Neutral Wire & Earth: Electricity Distribution

[asad1111]

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?

 

[I_am_learning]

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.

 

[Studiot]

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.

 

[Averagesupernova]

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.

 

[Evil Bunny]

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.

 

[Studiot]

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?