Residential Pole-Mounted Utility Transformer Grounding

[iwire]

Could you expand on your statement about personal protection on circuits 300 volts or less.

At least once a year we get links to stories about people getting electrocuted from metal street light poles.

Most times it will turn out these where installed by a utility company and the poles had no direct connection to a ground fault clearing path. They where simply connected to a local ground rod.

I picked 300 volts out of the air but my point is what works at utility distribution voltages does not work for the voltages used in premise wiring systems.

A ground rod can open the OCPD when 7.2KV goes to ground.

A ground rod can not open the OCPD when 120 volts goes to ground.

 

[don_resqcapt19]

ServiceTech,
I fully understand the need for utility grounding and low impedance grounding at the substations, I just have never ever seen a utility pole with multiple grounding electrodes, even where there are transformers on the pole. Also while the Megger company makes a "ground resistance tester" the use of the a "megger" to test for proper grounding is not correct. A "megger" is used to check insulation resistance not ground resistance, at least that is how most electricians understand the field use of that term.

In my area the AHJ says a 3 ft electrode below the service panel, no megger is required.

Then your AHJ is not looking at the NEC. The minimum length of an electrode is 8' and if the electrode is a single rod, it must be tested with a ground resistance meter to show 25 ohms or less. If the result is over 25 ohms, the only requires the installation of one additional rod at least 6' away from the first one. In most cases the installer does not test the ground resistance of the first rod, he just installs a second one. However this 25 ohm rule only applies driven rods, driven pipes and buried plate electrodes. If any other electrode is used, there is no testing required.

Could you expand on your statement about personal protection on circuits 300 volts or less.

Grounding electrodes provide no real safety benefit as far as shock protection on systems under 600 volts. They do not have a low enough impedance to case the OCPD to open the circuit. As long as the faulted circuit remains energized, the shock hazard remains.

I have resolved issues with several customers involved, where our system neutral has failed. Of those several (maybe 10) customers, only one had a severe enough problem to call us. The other nine were not so concerned. I talked to eight out of the ten and they all reported the same isuues, but were not severe enough to be concerned or they stated they were going to call an electrician later if it continued. Investigating the issue further, I found an Inadequate panel ground in the customers home whom complained. Because of that inadequate ground in the cust. panel they saw the worst of the effectcs of our ground failure.

By "ground" are you talking about the connection to earth or the grounded (neutral) conductor. The only time the customer's grounding electrode system can provide any real protection from an open or high resistance service grounded conductor is where the customers grounding electrode system has a very low impedance path back to XO on the utility transformer. The only electrode that has this low impedance is a common metal under ground water piping system, because the path back to XO is all metallic. If the path back to XO involves the earth, it would be a very rare case where the grounding electrode system at the building prevents damage.
Don

 

[electrofelon]

Originally Posted by mdshunk
The primary and secondary neutral is jumpered, and the grounding conductor is run down the pole to a rod or butt coil at the base of the pole. Most utilities will do this at each and every span.

My understanding is that a utility's primary usually consists of two phases, not one phase and a grounded neutral - at least in my area. Am I mistaken?

 

[jtester]

My understanding is that a utility's primary usually consists of two phases, not one phase and a grounded neutral - at least in my area. Am I mistaken?

If you have two phases and no neutral, it is likely a delta system. They are relatively rare. Usually it is a wye system, with one or more phases and a neutral which common to primary and secondary. If a section of triplex is run, for example, then there are two neutrals, and they are jumpered as noted.

I am very interested in the posts by Service Tech. I've also never heard of driving multiple rods at any location for the 25 ohm reading. The NESC requires 2 grounds at transformers, cap banks, lightning arresters, etc., but not for 25 ohms. I need a Code section to refer to.

Also, substation grounding is done in small part, to limit step and touch potentials within the substation for personnel protection. Neither would likely be achieved with a 10 ohm reading. I'd like to know more.

Jim T

 

[electrofelon]

If you have two phases and no neutral, it is likely a delta system. They are relatively rare. Usually it is a wye system, with one or more phases and a neutral which common to primary and secondary.

