Why ground the neutral bar in dwellings?

11Haze29

Member
I know its required, but what is the actual purpose. The utility brings both hots and a neutral to the service panel. They also ground the neutral at the pole with a solid #6 and a ground rod. So by grounding the neutral bar in the service panel, you effectively create a two paths for the neutral current to return to the transformer. One is the neutral conductor, but the other is the earth. Granted the neutral is the low resistance path, but the earth path is still in parallel. So if we had a hypothetical neutral current of 10 amps in our 240/120 single phase service panel, and the neutral cable to the transformer has 0.5 ohms resistance, and the earth path is 25 ohms, then the neutral is carrying 9.8 amps and 0.2 amps is flowing through the earth ground. You probably read the stories about cows being shocked on the farm, and flowing ground current is the culprit.

So in theory, you would NOT want current flowing through the earth. I've heard from fellow electricians that the reason is to act as a 'safety' in case the neutral service cable ever became disconnected. Higher voltages than 120 could develop on line to neutral loads, possibly causing a short or fire. So the neutral being grounded to the earth provide another path that will keep voltages at a reasonable level and also permit single pole breakers to continue to operate.

Does anyone have a better explanation.
 

retirede

Senior Member
Location
Illinois
Main reason is to clear a ground fault. Without the N-G connection, an L-G fault will not trip a breaker.

Your hypothetical 25 ohm resistance of the earth from the home ground to the utility ground is impossibly low. In reality, it will be high enough that under normal circumstances, the voltage gradient across points of the earth that could be touched simultaneously by a person will be imperceptible.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Does anyone have a better explanation.
I'll try. Talking only about the system neutral being earthed, and not about things like EGCs and GECs:

Electrical systems were not always grounded. There was much debate about which was safer for people.

The grounded system assures the risk of a 120v shock between a line conductor and grounded surfaces.

The floating system may reduce risk of a 120v shock, but increases the risk of a very high-voltage shock.

By earthing a system (neutral) conductor, you face a known-voltage risk versus an unknown-voltage one.
 

tom baker

First Chief Moderator
Staff member
As retirede pointed out its to clear a fault. But also See 250 A 1 and A 2.

"I've heard from fellow electricians that the reason is to act as a 'safety' in case the neutral service cable ever became disconnected"
No that is an electrical myth. Please see the last sentence of 250.4 A 5
Much of what the code has for grounding was worked out perhaps 100 years ago. Within the last 20 years article 250 has and continues to clarify the bonding portion of Art 250. Older electricians were taught wrong concepts, such as a ground rod will clear a fault.
 
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Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I think the op was curious as to why we grounded the neutral to earth twice. Once at the pole or trany and the other at the building.
 

kwired

Electron manager
Location
NE Nebraska
most but not all utility distribution systems are MGN, multi grounded neutral systems.

The service equipment needs to be bonded to the grounded conductor in either case.

MGN has advantages as well as disadvantages, but you can't just do it where you feel like it, it is all or none sort of situation. If none, then you would have either no grounded conductor(s) carrying normal operating current or you would at least have a separate EGC for everything and not just beyond the service equipment.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I think the op was curious as to why we grounded the neutral to earth twice. Once at the pole or trany and the other at the building.
Think of it as establishing a grounding electrode at each separate structure.

On the small scale, the earth interconnection is irrelevant; on the large scale, it acts as a ground plane.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I don't I think you could hurt anything by connecting the transformer neutral at the pole to a ground rod and then connecting the same neutral at the service disconnecting means to some other grounding electrode system. There is no equipment grounding conductor run from the pole so whatever current flow there is would flow through the neutral, with a tiny amount of flow through the Earth.

I don't think it makes a hill of beans a difference to be honest. Someone just decided to do it this way many decades ago and that's what we ended up with.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
As I see it, two possible upsides to reearthing the grounded conductor at the premises equipment:

1) If there ever were a potential difference between utility earth (wherever their grounding electrode is) and premises earth, the bonded equipment on the premises should be at local premises earth potential.

2) The premises grounding electrode is under control of the customer, so the customer can be sure that the grounded conductor is earthed regardless of what the utility does.

Cheers, Wayne
 

ramsy

Owner/Operator
Location
LA basin, CA
Occupation
Service Electrician 2017 NEC
Please see the last sentence of 250.4 A 5
And, 250.4(A)(1)
When lightning hits power lines nearby, anyone sitting on toilet in house without GES Main bonding to neutral, risks getting electrocuted by Million-volt bolts jumping across plumbing & gas line potentials not equalized to messenger-line lightning.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
As I see it, two possible upsides to reearthing the grounded conductor at the premises equipment:

1) If there ever were a potential difference between utility earth (wherever their grounding electrode is) and premises earth, the bonded equipment on the premises should be at local premises earth potential.

2) The premises grounding electrode is under control of the customer, so the customer can be sure that the grounded conductor is earthed regardless of what the utility does.

Cheers, Wayne

I disagree, you can still measure voltage to the earth in the back yard even if the neutral is grounded to earth at the main disconnect.
 

kwired

Electron manager
Location
NE Nebraska
I disagree, you can still measure voltage to the earth in the back yard even if the neutral is grounded to earth at the main disconnect.
And the majority of the time it is coming from the utility and not the service at the house.

Shut the main off in the house and it is still there. Disconnect the service neutral and it likely goes away, if it doesn't there is some other metallic path to other services likely involved, water pipe being one the first things to look for.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
And the majority of the time it is coming from the utility and not the service at the house.

Shut the main off in the house and it is still there. Disconnect the service neutral and it likely goes away, if it doesn't there is some other metallic path to other services likely involved, water pipe being one the first things to look for.


Yup, that 7,200 volts has Z going back its substation.
 
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