Simultaneous ground rods disconnection at utility pole and main panel

[tersh]

Supposed the ground rods at the utility pole and your main panel get disconnected lets say from seismic activity or roads works, etc. So the North American centertap transformer neutral is connected directly to your washing machine metal enclosure, and then you touch the metal enclosure and your bare feet touches the floor. Would you get a shock?

Or to make it simple. What would happen if the utility pole centertap neutral lost all its grounding, and you touch the hanging neutral with one hand and the other hand touching the disconnected ground rod. Would you get a shock?

Or since the transformer is an isolation transformer, then when the secondary lost connection with the ground. It becomes floating and just like the small isolation transformer you use at work bench to avoid connection with ground where you touch live and ground and not getting a shock. Then nothing would happen to you if the same thing happens to the utility pole case?

Of course I won't do it intentionally. Just what would happen and the principles involved. Thanks.

 

[LarryFine]

Simply, there should be no shock hazard unless another (a hot) wire becomes accidentally grounded instead.

 

[JFletcher]

Welcome to The Forum. There are times with a failed service neutral that the ground rod at the structure will become energized, or the unbalanced current will try to flow through the shield on the houses coaxial cable.

On older structures with all metal piping, that metal piping will basically carry the neutral current back through to a neighbor's service neutral.

I would not advise placing yourself in series between any two wires, especially under what is an abnormal condition by your descriptions.

Washing machines would simply not function without a service neutral as they are 120 volts. An older 3 wire dryer is probably mostly 240v anyway, and the few things like the interior light that are a hundred twenty volt, they wouldn't work with a severed neutral, although unless there is a fault in the machine, it is not really going to care what happens to any ground rods or service neutral.

And although I like to contemplate every scenario, losing ground rods at both the utility and the structure due to seismic activity is impossible here, and probably extremely rare even in seismically active areas.

 

[don_resqcapt19]

It becomes an ungrounded system and there may be a shock hazard based on the capacitance to ground. The larger the system the greater the capacitance and the greater the shock hazard. Large ungrounded industrial systems can have a 5-25 amps of current that can drive a shock to ground. Small systems may only have a few mA. If there is nothing other than the transformer, the service drop and the house wiring and equipment, it would likely only be micro amps.

 

[GoldDigger]

It becomes an ungrounded system and there may be a shock hazard based on the capacitance to ground. The larger the system the greater the capacitance and the greater the shock hazard. Large ungrounded industrial systems can have a 5-25 amps of current that can drive a shock to ground. Small systems may only have a few mA. If there is nothing other than the transformer, the service drop and the house wiring and equipment, it would likely only be micro amps.

:thumbsup:
And independent of the amount of current that could flow through a neutral to ground connection, a high impedance voltmeter could measure anything from a few volts (small imbalance of capacitance to ground in system) to close to the phase-neutral voltage (single phase equipment with higher capacitance to ground at the hot end of the internal circuitry.)

 

[romex jockey]

methinks the existing earthing system would be a factor....~RJ~

 

[ptonsparky]

Welcome to The Forum. There are times with a failed service neutral that the ground rod at the structure will become energized, or the unbalanced current will try to flow through the shield on the houses coaxial cable.

On older structures with all metal piping, that metal piping will basically carry the neutral current back through to a neighbor's service neutral.

I would not advise placing yourself in series between any two wires, especially under what is an abnormal condition by your descriptions.

Washing machines would simply not function without a service neutral as they are 120 volts. An older 3 wire dryer is probably mostly 240v anyway, and the few things like the interior light that are a hundred twenty volt, they wouldn't work with a severed neutral, although unless there is a fault in the machine, it is not really going to care what happens to any ground rods or service neutral.

And although I like to contemplate every scenario, losing ground rods at both the utility and the structure due to seismic activity is impossible here, and probably extremely rare even in seismically active areas.

FWIW, older dryer motors are 120 volt. IDK about newer.

 

[LarryFine]

FWIW, older dryer motors are 120 volt. IDK about newer.

Every dryer whose schematic I have seen has everything except the heater element wired from L1 to N. This makes them compatible for use on either 120/240v or 120/208v, with 3/4 of the full heat wattage when on 208v.

