Circuit completion : is it necessary?

[Studiot]

The 2011 National Electric Code paragraph 250.1(A)(1) states:

I hope I have clarified it a little better for you.

No not at all.

You claim to have quoted a paragraph from code regulations.

I am merely pointing out that the interpretation you have offered for this paragraph is fundamentally different from what the paragraph quite clearly says in plain English.

I am well aware that it is normal practice in the US to ground the centre point of the consumer's supply transformer.

I just don't agree that this has anything to do with lightning etc.

 

[rcgldr]

In the US, the black insulator is the "hot" or "live" wire. The neutral is white. The ground is either green or bare wire.

It was too late to edit and correct my post. The wiki link below shows some various standards for the wiring colors.

3 phase transmission lines

Note in the US, normally there is no 4th neutral line in higher voltage 3 phase distribution. There is a fourth grounded wire mounted the highest on the towers, but that is for lightning protection and does not go back to the generator station. For these systems, the step down transformers have a neutral connected to ground.

http://en.wikipedia.org/wiki/Three-phase_electric_power#Three-wire_versus_four-wire

 

[Evil Bunny]

No not at all.

You claim to have quoted a paragraph from code regulations.

Not only did I claim to have quoted it, I actually quoted it!

I am merely pointing out that the interpretation you have offered for this paragraph is fundamentally different from what the paragraph quite clearly says in plain English.

It's telling you how a grounding system SHALL BE installed. It SHALL BE installed in that manner so as to limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines.

To me... that's a declaration as to why the systems shall be grounded. Additionally, in other professional electrical forums (electriciantalk and mikeholt are a couple examples) where the focus is strictly code based discussion and topics on the electrician's profession, this is a well established and widely agreed upon explanation to the question, "why do we ground?"

I am well aware that it is normal practice in the US to ground the centre point of the consumer's supply transformer.

I just don't agree that this has anything to do with lightning etc.

You don't need to agree with it for it to be so...

I don't suppose you want to offer your opinion on why we ground?

 

[sophiecentaur]

If you don't ground in some way and somewhere, there is nothing to stop the average volts going to dangerously high values. The whole mains network could float up to 1kV or more, easily, and then break down someone's insulation on some just-to-spec domestic equipment. The total capacity of a local system is quite high - several uF - and probably a lot more. That could actually be lethal for some unsuspecting lady drying her hair.
Why do you think all planes are grounded before anyone touches them - all fuel pipes - all conveyor systems ? You ground things - natch. Where would you ground a mains distribution system? Not on one of the lives, for sure, so you choose the neutral.

 

[Studiot]

If you don't ground in some way and somewhere, there is nothing to stop the average volts going to dangerously high values.

And where & how do we ground industrial and site 110V supplies in the UK?


Firstly

Electrical systems that
are grounded shall be connected to Earth in a manner

This would be worded differently if all electrical systems were to be grounded.

As it is worded it allows for the possibility that some are and some are not (which I understand to be the case).

Secondly

I don't suppose you want to offer your opinion on why we ground?

Consider why there is a centre tapped secondary and how this is used.

As I understand it, doing this allows for independent wiring for say the 120V lighting and 120V low power outlets, given that the US practice does not use ring mains.

Now two questions:

Let us suppose that there a CT transformer is connected to (supplies) two such circuits in a building so that some lights are supplied from the 120V between one end terminal of the transformer and the centre and a 120V wall outlet is supplied from the other.

Now suppose that a device plugged into the wall outlet draws sufficient current to reduce the supply (perhaps a momentary switch on surge) by 10 - or even 20 volts.

What voltage would the circuit on the other half of the transformer see if

1) The CT was grounded
2) The CT was not grounded

Similarly suppose the wall outlet device draws sufficient fault current to trip its breaker.

Again what voltage would the circuit on the other half of the transformer see if

1) The CT was grounded
2) The CT was not grounded


I think if the CT were not grounded excess voltage could appear on the other circuit.

 

[sophiecentaur]

And where & how do we ground industrial and site 110V supplies in the UK?

