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Neutral vs Ground

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dknguyen

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What is the difference? From what I can gather neutral is the reference for the circuit and ground is...not necessarily that but seems to be the voltage the "environment" is at.

I ran into a paper from Rockwell and it said the neutral was a reference point and that ground represents an electrical path

**broken link removed**

But then it says something like all wires connected to the neutral should be non-current carrying, but if it's the reference point for the system how can current not flow through wires connected to the neutral point?
 
Ground should not carry current.
At some point neutral and ground are tied together. Normally in the main breaker box or where power comes into a building. All sub panels should not have ground and neutral tied together.
The hot wire and neutral carry the current. Neutral should be very close to 0 volts. Ground is more for safety and is not there to carry power. Years ago houses were wired with out ground.
 
The neutral is connected to the earth but it also carries current, and the fact that it carries current can cause it to become at a higher voltage than the earth , and for this reason another separate connection called ground is made to the earth that carries no current, and therefore does not suffer from this effect?

Right?

I think that article was talking about 3-phase power where neutral currents cancel out (or something like that).

For AC power systems , the neutral is connected to the earth very often so that the power lines do not float very high above the earth so that they become a hazard to people in contact with the earth due to things like charge build-up from the atmosphere? But ground is also there connected to the local earth because a remotely earthed neutral can still have too resistance such that a fault current might choose a person over the neutral. Is this also right?

My main interest in all this is common mode noise where the noise on common and +V neutral are measured with respect to ground and it's not necessarily related to AC power transmission and that's where I get a bit confused.
 
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I see your question now. As fare as high frequency noise is concerned; There is likely noise on hot and neutral that is not present in ground. Filters are designed to reduce this noise. Because ground is the only ‘quite’ line it is used in the filter. There will be high frequency noise currents on the ground wire. Not something you can measure with a simple meter.
 
Oh I see, neutral has current flowing in it and therefore can also have noise (or definately more noise than the zero-current neutral). I see. I edited my previous post about a few more questions about the logic behind sepearate neutral and grounds in AC power system though.
 
The neutral is typically bonded to ground at the transformer and often at the main service panel, although that can be optional. Generally, if the transformer is quite a distance from the main panel, bonding ground to neutral is done. But after that, never the twain shall meet -- whatever the heck that means.

Ground NEVER, EVER carries current except during a fault. But that's why the National Electrical Code specifies that the ground is to be the same gauge as the hot and neutral conductors.

Neutral should be at or near zero volts. It will rise up a bit (or a lot) for a few different reasons: 1) a really heavy electrical load causing a greater-than-normal IR drop in the neutral wire; 2) small-gauge wiring, e.g., the typical crappy 14-gauge circuits in a mobile home that can often make a 120-foot run from breaker to receptacle; 3) loose connections, either at the breaker, at the receptacle, at a plug or in a junction box. A neutral that's running high should be corrected.

The NEC confusingly refers to the neutral wire as the "grounded conductor" and the ground wire as the "grounding conductor". They should be shot for that crap.

Dean
 
dknguyen said:
The neutral is connected to the earth but it also carries current, and the fact that it carries current can cause it to become at a higher voltage than the earth , and for this reason another separate connection called ground is made to the earth that carries no current, and therefore does not suffer from this effect?

Right?

I think that article was talking about 3-phase power where neutral currents cancel out (or something like that).

For AC power systems , the neutral is connected to the earth very often so that the power lines do not float very high above the earth so that they become a hazard to people in contact with the earth due to things like charge build-up from the atmosphere? But ground is also there connected to the local earth because a remotely earthed neutral can still have too resistance such that a fault current might choose a person over the neutral. Is this also right?

My main interest in all this is common mode noise where the noise on common and +V neutral are measured with respect to ground and it's not necessarily related to AC power transmission and that's where I get a bit confused.



Neutral is the return path for the incoming current from the substation. And earth is an additional short cut for the Leakage current during equipment failure due to insulation leakage.In case if the current is leaked to the body of the equipment, it will always choose the shortest path to return and it will be definitely through the person's body to the ground , if he does not have proper insulation over the foot. If earth is connected to the equipment properly, this leakage current will not select the path through the person's body. It will directly get grounded.

Earth is a negative body, which can absorb "N" number of charge.

In the substation Neutral and Earth will be connected. To test this you can do a simple experiment, connect one bulb to phase and the other end to a Deep earth line, the circuit will get complete and the bulb will light normal. Earth resistance is Zero.

And the noise you have cited in the message, might be from an external High frequency source which will be near to the power line. Some devices and equipments which is not designed as per the EN standard and does not comply the CE & RE (Conducted emission & Radiated emission test) will inject noise to the power line either by conduction or by radiation. This noise you will be able to measure with respect to ground.
 
Noise on the mains can be reduced using a delta capacitor network,

Another more effective way is to use a balanced transformer which is just an isolation transformer the the centre tap bonded to earth. This effectively prevents noise from an appliance entering the power supply.
 
Ground is used for safety reasons, its hooked directly to a pole thats dug deep into the dirt. The whole reason behind ground is that if a short occurs due to bad wiring or any other issue, that the ground pulls the current instead of you.
 
speakerguy79 said:
Could you explain what you mean by a delta capacitor network?
Three capacitors are used, one connected from the live to neutral, another from neutral to earth and another from live to earth. The idea is to short circuit the high frequency currents between all the conductors.
 
Three capacitors are used, one connected from the live to neutral, another from neutral to earth and another from live to earth. The idea is to short circuit the high frequency currents between all the conductors.

This technique used to be used on 2-wire radio receivers, amplifiers and other item of electronic nature. It can be DANGEROUS on 2-wire equipment where the chassis is not grounded. If the cap from hot to chassis shorts or has high leakage, it can be lethal. With a grounded chassis, you're OK.

