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Why use a ground when the nuetral can do the same thing?

F

fishbulb

Jan 1, 1970
0
In a typical home breaker box the neutrals and grounds are on the same
ground bar.
So why if I install an outlet do I need to ground the box?
Why couldnt I just take a jumper from the neutral and ground the box
with it? They go to the same place.....if the hot wire touched the
outlet box with the nuetral grounding it, it would still short out and
trip the circuit......so why do we need the extra ground wire?
 
A

Andrew Gabriel

Jan 1, 1970
0
In a typical home breaker box the neutrals and grounds are on the same
ground bar.
So why if I install an outlet do I need to ground the box?
Why couldnt I just take a jumper from the neutral and ground the box
with it? They go to the same place.....if the hot wire touched the
outlet box with the nuetral grounding it, it would still short out and
trip the circuit......so why do we need the extra ground wire?

and what happens should your combined neutral and protective
ground wire break?
 
Z

Zathera

Jan 1, 1970
0
fishbulb said:
In a typical home breaker box the neutrals and grounds are on the same
ground bar.
So why if I install an outlet do I need to ground the box?
Why couldnt I just take a jumper from the neutral and ground the box
with it? They go to the same place.....if the hot wire touched the
outlet box with the nuetral grounding it, it would still short out and
trip the circuit......so why do we need the extra ground wire?

the neutral is a grounded conductor. it is established at the service. It
need to remain separate at all other locations.

Yes unless the box is non metallic. If there is no return path then the
breakers will not trip

What your describing is the way electricity was installed before circuit
breakers. Years ago there was no ground pulled with the conductors to the
location of use. People died hence we got smarter and started requiring
grounds.

If it is grounded properly then you will not die if there is a fault or
short circuit.
 
R

ripper

Jan 1, 1970
0
Get it right - wire is an electrical component!

w_tom said:
First and foremost is an erroneous assumption that wire is a
perfect conductor. Wire is electrically different at both
ends because even wire is an electronic component. Others
have further expanded on the concepts with correct answers.
But your first mistake is to assume wire is a perfect
conductor.
 
F

fishbulb

Jan 1, 1970
0
and what happens should your combined neutral and protective
ground wire break?

The same thing that would happen if my ground wire became seperated
from the box.....
 
F

fishbulb

Jan 1, 1970
0
the neutral is a grounded conductor. it is established at the service. It
need to remain separate at all other locations.

Yes unless the box is non metallic. If there is no return path then the
breakers will not trip

What your describing is the way electricity was installed before circuit
breakers. Years ago there was no ground pulled with the conductors to the
location of use. People died hence we got smarter and started requiring
grounds.


I am not saying to not ground the box. I am saying if I put a jumper
from my neutral wire to the box it would accomplish the same thing.

If the hot contacted the box the breaker would trip.

I know the way it is 'supposed' to be done .....I just dont know a
'technical' answer for why.

I am looking for a more detailed reason.....just trying to 'really'
understand the concept here.

Thanks.
 
A

Andrew Gabriel

Jan 1, 1970
0
The same thing that would happen if my ground wire became seperated
from the box.....

No.
In the former instance, a grounded appliance is left with a live case.
In the latter instance, the case has lost its earthing, but is not
live (unless a second fault also exists).
 
B

Ben Miller

Jan 1, 1970
0
fishbulb said:
I am not saying to not ground the box. I am saying if I put a jumper
from my neutral wire to the box it would accomplish the same thing.

If the hot contacted the box the breaker would trip.

I know the way it is 'supposed' to be done .....I just dont know a
'technical' answer for why.

Although they connect together and to the grounding electrode at the service
entrance, they perform different functions.

First, we have the "grounded conductor" (neutral), which normally carries
current. You do not use that to ground the outside of enclosures, conduit,
etc, because you don't want current flow through the surfaces that peo[ple
can contact. As was pointed out, if the neutral opened up somewhere in the
system, everything on the side of the break away from the service panel
would rise to line voltage with respect to earth. Even a high resistance can
result in lethal potentials.

Next, we have the "grounding conductor" (green wire) that is used to provide
the equipment ground throughout the system. It does not normally carry
current, other than small leakage currents from the connected equipment.
Therefore, an open equipment grounding conductor would have no negative
effect, unless an additional fault occured.