Good question but I think there are several answers to it. Firstly, industrial equipment for use outside is very highly spec'd (it's all double insulated, for a start). Secondly, the ungrounded networks are small in extent (in the cases I have seen, at least- with one or two appliances fed from a transformer; you could perhaps tell me different - I just don't know) but one supply is definitely limited to one premises. Thirdly, you can't guarantee that all exposed metal will be also grounded (very relevant when there is high risk of temporary cables being damaged during construction work)

It is a compromise between various factors which makes grounding produce possibly a greater risk than not grounding - in those particular circumstances. I'd bet that a lot of thought went into the decision to have two different standards.

 

[sophiecentaur]

Consider why there is a centre tapped secondary and how this is used.

As I understand it, doing this allows for independent wiring for say the 120V lighting and 120V low power outlets, given that the US practice does not use ring mains.

Why is that an advantage, except that it (as much as) halves the current demand on one incoming circuit (which would be less, in any case if they used a higher voltage system)? UK systems just use several independently fused circuits. The only reason for using ring mains is that it is cheaper in copper.

 

[sophiecentaur]

Again what voltage would the circuit on the other half of the transformer see if

1) The CT was grounded
2) The CT was not grounded


I think if the CT were not grounded excess voltage could appear on the other circuit.

Absolutely - which is what I said about two pages back - only with regard to a proper three phase system.

 

[Studiot]

see also this thread

https://www.physicsforums.com/showthread.php?t=494343

 

[sophiecentaur]

Does that thread actually add anything to this one?
As far as I can see, the Federal System in the US probably means that it is harder to introduce a tough regulatory system for electrical supply and use than in European (UK) authorities. I can see how a system with 110V was more attractive to the US government because it meant some inherent safety. The UK system was more 'tight',with many more restrictions on systems and could relax the need for an inherently safer voltage - going more for a cheaper (cabling) system.
Standards are a compromise between political, economic and health-and-safety issues. The environment will affect the final choice (as on UK building sites).

 

[Evil Bunny]

This would be worded differently if all electrical systems were to be grounded.

As it is worded it allows for the possibility that some are and some are not (which I understand to be the case).

Not every electrical system in existence needs to be grounded. Residential dwellings are... but there are certainly scenarios where grounding is not mandatory. Probably some industrial applications would not have grounding requirements, but I'm not sure of any specific examples. It suffices to say that ungrounded systems are certainly here... they are just uncommon.

Consider why there is a centre tapped secondary and how this is used.

As I understand it, doing this allows for independent wiring for say the 120V lighting and 120V low power outlets, given that the US practice does not use ring mains.

I don't know what ring mains are...

Now two questions:

Let us suppose that there a CT transformer is connected to (supplies) two such circuits in a building so that some lights are supplied from the 120V between one end terminal of the transformer and the centre and a 120V wall outlet is supplied from the other.

Now suppose that a device plugged into the wall outlet draws sufficient current to reduce the supply (perhaps a momentary switch on surge) by 10 - or even 20 volts.

What voltage would the circuit on the other half of the transformer see if

1) The CT was grounded
2) The CT was not grounded

The voltage would be the same whether the center tap was grounded or not... The grounding of the center tap has absolutely nothing to do with how the circuit operates. You could rip the grounding rod out of the Earth and you would see no difference at all in the operation of the circuits.

Now to answer what would happen on the other half of the transformer... I don't believe the other half would "notice" the voltage drop on the first half at all. I believe this because I have heard of overloaded transformers where only one half of the windings were damaged... But this is a bit off topic, because even if the other side did "notice" the voltage drop, whether or not we attached a ground rod to the center tap and pounded it into the ground would have no bearing on this outcome whatsoever.

Similarly suppose the wall outlet device draws sufficient fault current to trip its breaker.

Again what voltage would the circuit on the other half of the transformer see if

1) The CT was grounded
2) The CT was not grounded


I think if the CT were not grounded excess voltage could appear on the other circuit.

This happens all the time. The systems are designed this way on purpose! Nothing happens to the other side of the transformer whatsoever.

And again, if something did happen, a grounded center tap would not affect this outcome.