 

[kwired]

Supposed the ground rods at the utility pole and your main panel get disconnected lets say from seismic activity or roads works, etc. So the North American centertap transformer neutral is connected directly to your washing machine metal enclosure, and then you touch the metal enclosure and your bare feet touches the floor. Would you get a shock?

Or to make it simple. What would happen if the utility pole centertap neutral lost all its grounding, and you touch the hanging neutral with one hand and the other hand touching the disconnected ground rod. Would you get a shock?

Or since the transformer is an isolation transformer, then when the secondary lost connection with the ground. It becomes floating and just like the small isolation transformer you use at work bench to avoid connection with ground where you touch live and ground and not getting a shock. Then nothing would happen to you if the same thing happens to the utility pole case?

Of course I won't do it intentionally. Just what would happen and the principles involved. Thanks.

Majority of single phase services are supplied with line to MGN on the primary and secondary center tap is bonded to the MGN as well. If all that gets disconnected is one ground rod, you still have a large MGN network and many electrodes- you likely never even know that one ground rod got disconnected in most cases.

If you were completely separated from grounding at the pole, you still have a grounded neutral at your service equipment, it just doesn't have as large of a network of electrodes without the MGN being involved, though many times with pole mounted transformers your service neutral connects to the MGN conductor and not directly to the secondary center tap, a separate jumper connects MGN to secondary center tap - should that become compromised you will have more issues with "open neutral" problems (unbalanced voltage issues) than you will have with grounding problems in most instances.

 

[tersh]

It becomes an ungrounded system and there may be a shock hazard based on the capacitance to ground. The larger the system the greater the capacitance and the greater the shock hazard. Large ungrounded industrial systems can have a 5-25 amps of current that can drive a shock to ground. Small systems may only have a few mA. If there is nothing other than the transformer, the service drop and the house wiring and equipment, it would likely only be micro amps.

For a regular distribution transformer with 13,800 volts at the primary and split phase 120/240v at the secondary, and assuming the transformer has "100" rated capacity (see picture below). What could be the capacitance coupling current.. is the current continuous? And this will only appear when the secondary become floating from ground?

 

[tersh]

I read this at Quora which made me concerned:

"Never assume that it is safe to touch a neutral wire. When I started at my utility the veteran who was training me told of a worker new to the job who touched a ground connection coming from an overhead transformer bank. It should have been at ground potential due to being hooked to a ground rod at the base of the pole. But someone had cut and removed the copper wire near the base of the pole. The unbalance current from the transformer primaries went through the worker and he died, even though it was not the massive flow of current that would result from touching a primary conductor. One aggravating factor was that there would be no fuse protection to interrupt the current. If he had used all protective gear it would not have happened unless the end of a wire whipped around and caught him somewhere there was no protection."

Did the above person died from static discharge or capacitive coupling current? What do you think? What component in the primary is so dangerous that when the ground rod got removed, it suddenly changed potential. I thought static discharge is not enough to kill? Why couldn't it happen in the secondary assuming the pole got isolated from say service and all multi grounding were disconnected for maintenance and the person touch the hanging neutral and the ground rod (assuming this is the only ground.. just hypothetically speaking as we are interested in the principles and not whether it would happen or not)?​

 

[GoldDigger]

I read this at Quora which made me concerned:

"Never assume that it is safe to touch a neutral wire. When I started at my utility the veteran who was training me told of a worker new to the job who touched a ground connection coming from an overhead transformer bank. It should have been at ground potential due to being hooked to a ground rod at the base of the pole. But someone had cut and removed the copper wire near the base of the pole. The unbalance current from the transformer primaries went through the worker and he died, even though it was not the massive flow of current that would result from touching a primary conductor. One aggravating factor was that there would be no fuse protection to interrupt the current. If he had used all protective gear it would not have happened unless the end of a wire whipped around and caught him somewhere there was no protection."

Did the above person died from static discharge or capacitive coupling current? What do you think? What component in the primary is so dangerous that when the ground rod got removed, it suddenly changed potential. I thought static discharge is not enough to kill? Why couldn't it happen in the secondary assuming the pole got isolated from say service and all multi grounding were disconnected for maintenance and the person touch the hanging neutral and the ground rod (assuming this is the only ground.. just hypothetically speaking as we are interested in the principles and not whether it would happen or not)?​

The situation described is neither static nor capacitive current. The mention of primary current (and voltage) means that the utility did not have a primary side neutral wire but instead relied on the earth connection for the return of unbalanced primary current. That would generally NOT be found in the US where a multiply grounded wire neutral is used on the primary side.