The concept is to not mix the current carrying and safety grounding
functions on the same conductors.

Ben Miller
 
L

Louis Bybee

Jan 1, 1970
0
fishbulb said:
I am not saying to not ground the box. I am saying if I put a jumper
from my neutral wire to the box it would accomplish the same thing.

If the hot contacted the box the breaker would trip.

I know the way it is 'supposed' to be done .....I just dont know a
'technical' answer for why.

I am looking for a more detailed reason.....just trying to 'really'
understand the concept here.

Thanks.

In theory you are partially correct. Using a jumper, and "grounding" the box
to the Neutral in the box, in most cases, would cause the breaker to trip if
the hot touched the box. There are four reasons I'm aware of for not doing
the above.

1) Most current electrical codes I'm aware of prohibit it.

2) Connecting the Neutral to the metallic frame of a piece of equipment, or
other metal surfaces, allows the metal surfaces to rise above ground
potential where high current flow in the Neutral results in elevated
impedance of the Neutral Conductor.

3) Having a very low impedance path on the ground wire, not normally a
current carrying conductor, usually results in quicker tripping of the over
current device in the event of a short to ground.

4) Electronic devices plugged into a properly grounded circuit are less
likely to experience difficulties from an elevated ground reference, and
more immune to common mode noise.

I'm sure others here can offer additional reasons why it would ill advised.

Louis--
*********************************************
Remove the two fish in address to respond
 
R

Rowbotth

Jan 1, 1970
0
fishbulb said:
In a typical home breaker box the neutrals and grounds are on the same
ground bar.
So why if I install an outlet do I need to ground the box?
Why couldnt I just take a jumper from the neutral and ground the box
with it? They go to the same place.....if the hot wire touched the
outlet box with the nuetral grounding it, it would still short out and
trip the circuit......so why do we need the extra ground wire?

In most boxes, the Ground Bus and the Neutral Bus are already tied
together.

But you ground the box because your transformer ground goes to earth,
and as ground fault current wants to get back to source, the most direct
path is via the two grond rods through earth. And also, usually the
grond fault current which is available is far in excess of the rating of
your neutral conductor, and the last thing you want to do is melt your
life-line. Because you are then literally up the proverbial creek.


HR.
 
D

daestrom

Jan 1, 1970
0
fishbulb said:
I am not saying to not ground the box. I am saying if I put a jumper
from my neutral wire to the box it would accomplish the same thing.

If the hot contacted the box the breaker would trip.

I know the way it is 'supposed' to be done .....I just dont know a
'technical' answer for why.

I am looking for a more detailed reason.....just trying to 'really'
understand the concept here.

What everyone else has said so far, I agree with.

ANOTHER thing to consider is: Grounding via the neutral means a single
failure (open neutral) can create a hazardous condition (voltage between box
and consumer). With a separate 'grounding conductor' it would take two
failures (open grounding conductor *and* hot wire touching the box).

Two failures to create a hazardous condition is less likely to happen than
one. Not 'perfect', but much better.

daestrom
 
D

daestrom

Jan 1, 1970
0
Rowbotth said:
In most boxes, the Ground Bus and the Neutral Bus are already tied
together.

But you ground the box because your transformer ground goes to earth,
and as ground fault current wants to get back to source, the most direct
path is via the two grond rods through earth. And also, usually the
grond fault current which is available is far in excess of the rating of
your neutral conductor, and the last thing you want to do is melt your
life-line. Because you are then literally up the proverbial creek.

You know, I've read this from many folks and just have a hard time with it.
I think one of the main reasons to connect the 'grounding conductor' and the
'grounded conductor' at the service entrance is so current does *not* have
to flow through the grounding rods to trip the breaker.

A fault in equipment connecting 'hot' to the casing will allow current to
flow back through the grounding conductor to the service entrance. There,
it is connected *directly* to the neutral and flows through the neutral back
to the pole transformer. The resistance of the grounding rods is not a
factor in tripping the breaker.