I have seen many situations where people have stolen copper wire (to sell to the recycling shop) off of ground rods and nobody ever noticed the difference. This will not affect the operation of the circuits at all.

 

[Evil Bunny]

Why is that an advantage, except that it (as much as) halves the current demand on one incoming circuit (which would be less, in any case if they used a higher voltage system)? UK systems just use several independently fused circuits. The only reason for using ring mains is that it is cheaper in copper.

I hope someone explains what ring mains are because I'm at a loss on this...

At any rate... I don't know if a center tapped transformer has any advantages over a straight 240V system. I think (but I'm not sure) some suit in an office somewhere decided long ago that 120V was safe and 240V was not safe, so we decided to run with it and have been stuck with this decision.

We use several independently fused circuits as well (circuit breakers, not fuses)... The only difference is that ours are mostly 120V circuits and they are distributed over two halves of a 240V service transformer. We use the full 240V for larger loads, such as kitchen stoves, electric water heaters, clothes dryers, etc...

 

[sophiecentaur]

Not every electrical system in existence needs to be grounded. Residential dwellings are... but there are certainly scenarios where grounding is not mandatory. Probably some industrial applications would not have grounding requirements, but I'm not sure of any specific examples. It suffices to say that ungrounded systems are certainly here... they are just uncommon.



I don't know what ring mains are...



The voltage would be the same whether the center tap was grounded or not... The grounding of the center tap has absolutely nothing to do with how the circuit operates. You could rip the grounding rod out of the Earth and you would see no difference at all in the operation of the circuits.

Now to answer what would happen on the other half of the transformer... I don't believe the other half would "notice" the voltage drop on the first half at all. I believe this because I have heard of overloaded transformers where only one half of the windings were damaged... But this is a bit off topic, because even if the other side did "notice" the voltage drop, whether or not we attached a ground rod to the center tap and pounded it into the ground would have no bearing on this outcome whatsoever.



This happens all the time. The systems are designed this way on purpose! Nothing happens to the other side of the transformer whatsoever.

And again, if something did happen, a grounded center tap would not affect this outcome.

I have seen many situations where people have stolen copper wire (to sell to the recycling shop) off of ground rods and nobody ever noticed the difference. This will not affect the operation of the circuits at all.

I think that you do not understand some of the basics E-B.
The grounding or not of the neutral makes, as you say, not difference to the operation of the circuits. It's the fault condition that makes the difference. (Your copper thieves would have no immediate effect on the system - just its behaviour after a fault)
If a neutral is not grounded then a fault to Earth (i.e. a conductive connection to Earth) on one of the live conductors will cause the other 'live' conductor to have up to twice the volts wrt Earth (this is really basic theory). The PD across any equipment on the other side will not be affected but the PD relative to Earth will go up (a fuse or CB may disconnect the faulty side but can you rely on it with a low current leak?). However, if you ground the neutral then, whatever fusing or breaker you have on the faulty side,the peak volts on the other side will be unaffected. This MAY not worry you, but it means that what you may have thought was a neutral at near zero volts, is now a neutral at 110V (or whatever), with a potentially lethal current available. If you can live with that, OK but you may not be the only one involved.

A ring main is a system which uses a pair of conductors (L&N) arranged in a loop with both connected at each end to the fused (32A) outlet on the consumer unit. There are two paths for the currents to take. Look it up. It is a cheap and safe way to feed a large number of outlets (a large number of low loads connected in parallel across the cables - usually on one floor of a house and involves thinner and less cable than a star system. The philosophy is that there is a limit to the total amount of load that it will have to support (basically, the place would get too hot with 7kW of load in a relatively small area). It's a system that works well in homes and offices in the UK. Each plug / connector is fused, btw.

 

[sophiecentaur]

I hope someone explains what ring mains are because I'm at a loss on this...

At any rate... I don't know if a center tapped transformer has any advantages over a straight 240V system. I think (but I'm not sure) some suit in an office somewhere decided long ago that 120V was safe and 240V was not safe, so we decided to run with it and have been stuck with this decision.

