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#590 - 02/10/01 05:28 PM Fictional Scenario Discussion
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
File = [PC#2] C:\Wordstrc\Wsc2\Scenario Discussion.doc


Topic: Electrical Installations Fictional Scenarios For Discussions.

The following items are fictional setups, posted to this forum for "What If" discussion purposes only. Although they are intended to be fictional [as in the listed methods being done], the fundamentals and theories will still apply to a normal systems' operation.

Please feel free to reply to the thread with any answers, questions, or comments you have. There are no "wrong" answers to these situations, only each persons' interpretation of what will happen and why. The combined responses from members will be the primary methods of determining an overall correct answer.

The member with the most in depth and conclusive replies wins the Grand Prize, which is the Coveted "Sparky Of The Year" award, in the field of Usenet replies

This message's fictional location will be a single family dwelling located in a Suburbial Residential Tract.
If this scenario type discussion message is of interest, and you want more, let me know.

This message will be posted to the following forums:
- ECN Discussion Forums / General Discussion Area / Fictional Scenario Discussion
[Electrical Contractor.net],
- National Electrical Code Forum / NEC Message Forum / Fictional Scenario Discussion
[Mike Holt.com]

__________________

Fictional scenario:

System data: Pole mounted Transformer [supplied by local utility company] = 13.2KV x 240/120 VAC, 1 Phase 3 Wire, 25 KVA, Z = 2.0% [+/- 10%], Split-Coil Secondary. Ground wire included and mounted on pole for transformer grounding.
Service: 200 amp main distribution panel, 200/2 main, service feeders = 3 - 3/0 THHN cu. Main panel feeds one subpanel, plus branch circuits. Distance from transformer to main service = 200 feet. Soil = slightly damp, previously used for orange groves [non-rocky soil].
Grounding Electrode: Concrete-Encased reinforcement steel rods in building's foundation [AKA UFER ground], GEC = #4 cu. No cold water bonding or inclusion to electrode system.
This transformer feeds this house only. No other residences connected to the secondary circuit.

Situation 1:

In your opinion, what would be the results to the system, connected equipment, and loads, if the grounding connection at the transformer was made to a terminal of one end of the coil's winding, instead of to the center tap [on X1 instead of at the X2/X3 jumper] and the grounding electrode conductor was also connected to the same line feeder?
- Would the system operate normally,
- Would the overcurrent devices function properly for line to line and ground faults,
- Would the transformer's maximum KVA need to be derated, or "double" the KVA rating for L-N loads,
- Would there be a ground fault path from the service to the source,
- Would there be a change in the voltage to ground,
- Would Multiwire branch circuits function properly,
- Would L-L loads function properly,
- Would GFCI devices work properly

Situation 2:
On the same situation, what would happen if I decided to remove the ground connection at the transformer and disconnected the grounding electrode conductor from the service, leaving the enclosures and eq. grounds still bonded to the system?
- would the system still function normally,
- would OCPD work,
- would there be a ground fault path from a load to the transformer,
- would there be a voltage to ground,
- would GFCI devices work,
- would multiwire circuits work properly

Situation 3:
If the transformer's ground was connected to the center tap and the service's grounded conductor was disconnected from the grounding electrode system, what would result from:
- ground faults [would this trip a OCPD],
- L-L loads,
- normal operation,
- will GFCI devices work,
- L-N loads,
- L-N currents

Situation 4:
What would be the results of Lightning Discharges and related surges to the system under these listed conditions?


I'll end this one here and add more situations later after some input.

I hope this will be interesting for members and possibly help educate, or explain, certain areas to those that are new to the trade, or just beginning to study in depth theories and/or systems design.

Once again, there are no "wrong" or "bad" answers to these items. I have purposely "loaded" these questions to get variable responses. I'll reply with comments as they appear.

If you like this, let me know if you want more.

Thanks for your time!!


Scott "S.E.T."
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#591 - 02/10/01 08:42 PM Re: Fictional Scenario Discussion
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
Scott,
Scenario #1
1) There would be no change in the normal operation of the system
2) The over current devices would still react to overloads in the to conductors that are normally hot. There is no possibility of a ground fault in the conductor that is grounded. A fault between the "neutral" and ground would result in a ground fault and the only overcurrent protection would be on the line side of the transformer.
3) The transformer KVA would not be affected.
4) There is ground fault path via the conductor that is connected to X1 for ground faults that occur on either of the other 2 conductors.
5) The voltage to ground becomes 240 volts in place of the normal 120. The conductor connected to X2 will be at 240 volts to ground while the conductor connected to X0 will be at 120 volts to ground.
6) Multiwire circuits do not care what conductor is connected to ground so they will function normally.
7) L-L loads will function normally
8) L-N loads will function normally.

