Residential Pole-Mounted Utility Transformer Grounding

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iwire said:
Service tech in many cases the grounded conductor is not bonded to the pole at all.
The two circuit conductors supply the fixture and the pole is only bonded to the rod.
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Iwire,
You are correct. NESC 97A states "Except as permitted '(to which you refer is not permitted)' in Rule 97B, grounding conductors from equipment and circuits of each of the following classes shall be run separately to the grounding electrode for each of the following classes:
1. **********
2. Lighting and power circuits under 750 V
3. ***********
 
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NoVA Comms Power said:
Fair enough.

The point we're all trying to make is that at the voltages most light poles operate ... a line-to-structure fault probably isn't going to trip any OCPD if the only path back to the source is the "earth" via a grounding electrode.

An effective ground fault clearing path (e.g. EGC) needs to be provided for this to occur rapidly.

Otherwise, the pole (or cover) will remain "hot" and the "step voltage" (created by the voltage gradient across the physical soil) can create a fatal shock hazard.

[Edit to add]: Anybody got Mike Holt's "Hot Pole", or "Step Voltage" diagram they could post?
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I agree with the premise that a line-to-pole fault isn't going to trip a OCPD. But, the specific point I am defending here is that I have not read nor seen
any instance of an electrocution by step voltage or incidental contact on a metal street light pole. If you can direct me to some article or public instance, I will conform.
It was once said by an unknown author, "the mind of a bigot is like the pupil of the eye, the more light you put in it the smaller it gets."
 
ServiceTech,
You are correct. NESC 97A states "Except as permitted '(to which you refer is not permitted)' in Rule 97B, grounding conductors from equipment and circuits of each of the following classes shall be run separately to the grounding electrode for each of the following classes:
1. **********
2. Lighting and power circuits under 750 V
3. ***********
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If that section is saying that a metal light pole can be grounded to its own grounding electrode, can you explain how that can make it safe in the case of a hot conductor faulting to the pole? The impedance will be too high to open the OPCD and there will be a potential between the pole and the earth around the pole that can be fatal.
Don
 
NoVA Comms Power said:
While this is a "circular reference" (i.e. it's on the Mike Holt web site vice an independant source) ... I wanted to get something back quickly.

I have no reason to think that the article posted within the page is anything but genuine.

http://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/GirlKilledatMetalLightPole~20020429.htm

I've not yet combed thru all the other "Google hits"
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Very good reading, and I thank you. I see reference to the incident. Now if I were to correct this situation, looking at NEC 250.4(A)(5), I would install a ground from the tx to the pole, do you agree? And again thank you for the URL
 
ServiceTech1/c said:
Iwire,
You are correct. NESC 97A states "Except as permitted '(to which you refer is not permitted)'
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No, I did not say if it was permitted or not.

What I have been saying is it is a unsafe practice.
 
ServiceTech1/c said:
... Now if I were to correct this situation, looking at NEC 250.4(A)(5), I would install a ground from the tx to the pole, do you agree?
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A grounding conductor should be run back from the pole to the same point where the grounded conductor (neutral) is bonded to it's ground at the source ...

... which is generally the X0 or -- if there's no transformer involved -- the service disconnect.
 
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ServiceTech1/c said:
Jim,
I must correct you with the appropriate NESC Section. In reference to NESC Section 9.96A, "Single Grounded Systems", it states"Individual made electrodes shall, where practical, have a resistance to ground not exceeding 25 0hms."
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This would apply if you are in a single grounded system, or a delta system. Where do you operate one of these systems? They are extremely rare compared to the multi grounded systems.

Jim T
 
jtester said:
This would apply if you are in a single grounded system, or a delta system. Where do you operate one of these systems? They are extremely rare compared to the multi grounded systems.Jim T
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Jim,
Nearly 99.9% of our residential distribution is single phase WYE-DELTA. The other 0.10% is open WYE-WYE connected.
Our commercial and industrial customers are connected according to load. If more power than lighting load, we connect WYE-Delta with a floating neutral. If more lighting is needed than power, we connect WYE-WYE. In any connection we still maintain the 25 ohms or less.
 
ServiceTech,
Nearly 99.9% of our residential distribution is single phase WYE-DELTA.
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How do you feed dwelling units with a delta secondary?
If more power than lighting load, we connect WYE-Delta with a floating neutral.
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What is a "floating neutral"? Any neutral used as a circuit conductor must be a grounded conductor.
Don
 
ServiceTech1/c said:
Jim,
Nearly 99.9% of our residential distribution is single phase WYE-DELTA. The other 0.10% is open WYE-WYE connected.
Our commercial and industrial customers are connected according to load. If more power than lighting load, we connect WYE-Delta with a floating neutral. If more lighting is needed than power, we connect WYE-WYE. In any connection we still maintain the 25 ohms or less.
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I'm pretty familiar with utility systems, and I have never heard of a distribution system described as wye-delta. That might be a transformer connection, but the distribution system is either delta or wye.

