Neutral wire and earth

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  • #51
sophiecentaur
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Further there is a definite danger in the british system of unwittingly grabbing the wrong wire.
This is because the wiring colour for neutral is also used for line voltage in certain circumstances (light switches).
So a wire carrying the neutral colour will be either at line voltage or disconnected if part of the light switch circuit.
This is very annoying, bad practice and there is no excuse why a professional electrical installer shouldn't use a different pair of colours in the cable that runs from the ceiling rose to the light switch. A different cable with yellow down to the switch and a brown up to the lamp would avoid any problem. But this cannot be a problem, only with the UK system. It will happen anywhere where they are too stingy to use a different cable colours for light switch cirtcuits.
The only time I have ever come across anything other than brown and blue (or red and black, in the past) has been on three core cable, used for 'hall / landing' switching which uses more than just one light switch.
 
  • #52
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This is very annoying, bad practice and there is no excuse why a professional electrical installer shouldn't use a different pair of colours in the cable that runs from the ceiling rose to the light switch. A different cable with yellow down to the switch and a brown up to the lamp would avoid any problem. But this cannot be a problem, only with the UK system. It will happen anywhere where they are too stingy to use a different cable colours for light switch cirtcuits.
And also illegal to do anything else.
 
  • #53
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okay another question we donot have underground wiring system so on street when i saw a pole there are three horizontal conductors which i persume are high voltage delta connected and below them are five vertically connected wires which i persume are low voltage y connected conductors but what i donot understand is that what is the fifth wire is for?is'nt the y connection requires only four conductors is this 'protective conductor?'
 
  • #54
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Difficult to comment on your local wiring system as I know nothing about it.

Are you sure they are all conductor wires, not supports?
 
  • #55
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"Why don't we connect the neutral itself to the equipment body and totally eliminate the use of ground wire?"
Because we don't normally want current flowing through our equipment bodies. We would rather keep it in the wires.
 
  • #56
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Because we don't normally want current flowing through our equipment bodies. We would rather keep it in the wires.
Perhaps a Diagram would help.
scheme.jpg

The Upper Diagram is what I am asking about. The Lower Diagram is the usually practiced scheme.
In the upper diagram, connecting neutral to the equipment body don't make the current flow through the equipment body as you were thinking.

So, I was asking what are the bad things about the protection scheme of the Upper Diagram?

Edit: Updated the Image as per suggestion of Evil_Bunny
 
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  • #57
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So, I was asking what are the bad things about the protection scheme of the Upper Diagram?
Good question...

First of all, the drawings are incomplete. The neutral and the "seperate ground wire" are attached together at the distribution panel. This would be located in between the equipment body and the distribution transformer (more specifically, at the service entrance to your house, at the distribution panel). This creates a parallel path for the return (the normal path would be the neutral and the alternate parallel path would be the grounding conductor).

If this part were included in the drawing, you could see the problem more clearly.

One "bad thing" that could happen in the upper diagram is an open neutral. In this case, your equipment would be at full potential. If you were touching the equipment, the return path would be through you and into the ground you're standing on (as the current will try to make it's way back to the transformer)... a dangerous situation.

With the grounding conductor attached (as would be illustrated in the lower diagram if the distribution panel were included), the current would flow safely through the alternate path (the grounding conductor) and back to the transformer.

It's just one more layer of safety built into the design.

Under normal conditions, as you seem to realize, there would be little danger with the upper diagram. The danger arises when there is a problem... such as an open neutral.
 
  • #58
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Good Answer. :)
I Totally Understood the dangers involved in the top drawing, that the neutral conductor should breaks somewhere up-stream then the equipment body comes LINE. A Dangerous Situation.
So, I was thinking of modifying that drawing to avoid that situation.
Another%252520Alternative.jpg

But, I now realize that it is still less safer than the separate ground wire scheme, because, should the neutral wire break in in-between the Distribution-Board and the equipment, its again the same thing as the previous case. So, even this modified scheme can only protect from up-stream neutral break, Should the neutral Break in our house, its still Dangerous.
Hence, the Separate ground Wire Scheme.

Thanks a Bunch. This cleared up my long standing confusion.
 
  • #59
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Now, question on the Industry practice.
What is the relative Size of the ground wire? Form what I have learned, I think that, it can be much smaller than the line and neutral conductor because current flows in it for only brief amount of time.
I think it only needs to be big enough so as to have small enough resistance to earth so that at fault condition big enough current flows to trip the circuit breaker. Am I right?

But from what I see,
10m-iec-power-cable.jpg

such as this Computer Plug, the grounding Plug looks even bigger than the other two. I think there must be a reason for it, but what?
 
  • #60
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Well safety is a developing art, not a static one and some would say the direction is not always forward or that some areas are neglected.