Also in the US, on the secondary (user) side of the transformer the center tap of the secondary would be wire connected to the neutral point of the service disconnect where there would also be a neutral/ground bond. So the voltage offset of the transformer center tap from the earth ground would be minimal (not hazardous) even if the ground wire at the pole were interrupted.

 

[tersh]

The situation described is neither static nor capacitive current. The mention of primary current (and voltage) means that the utility did not have a primary side neutral wire but instead relied on the earth connection for the return of unbalanced primary current. That would generally NOT be found in the US where a multiply grounded wire neutral is used on the primary side.

Also in the US, on the secondary (user) side of the transformer the center tap of the secondary would be wire connected to the neutral point of the service disconnect where there would also be a neutral/ground bond. So the voltage offset of the transformer center tap from the earth ground would be minimal (not hazardous) even if the ground wire at the pole were interrupted.

But supposed for sake of discussions, there is no ground/bond in the service disconnect, and the pole ground rod was disconnected. What would happen if there is a voltage offset that is huge compared to ground. In the concept of isolation transformer, the secondary is isolated to ground. So any voltage offset won't affect the disconnect ground. Here is an illustration.

Won't the utility pole behave like it when all the ground rods were disconnected (for sake of discussions)?

 

[kwired]

I read this at Quora which made me concerned:

"Never assume that it is safe to touch a neutral wire. When I started at my utility the veteran who was training me told of a worker new to the job who touched a ground connection coming from an overhead transformer bank. It should have been at ground potential due to being hooked to a ground rod at the base of the pole. But someone had cut and removed the copper wire near the base of the pole. The unbalance current from the transformer primaries went through the worker and he died, even though it was not the massive flow of current that would result from touching a primary conductor. One aggravating factor was that there would be no fuse protection to interrupt the current. If he had used all protective gear it would not have happened unless the end of a wire whipped around and caught him somewhere there was no protection."

Did the above person died from static discharge or capacitive coupling current? What do you think? What component in the primary is so dangerous that when the ground rod got removed, it suddenly changed potential. I thought static discharge is not enough to kill? Why couldn't it happen in the secondary assuming the pole got isolated from say service and all multi grounding were disconnected for maintenance and the person touch the hanging neutral and the ground rod (assuming this is the only ground.. just hypothetically speaking as we are interested in the principles and not whether it would happen or not)?​

Unless it is a single wire earth return (SWER) on the transformer primary, which would yield full primary voltage to earth if you opened the conductor to the electrode, most MGN systems have an electrode at nearly every structure as well as a solid conductor between structures. Losing an electrode at one structure only gives you whatever voltage drop occurs between your point and the last structure with an electrode, probably less then a volt to even less than a tenth of a volt in most cases. the more current there is on the neutral the more that voltage drop will be.

 

[tersh]

If the mains voltage is 13,000 volts and the secondary the split phase 120/240v. What is the formula to compute the capacitive coupling current for worse case insulation (and supposed the secondary is floating)?

 

[tersh]

The situation described is neither static nor capacitive current. The mention of primary current (and voltage) means that the utility did not have a primary side neutral wire but instead relied on the earth connection for the return of unbalanced primary current. That would generally NOT be found in the US where a multiply grounded wire neutral is used on the primary side.

Also in the US, on the secondary (user) side of the transformer the center tap of the secondary would be wire connected to the neutral point of the service disconnect where there would also be a neutral/ground bond. So the voltage offset of the transformer center tap from the earth ground would be minimal (not hazardous) even if the ground wire at the pole were interrupted.

About the anecdote of the person who touched the disconnected primary grounding conducted and he got electrocuted. Supposed he really touched a disconnected neutral, would the primary capacitance be enough to electrocute him?

An electrical engineer who worked at nuclear power plants told me:

"so he was on the PRIMARY side.... where voltage is much higher.
In absence of a solid ground
there is no direct return path for the relatively small current that flows to ground through the distributed capacitance of the system.
On primary side that current might amount to several amps because the wires are quite long , you have the capacitance of the transformer, and the voltage is pretty high. In short the 'system' is large.
On secondary side you have only the distributed capacitance inside your house and the voltage is low
so i'd expect a tenth of an amp or so which could still be lethal but might not. They 'system' is small.
If you provide that return path you'll feel it".