We use several independently fused circuits as well (circuit breakers, not fuses)... The only difference is that ours our mostly 120V circuits and they are distributed over two halves of a 240V service transformer. We use the full 240V for larger loads, such as kitchen stoves, electric water heaters, clothes dryers, etc...

I hope my last post helped with the ring main thing.
Yes- I agree that it was probably just someone's decision.
The problem with the 110V choice is that it was made on the assumption that people would just want lighting and a radio / TV. Once people started needing washing machines and cookers, the thickness of cable needed for 110V was ridiculous. Using a 'two phase' system gave twice the volts for high power devices without too much current / thick wires yet didn't involve new lightbulbs and electronic goods. As I have said before - it's a bit of "dog's dinner" as we say, in the UK - who possible got it right (for once).

 

[Evil Bunny]

I think that you do not understand some of the basics E-B.
The grounding or not of the neutral makes, as you say, not difference to the operation of the circuits.

Wow... Studiot asked me a couple questions about what would happen to the "other" side of a CT transformer if it was grounded or not.

I answered them (correctly), and now you're telling me I don't understand basic electricity?

At any rate... You're saying that if we ground a hot (unintentionally), then we now have 120V between Earth and the neutral and 240V between Earth and the other hot.

Good point, and certainly not a desirable situation. And another good reason to ground the neutral... in addition to the other reasons mentioned above.

 

[westom]

However my understanding of the above wording is that it does not constitute a reason.
Nor does it require grounding.
It merely states a manner of grounding if the electrical system is grounded.
So that is not the answer to my question.

I am not going to read all 100 previous posts. But in the last tens of posts, you are correct. They told you what to do (according to code) and did not say why. Appreciate that most electricians are taught what must connect to what. But learn few of the reasons why.

Let say the transformer primary is 33,000 volts. The secondary is 240 volts with a center tap. That means the house has 120 volts from one side of the transformer to the center tap. And 240 volts if connected from one side of the transformer to the other side.

At the same time, all three wires can be 33,000 volts to earth. Notice what is important. No wire alone has a voltage. Voltage is always between "X and Y". The same wire that can be 240 volts may also be 33,000 volts. Depending on what is referenced.

So that the same wire is only 240 volts or less compared to all other conductors, one wire from the secondary side must be earthed. Not just safety grounded. Earthed.

Earthing performs numerous other functions. Too many to discuss in this post. Some include transistor safety and various human safety issues. But if one end or the center tap of that transformer secondary is not earthed, then up to 33,000 volts can also exist on the 120 or 240 volt wires.

 

[sophiecentaur]

With only magnetic coupling between primary and secondary, how could kVs appear on the secondary?

 

[Dmytry]

by insulation breakdown, leakage, or by capacitive coupling. When any tap of the secondary is earthed, insulation breakdown would result in enormous current through secondary (keep in mind that secondary is built to handle much larger current than primary), which would open the circuit breaker or blow the fuse on primary.
The thing is, there's also a lot of equipment which will happily bring up mains to many kilovolts. A tv for example. There's also lightning strikes.
When you do stuff like those high voltage inverters using flyback transformer from TV, you need to ground the power supply's - or + else you risk arcing over the transformer in it. (its usually build to hold off much more than rated voltage though, so usually it withstands such abuse)

 

[sophiecentaur]

by insulation breakdown, leakage, or by capacitive coupling. When any tap of the secondary is earthed, insulation breakdown would result in enormous current through secondary (keep in mind that secondary is built to handle much larger current than primary), which would open the circuit breaker or blow the fuse on primary.

My post was specifically about the 'transformer statement'. I, of course, go along with the notion that grounding is useful for all the other reasons you give (I have written as much already).
However, I have a feeling that any high power transformer that is likely to be used in a serious power distribution system will be designed 1. With the HV ends of the primaries away from the secondary windings and 2. with a grounded electric screen in between the primary and secondary windings (eliminating capacitative coupling). Even a half decent transformer in a low power equipment power supply will have an electrostatic screen.

 

[sophiecentaur]

now you're telling me I don't understand basic electricity?

