More 3 phase 3 ungrounded systems

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thunder15j

Member
Location
Cali
We have been upgrading some existing electrical systems that are/ were 3 phase 3 wire systems that feed irrigation pumps for agricultural water pumping apllications (25hp to 250hp). Added load has neccessitated service upgrades from 100 amp to 200 amp or 200 to 400 amp services. All loads are motor load only. All new services are upgraded to 4 wire from existing 3 wire. Ground rods are driven and tied/bonded into the incoming grounded conductor at the main switch.

I've been noticing at some existing 480 volt installations that I'm picking up voltage to ground from the 3 ungrounded conductors at 277 volts. Most services are overhead so it is quite simple to look up the utility owned pole where the transformer(s) are mounted. On underground units we're in the dark. I'm seeing that the utility has run a ground wire down the side of their pole probably to a plate or ground rod. Also they run an overhead drop consisting of 3 hot (ungrounded) alum conductors wrapped around a bare wire (quad) to supply the customer. However the bare conductor is not connected at either end. At the customer end there is nothing to connect to since there are only 3 wires coming out of the weather head (this is a 3 phase 3 wire ungrounded system originally). Meter socket is a 5 jaw configuration.

The utility transformer usually looks kind of new. The utility probably changed it out due to either failure or distribution voltgage changes. The quad drop probably changed from original 3 wire open spool installations. The problem I see here is that an existing customer owned 3 phase/ 3 wire installation is now fed by a 3 phase 4 wire system utility modified installation. There is not an ungrounded conductor connecting the customer serive to the utility transformer. A ground fault would have to travel from ground rod to ground rod via the earth. Not a good scenario. Even if a low resistance ground rod (less than 25 ohms) and very conductive soil existed, there is no way of knowing that the utility ground rod and conductor is satisfactory.

Fortunatly, we are eliminating these potential problems with the 4 wire upgrades. What really surprises me is that a utiltiy would connect a newly reconfigured grounded transfomer to an existing ungrounded system, that was formerly fed by a transformer that was ungrounded. Also not installing a 4th wire from transformer to main switch or overhead riser. Is it possible that the readings are coming from the electrical meter connectins, which I kind of doubt?

Please provide your thoughts......
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
If the power company grounds the secondary of their transformers then, for reasons you state, and per 250.24)C) a grounded conductor must be brought to the service equipment.
I have seen cases where on existing ungrounded systems, POCO elects not to ground at the transformers.
In those cases you would not read 2777v to ground on all 3 phases (voltage readings would be sporadic)
 

thunder15j

Member
Location
Cali
Just ran into another similair installation. 40 hp pump motor running @ 240 volt; 3 hot wires in service riser with no grounded wire in riser. However, poco OH transformer bank was grounded. I get 120V; 120V; and 208 to ground at main switch on a 3 wire setup. No 4th wire connection between xformer and switchgear.

Poco probably changed out transformer and decided to ground it; the former transformer more than likely had been ungrounded. The power customer (end user/grower) probably was not notified of the change. In my opinion, the poco is creating a hazardous situation by their actions which basically creates an earth return to clear a ground fault and not warning the customer.

I feel a grounded 4 wire, 3 phase system is much safer and required to be so by NEC. However a 4 wire system feeding an existing 3 wire system with an earh return for a ground reference is potentially more hazardous than a 3 wire ungrounded system. Better to put it back the way it was or advise the customer to bring the existing installation up to code. An installation that was up to code until the poco made their chnage. The customer will probably respond with something like "it worked the way it was, so put it back that way". Utility companys probably should never have installed those three wire, ungrounded systems eons ago
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
Can you post a pic of the bank?

Id call the poco and have them explain. What they are doing is a huge hazard and a NESC violation. Relying on the earth alone is a poor choice where a large current is needed to clear protective devices such as standard thermo-magnetic OCPs. During a ground fault on your side of the system ocps may not open and all metal parts will remain energized.

Secondly how thick is the ground wire from the transformer to the utility ground rod? Chances are its not very large (probably 10 to 6 g). During a fault on your side of the system the wire can easily overheat and melt if sufficient fault current passes.

My guess it is a lineman mistake. (Not aware of your system)
 

thunder15j

Member
Location
Cali
I was recently on a call at another facility. This time a quarry. Another 3 wire, 3 phase ungrounded 480 volt system. Put my voltage meter on L1, L2, and then L3 testing to ground. Got 277 volts on all three readings. Looked up at pole mounted transformer and did not see a wire landed on the 4th bushing on the transformer nor did I see a grounded wire coming down the side of the pole. However, on this very old system, the metering is accomplished by pole mounted CT's. Wires are then run down the pole in a conduit that go to a meter. I saw a ground wire coming out of the sealed metering section going down the pole.

