Why only one path to ground?

[SmithBuilt]

At the service why is only one path to ground allowed? What is the reasoning for this?

 

[charlie b]

I don?t think that is true. Or perhaps we are not using the same words in the same manner. You can, and often do, have more than one grounding electrode (e.g., water pipe, UFER, ground rod). That makes for several ?paths to ground,? as you put it. But those paths seldom, if ever, carry current. That is not their job.

Are you speaking about the fact that the ground bar and the neutral bar are bonded to each other at the service point, and nowhere else?

 

[bphgravity]

I agree with Charlie. I think the submitter is refering to 250.24(A)(5) ?

 

[SmithBuilt]

I'm referring to the grounded conductor from the meter to the service disconnect. Why is it so critical to only have one path to ground here?

edit due to wrong term.

 

[winnie]

Well, if you connect your _ungrounded_ conductor to a grounding electrode, you will kill lots of worms. *grin*

The issue with the _grounded conductor_ (also called the neutral) having multiple connections to 'Earth' is _not_ the connection to the soil. Multiple connections between the grounded conductor and soil are not considered a problem. (More below.) The issue is that we intentionally bond together (and additionally connect to soil) _all_ of the metal in a structure that is not expected to carry current. Conduit, water pipes, building steel, etc. All of this metal is electrically connected together, to provide protection from unintentionally energizing it.

The grounded conductor is part of the circuit, and is _expected_ to carry current. If you now make multiple connections between the grounded conductor and the bonded, grounded metal, then the bonded, grounded metal becomes a 'parallel path'. This means that some fraction of the current that should be flowing in the neutral is flowing through the bonded metal.

How much problem is cause by this ill defined current flow through unintentional conductors is not easily known. If the current path is large and solid (building steel) then it probably isn't much of a safety issue. Welders use building steel all the time as part of their circuit. But it is an uncontrolled current path, one with unknown safety issues.

On multiple connections to the Earth itself: this is actually common practise in the utility issue (search for "Multiple Earth Neutral"). It is also common practise for multiple customers sharing the same service transformer to ground the neutral at each service entrance, again multiple connections to Earth itself. Generally this is not a problem, however sometimes this shows up as a cause of 'stray voltage'.

-Jon

 

[SmithBuilt]

I can see the load side issues. Then you bond at the service the neutral and ground. My question (poorly stated I now see) is just between the meter and the disconnect. For example I could not run the neutral(bonded to both the meter and mdp) and emt between the meter can and the mdp. That would provide two paths to ground.

It looks like more paths to ground in this case would be better or certainly not hurt anything. Just trying to understand why that is not true.

 

[bphgravity]

No, that is not correct. You can create as many parallel paths of grounded (neutral) current on the line side of the main service disconnects as you would like however risk violating 250.6 of the NEC.

 

[SmithBuilt]

I read over 250.6 but don't understand how that would cause objectionable current. After all they are bonded at this point.

 

[bphgravity]

Perhaps you would benefit from a basic grounding and bonding lesson. There are lots of good topics on this forum regarding grounding and bonding basics. Your confusion is understood as most are confused with nthe requirements and concepts in Article 250.

 

[al hildenbrand]

I could not run the neutral. . .and emt between the meter can and the mdp.

Yes you can.

In my area (Minneapolis, St. Paul and surrounds) there is very little cable used for service entrance.

My area's common residential service is metal raceway (EMT, GRC, etc.) between the meter socket and the service disconnect. When PVC became available, that was, and still is, used as well.

 

[al hildenbrand]

Smithbuilt,

Consider the "unbalance" current of all the load in a single family residence.

Let's say we have 50 amps on L1 of the service disconnect.
And say we have 75 amps on L2 of the service disconnect.

That means the total "unbalance" current at the neutral bus is 25 amps.

On the load side of the Main Bonding Jumper (tying together the load neutrals with the Grounding Electrode System and the Service Grounded Conductor), the 25 amps coming from the load SPLITS into smaller currents as it heads back to the local Power Company transformer.

Some of the current is in each (if present) of the:

1. water pipe,
2. concrete encased electrode,
3. ground rod,
4. grounded service conductor,
5. the metal raceway containing the grounded service conductor,
6. AND any other conductive path that exists between the Main Bonding Jumper and the transformer neutral terminal.

The current that flows in any one of these is a function of the resistance of that particular path.

The current is not "objectionable", and in fact is expected to be in all these places.

 

[celtic]

Grounding Presentation

I have a MS Powerpoint presentation (*.*ppt) that describes the problem of parallel ground paths in detail ~ but not overly technical, has LOTS of graphics, easy to understand .
It was sent to me by another forum member.



Here's the deal:

If ANYONE else would like a copy of it send me a PM with your email address in it.

The presentation is 738kb
FYI:
The presentation is a Microsoft Power Point file. If you do not have Power Point (like me), you can download a Power Point viewer from Microsoft....or I can send the viewer under a seperate email
The Power Point viewer is 2,837KB

Did I mention both are free ~ just for asking :grin:


(This is a re-post from a thread on 10/06/06 ~ post #8 & 13 http://www.mikeholt.com/code_forum/s...ad.php?t=80488)

 

[SmithBuilt]

Thank you all for the responses and Celtic for the presentation.

You have answered my initial question.

bph mentioned 250.6 Objectionable Current over Grounding Conductors

I want to understand the psychics of how more than one grounding path would cause objectionable current, a violation of 250.6. I realize if a low impedance path was not provided by the service neutral there would be problems. I can also see how current would split and use more than one path to the utility.

 

[bphgravity]

Perform a search on this Forum with the terms "objectionable current". You will find plenty of discussion on the issue. Objectionable current is not an exact science. It is not defined or quantified by the NEC. Interpretations of what objectionable current is and the methods permitted to mitigate objectionable current vary widely among AHJ's and other NEC authorities.

 

[al hildenbrand]

The 2005 NEC Handbook includes the following explanatory text in 250.6(B)

An increase in the use of electronic controls and computer equipment, which are sensitive to stray currents, has caused installation designers to look for ways to isolate electronic equipment from the effects of such stray circulating currents. Circulating currents on equipment grounding conductors, metal raceways, and building steel develop potential differences between ground and the neutral of electronic equipment.
A solution often recommended by inexperienced individuals is to isolate the electronic equipment from all other power equipment by disconnecting it from the power equipment ground. In this ill-conceived corrective action, the equipment grounding means is removed or nonmetallic spacers are installed in the metallic raceway system. The electronic equipment is then grounded to an earth ground isolated from the common power system ground. Isolating equipment in this manner creates a potential difference that is a shock hazard. The error is compounded because such isolation does not establish a low-impedance ground-fault return path to the power source, which is necessary to actuate the overcurrent protection device. Section 250.6(B) is not intended to allow disconnection of all power grounding connections to the electronic equipment. See also the commentary following 250.6(D).

And the commentary following 250.6(D) is:

Section 250.6(D) indicates that currents that result in noise or data errors in electronic equipment are not considered to be the objectionable currents referred to in 250.6, which limits the alterations permitted by 250.6(C). See 250.96(B) and 250.146(D) for requirements that provide safe bonding and grounding methods to minimize noise and data errors.