Don(resqcapt19)
_________________________
Don(resqcapt19)

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#592 - 02/10/01 08:47 PM Re: Fictional Scenario Discussion
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
Scott,
In situation #2 is the main bonding jumper installed at the service panel?
Don(resqcapt19)
_________________________
Don(resqcapt19)

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#593 - 02/11/01 04:27 AM Re: Fictional Scenario Discussion
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
Hmmmm, this is really gonna work out that grey matter...

OK Sit #1-
i'm trying to imagine metering the panel here.
I'll wager i'll get 240/120 , with maybe the possibilty of one side higher say 135/95 . I say this as i am wondering here about the distance back to the transformer, and the solidity of the UFER, or 0 reference ( all of which sounds like a good ground ref)

The fact that there is no cold h20 bonding means, to me, that we cannot "share" this point of reference with the nieghbors who assumably use the same utility.( but not secondary as you've stated) I see this as an important point.If anything else, we've saved the water department some grief!!

I'm willing to wager that things would operate normally until a heavy load is introduced, were current needs to return.

I am thinking in terms of ground-faults & overloads here for OCPD's

I have to think more on this one...

Sit#2--
no grn @ the trans, lift the GEC @ the panel, leave all the bonds.
that can't be good....
we are relying on the bonds only for a 0 point reference.
this may still meter out ok, until a L-N load is introduced, then how does it return?
Also the OCPD is dependent on a 0 point reference for L-N , so there will probably be worse performance on ground-faults than in #1.

This is deja-va , as i was on a call just like this. There happened to be a very poor GES, and a bad noodle at the trans.

A little current could return, so the kitchen light was ok... i was getting "bit" from the panel as it was trying to use me as a nuetral.( I was leaning over the stove)

Add a heavy L-N load, and everything screwed up...

Sit #3-
No Trans connection to N @ panel....well this can't be good, but we still have that UFER !
In all situations i see a difference in OCPD performance. This would be pertinent to either ground-fault or overload.
one can function, one cannot...

This senario is commonlly the "220v ate my TV, Stero, etc" disaster. The saving grace being the UFER, and 200' of good conducatble soil, and the probability of a utility ground at the pole.

Sit #4--
well , i guess it wuld depend on the area the lightning strikes?

I'll add more, my brain hurts right now...
damn good senario here Scott!

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#594 - 02/11/01 12:09 PM Re: Fictional Scenario Discussion
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
ok, more fun...

Sit #1 has me admittedly rattled, i can't recall a situation, or cough up an example i've heard. I've drawn this out now a # of times...

Sit# 2 & 3 are more the norm,
i can see the OCPD's being responsive to overloading, yet having a problem with ground faulting due to a poor return.

I imagine that these two scenario's depend on the R value of the return path, 200 feet, the soil conditions, the Z factor of the trans... and that the GES would constitute the neutral in absence of one.

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#595 - 02/12/01 12:32 AM Re: Fictional Scenario Discussion
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Hi Don and Sparky

Thanks for replying to this message. I thought that no one would want to, so when I saw your replies, it made my day! Hopefully more people will reply, but even if they don't it's great you guys did

Let me know if I should continue with more scenarios.......

Don hit Scenario 1 great. The only thing that would be different is the voltage to ground. Instead of being a maximum of 120 VAC to ground, it's either 120 VAC to ground from the center tap, or 240 VAC to ground from the 3rd conductor. Everything else works the same.

FYI: For scenario 2, the system is still connected to the enclosure in the main service and all eq. grounds remain connected as usual, but nothing is bonded to the grounding electrode. Also, the transformer is ungrounded.

Sparky: If after a few more days these examples are still unexplainable, I'll follow up with explainations, or E-Mail you.
You are correct that the reason there are no other houses connected to the secondary, has an effect to this scenario. In fact, it has a major part!
Please also note that there is no physical connection of the electrical system to either city water or gas pipes.

I thought up these ficticious scenarios to pose questions and responses that would explain [or help to explain] the basics of grounded AC systems' operations.
In other words, these 4 situations should help "shed some light" on the reasons of and methods to ground AC systems, plus how ground faults work. I figured they would have great impact, as they are primary functions.

Hopefully, this gets covered enough to be beneficial to everyone.

Let me know if you like it.


Scott "S.E.T."
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#596 - 02/12/01 04:13 AM Re: Fictional Scenario Discussion
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
I have many pages of scribbles here....and more Q's than A's .....

one thing is that Bussman link , that L-N 120v fault calc....

i have nothing but speculation concerning GFI's and Lightning strikes.

i guess i'll be in a holding pattern...