I checked with a metering tech from the utility and he stated that on some installations that are ungrounded, a voltage stabilizer is installed that could provide the ground reference. I'm wondering if this stabilizer installation, if it is causing the readings to ground, could cause a hazard. On a high amperage ground fault I think it would burn clean and the potential to ground would go away. However, on a small fault it might just pass a small amount of current and therefore create a hazard. The hazard being that someone might come in contact with an energized item and get in the fault path since there is no low impedance path to clear a fault as there would be in a properly installed 4 wire system.

Are my concerns justified?
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
We have been upgrading some existing electrical systems that are/ were 3 phase 3 wire systems that feed irrigation pumps for agricultural water pumping apllications (25hp to 250hp). Added load has neccessitated service upgrades from 100 amp to 200 amp or 200 to 400 amp services. All loads are motor load only. All new services are upgraded to 4 wire from existing 3 wire. Ground rods are driven and tied/bonded into the incoming grounded conductor at the main switch.

I've been noticing at some existing 480 volt installations that I'm picking up voltage to ground from the 3 ungrounded conductors at 277 volts. Most services are overhead so it is quite simple to look up the utility owned pole where the transformer(s) are mounted. On underground units we're in the dark. I'm seeing that the utility has run a ground wire down the side of their pole probably to a plate or ground rod. Also they run an overhead drop consisting of 3 hot (ungrounded) alum conductors wrapped around a bare wire (quad) to supply the customer. However the bare conductor is not connected at either end. At the customer end there is nothing to connect to since there are only 3 wires coming out of the weather head (this is a 3 phase 3 wire ungrounded system originally). Meter socket is a 5 jaw configuration.

The utility transformer usually looks kind of new. The utility probably changed it out due to either failure or distribution voltgage changes. The quad drop probably changed from original 3 wire open spool installations. The problem I see here is that an existing customer owned 3 phase/ 3 wire installation is now fed by a 3 phase 4 wire system utility modified installation. There is not an ungrounded conductor connecting the customer serive to the utility transformer. A ground fault would have to travel from ground rod to ground rod via the earth. Not a good scenario. Even if a low resistance ground rod (less than 25 ohms) and very conductive soil existed, there is no way of knowing that the utility ground rod and conductor is satisfactory.

Fortunatly, we are eliminating these potential problems with the 4 wire upgrades. What really surprises me is that a utiltiy would connect a newly reconfigured grounded transfomer to an existing ungrounded system, that was formerly fed by a transformer that was ungrounded. Also not installing a 4th wire from transformer to main switch or overhead riser. Is it possible that the readings are coming from the electrical meter connectins, which I kind of doubt?

Please provide your thoughts......

Some of this doesn't add up... 5 terminal meter for three phase 480? you never mentioned the CT's. Where are you checking it to "ground"? Is it earth, or a wire? Are you sure the end of the neutral wire isn't grounded at the pole somewhere? Maybe tied into the tank grounds?
 

thunder15j

Member
Location
Cali
Some of this doesn't add up... 5 terminal meter for three phase 480? you never mentioned the CT's. Where are you checking it to "ground"? Is it earth, or a wire? Are you sure the end of the neutral wire isn't grounded at the pole somewhere? Maybe tied into the tank grounds?
Answers:
100 and 200 amp 3 phase- 3 wire 480 and 240 volt systems are metered by 5 jaw meters. Our utility only meters 2 of the 3 phases.

I am measuring voltage from each of the hot lines/wires to the grounding bar, which is connected to the customer's groundrod.

On a 3 phase- 3 wire system there is no nuetral provided by the customer. A service riser would only have 3 wires (L1,L2, L3).

There are no tanks installed on these irrigation systems. The water is pumped directly into pipelines at 1000-2500 gallons per hour.
 

thunder15j

Member
Location
Cali
GFP schemes are set up different for 3 and 4 wire systems, hope you changed those as well.

Please advise. Does GFP mean ground fault protection?

If the utility comes out and changes things without notifying the customer, what can a contactor do about it?
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Answers:
100 and 200 amp 3 phase- 3 wire 480 and 240 volt systems are metered by 5 jaw meters. Our utility only meters 2 of the 3 phases.

I am measuring voltage from each of the hot lines/wires to the grounding bar, which is connected to the customer's groundrod.

On a 3 phase- 3 wire system there is no nuetral provided by the customer. A service riser would only have 3 wires (L1,L2, L3).

There are no tanks installed on these irrigation systems. The water is pumped directly into pipelines at 1000-2500 gallons per hour.