Lately I have been interested in the utility side of things so I am always looking up at wires and talking to the linemen when we meet them for a service upgrade, and around here it seems to be the opposite - mostly delta and few wyes (assuming 3 wires is a delta and 4 wires is a wye). Also a lineman told me that the two wires in rural areas are usually two phases. Perhaps since I am in the "old" part of the country (ny) and in a very rural area, deltas are much more prevalent?

 

[suemarkp]

Whether its wye or delta, hot-hot or hot-neutral, isn't it required that they tie one of the conductors to ground? If not, where's their lightning protection?

If one conductor is grounded, won't it also be directly be bonded to the 120V neutral since they are usually interconnected on the transformer case?

 

[jtester]

Lately I have been interested in the utility side of things so I am always looking up at wires and talking to the linemen when we meet them for a service upgrade, and around here it seems to be the opposite - mostly delta and few wyes (assuming 3 wires is a delta and 4 wires is a wye). Also a lineman told me that the two wires in rural areas are usually two phases. Perhaps since I am in the "old" part of the country (ny) and in a very rural area, deltas are much more prevalent?

As an apprentice lineman in the early 1970's I worked on line crews that converted the last of the 2400v delta systems to 4160wye in our service territory in western New Mexico. I didn't think there were places more backwards than here.

The plus side of a delta is it only takes 3 wires to provide an entire 3 phase system, vs 4 on a grd wye. The drawbacks include less capacity and more hazardous conditions with a delta distribution system.

If there are only 2 wires on the crossarm, look at the insulators. If one is white or if it is smaller than the other your system is wye, if they are the same size, you'll have to look at transformer installations. If all transformers have 2 bushings and lightning arresters on both wires, it is a delta. One bushing or one arrester will indicate a wye.

Jim T

 

[jtester]

Whether its wye or delta, hot-hot or hot-neutral, isn't it required that they tie one of the conductors to ground? If not, where's their lightning protection?

If one conductor is grounded, won't it also be directly be bonded to the 120V neutral since they are usually interconnected on the transformer case?

Utilities won't tie a phase wire directly to ground on purpose in a delta system. They will install lightning arresters from each phase to ground, often the secondary neutral, or the butt wraps and rods if no secondary neutral is there. All phase wires are fully insulated at each pole, transformer, etc. They don't use a "corner" ground like we do.

Jim T

 

[ServiceTech1/c]

I am very interested in the posts by Service Tech. I've also never heard of driving multiple rods at any location for the 25 ohm reading. The NESC requires 2 grounds at transformers, cap banks, lightning arresters, etc., but not for 25 ohms. I need a Code section to refer to.

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."

 

[ServiceTech1/c]

I fully understand the need for utility grounding and low impedance grounding at the substations, I just have never ever seen a utility pole with multiple grounding electrodes, even where there are transformers on the pole. Also while the Megger company makes a "ground resistance tester" the use of the a "megger" to test for proper grounding is not correct. A "megger" is used to check insulation resistance not ground resistance, at least that is how most electricians understand the field use of that term.

You are correct, when I am talking to associates in the electrical field, we refer to 'megger', in two areas of thought. One, is a resistivity measurement (ground resistance tester), whereas when we are talking of grounds and the grounding system.
Two, is when we are talking of the ampacity or tx coil insulation test (insulation resistance).
With that in mind, I believe we were talking of ground resistance. I stated we megger to a certain degree. I stand corrected to state we were talking of ground resistance using a ground resistance tester, such as the type manufactured by the Megger Company.

 

[ServiceTech1/c]

At least once a year we get links to stories about people getting electrocuted from metal street light poles.

Really? In my state I have many years in the industry, don't recall one instance of electrocution (Wikipedia defines electrocution as "A fatal electric shock") as a result of metal street light poles.

 

[ServiceTech1/c]

A ground rod can open the OCPD when 7.2KV goes to ground.

A ground rod can not open the OCPD when 120 volts goes to ground.

Could you expain your thoughts on 'A ground rod can not open the OCPD when 120 volts goes to ground.'

 

[tallgirl]

Could you expain your thoughts on 'A ground rod can not open the OCPD when 120 volts goes to ground.'

What's the current? What's the size of the OCPD?