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#597 - 02/12/01 05:33 AM Re: Fictional Scenario Discussion
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
Situation #2
I'm still not clear on whether the main bonding jumper is installed, but am assuming that it is.
1) The system would function normally
2) The OCPDs would function normally
3) The ground fault path for ground faults on the load side of the service would be the equipment grounding conductor to the main bonding jumper to the neutral conductor and back to the transformer. The only ground fault path on the line side of the service would be through the earth itself.
4) The voltage to ground would be 120. Some of the equipment bonding conductors would be connected to things that would act as grounding electrodes and make the voltage readings normal.
5) GFCIs would work fine.
6) Multiwire circuits would work as normal.
Don(resqcapt19)
_________________________
Don(resqcapt19)

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#598 - 02/12/01 05:40 AM Re: Fictional Scenario Discussion
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
Situation #3
1) Ground faults would not be likely to open the OCPD in this case as the only return path is the earth itself and this will not normally conduct enough current to open even a 15 amp OCPD.
2) L-L loads would function normally
3) everything would work normally except for ground faults as in item #1
4) GFCI devices will work fine, they do not require a grounded system to operate, and the earth should provide a low enough impendence to allow 5mA to flow to open the GFCI. If the impendence is do high that 5mA won't flow the GFCI won't open, but the shock level would be too low to cause harm to most people.
5) L-N loads would function normally
6) L-N currents would be normal
Don(resqcapt19)
_________________________
Don(resqcapt19)

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#599 - 02/15/01 01:27 PM Re: Fictional Scenario Discussion
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Hi everyone, and thanks for the interest on this thread. I'll create another scenario this weekend and post to this group, possibly to the Mike Holt group also ??

Anyhow, the reasons of what I based these scenarios on are listed below:

Situation 1: This would cover the fundamentals of how a grounded conductor works, along with the infamous 240/120 VAC 1 phase 3 wire system. From this part, I was hoping to explain that the multiwire transformer secondary is primarily 240 volts, with the "neutral" coming from the center tap , meaning that the nominal voltage is 240 volts, but is cut in half when using the center tapped neutral conductor.

On the same subject, the conductor that gets grounded has no bearing on system operation, only on the maximum voltage to ground.

I have heard people insist that this connection would require a derated transformer, others arguing that this would short out the transformer, still others saying that nothing would work at all!!

This part is a basic theory area.


Situation 2: This was intended to explain that even though there is no physical connection of the system to the earth at any point, the system will still operate correctly. The hazard of ground fault to personnel touching a metallic enclosure that is connected this way can be low or high, depending on if there is any Capacitive Coupling between the system and the transformer's secondary. I thought this would cover the reasons why an earth bond is needed.


Situation 3: This one was added to cover the reasons for having the earth connection and grounded conductor at both the transformer and the service. I have seen older open delta "V" connected 3 wire systems that have only the center tap on one transformer grounded at the pole, then the service equipment is grounded at the main. No grounded conductor was brought to the service from the pole.
This one has odd voltage readings and most certainly will be a hazard for any short to ground.
The G.C. I work with has this for the old shop in the back of the Office. This property was bought about 4 years ago. We designed a new Office renovation on the property, but left the existing shop as is - including the separate-existing 3 wire delta service for it. Prior to this location being completed, the Office was at another location [this was the third new office in 10 years].

When I was designing the Electrical systems and such, I was planning to connect the location to a newly installed and dedicated 75 KVA 208Y/120 VAC 3 phase 4 wire Wye system. The existing power was on a 50 KVA 1 phase xfmr, and the open delta was two 5 KVA xfmrs - total of 8.3 KVA, but only 3 wire.
When I met with the SCE design Engineer to discuss this, we both took a good look at the existing delta system. SCE wanted to remove as many loads from the existing 4 KV primary feeders as possible, and we were trying to figure a way around the cost of a new 4 wire Wye system to the customer. As we were studying the pole, we both simutaniously spotted the point of grounding!! Was quite funny, so that's the basis of the 3rd situation.

As for situation 4 [Lightning], it will work out better for the first situation. One reason I eliminated the connection of the cold water lines wasn't to be cute , but to limit the influences of surges sneaking in through the city water. In this case, the surges will be spread across the building's foundation and limited to intensity due to the fact that the flow is restricted by a somewhat centralized spot of concentration.

The amount of flow that is added to the discharge is increased by a direct connection of the electrical system to city water. The surges can easilly connect through the primary feeders, flash across the transformer[s]' core and coil, and down through the secondary feeders. Positive charges can be brought up easilly from a multitude of locations.

The least protected system would be the one that has no earth grounding to the system [it is more hazardous to personnel], whereas the one grounded only at the pole would have the transformer be the "sacrificial" component and would most often cause high surge values to the secondary - creating further hazard to personnel.

So to end this, I am glad to see some interest here. We can discuss these things more indepth later, and/or in another thread.

I'll come up with some more topics that would [hopefully] contain an equal amount of theory, code and design issues.

Any ideas?? send them my way!!


Once again, Thank you for the interest!!


Scott "S.E.T."
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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