By tank grounds, I meant the grounds on the actual transformer tanks themselves. These have to be tied to neutral, the end of the bare wire should also be tied to neutral also to meet NESC rules. The NESC section 9 (092) (B) (1) says that the service should be seperately grounded or phase grounded, whichever they chose.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
By tank grounds, I meant the grounds on the actual transformer tanks themselves. These have to be tied to neutral, the end of the bare wire should also be tied to neutral also to meet NESC rules. The NESC section 9 (092) (B) (1) says that the service should be seperately grounded or phase grounded, whichever they chose.

The the NESC not allow for ungrounded systems ?
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
That is the way I take it, See what you think...
 

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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I think the OP's point is, even if the POCO meant to give the customer ungrounded service because that is what they had before from the olden days, it is not compliant with NESC or the NEC because the new transformer bank has a grounded conductor. If they wanted a true ungrounded system they can't ground the service at ANY point. If I understand the OP correctly as to what he has, this is a bad hazard and could kill somebody.
 

kwired

Electron manager
Location
NE Nebraska
Can you post a pic of the bank?

Id call the poco and have them explain. What they are doing is a huge hazard and a NESC violation. Relying on the earth alone is a poor choice where a large current is needed to clear protective devices such as standard thermo-magnetic OCPs. During a ground fault on your side of the system ocps may not open and all metal parts will remain energized.

Secondly how thick is the ground wire from the transformer to the utility ground rod? Chances are its not very large (probably 10 to 6 g). During a fault on your side of the system the wire can easily overheat and melt if sufficient fault current passes.

My guess it is a lineman mistake. (Not aware of your system)

Then they should have been wondering where the neutral was when it was time to connect the existing conductors up.

I was recently on a call at another facility. This time a quarry. Another 3 wire, 3 phase ungrounded 480 volt system. Put my voltage meter on L1, L2, and then L3 testing to ground. Got 277 volts on all three readings. Looked up at pole mounted transformer and did not see a wire landed on the 4th bushing on the transformer nor did I see a grounded wire coming down the side of the pole. However, on this very old system, the metering is accomplished by pole mounted CT's. Wires are then run down the pole in a conduit that go to a meter. I saw a ground wire coming out of the sealed metering section going down the pole.

I checked with a metering tech from the utility and he stated that on some installations that are ungrounded, a voltage stabilizer is installed that could provide the ground reference. I'm wondering if this stabilizer installation, if it is causing the readings to ground, could cause a hazard. On a high amperage ground fault I think it would burn clean and the potential to ground would go away. However, on a small fault it might just pass a small amount of current and therefore create a hazard. The hazard being that someone might come in contact with an energized item and get in the fault path since there is no low impedance path to clear a fault as there would be in a properly installed 4 wire system.

Are my concerns justified?

You measured 277 to ground - did you use a low impedance meter to see if it would sustain voltage while under load?



If they are indeed converting these systems as you say, someone with some kind of authority needs to be notified before someone gets killed. If that was happening around here I would like to think and am pretty sure we could notify our State Electrical Division. They don't have direct authority over the POCO's but they would find some way to intervene. At the very least they could show up at the site and find an ungrounded service connected to a grounded supply and see that as a serious enough hazard to order the service disconnected until it is made safe.
 

wirenut1980

Senior Member
Location
Plainfield, IN
That is the way I take it, See what you think...

I disagree. The section is titled, "Alternating current systems that are to be grounded."

To me, that means if the system is supposed to be grounded, this is how you can do it.

To the original poster: Are you sure the utility transformer bank secondaries are constructed for grounded service? If the bare grounded conductor is not connected to either the transformer or at the service, then that is consistent with an ungrounded service delivery and matches what you have (it might just be used as a messenger). If one phase faults to ground, the fault current flow will be low, since as you have said, the earth will be part of the return path. So with low current fault current flow, no shock hazard should exist and no small ground conductors should burn up. I guess I am missing the danger to this setup (maybe due to a brain fart on my part).


Reading close to 277 V phase to ground does not automatically suggest the transformer bank is grounded, since with only 3 phase loads, very little voltage unbalance should exist on an ungrounded system.

Ungrounded services are not built anymore for new service, but there are many still in existence in Indiana that were built back in the day.

I would suggest talking with the utility to determine whether the transformer banks are grounded or not. This conversation should clear things up and may alert them to a mistake they have been making.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
480V 3Wire Delta....

480V 3Wire Delta....