Dean
 
I'd imagine that doing such a thing on a circuit powered directly from the mains violates the electrical code in most countries.

However, it isn't dangerous if it's done on the secondary side of an isolation transformer.
 
Salgat said:
Ground is used for safety reasons, its hooked directly to a pole thats dug deep into the dirt. The whole reason behind ground is that if a short occurs due to bad wiring or any other issue, that the ground pulls the current instead of you.

can someone explain to me why it goes to ground? how can being connected to ground complete the circuit?

thanks.
 
THis article is really good at describing why:
http://amasci.com/amateur/whygnd.html

THe gist of it seems to be:

First we start off with just the two AC lines- since it's AC the lines are symmetric and there is no positive or negative line. They isolated from ground and everything else. Things work fairly well...but...

Why you connect one AC line to ground (the neutral)-
so charge doesn't build up on the power grid at end up floating both AC lines high above earth voltage (which are approximately equal to since you are on the ground). If enough charge builds up you occasionally get a lightning bolt jumping out at you from the power grid. THis line connected to ground is now called the neutral and carries thre turn current from the other "hot" AC line.

Why you have a 3rd line connected to the earth (the ground)-
Now that you've connected one line to the earth, whenever someone touches the hot-wire since the hot-wire has now been referenced to the earth to prevent the charge-build up mentioned earlier. Before you had to touch both wires at the same time for anything to happen (and probably the path wouldn't flow through your heart unless you touched one line with each hand so the current path is through your heart. So you add in a 3rd line for safety.

THis line can do a few things:
If the neutral gets cut, there is still a connection to ground so the entire piece of equipment doesn't float at the same voltage above ground (which will use you as a current path if you touch it).

If you connect this ground to the metal case of equipment, it also means that if the hot wire gets loose and touches the case it has the ground path for the current to flow (rather than you if you touch the case- it's like two resistances in parallel when one is much less than the other). This also makes a short-circuit which blows the fuses if the hot wire does touch the case.

Also, current flowing through the neutral can raise the voltage of the neutral too high above ground to be safe even if it is connected to the earth somewhere remote. Since the voltage of the neutral at your location may be above the earth due to the current flow. So you have a ground connection that doesn't carry current and does't have this problem of raising the voltage above the earth due to current.

And from what someone else mentioned the noise due to current flow in the neutral can cause problems. Ground has no current flow (normally) and thus doesn't have this noise problem for things like PCs, etc. So it seems the reason there is a separate ground and neutral is mainly to solve some new problem caused by solutions to other problems related with the fact that everything has a voltage relative to everything else and charge is always flowing around the earth.

NOT IN THE LINK:
And I guess an isolation transformer allows you to get back the "you have to touch both lines" since in the regular electrical grid you can still get shocked if you touch the hot-line. Then you get the best of both worlds- needing contact with both lines to be shocked and being able to leak away charge build-up in a widespread power grid. I guess this charge absorption problem isn't a problem for small devices but something like a wide-spread power grid intercepts enough earth-atmosphere charge exchange to build up the voltage to dangerous levels above the earth.
 
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HerbertMunch said:
can someone explain to me why it goes to ground? how can being connected to ground complete the circuit?

thanks.

Grounding in house wiring is only meant to make all housing, such as the casing that holds the wire, the same potential as you. Grounding should never carry current unless there is a short or a break in the wire or some other issue. Since the casing is grounded at the same potential as you (or in many cases, a lower potential), it will carry most of the current, and usually trip a breaker before it ever harms you or starts a fire. Earth Ground is only used for safety reasons when something goes wrong, never for completing a circuit under normal circumstances, since that is neutrals purpose.
 
Salgat said:
Grounding in house wiring is only meant to make all housing, such as the casing that holds the wire, the same potential as you. Grounding should never carry current unless there is a short or a break in the wire or some other issue. Since the casing is grounded at the same potential as you (or in many cases, a lower potential), it will carry most of the current, and usually trip a breaker before it ever harms you or starts a fire. Earth Ground is only used for safety reasons when something goes wrong, never for completing a circuit under normal circumstances, since that is neutrals purpose.

yeah but why does it flow to ground?

thanks.
 
One weird thing here in Australia is that the Neutral can wander from ground potential. As the ground is (was - we have had lots of rain) so dry the earth rod at our house has become ineffective. If I now work on a circuit where the live (active) is disconnected, I can still trip the breaker by shorting neutral and earth. Obviously, more than 35mA flows and trips the breaker.

The estate I live on is large and so the properties are well spaced. They are also all on 3 phase. The result is that there is an imbalance in the phase loadings and this results in the neutral drifting away from 0V.

Just thought I would add this little snippet of information because I know I would have been amazed had I come across this in the UK.

Mike.
 
Pommie said:
One weird thing here in Australia is that the Neutral can wander from ground potential. As the ground is (was - we have had lots of rain) so dry the earth rod at our house has become ineffective. If I now work on a circuit where the live (active) is disconnected, I can still trip the breaker by shorting neutral and earth. Obviously, more than 35mA flows and trips the breaker.

The estate I live on is large and so the properties are well spaced. They are also all on 3 phase. The result is that there is an imbalance in the phase loadings and this results in the neutral drifting away from 0V.

Just thought I would add this little snippet of information because I know I would have been amazed had I come across this in the UK.

Not really, it's perfectly normal to read a voltage difference between earth and neutral, usually a few volts - you can even run small torch bulbs off of it, and it doesn't register on your meter! :p

13A switched sockets in the UK only switch the live, so if you're soldering on a live chassis TV set with an earthed soldering iron, and only switch it off at the socket - then as soon as you touch the chassis with the soldering iron it blows the earth leakage trip! - as you say, neutral to earth provides enough current to take the RCD out :D
 
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