A lot packed in here...
RECS wrote:
First. It does matter that is an outdoor transformer because the NEC has a specific article for outdoor transformers. See NEC 2017 Article 250.24 (A) (2). Here it states you have to have an additional connection to earth in case the transformer is outdoors. If this transformer is indoors, it would NOT require an additional connection to earth. Additional because there is already a connection to earth in the service equipment. By the way, this is the service transformer, not a transformer that provides a separately derived system.
Yes, I stand corrected. I work almost exclusively on industrial systems and rarely in commercial installations so most of the time the installations I work on never service entrance equipment or if they are, I'm probably working from the "other side" of the service point in which case this is all meaningless...we're using NESC and OSHA 1910.269 anyways. As per the Handbook edition from 2014 (I don't have a 2017 version yet), "Outdoor installations are susceptible to lightning as well as accidental primary-to-secondary crossovers. This requirement for a connection uutside of a building helps mitigate the effects of these influences on the interior portion of the premises wiring system."
Quote:
Second. You mentioned that because the transformer (neutral terminal) is bonded (to the frame of the transformer) and then this bonding is grounded, it is wrong according to the NEC. I do not agree. Please refer a specific article that applies to thus specific case that stipulates that this is not allowed.
As service entrance installations, obviously we're really looking for lightning protection for the most part. And as such, they don't get really picky about how to install it. Appealing to NFPA 780 or the IEEE standards for lightning, the general concept is to provide TWO different paths to ground where a lightning strike can occur and the standards are not particularly picky about maintaining the kind of conductivity that is necessary for shock and EMF protection. I'm stating this as a general principle here because the prescriptive material in the standards doesn't give any kind of ohmic requirements.
Quote:
Third. You argue that the connection to earth at the transformer will have a resistance higher than with a copper conductor because the current is flowing through steel. This is also debatable, because the large area of the steel of the frame of the transformer will probably reduce the resistance to a value lower than the small area of the copper conductor.
Again, we're talking service entrance transformers here vs. system bonding jumper. As to the actual electrical resistance and treating it as resistance and not frequency-dependent impedance...
I've dealt with this meany times when dealing with ground sensing circuits that are government regulation required on mining equipment but defy basic engineering. If dimensionally you have what is basically a solid conductive object where the length is substantially larger than the length then you can use the common method for calculating resistance of R=rho*A*L where A is the area of the conductor and L is it's length. This also works for the "sheet" case commonly published ASSUMING that the electrical connection is low resistance along it's edge so that in this case the electrical current is distributed evenly. We are constraining current flow to effectively a 1 dimensional space. Intuitively so far this makes your argument.
However in the case of a large uniform plate with two electrodes attached to it somewhere, there are a large number of potential pathways for the current but the resistance along each path is not the same because it is proportional to length. It becomes a two-dimensional problem rather than a 1 dimensional one. This is exactly what happens when looking at the resistance between two. The formula in this case is R=rho/(2*pi*L), assuming the plate isn't terribly thick. So curiously enough your logic holds up very well over long distances between electrodes but falls apart at short distances. Intuitively the reason for this is that even though the linear (1 dimensional) resistance is increasing with length between electrodes, the area over which the number of paths exist is increasing at the square of the length, so it eventually overcomes them, despite the fact that for the 2-dimensional case we are working with steel which has a resistance of 16*10^-8 ohm-meters compared to copper at 1.68x10^-8 ohm-meters.
Quote:
Fourth. As a reference. The NEC 2017 in Article 250.24 (A) (4) allows the grounding electrode conductor to be connected to the frame of the service equipment (i.e. the ground bar) instead to be connected directly to the neutral bar ot terminal of the service equipment. So, you have the grounded conductor from the outdoor transformer terminating on the neutral bar of the service equipment, then a main bonding jumper to the frame (i.e. the ground bar at the service equipment) and then the grounding electrode conductor connects to this ground bar in one side and in the other to the grounding electrode. This set up in the service equipment is exactly as the one I am referring on my original setup in the outdoor transformer. If it is acceptable for the NEC in the service equipment, I assume it is also acceptable for the additional grounding connection at the outdoor transformer.
It really just says that it is connected to your service entrance bonding. The illustration from the 2014 Handbook attached shows a ground electrode jumper tied to a conduit and the conduit is tied to the transformer neutral so yeah....you could do almost anything you want to with this particular ground.
Quote:
I would like to hear any comments regarding if the grounding connection at the outdoor transformer satisfies the NEC requirements or not, but please refer to the NEC specific articles in your answers. If it does, I can use the grounded parameter in the IEEE equations for AFIE.
Actually it really doesn't matter. You have a system bonding jumper at the panel. Since you have a service entrance ground then I'm assuming that the system is not resistance grounded or ungrounded because as per NEC you would not have one in those cases (because it would short out the resistance or ungrounded system). This gives a low resistance path to ground that is connected to the transformer neutral. The extra service entrance ground is just an extra. The IEEE 1584 empirical equation gives slightly higher incident energy ratings when a transformer is resistance grounded or ungrounded. This value is carried throughout the entire system because indeed we're talking about a single ground plane and not a separately derived system and at least in principle we have a very low impedance path back to the system neutral. So I would still defer to the type of system grounding you have and not worry about the extra service entrance ground, at least on the secondary side of the transformer. The primary side would depend on the type of grounding that is present on that side of the transformer, with one possible exception. Depending on the type of transformer, there may actually be a connection through the transformer. For instance with wye-wye transformers, zero sequence currents on the primary side are seen by the secondary side and vice versa. This would not be the case with a delta-wye transformer.
| Attachments: |
|
service entrance transformer photo.png [ 143.13 KiB | Viewed 4007 times ] |