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 Post subject: Outdoor Transformer Grounding and Arc Flash
PostPosted: Thu Jan 26, 2017 11:51 am 
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Posts: 104
Location: San Antonio, TX
This is an Electrical Grounding questions (NFPA 70 2017) that affects the arc flash calculation.

I have a customer that states that the following installation is NOT grounded and therefore I should not consider the Grounded option in the calculation of the arc flash incident energy.
 
I have an OUTOOR Delta-Wye transformer feeding a building.
 
The neutral terminal at the secondary is connected to the metallic frame of the transformer through a copper conductor and using a good connector at the frame.
 
There is another connector attached to the frame. An uninsulated copper conductor is connected to the frame and to a grounding electrode in close proximity of the transformer.
 
Therefore, the neutral terminal is indirectly connected to the grounding electrode.
 
In other words, the neutral terminal it is not directly connected to the grounding electrode through a continuous "grounding electrode conductor".
 
Question:  Is this installation satisfying the NEC Article 250 requirements? If it is, I should consider the grounded option in the arc flash equations.
 
Just to clarify this question.
 
This installation is in an OUTDOOR transformer.
The installation requirements of the "grounding electrode conductor" that are specified for the service equipment do not apply to this outdoor location.
 
Thanks.


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 Post subject: Re: Outdoor Transformer Grounding and Arc Flash
PostPosted: Fri Jan 27, 2017 7:15 am 
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Location: North Carolina
Whether the transformer is an outdoor or indoor installation does not matter. NEC does not differentiate between the two. It does matter as far as the arc flash calculation goes in some cases (see below) but not drastically.

What you have is that the transformer is BONDED and the bonding is grounded. In terms of the Code, it's not right. The goal to be a solidly grounded system is to have the resistance as low as possible...e.g. a copper conductor. In this configuration the resistance is a bit higher than that because some of the path to the ground stake is steel. It's not unheard of since the other two alternatives are an ungrounded system and a resistance grounded system.

An ungrounded system would mean that nOTHING is connected to the wye. If that's the intent then you'd need phase lights or something equivalent to alarm in the event of a ground fault as per NEC.

A resistance ground means that there is an intentional resistance mounted between the transformer neutral and the ground. In this case, ground fault current monitoring and alarming or tripping is required by NEC.

In either case below 250 V, you can't have anything other than a solidly grounded system or connecting the neutral back to the system neutral of the upstream transformer making it not a separately derived system. Ungrounded and resistance grounded systems are only available for 480+ V systems.

That brings us finally to the arc flash. NEC (NFPA 70) has the status of being a regulatory requirement by state law in every state, although nearly every state slightly modifies the Code and they don't all keep up with the current version. NFPA 70E has a tabular based arc flash analysis method that does not get concerned with whether or not the transformer is grounded or not. Of the calculation methods in IEEE 1584, the empirical model in that standard gives a different incident energy rating (slightly lower) if a transformer is solidly grounded but a higher rating for all other cases. This is basically almost an anomaly in the underlying data and when the next version of IEEE 1584 comes out (probably in the next year or two), it is possible that this calculation factor will disappear. I'm not on the committee so all I can comment on is based on published studies.

IEEE 1584 is based on laboratory data that simulates a three phase arcing fault across all three phases. It uses as an input the fault current. The means of calculating the fault current is NOT set by IEEE 1584. This is typically done using an IEC short circuit calculation instead. ALL faults if they don't self extinguish (and high resistance grounding and/or low voltage are probably the only cases that qualify) will rapidly turn into 3 phase bolted faults in most industrial equipment, typically within 1 cycle. So the fact that it starts out as a ground fault or a line-line fault is pretty much immaterial because it will be a full 3 phase arcing fault.

You CAN calculate the incident energy from a grounded fault (divide the voltage by the square root of 3) but from there it would only be valid for a true single phase case where the conductors are separated by such a large distance (a couple feet) that it can't turn into a three phase arcing fault, and right now except for ArcPro we don't have any way to calculate single phase faults anyways. But with any other equipment, it will turn into a 3 phase fault so it's kind of pointless.


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 Post subject: Re: Outdoor Transformer Grounding and Arc Flash
PostPosted: Fri Jan 27, 2017 9:32 am 
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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.

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.

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.

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.

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.

Thanks.


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 Post subject: Re: Outdoor Transformer Grounding and Arc Flash
PostPosted: Sat Jan 28, 2017 10:24 am 
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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.


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 Post subject: Re: Outdoor Transformer Grounding and Arc Flash
PostPosted: Mon Jan 30, 2017 4:32 pm 
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If the service transformer is utility owned/operated, NEC does not apply.


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 Post subject: Re: Outdoor Transformer Grounding and Arc Flash
PostPosted: Tue Jan 31, 2017 6:04 am 

Joined: Tue Dec 31, 2013 12:19 pm
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You didn't differentiate whether the transformer was a utility or a owner transformer in a campus type set up or simply a separately derived system fed from another building.

The short answer to your question is: Per NEC 250.20(B) your system is required to be a grounded system.

Service Transformers & Campus Type Setup: At the outdoor service transformer, there typically is a bonding jumper from the X0 lug to the case and a grounding electrode conductor (GEC) from the case to an electrode. However, from the transformer to the meter or service disconnect, all you run is ABCN with no grounding conductor. At the meter or service disconnect you are required to have a main N-G bonding jumper per NEC 250.24. At that point, you're required to also have a GEC to a grounding electrode.

Separately Derived System fed from another building: Follow NEC 250.32(B)(2). The System Bonding Jumper may be at the transformer or the first downstream overcurrent protective device/disconnect. Where the system bonding jumper is determines where the GEC to the grounding electrode goes.


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 Post subject: Re: Outdoor Transformer Grounding and Arc Flash
PostPosted: Tue Jan 31, 2017 8:18 am 
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Location: San Antonio, TX
Yes, it is a service transformer.

Thanks for your response. I confirmed that this installation meets NEC requirements for grounding outdoor transformer with an NFPA expert. Thanks.


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