If there are 25 ohms resistance to ground, and the available current is infinity, 120v to dirt will provide no more than 5A current. That's just Ohm's Law.

 

[roger]

Service Tech, as Julie points out, it is simple Ohms Law. A ground rod at our low voltages does very little, they only help if we are talking about HV surges and Lightning strikes.


As far as not hearing about electrocutions caused by utility or privately wired metal pole lights, just keep searching, these occurrences have a way of being swept under the rug so to speak.

Roger

 

[ServiceTech1/c]

As far as not hearing about electrocutions caused by utility or privately wired metal pole lights, just keep searching, these occurrences have a way of being swept under the rug so to speak.

Roger,
Ahh..yes.. swept under the rug. With every attempt and success at concealment, comes the inevitable broad stroke of the brush. When an electrocution occurs, OSHA is called and a full detailed investigation is undertaken by both gov't and private investigators. The results of those investigations are sometimes less detailed than we would like. But aside from that, all electrocutions are summarized as event driven and may be as vague as 'direct contact with energized conductors'. But, within my company, the events are detailed and self explanatory, we have had electrocutions of contractors and employees setting poles, removing poles, rewiring in and on poles, thus the many areas of hazard.
Now to the safety of the general public in relation to metal poles. I agree more could be done.
For example lets assume a metal street light pole which has a 120v fixture is fed from a #4 cu UF direct buried underground cable approximately 20 feet from a 25KVA tx feeding several lights. Let's look inside the hand-hole where the wire terminates. We have 2-conductor (Hot and Neutral) #4 as the source, and 2 conductor with ground #14 as the load. We install an electrode and a bond of #4 cu to the pole ground lug and then ground electrode.
Now as you can see we have the pole grounded. Lets assume the 120v leg comes loose from the termination point and faults against the body of the pole. What happens next, is the fault current can create several different forms of outcome. Let's focus on the high impedance fault. Say the fault current generated is enough to open the #4 sol ground to the electrode, but not of enough magnitude or time to open the OCPD in the 25KVA tx (25,000 watts). What we are left with is 120v leg lying on a high impedance metal pole. An unsuspecting person can theoretically be electrocuted, because the only protection is the OCPD of the 25KVA tx. If a GFCI of 15 amperes were installed, yes it would be practically safer. IMO if one person in the general public were to be electrocuted in this fashion, I believe alternatives would be implemented. Therefore, I will consider metal poles a shock hazard, and as for electrocutions, I have no knowledge of any.

 

[NoVA Comms Power]

... don't recall one instance of electrocution (Wikipedia defines electrocution as "A fatal electric shock") as a result of metal street light poles.

While it wasn't a metal "light pole" ...

... the same inherent flaw (lack of an effective ground fault path to the source/OCPD) which caused this particularly public death -- "public" as it occured in Manhattan in full-view of numerous onlookers -- is not limited to NYC.

http://topics.nytimes.com/top/reference/timestopics/people/l/jodie_lane/index.html

 

[iwire]

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.

An unsuspecting person can theoretically be electrocuted,

It is not just theory, it happens, the news reports have been up on this forum.

IMO if one person in the general public were to be electrocuted in this fashion, I believe alternatives would be implemented.

We all would think that but it is not what is happening.

 

[roger]

IMO if one person in the general public were to be electrocuted in this fashion, I believe alternatives would be implemented. Therefore, I will consider metal poles a shock hazard, and as for electrocutions, I have no knowledge of any.

To bad those in power aren't listening to your opinion.

Roger

 

[ServiceTech1/c]

While it wasn't a metal "light pole" ...

... the same inherent flaw (lack of an effective ground fault path to the source/OCPD) which caused this particularly public death -- "public" as it occured in Manhattan in full-view of numerous onlookers -- is not limited to NYC.

http://topics.nytimes.com/top/reference/timestopics/people/l/jodie_lane/index.html

While I appreciate the url to the story, (she stepped on electrified metal plate on Manhattan street) the focus is on metal poles. I don't have any argument for the lack of workmanship, eng. design, or lack of concern for safety some have in the power industry.

 

[NoVA Comms Power]

While I appreciate the url to the story, (she stepped on electrified metal plate on Manhattan street) the focus is on metal poles.

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?