I'm not saying a good earth grounding system isn't important, just that it
isn't what trips a breaker when there is a fault from 'hot' to the
'grounding conductor'. Certainly the tie in the service panel and the
neutral conductor from the service panel back to the pole transformer is
*much* lower resistance than150 feet of even wet mud (much less dry sand).

daestrom
 
T

Tim Jackson

Jan 1, 1970
0
I am looking for a more detailed reason.....just trying to 'really'
understand the concept here.

Thanks.
Simple really.

1. If you don't connect exposed metalwork to anything, then leakage currents
will flow to it, and potentially through you to ground. So if an insulator
breaks down, or leaks badly, it can kill you.

2. If you connect exposed metalwork to neutral then it won't be at ground
potential, it will be a couple of volts up because of the voltage drop in
the neutral. This won't kill you but it will cause corrosion if wet and it
could under some circumstances cause fire, it can certainly light a torch
bulb because a low resistance connection from neutral to ground will carry
the full circuit current albeit at low voltage. What can kill you is the
current surge when a fuse or breaker trips. You get a momentary very large
current before the trip, in which the whole supply voltage to be dropped
half each in live and neutral leads. This short pulse can stop hearts,
rarely, although with modern overcurrent protection the risk is not as
serious as it once was.

3. Finally, when the breaker does go out there is an inductive pulse, which
is what makes it arc. How high and how long depends on lots of things, but
that can jump fuses, jump breakers, raise your equipment up to kilovolts for
milliseconds and generally create havoc. It also rings at high frequency so
can be coupled capacitatively. This pulse is probably more likely to be
responsible for deaths. The worst case is probably when there is no load on
the supply transformer, a colleague of mine put accidentally put an ohmmeter
across a 240V 15A spur under these conditions. His test meter exploded
despite all the fuses and breakers in the circuit!

If instead you return the exposed metalwork through a non-current-carrying
conductor right back to the electromotive source, usually a transformer, to
a point which is electrically connected to the groundwater, then you only
have the leakage current to create voltage drop, not the fault current, and
all the mayhem stays safely inside its Faraday cage, and you can't get
killed by conduction from exposed metalwork through you to the groundwater
and back to the source.

Tim Jackson
 
B

Ben Miller

Jan 1, 1970
0
Rowbotth said:
But you ground the box because your transformer ground goes to earth,
and as ground fault current wants to get back to source, the most direct
path is via the two grond rods through earth. And also, usually the
grond fault current which is available is far in excess of the rating of
your neutral conductor, and the last thing you want to do is melt your
life-line. Because you are then literally up the proverbial creek.

Most of the fault current will return to the transformer through the copper
grounded service conductor, which is in parallel with the path through the
earth and has perhaps 50 to 100 times lower resistance!

The main reasons for grounding have to do with stabilizing the system
voltages, especially in cases of lightning or contact with higher voltage
conductors, and allowing protective devices in the distribution system to
function.

Ben Miller
 
R

Ryan Evans

Jan 1, 1970
0
If current is allowed to go to ground without using the neutral conductor,

EMFs around the unbalanced cables and raceways could cause problems.

....not to mention making GFIs and their safety characteristics useless.

R.E.
 
R

Rowbotth

Jan 1, 1970
0
Ryan Evans said:
If current is allowed to go to ground without using the neutral conductor,

EMFs around the unbalanced cables and raceways could cause problems.

...not to mention making GFIs and their safety characteristics useless.

R.E.

OK, as long as you are talking about downstream of the main panel. But
if you are talking about between the service transformer and the main
transformer, these comments do not apply.

HR.
 
L

Lou K

Jan 1, 1970
0
The only place the neutral is to be bonded to ground is at the service
entrance. Neutrals carry current ! That is the return path for your single
phase circuits. You don't want ground carrying current. If you take your
neutral to ground at your outlet, this creates a parallel current path
through your ground. What would happen if you lost the ground connection at
your outlet from the rest of the system? Now the box (If metal) can be
carrying current. You could get bit. The neutral is safe to touch as long as
it is at ground potential. !!!!! Don't go grabbing the neutral !!!!!! Just
trust me !!!

If the neutral at your outlet is isolated from the neutral going all the way
back, and let say there is a neutral from a lighting circuit tied into it,
if someone were to turn on the light switch, you would read almost full
voltage on the neutral. This could be a very dangerous situation.
 
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