Grounding explained (please)

Reply

  #1  
Old 04-20-16, 12:07 PM
Member
Thread Starter
Join Date: Apr 2016
Location: Usa
Posts: 6
Grounding explained (please)

Just some fun for the experts here, this is for my own education, here’s how I understand it.

The utility pole is grounded by a wire that connects to the step down transformer neutral and runs down the pole to the base of the pole and deep into the earth. The transformer puts out two hots and a neutral. In most contexts ground and neutral are not the same, but it seems to me at the transformer they actually are.

The 2 hots and neutral run to the main breaker box, where the neutral is bond to the house ground which is a long copper pole driven into the ground. The ground at the main box is there ‘just in case’ and tries to guaranty that current has a clear path to ground at the box. Electricity will take the path of least resistance and the neutral back to the pole ‘should’ be the least resistance (given the gauge of the wire).

Now code says that no other ground at any sub-box can exist, and I suppose this is to ensure current has to pass back through the box to get to the one-and-only true ground.

The ground on outlets is a safety feature that mechanically connects the case of an appliance to the ground wires which as noted eventually tracks back to the ground-neutral bond at the main box. The purpose is to allow any 'short' to the case to find an easier path to ground than through your heart.

I don’t know why additional grounds at sub-boxes would be a problem but I do know that older code says this can only happen if no other conductor (pipes, cable, phone lines, etc) exist, say to an unattached garage. Current code in my area does not even allow that.

So what did I say above that made someone cringe?
 
Sponsored Links
  #2  
Old 04-20-16, 12:46 PM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,372
So what did I say above that made someone cringe?
It is what you WROTE that makes me cringe.

The utility pole is grounded...
Unless the utility pole is metallic the term "grounded" is meaningless in regard to the pole.

The transformer puts out two hots and a neutral. In most contexts ground and neutral are not the same, but it seems to me at the transformer they actually are.
Again, your terminology is incorrect. When dealing with specific concepts you MUST use the proper terminology so that all people discussing the issue are using a common language. The terms "hot" and "neutral" are only borderline correct; proper terminology for the secondary of a distribution transformer would be "ungrounded" and "grounded" conductors.

The 2 hots and neutral run to the main breaker box, where the neutral is bond to the house ground which is a long copper pole driven into the ground. The ground at the main box is there ‘just in case’ and tries to guaranty that current has a clear path to ground at the box.
The "service conductors" run to the meter (or current transformers in the case of a large service) and then to the first "overcurrent protective device" (fuse or circuit breaker) and at this point the "grounded conductor, which is also the center-tap of the distribution transformer secondary, is "bonded" to the "grounding electrode conductor" which in turn runs to and connects to the "grounding electrode" (ground rod). There is no "just in case" and it has nothing to do with ordinary currents having a "clear path to ground" for any purpose.

Electricity will take the path of least resistance and the neutral back to the pole ‘should’ be the least resistance (given the gauge of the wire).
ABSOLUTELY FALSE! Electricity follows ALL paths back to the source. The current flow is inversely proportional to the impedance (AC resistance) of the path which means that the lower the impedance the higher the current flow but current WILL flow on all paths, regardless of their impedance.
Now code says that no other ground at any sub-box can exist, and I suppose this is to ensure current has to pass back through the box to get to the one-and-only true ground.
Your use of the terms "ground" and "box" are vague and misleading. The "equipment grounding conductor" is for one purpose and one purpose only. That purpose is to provide a low impedance path from the point of utilization back to the source for FAULT currents to facilitate the operation of overcurrent protective devices. The equipment grounding conductor plays no part in the normal operation of any electrical device. For example, if the ungrounded conductor (known as the hot conductor) were to come into contact with a metallic case of a tool the equipment grounding conductor would provide the return path to the source of the electricity, cause a large current flow (it would be a short circuit flow) and trip the circuit breaker or blow the fuse almost instantly and thereby protect the user from having a lethal current pass from the tool case through his body and into the earth where the current would be high enough to kill but too low to activate the overcurrent protective device.

I don’t know why additional grounds at sub-boxes would be a problem but I do know that older code says this can only happen if no other conductor (pipes, cable, phone lines, etc) exist, say to an unattached garage.
The problem is not additional "grounds" but the bonding of groundED (neutral) and groundING (equipment grounding) conductors at any point other than the first overcurrent protective device. If you were to bond the neutral and ground at any other point the equipment grounding conductor would then become a parallel path for the return currents carried by the grounded (neutral) conductor. Since all exposed metallic parts are bonded to the earth and to the equipment grounding conductor having the parallel path would mean that some current, depending on the impedance of the path, would be returning on these exposed paths. This would set up varying potential differences in both voltage and current between various parts of the installation and the earth and this would be a hazard.
 
  #3  
Old 04-20-16, 12:51 PM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,296
Overall you got it pretty close in colloquial terms, but precision is important in this case. Part of the confusion is that the common term "ground" in reality means two or three different things. When having a discussion like this, we have to be clear on the language.

1. grounding electrodes - physically connecting to the earth. These are mainly to protect against large events like lightning and to provide a reference ground which is common throughout the power grid. They do also allow some current to flow through the earth.

2. equipment grounding - a low impedance safety path for current to travel if there's a fault in the circuit. This provides a path for the vast majority of the fault current to go back to the panel and allow the breaker to trip without causing too much damage or injury. A common misconception is that current takes only the path of least resistance. It actually takes all available paths, inversely proportional to the impedance of the path; meaning the majority of the current flows on the best-connected path highlighting the importance of good equipment grounding.

3. bonding - connecting metal items together with a wire so they stay at the same potential / voltage. This is both a safety and equipment protection item. If adjacent metal surfaces are not bonded, and one becomes energized, a person who touches both (example: a washer and a faucet) could experience a shock as the potential equalizes through the body. The previous two types of grounding also accomplish bonding and we also explicitly bond some things like swimming pool frames.

The overall concept you're approaching with sub panel grounding is what we like to call single-point earth grounding. This is a protection against lightning damage. If you have multiple points of earth grounding for the same building, this can cause high voltage gradients to develop as a lighting strike dissipates through the soil around the building. Without single-point grounding your cable TV, phone, satellite receiver and similar devices which connect to power the power line and a data line can be severely damaged if lightning strikes nearby.

The four wire feed to outbuildings will still allow a breaker to trip if the neutral wire has burned off or broken (which is remarkably common).
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
'