Sorry. On re-reading that, it sounded a bit aggressive. But my point about 'stacking up voltages' still stands.

 

[westom]

Even a half decent transformer in a low power equipment power supply will have an electrostatic screen.

Why does that elecrostatic scrreen exist? Because electric fields (and other reasons) are never eliminated. The screen only reduces that current. Primary currents leak by various means into the secondary. An electrostatic shield only reduced currents from the 33,000 volts side appearing on the secondary. Does not eliminate it.

Second, all wires can have two different voltages. Demonstrated were different currents creating 240 volts and 33,000 volts on the same secondary wire. To better understand this, learn two basic electrical concepts - longitudinal and transverse currents.

So that 33,000 volts does not appear on the 240/120 secondary, the secondary must be earthed somewhere.

Third, another example. Suppose lightning strikes the primary (33,000 volt) system. This is a lower energy event. That lightning strike can short transformer primary to secondary. The resulting plasma leaves utility 33,000 volts shorted directly to 240/120 volt power consumers. This 'follow-through' current means high energy connects into a homes and factories. Lightning is the low energy event. A high energy source that may cause much more damage exists if the transformer is not properly earthed.

If a plasma short is not created, then currents on the primary side are not shorted to wires on the secondary side. Another example of why earthing is essential for protection - in this case for lightning protection.

Fourth, earthing a secondary is necesary for numerous reasons both for normal operation and for safety during fault conditions. Elecricians, for example, would not know this. These installers spend years learning only what must be connected to what. Code also only says what must be connected to what. Neither would learn or say "why" this connection is made. Neither would know or discuss two important concepts - longitudinal and transverse currents.

Studiot kept asking "why" a transformer must be earthed. Instead, with frustration, he got "what" must be earthed. If a secondary is not earthed, then 33,000 volts will exist on same wires that also provide 120 volts.

 

[Studiot]

Third, another example. Suppose lightning strikes the primary (33,000 volt) system.

Just how could this happen without striking the casing first?

Surely the casing of a 33Kv txfmr will be earthed in its own right, regardless of other earthing arrangements?

 

[Dmytry]

My post was specifically about the 'transformer statement'. I, of course, go along with the notion that grounding is useful for all the other reasons you give (I have written as much already).
However, I have a feeling that any high power transformer that is likely to be used in a serious power distribution system will be designed 1. With the HV ends of the primaries away from the secondary windings and 2. with a grounded electric screen in between the primary and secondary windings (eliminating capacitative coupling). Even a half decent transformer in a low power equipment power supply will have an electrostatic screen.

well you won't need a screen if you ground the secondary, and you get added benefit of protection against any equipment (or thunderstorm) leaking high voltage onto secondary. But yes it is not too hard to prevent primary --> secondary strike. Just extra protection (and its validation) is much less necessary if secondary itself is grounded.

 

[westom]

Just how could this happen without striking the casing first?

Kilometers of wire down many streets. A lightning strike to those wires (that are often highest and most exposed) is a direct lightning strike to the transformer - inside its case. Transformer mounted on Earth must have a dedicated and short connection to a superior Earth electrode. Similar to what transformer atop poles require.

A late 1970s IEEE paper discusses this also with a figure. Figure shows an extremely rare 100 kA strike to distant wires. 40 kA of that surge go to Earth via transformer grounding. 20 kA attempts to destroy home appliances. Another 40 kA is earthed by other consumers. That earthing is essential even to protect appliances and factory machines.

Lightning is only one reason why transformers are earthed. Primary and secondary must be as close as possible to make the transformer efficient. Lightning that cannot be stopped by 3 miles of sky will just as easily short 33,000 volts (utility power) into any nearby house IF that always required and critically important earthing does not exist.

Again, most techs (ie electricians) are only told what must be connected. Most would never understand why a utility transformer (or one inside some factory for a special machine) also must be earthed. Most only know "what" must be done. Never learn "why" it must be done. And never learn the difference between "what" and "why". Even utility linemen will claim that earthing does nothing because they never learn the difference between "observation" (also called junk science) and "why".

