EGC at utility xfmr
- Thread starter faresos
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Primarily because it is not needed.
But also because it would create a parallel path for current to return to the utility transformer. At the main service equipment, we bond the Neutral and Ground bars. This provides the path for fault current (i.e., from a short circuit to the case of some equipment in the building) to return to the main panel, and from there to the utility source. The high fault current will cause the breaker to trip, thus terminating the event.
But if there is a connection between N and G both at the main panel and at the utility transformer, then all of the neutral current will travel back to the utility transformer via both the neutral wire and the ground wire.
But also because it would create a parallel path for current to return to the utility transformer. At the main service equipment, we bond the Neutral and Ground bars. This provides the path for fault current (i.e., from a short circuit to the case of some equipment in the building) to return to the main panel, and from there to the utility source. The high fault current will cause the breaker to trip, thus terminating the event.
But if there is a connection between N and G both at the main panel and at the utility transformer, then all of the neutral current will travel back to the utility transformer via both the neutral wire and the ground wire.
ecg
ecg
an opinion:
250.28 requires a bonding jumper between the grounded and grounding conductors WITHIN the enlosure for the service disconnect.
If you had equipment ground conductors along with grounded conductors to the pad, you would violate 310.4
(sorry charlieb, I was confirming my references as you posted , I guess)
ecg
an opinion:
250.28 requires a bonding jumper between the grounded and grounding conductors WITHIN the enlosure for the service disconnect.
If you had equipment ground conductors along with grounded conductors to the pad, you would violate 310.4
(sorry charlieb, I was confirming my references as you posted , I guess)
Thanks you all for the reply; I thought the equipment ground conductor will be bonded to the case of the utility transformer only, since we are bonding the neatral and the ground only at the main serivice. Would that still be a problem (bonding to the case of utility transformer)? One more thing, does the utility bond their EGC from the primery side and that would be suffitient?
thanks again,
thanks again,
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
Actually, we do not install an equipment ground conductor from the utility transformer to the building service, because the utility company would become responsible, and they do not want to incur the extra cost. That is why we now have to use the neutral conductor on the line side of the service disconnecting means as both the "neutral" and the "equipment ground conductor"/effective ground fault current path.
If we did install the "extra" conductor, it would be a better installation in my opinion. :wink:
If we did install the "extra" conductor, it would be a better installation in my opinion. :wink:
- Location
- Massachusetts
Pierre C Belarge said:
I agree providing we also eliminated the requirement to bond it all together at the SD. :smile:
peter d
Senior Member
- Location
- New England
Pierre C Belarge said:
Perhaps....but I doubt the poco's are ever going to change.
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
iwire said:
That is correct, as wiring methods would have to change
Peter
I agree that it may not happen, but it would be some bonanza for us all if it did happen.:grin:
peter d
Senior Member
- Location
- New England
sparkydon said:
I don't know if you're joking or not, but I'm sure you know they are not bound by the NEC.
I think we just have to accept the fact that utilities use a MGN system and that isn't likely to change. We are not linemen so we don't have to deal with it. I don't see the issue, to be honest.
- Location
- Illinois
- Occupation
- retired electrician
Peter,
Don
The lack of an EGC between the utility supply and the building results in code required parallel paths for the grounded conductor current...the most common of these are a metal underground water pipe system, the shield of the TV cable coax system, and the metal jacket of the underground telephone service cable. If parallel paths for grounded conductor current are such a hazard that we have to make code rules to prevent them, why do we have code rules that require the same types of paths on the line side of the service disconnect. Either this current is a hazard on both side of the service disconnet or its not a hazard on either side, but there is no way that it is safe on one side and a hazard on the other side.
Don
peter d
Senior Member
- Location
- New England
don_resqcapt19 said:
Understood. The point I was trying to make is that it's pretty late in the game to really do anything about it, hence the "I don't see how it matters" comment since there is nothing we can do to change utility practice.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
don_resqcapt19 said:
I don't know that this is necessarily true.
We have two approaches: single point bonding of the grounded conductor to earth, and multiple point bonding. Multiple point bonding would seem to produce a lower impedance fault current path, and probably lowers impedance to 'electrostatic insults', but introduces risks associated with parallel paths
It is entirely possible that _both_ approaches have risks, and that rather then calling either approach a hazard or safe, one has to _balance_ the risks of one approach against the other.
With this in mind, it is possible that the balance of risks tilts one way or the other at different scales; that multiple bonding is safer in the domain of the utility distribution system, but that single point bonding is safer at the level of an NEC controlled facility.
-Jon
- Location
- Illinois
- Occupation
- retired electrician
Jon,
Don
Maybe, but the use of the multigrounded neutral on the utility side is the source of most tingle voltage associated with pools (the voltage is the voltage drop on the primay neutral). It also has risks associated with the fact that the utility neutral is often a common conductor for both the primary and secondary voltage systems.
Don
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
As I presume you know, for grounded electrical system, the NEC restricts the location and number of bonds between ground and neutral. Generally this means a single bond in the main disconnect, and no others.
However the NEC does not apply to the power company distribution network. They follow a different set of rules.
A standard utility practise is something called a Multi Earth Neutral (or Multi Ground Neutral) . This is exactly what it sounds like; the distribution system neutral is connected to Earth all over the place; usually with grounding electrodes at many poles, as well as being connected to the neutral conductors heading in to various services (thus using the structure's grounding electrode as part of the utility grounding electrode network.
Because there are multiple connections between a current carrying conductor and Earth, some fraction of the utility neutral current will flow through the Earth. As Don correctly points out, this current flow through the Earth is a significant source of low level shocks and stray voltage; not trivial problems.
By 'electrostatic insults', I mean any transient high voltages caused by the mechanical movement of charged particles in the environment. The heavy duty version of this is lightning; however it is very common to develop extreme voltages with very little charge (and very little total energy) simply from stuff moving around. Helicopter blades moving through the air can develop a significant fraction of a milliamp of charging current, for example. The energy in these transients may be quite small, but it can damage insulation.
I contend (but cannot prove) that the MEN (MGN) system would reduce the impedance to any such transient events, thereby helping to maintain the conductor insulation system. It is an open question if the benefits of MEN actually outweigh the very real problems; however it is the installed system, and not likely to change anytime soon.
-Jon
However the NEC does not apply to the power company distribution network. They follow a different set of rules.
A standard utility practise is something called a Multi Earth Neutral (or Multi Ground Neutral) . This is exactly what it sounds like; the distribution system neutral is connected to Earth all over the place; usually with grounding electrodes at many poles, as well as being connected to the neutral conductors heading in to various services (thus using the structure's grounding electrode as part of the utility grounding electrode network.
Because there are multiple connections between a current carrying conductor and Earth, some fraction of the utility neutral current will flow through the Earth. As Don correctly points out, this current flow through the Earth is a significant source of low level shocks and stray voltage; not trivial problems.
By 'electrostatic insults', I mean any transient high voltages caused by the mechanical movement of charged particles in the environment. The heavy duty version of this is lightning; however it is very common to develop extreme voltages with very little charge (and very little total energy) simply from stuff moving around. Helicopter blades moving through the air can develop a significant fraction of a milliamp of charging current, for example. The energy in these transients may be quite small, but it can damage insulation.
I contend (but cannot prove) that the MEN (MGN) system would reduce the impedance to any such transient events, thereby helping to maintain the conductor insulation system. It is an open question if the benefits of MEN actually outweigh the very real problems; however it is the installed system, and not likely to change anytime soon.
-Jon
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