With regards to the shiny new plug in the photo:

One principle is that the earth should be the first connection to be made and the last to be disconnected.

So the earth connection is longer than the others.

Further, internally, the plug should be wired with more slack in the earth wire than in the others so that if the wires are mechanically tugged out again the earth will be the last connection lost.

I said shiny new plug because, of course, after years in service the contact surfaces become corrroded and/or dirty.

So, in line with the above principle, the contact surface area of the earth connection is larger than that of the others.to counter this ageing.

Note also in the photo that the L and N prongs are partially shrouded.
This was not the case when this type of plug was first introduced. In those days it was possible to short across the L and N when the plug was partially in or out of the socket, but still connected to the supply. It was also possible to grab the plug by its base and also touch the L and N prongs when inserting or withdrawing the plug.

go well
 
  • #61
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Difficult to comment on your local wiring system as I know nothing about it.

Are you sure they are all conductor wires, not supports?
yes iam pretty sure they are conductors so can the fifth wire be the 'protective conductor' because in two phase supply someone said that there are three wires and the third wire is 'protective conductor' so can in three phase fifth wire is protective conductor?
 
  • #62
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Thanks Studiot.
That makes sense.
What about the size of grounding wire? Are they also selected more thicker to increase factor of safety? But, I don't think that will be necessary because the current flows only for very short time.
 
  • #63
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What about the size of grounding wire? Are they also selected more thicker to increase factor of safety? But, I don't think that will be necessary because the current flows only for very short time.
It's actually more complicated than that.

Protective disconnection devices take finite time to operate. This time depends upon the current. The greater the current the faster the operation.

The UK regulations require the electrician to establish what is known as the 'prospective earth fault current'. This is a value that operates the design disconnection devices within specified time frames. One factor in this calculation is the 'earth loop resistance'.

The resistance of the earthing has to be such as to sink the prospective fault current within the time allowed in the code.

The code disconnection times vary with location, being shortest at locations of greatest risk.

In theory each location should use the actually measured earth resistance in this calculation.
In practice a value base on experience is often assumed.

In theory individual earthing cables should be sized according to this process. It is impracticable to size every cable individually so the worst case is often taken and used for the whole wiring scheme or the scheme divided into a few size groups.

These principles also apply to equipotential earth bonding.

Take a look at this thread for an explanation.

https://www.physicsforums.com/showthread.php?t=520792

Finally here is an extract about transformer earthing from the electricians guide to regualtions.
 

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  • #64
Averagesupernova
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AS, you are not understanding.

It is a statutory requirement for the power company to deliver 230volts (within tolerances) to the supplier/consumer interface at the property meter.

The capacity of the three phase supply running past not only one property but the perhaps hundreds in that road, has to be sufficient to supply all the property spurs.

Yes the consumer may suffer voltage drop if his own wiring runs are too long - there are standard formulae to determine this.

You effectively stated this by saying 'relocate the transformer' ie it is the power companies responsibility to size the feed cable/voltage to the transformer primary to achieve the desired secondary output.
No I understand perfectly. Always have. It just seems to me that it more impractical to run low voltage (by this I mean 240 volts) the long distances you tell me they are run in the UK compared to the shorter distances we run here in the USA. I'm not saying it cannot be done and I am not saying that you are wrong. I'm just telling it the way I see it. BTW, in my case of running secondary wire 750 feet, this is NOT the responsibility of the consumer. It is the responsibility of the power company to size this wire up until the meter and to maintain a reasonable stable voltage at the meter.
 
  • #65
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The National Electric Code (in the US) states:

...in no case shall
they (equipment grounding conductors) be required to be larger than the circuit conductors supplying
the equipment.
This is from article 250.122(A) of the code. I added the part in parenthesis for clafification.

So, the answer is no... The "ground wire" doesn't need to be any bigger than the line and neutral wires. And this makes perfect sense, since it would only be wired in parallel with the neutral anyway... no reason it should ever see any load bigger than what would normally be carried on the neutral.
 
  • #66
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It just seems to me that it more impractical to run low voltage (by this I mean 240 volts) the long distances you tell me they are run in the UK compared to the shorter distances we run here in the USA.
By impractical I assume you mean uneconomic?

It is clearly practical since it is done.

Actually even the economics are not straightforward since you have to balance the cost of purchase, installation and maintenance of a cable at kilvolt levels against a 240 volt one.
Further you have to balance the additonal cost of many local transformers against a few big ones in a substation.

So, the answer is no... The "ground wire" doesn't need to be any bigger than the line and neutral wires.
IAL posted a photo of a UK cable and asked about earthing cable sizes.

It is therefore reasonable to answer in terms of UK regulations.

As a matter of interest how do you apply that regulation to equipotential bonding in the US or do you not go in for that measure?
 

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