Demonstrated is only one reason "why" transformers must be earthed. Another reason "why" was presented previously. Your question demonstrates one who seeks "why" as well as "what".

 

[Studiot]

Thank you for the information.

 

[Evil Bunny]

The NEC clearly states that grounding SHALL BE done in such and such a way in order to protect against lightning, etc...

I interpret that as an answer to "why" we do it... but I guess your mileage may vary.

And also, I don't think anybody was saying that grounding was unimportant... The point that was being made was that it doesn't affect how the circuit works under normal conditions... There always has been and always will be confusion on this. People tend to think that electricity from power lines (as opposed to static electricity, like lightning) has this inherent tendency to "dissipate" itself into the earth, when in actuality the tendency is just to return to the source any way it can. There is no giant electron sink in the earth...

 

[westom]

I interpret that as an answer to "why" we do it... but I guess your mileage may vary.

That only says "what" must be done. Where, for example, does it discuss wire impedance? Where does it discuss "why" a grounding wire must be shorter rather than thicker? Where does it discuss "why" a grounding wire must not be inside metallic conduit for lightning protection? Where does it even say "why" grounding is so important to lightning protection? All examples of the "why" that is not found in code.

Also not discussed is "why" both longitudinal and transverse currents are relevant. Does either relevant word appear in the code? Basic electrical concepts are required to understand "why". But not required for "what".

If code says “why”, then each above question is answered by quoting directly from the code.

 

[sophiecentaur]

Why does that elecrostatic scrreen exist? Because electric fields (and other reasons) are never eliminated. The screen only reduces that current. Primary currents leak by various means into the secondary. An electrostatic shield only reduced currents from the 33,000 volts side appearing on the secondary. Does not eliminate it.

Second, all wires can have two different voltages. Demonstrated were different currents creating 240 volts and 33,000 volts on the same secondary wire. To better understand this, learn two basic electrical concepts - longitudinal and transverse currents.

So that 33,000 volts does not appear on the 240/120 secondary, the secondary must be earthed somewhere.

AN electrostatic screen is a sheet of conductor wrapped around (not joined - so it's not a shorted turn) the inner windings and earthed. Any electric field on the secondary winding will be a severely potted down version of any field due to the primary - your 33kV wouldn't appear by capacitative coupling. The screen cuts out common mode signals on the primary - such as interference or unbalanced volts due to lightning. The core, itself, is grounded and also acts as an electric screen between windings.

You will need to explain "longitudinal and transverse currents".

You have not explained any mechanism for this 33kV appearing on the secondary - you have merely asserted that it will be there "and other reasons". You could say, with justification, that there may be a DC (resistive) path between the windings but how much current is likely to get through this Earthed screen and appear on the secondary? Do you some actual numbers to back up your assertion?

There are many reasons for earthing a supply to a house, of course, but the workings of the transformer seems to be one of the less relevant ones.

 

[RedX]

Going back to the discussion on parasitic capacitance:

Assume you have a 12 V battery connected to a parallel-plate capacitor whose plates are 12 meter aparts. Take a voltmeter, and put its lead on the wire leading to one plate, and its other lead you put on the other wire leading to the other plate. We all agree we measure 12 V. Now take one of the leads of the voltmeter off of one wire, and instead place it in the empty space halfway between the plates. Do we all agree there should be 6 volts [ V=Ed=(12V)/(12 m)*(6m)=6V ] , but the voltmeter instead reads 0 volts?

Now consider 120 volts AC that has not been grounded. Would there be 60 volts between each wire and the ground? However, if you touch one of the wires and the ground, not much current will flow through you to the ground, just as the case that no current flows in the voltmeter in the above example because there is no completed circuit?

This might be a dumb question, but if there is 60 volts between you and the ground, but the ground doesn't complete a circuit with the source, how do you know how much current will flow through you in that brief instant you touch the wire and the ground? Is it enough to be dangerous?

 

[sophiecentaur]

If you have 6V/m then you would measure 6V half way across. This assumes the instrument is ideal.

Edit...Owch, that's rubbish. I mean 1V/m.