Understanding the Role of Grounding in AC Single Phase Systems

[russ_watters]

Do you really not understand the significance of the difference?

I don't. Please explain.

If the neutral and ground are tied together at the substation, the neutral will have approximately zero potential art the ground, everywhere. As it pertains to the OP's question, I don't see why it matters where they are connected, only that they are.

 

[Studiot]

I don't. Please explain.

Good evening Russ.

First who said the neutral and ground are tied together at the substation?
I didn't.

In the US, a pair of transmission wires, single phase no Earth (the company feed) are brought to the company side of the consumer supply transformer (actually owned by the company I believe but I'm not sure about that). That is the primary.

The consumer is supplied from a centre tapped secondary on this transformer. The centre tap is earthed locally. The two halves of the 'split phase' are used to supply the consumer 120 outlets and the whole secondary used to supply any 240 heavy demand installations.
As far as possible the split phases are balanced but in any event their current draw does not affect the current through the primary circuit terminals, only its magnitude.

The other point of this is that the consumer is electrically isolated from other consumers and cannot unbalance their supplies, since the consumer is in the secondary and the feed supply in the primary.

In British practice, on the other hand, imagine the effect of multiple grounding one of the two main conductors of a phase. All the phase current would be routed directly to local ground, leaving serious unbalance in the (neutral) return conductor.

The British consumer is not electrically isolated from his neighbour and what he does affects the neighbours supply.

 

[russ_watters]

That's all fine, but what does that have to do with the OP's question? To be more precise: if you stand in a bathtub full of water and grab a neutral wire in London, should you expect a shock?

 

[Studiot]

That's all fine, but what does that have to do with the OP's question? To be more precise: if you stand in a bathtub full of water and grab a neutral wire in London, should you expect a shock?

I already answered that in my first post

mickybob is on the right lines, the actual current flowing in the wires does not present the danger. It is the voltage difference between yourself and something else.
The power company maintains the neutral at somewhere near Earth potential (mine is often about 30 volts) and the phase at line voltage above that. It is this potential difference the delivers the electrical power to your appliances.

You are also at approximately Earth voltage so if you touch the neutral of the supply wire you are probably safe, but don't try it to see.

 

[Martinaston]

In a polyphase system if all three phases are not equally balanced the voltage on the Neutral can increase.

If you have three houses in a row where each house is on a separate phase from the substation, two houses are empty and the third is consuming power. The return voltage of the Neutral will rise.
This will not rise in just the occupied house, it will also rise in the other two houses because they all share the Neutral conductor back to the substation.

This voltage should be fairly low and can be felt if you stick your hand between the Neutral and Earth conductors to give you a "tinkle"(so it only passes through your hand, NOT from one hand to the other)
It only takes 50mA to interfere with your heartbeat.

Here in the UK we now use RCD's on all new installations that trip at 30mA, not sure how the US does it ?

This Neutral current can be very dangerous in commercial installations if you have machinery with very high start up currents that unbalance the three phase system while your working on the Neutral condutor.

If you call the power company concerning the 30v on your Neutral they would probably thank you because it means they are earning a fortune.

If the power company had a way of continuously,instantaneously balancing all three phases from the power station they could then get money for old rope.(like they don't already)

To whoever made the comment about the Neutral being connected at the houses in the US, please explain that to me.

As for the generator part, ALL generators should be Earthed.
If its not its giving you a potential difference of 110v or 230v between Live and Neutral but the peak voltage on the Live conductor measured to Earth could be absolutely anywhere and dependent on things like the amount of moisture in the air that day.
If it gets high enough your camper van could look like David Blain was visiting.

 

[sophiecentaur]

The British consumer is not electrically isolated from his neighbour and what he does affects the neighbours supply.

His neighbours three doors either side of him, usually (WYE transformer). But his neutral is shared with everyone. If the neutrals and Earth's were joined at both ends, there could be some really hefty Earth currents flowing constantly from induction. It's just like a Hifi system, where you should connect all grounds to one point.

 

[Studiot]

Hello martinaston, you have pretty well confirmed what I was saying about British practice.

What do you not understand about US practice?

It is indeed a US code requirement that the consumer 'neutral' should be earthed to a local Earth at the entry point to the premises. US power companies do not provide Earth terminals, as a rule.

The transformer is often in a service cupboard these days. Formerly it was on poles.

 

[jbriggs444]

Here in the UK we now use RCD's on all new installations that trip at 30mA, not sure how the US does it ?

In the U.S. individual wall outlets are sometimes protected with "ground fault interruptors" (GFI's). You will generally find such outlets in the wash room or kitchen. There are typically two buttons on such a device. One to reset it in the event of a an accidental trip and one to test it, thereby causing it to trip.

Downstream outlets on a circuit are sometimes protected by the interruptor. An outlet will stop working and you walk into the wash room to reset the GFI.

There is a picture of a U.S. device and some additional details in the North America section of http://en.wikipedia.org/wiki/Residual-current_device.

[I am not an electrician]

 

[rollingstein]

If you call the power company concerning the 30v on your Neutral they would probably thank you because it means they are earning a fortune.

I didn't get that remark. Can someone elaborate?

 

[rollingstein]

This voltage should be fairly low and can be felt if you stick your hand between the Neutral and Earth conductors to give you a "tinkle"

Sounds like a dangerous thing to try!

 

[Studiot]

For example, a crane that was grounded to the system neutral and that contacted an energized line would expose any person in contact with the crane or its uninsulated load line to a touch potential nearly equal to the full fault voltage.

Here is an excerpt from dlgoff's link in this thread.

Note the effect of grounding the neutral.

https://www.physicsforums.com/showthread.php?p=4249959&posted=1#post4249959

 

[rollingstein]

Note the effect of grounding the neutral.

For something mobile like a crane what does "grounding the neutral" mean?

"grounded to the system neutral " to be precise.