What system voltages are you dealing with when you do this grounding?

You've piqued my interest.

Jim T
 
don_resqcapt19 said:
ServiceTech,
How do you feed dwelling units with a delta secondary?
Don
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Don,
You can not derive 120/240v from a WYE tx bank connection.
Therefore if you have 120/240v you have a DELTA connection. 99.9% of our customers have a 120/240v service, thus a DELTA connection. Also, I might add this is not a true DELTA connection. A true DELTA does not have a ground. We have not had a true DELTA in years.


don_resqcapt19 said:
What is a "floating neutral"? Any neutral used as a circuit conductor must be a grounded conductor.Don
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In industrial areas where the demand is for power and less for single phase loads, we supply a WYE-DELTA with a floating neutral. What that means is that we isolate the primary ground of the H2 bushing "float" and connect it to an open disconnect. When this arrangement is installed we have three tx's connected DELTA on the secondary which supplies equal balanced circulating current to each phase.

The impedance of the three tx's is critical at this stage, there can be no more than (if I remember correctly) 1.5% deviation of impedance between the three tx's. If one tx fails, a noticeable problem will be observed by the industry supplied, whereas we can respond and repair before the other tx's or cust equipment are damaged. If the "floating" neutral were directly connected, and a tx went off line, the customer would not see a problem. Because, the DELTA connection allows service to continue uninhibited without noticeable effects, until the other two tx's, which at this stage have been supplying the demand of the failed tx off line, fail due to overload and/or core meltdown.

Therefore for service reliablity and cost associated with a WYE-DELTA, the neutral is "floated". The reason the fourth (as we call it) disconnect is in place, is so we can safely trouble shoot this extremely hazardous arrangement.
 
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jtester said:
I'm pretty familiar with utility systems, and I have never heard of a distribution system described as wye-delta. That might be a transformer connection, but the distribution system is either delta or wye.
Jim T
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Jim,
Let me start by saying the transmission portion of the "SYSTEM" is delivered DELTA, in the voltage range of 110kv, 115kv, 230kv, and 500kv three phase with a 120 degree shift. When that transmission voltage is tapped at a substation, we step down to a distribution voltage of 34.5kv, 13.2kv, or 4.1kv. At this point it is DELTA three phase that leaves the station after regulation and protection schemes are in place.

Let's look at three phase primary running down the road on x-arms. Lets stop at the first customer on line. At the pole you see a tx (obviously OH construction). The primary of the three phases are DELTA. The tx requires 19.9kv, (34.5 / sq rt of 3) = 19942 or 19.9kv. So in order to achieve this, we ground the primary coil at the tx (H1 bushing connected to one phase of 34.5kv, H2 bushing to ground). This connection gives us a primary connection which is WYE connected. The secondary is DELTA connected, thus the WYE-DELTA connection. Now if we were to utilize the 34.5kv, we would connect the primary side of the tx DELTA (H1 connected to one phase, and H2 connected to one of the other two phases). The secondary is DELTA connected, thus we have a DELTA-DELTA connection. The WYE-DELTA or DELTA-DELTA is referring to the tx connections only, not the main line delivery primary.
 
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ServiceTech,
You can not derive 120/240v from a WYE tx bank connection.
Therefore if you have 120/240v you have a DELTA connection.
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120/240 is a single phase voltage, there is no wye or delta involved. You only need a single transformer to supply this voltage.
don
 
ServiceTech,
At this point it is DELTA three phase that leaves the station after regulation and protection schemes are in place.

Let's look at three phase primary running down the road on x-arms. Lets stop at the first customer on line. At the pole you see a tx (obviously OH construction). The primary of the three phases are DELTA. The tx requires 19.9kv, (34.5 / sq rt of 3) = 19942 or 19.9kv. So in order to achieve this, we ground the primary coil at the tx (H1 bushing connected to one phase of 34.5kv, H2 bushing to ground).
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You can't get power from a delta system by using one phase and a grounded conductor. There is no grounded circuit conductor in delta distribution. If you are supplying the transformer with one phase and a grounded conductor, then your distriution system is a wye system.
Don
Don
 
don_resqcapt19 said:
... If you are supplying the transformer with one phase and a grounded conductor, then your distriution system is a wye system ...
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Servicetech:

Don's post accurately describes how the Dominion 240/120 residential distro transformers I'm familiar with appear to be wired -- at least from the ground looling up.
 
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