section's resistance.

For example, if you have a 10 ohm and a 2 ohm resistance in series, and a 120-volt source, there will be 100 volts across the former, and 20 volts across the latter.

Guess which has a higher resistance: a light bulb or a human body! And, the proportion is a lot more than 5:1!

 

[Paul B]

Re: neutral vs ground

Larry I am in no way trying to make light of your explanation. I like the analogy. The resistance through the human body is much greater then the 10 and 2 ohms of resistance in your example. So the current through you would be much less then the 10 amps. I guess that is why there is so many electricians still kicking.

I learned in electronics school that it only takes 140 mA to kill you through one hand and out the other.

Just food for thought.

Paul

 

[roger]

Re: neutral vs ground

Larry, look at the OP's profile, hence my very basic and simple series explanation. (actually I like simplicity anyways )

Originally posted by LarryFine:

Originally posted by roger:
...but the voltage only flows to the load on the hot conductor and is used up going across the fillament dropping to zero.

This gives the impression that there is not an equal amount of current on the neutral conductor, which I know is not wnat you mean.

You need to include the whole sentence, I think you are confusing voltage and current.

For instance, a 120 watt lamp (120 watts for simplicity) fed with 120 volts will have 1 amp flowing through the complete circuit trying to get home (source), but the voltage only flows to the load on the hot conductor and is used up going across the fillament dropping to zero.

Notice the current is the same through out the circuit, voltage is not, Kirchoff's voltage and current laws.

Roger

[ January 06, 2006, 06:29 AM: Message edited by: roger ]

 

[charlie b]

Re: neutral vs ground

Originally posted by Paul B: The resistance through the human body is much greater then the 10 and 2 ohms of resistance in your example. So the current through you would be much less then the 10 amps. I guess that is why there is so many electricians still kicking.

So presume that the resistance of a human body is 1000 ohms, and put that unfortunate body in series with the 10 and 2 ohm resistors. The total resistance is 1012 ohms. With a 120- volt source, the current will be 119 milliamps. That is enough to kill, and is the reason that there are far, far too many electricians (and homeowners) who are not kicking any more.

 

[charlie b]

Re: neutral vs ground

Originally posted by roger:. . . I think you are confusing voltage and current.

. . . but the voltage only flows to the load on the hot conductor

It would help prevent confusion if you did not use the verb "flow" in the context of voltage. Current "flows"; voltage "drops."

 

[roger]

Re: neutral vs ground

Originally posted by charlie b:
It would help prevent confusion if you did not use the verb "flow" in the context of voltage. Current "flows"; voltage "drops."

Good point.

Roger

 

[LarryFine]

Re: neutral vs ground

Originally posted by roger:
Larry, look at the OP's profile, hence my very basic and simple series explanation. (actually I like simplicity anyways [Wink] ) ... You need to include the whole sentence, I think you are confusing voltage and current.

I definitely am not, but I feared that someone who needs the simplistic explanation may have been confused by those very terms, which are of course not synonymous.

For instance, a 120 watt lamp (120 watts for simplicity) fed with 120 volts will have 1 amp flowing through the complete circuit trying to get home (source), but the voltage only flows to the load on the hot conductor and is used up going across the fillament dropping to zero.

This line, in my opinion, is more confusing to someone with less of an understanding of electrical theory than to us.

Notice the current is the same through out the circuit, voltage is not, Kirchoff's voltage and current laws.

Of course, we understand this stuff, whereas some others may not.