His subscription to the site expired before I had chance to question him something important.

The question is whether the capacitance of the primary is enough to electrocute someone if it's really a disconnected neutral with the scenario it's not multiply connected?

Second, on the issue of disconnected ground rods at secondary where all grounding backup is lost. why didn't he mention the capacitive coupling of the transformer itself when describing the secondary? Why did he only talk about the distributed capacitance inside the house, and not the capacitive coupling current from the transformer?

​

​

 

[kwired]

About the anecdote of the person who touched the disconnected primary grounding conducted and he got electrocuted. Supposed he really touched a disconnected neutral, would the primary capacitance be enough to electrocute him?

An electrical engineer who worked at nuclear power plants told me:

"so he was on the PRIMARY side.... where voltage is much higher.
In absence of a solid ground
there is no direct return path for the relatively small current that flows to ground through the distributed capacitance of the system.
On primary side that current might amount to several amps because the wires are quite long , you have the capacitance of the transformer, and the voltage is pretty high. In short the 'system' is large.
On secondary side you have only the distributed capacitance inside your house and the voltage is low
so i'd expect a tenth of an amp or so which could still be lethal but might not. They 'system' is small.
If you provide that return path you'll feel it".

His subscription to the site expired before I had chance to question him something important.

The question is whether the capacitance of the primary is enough to electrocute someone if it's really a disconnected neutral with the scenario it's not multiply connected?

Second, on the issue of disconnected ground rods at secondary where all grounding backup is lost. why didn't he mention the capacitive coupling of the transformer itself when describing the secondary? Why did he only talk about the distributed capacitance inside the house, and not the capacitive coupling current from the transformer?

​

If you completely open the primary neutral, say on a 7200 volt "single phase, line to neutral" segment of distribution, then you will have 7200 open circuit voltage at that open point. If someone gets themselves across that open circuit path, one side of which is at or very near ground potential, then they are subjected to all or nearly all of the 7200 volts.

If you have just one grounding electrode lead at one pole that has been cut, you did not open the neutral, neutral current still flows on the neutral conductor. You may be subject to whatever voltage drop is present on said neutral to local earth though, probably going to be less than a volt in most cases where just one electrode has been disconnected. If copper thieves are taking out sections of grounding electrode conductor over a large area of distribution, you are losing connection to many electrodes and may see even more voltage drop because of less grounding electrodes - just depends on circumstances.

If you had 7200 volts with 10 amps of current, and 300 feet of conductor between poles, and disconnected the grounding electrode at one pole, voltage drop on a 1/0 aluminum neutral would only be about .6 volts over that distance.This is enough to be an issue in a swimming pool if there is no equipotential bonding, but someone wearing average shoes and touching that disconnected grounding electrode conductor with their hand - they probably don't feel a thing.

 

[tersh]

If you completely open the primary neutral, say on a 7200 volt "single phase, line to neutral" segment of distribution, then you will have 7200 open circuit voltage at that open point. If someone gets themselves across that open circuit path, one side of which is at or very near ground potential, then they are subjected to all or nearly all of the 7200 volts.

If you have just one grounding electrode lead at one pole that has been cut, you did not open the neutral, neutral current still flows on the neutral conductor. You may be subject to whatever voltage drop is present on said neutral to local earth though, probably going to be less than a volt in most cases where just one electrode has been disconnected. If copper thieves are taking out sections of grounding electrode conductor over a large area of distribution, you are losing connection to many electrodes and may see even more voltage drop because of less grounding electrodes - just depends on circumstances.

If you had 7200 volts with 10 amps of current, and 300 feet of conductor between poles, and disconnected the grounding electrode at one pole, voltage drop on a 1/0 aluminum neutral would only be about .6 volts over that distance.This is enough to be an issue in a swimming pool if there is no equipotential bonding, but someone wearing average shoes and touching that disconnected grounding electrode conductor with their hand - they probably don't feel a thing.

Back to the transformer secondary. What if all grounding rods and multi-grounding network of 2 poles in isolated town were all cut by thiefs. And there is lightning for succeeding days many hitting or affecting the poles?

Or in general, what damages would occur to transformers if there was no grounding and many lightning strike?