What you are talking about is an ungrounded 480V 3 wire delta configuration. The bare "neutral" in the overhead "quad" service drop from the transformer bank is only there to support the weight of the wire. It is ACSR, which has a steel reinforced center conductor for strength. The pole ground is only for clearing ground faults on the primary circuit. There is no connection from the system "primary " neutral and it's associated ground to the transformer secondary bushings. A reading of 277 volts to ground is due to induction or capacitance, and usually only shows up on digital meters with high input impedance. 480 volt 3W Delta services (unlike 240 volt 3W Delta services) are seldom "corner grounded" due to safety considerations. In an ungrounded service, any ground on a single phase will not cause breaker trip. But...a second ground or an unintentional contact with an ungrounded phase will be 480V phase to phase. Upgrading 3W to grounded 4W is a safer method, but the extra cost of running a neutral to the service disconnect panel (especially on underground) required by Code can be very high, especially if there is not enough room in the existing conduit. Our utility no longer offers 3W delta service for new customers, The meter for 3W Delta without CT's is a Form 12, which is 5 jaw with two current circuits and two voltage circuits.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
What you are talking about is an ungrounded 480V 3 wire delta configuration. The bare "neutral" in the overhead "quad" service drop from the transformer bank is only there to support the weight of the wire. It is ACSR, which has a steel reinforced center conductor for strength. The pole ground is only for clearing ground faults on the primary circuit. There is no connection from the system "primary " neutral and it's associated ground to the transformer secondary bushings. A reading of 277 volts to ground is due to induction or capacitance, and usually only shows up on digital meters with high input impedance. 480 volt 3W Delta services (unlike 240 volt 3W Delta services) are seldom "corner grounded" due to safety considerations. In an ungrounded service, any ground on a single phase will not cause breaker trip. But...a second ground or an unintentional contact with an ungrounded phase will be 480V phase to phase. Upgrading 3W to grounded 4W is a safer method, but the extra cost of running a neutral to the service disconnect panel (especially on underground) required by Code can be very high, especially if there is not enough room in the existing conduit. Our utility no longer offers 3W delta service for new customers, The meter for 3W Delta without CT's is a Form 12, which is 5 jaw with two current circuits and two voltage circuits.

I agree with all you are saying, but the understanding I had from the OP was that the new xformer bank had the neutral terminal grounded. If that were the case I don't believe it is safe or compliant with NESC or NEC.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
What you are talking about is an ungrounded 480V 3 wire delta configuration. The bare "neutral" in the overhead "quad" service drop from the transformer bank is only there to support the weight of the wire. It is ACSR, which has a steel reinforced center conductor for strength. .

And according to rule 215 (C), it is required to be grounded.
The pole ground is only for clearing ground faults on the primary circuit. There is no connection from the system "primary " neutral and it's associated ground to the transformer secondary bushings. A reading of 277 volts to ground is due to induction or capacitance, and usually only shows up on digital meters with high input impedance. 480 volt 3W Delta services (unlike 240 volt 3W Delta services) are seldom "corner grounded" due to safety considerations. In an ungrounded service, any ground on a single phase will not cause breaker trip. But...a second ground or an unintentional contact with an ungrounded phase will be 480V phase to phase. Upgrading 3W to grounded 4W is a safer method, but the extra cost of running a neutral to the service disconnect panel (especially on underground) required by Code can be very high, especially if there is not enough room in the existing conduit. Our utility no longer offers 3W delta service for new customers, The meter for 3W Delta without CT's is a Form 12, which is 5 jaw with two current circuits and two voltage circuits.
Must be an IOU. There is no added cost on overhead, (the neutral wire is already there, it is the ACSR) and on underground, the added cost would be removing the neutral from the premade QPX, not adding it, unless you pull in single runs to make the triplex the same size.
 

kwired

Electron manager
Location
NE Nebraska
And according to rule 215 (C), it is required to be grounded.
Must be an IOU. There is no added cost on overhead, (the neutral wire is already there, it is the ACSR) and on underground, the added cost would be removing the neutral from the premade QPX, not adding it, unless you pull in single runs to make the triplex the same size.

I think for the underground he was referring to added cost of installing a fourth conductor if only three were originally present.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Added cost...

Added cost...

Well, first off, not an IOU, just a co-op. Any profit goes back to the customers as capital credits. Secondly, the added cost is not so much the wire as the new transformers. As I assume you know, a 480 delta 3 wire bank uses three 480 volt transformers (or two if it's an open delta bank). A 480 4 wire grounded wye bank uses three 277 volt transformers. So....who pays the replacement cost? The rate payers or the customer. On underground, it's the same story. New transformer, unless the old one was not using the neutral and was running ungrounded wye, which would be pretty strange. If the transformers have a grounded neutral, a neutral wire MUST be run to the service disconnect, per 250.24 (C). Otherwise, there is no low impedance path back to the neutral of the bank to trip the main breaker on a ground fault.
 
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