Neutral wire and earth

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  • #26
sophiecentaur
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Where would you put the ammeter?
 
  • #27
Averagesupernova
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Not sure what you mean by double pole isolator. The neutral all the way from the center tap of the transformer is NEVER EVER EVER under ANY circumstance interuptable in any way. We have double pole circuit breakers for our appliances that run on 240 volts. Supposing we have an electric heat panel that runs off of 240 volts. Each end of the secondary transformer winding would be hooked to the heater. Of course there is a double pole circuit breaker in between. If there is a fault on one line between the breaker and heater causing excessive current to flow on only one line, the double pole breaker will open both lines. But if the heater were running on 120 volts using the neutral wire it would only use a single pole breaker. The neutral is NEVER opened.
 
  • #29
uart
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well, in series with earth
The problem with Earth faults is that you don't know exactly what path they will travel as there are almost always multiple ground paths. Whether you choose to call them GFCI's or RCCB's or RCD's (residual current devices) or my favorite ELCB's (earth leakage circuit breakers) it's all the same, they all work on detecting a current difference between the live and neutral wires as Sophie previously described.
 
  • #30
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The problem with Earth faults is that you don't know exactly what path they will travel as there are almost always multiple paths. Whether you choose to call them GFCI's or RCCB's or RCD's (residual current device) or my favorite ELCB (earth leakage circuit breaker), they all work on detecting a current difference between the live and neutral wires as Sophie previously described.
i understand thats how RCDs, GFCI's etc work and that's obviously the right way for that reason. What i'm saying is that if you want to measure the amount of leakage your device inherently has, it should be sufficient to measure earth current (provided that the *device is isolated from ground* which eliminates all other paths that you are talking about) because earth current should theoretically be equal to the diff between line current and neutral current.

Shouldnt it?
 
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  • #31
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Suppose neither line is grounded. As wire insulation ages it may crack or become worn and allow a small amount of current to flow to ground. If hundreds of homes are connected to the same transformer there could be appreciable current flowing to ground even if all the lines have fuses or circuit breakers.

Someone who knows that both lines are floating, confidently if naively, sits in a bathtub full of water using a hair dryer. As the hair dryer gets wet, a small amount of current may pass from one line in the hair dryer through the person's body to ground where due to the leakage current from many other homes, the current finds a path back to the other line. Where would you put the ammeter in series with ground to prevent such an occurrence?
 
  • #32
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Suppose neither line is grounded. As wire insulation ages it may crack or become worn and allow a small amount of current to flow to ground. If hundreds of homes are connected to the same transformer there could be appreciable current flowing to ground even if all the lines have fuses or circuit breakers.

Someone who knows that both lines are floating, confidently if naively, sits in a bathtub full of water using a hair dryer. As the hair dryer gets wet, a small amount of current may pass from one line in the hair dryer through the person's body to ground where due to the leakage current from many other homes, the current finds a path back to the other line. Where would you put the ammeter in series with ground to prevent such an occurrence?
i feel like im being misunderstood. Forgetting hypotheticals and taking one specific example. I have a lamp, plug it into a wall and float it from the middle of a ceiling. Can I determine the leakage of the lamp (diff between line and neutral) by measuring earth current?
 
  • #33
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This thread has wandered rather off topic.

Further some less than wholly accurate statements have been made.

I hope no electrician would recommend making an earthing connection through an ammeter. The object of any earthing connection is to make the resistance as small as practicable.

There is a fundamental difference between American and British practice in that the american wiring system makes two distinct uses of the Earth or Ground wire, whilst the British system makes only a single use. The Canadian system is different again.

The British system is simpler, so I will deal with it first.

Domestic single phase supply is taken from the phase to neutral voltage of a three phase supply cable, belonging to the supply company. It has a nominal 230 volts AC. There is no transformer.

Both these conductors (they are not called wires) are considered 'live'.
The neutral is actually earthed by the power company upline somewhere, perhaps at the power station, perhaps at a substation.
In most circumstances the company also supplies a third conductor called officially called the 'protective conductor' otherwise known as the 'earth'.
The company also connects this to earth so that the consumer does not need to do so.

I stress again

The sole purpose of the protective conductor is to provide - yes- protection. Protection against, gas explosion, electric shock, fire hazard.
It is not used as a current return conductor or for protection of connected apparatus.
It is not connected by the consumer to the neutral.

One use of the ground connection in American practice in the protection mode as above.

It is also used (wisely) as a reference in the event of certain faults that cannot occur in the British system.

However American supply is different. The phase and return conductors from the power company are connected to a transformer which outputs two opposite phase line connectors and a centre tap neutral to the consumer.

The voltages are such that conventional lower powered apparatus is supplied between one or other line and the centre tapped 'neutral' at 120 volts AC.
Higher powered apparatus is connected between the two lines at 240 volts AC.

This centre tap is grounded because in the event of a fault (such as a shorted turn) causing the centre tap to shift from the mid voltage the centre tap is pegged to ground. Thus the line to ground voltage can never rise above 120 volts. If the centre tap were not grounded a condition could arise where the centre to one line voltage was 130 , 140, 160 or whatever.
This would clearly offer danger via and to apparatus connected to that circuit.

Averagesupernova, I still don't see any argument.
 
  • #34
Averagesupernova
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Here in the USA we commonly have 200 amp 240 volt residential services. What size do you have in the UK? You say there is no transformer. Just how far away is it from the residence to the first transformer? And fine, no argument, whatever...
 
  • #35
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Averagesupernova you win
 
  • #36
Averagesupernova
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Lenny, I'm just curious about the practices in the UK.
 
  • #37
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That's why you win
 
  • #38
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Here in the USA we commonly have 200 amp 240 volt residential services. What size do you have in the UK? You say there is no transformer. Just how far away is it from the residence to the first transformer?
Most average properties would go for a 100 amp supply, single phase 230 volts.

If you wanted more you would probably use 3 phase and maybe the interphase voltage (415).

Distribution is by 3 phase supply cables, so part of a street might be on one phase and part on another and the streeet lights on the third.

Even in a town you could be half a mile or more from the local substation transformer (this would be a 3 phase industrial quality jobby )

It is decades since supplies went underground. Very few are now supplied from overhead cables.
This is the reason it is so easy for the supplier to provide the local earth. The supply cable armoured sheath is well buried locally, unlike an overhead cable.

Building wiring make great use of 'diversity' and ring main wiring for power, rather than star wiring as in the US.
Lighting wiring is also different, being distributed daisy chain wise via special ceiling roses rather than via light switches.

What else would you like to know?
 
  • #39
Averagesupernova
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It seems to me that a half mile is a LONG way to run what we call in the USA secondary wire. Meaning that the 240 VAC wires between the transformer and the service panel in your house are called secondary wires.
-
I looked at buying some property here that would have had a driveway about 750 feet long. The transformer was on a pole at the end of the driveway next to the public road. The house that had been there (gone when I looked at) had a 100 amp service. I called the power company and asked them about this. I told them what I had in mind and that it required a 200 amp service. I don't recall whether the wires from the transformer to the meter were large enough to carry it but even if they were the engineer from the power company said that is too far of a run for secondary wiring. In fact he thought it was quite a distance even for a 100 amp service. The central air unit alone would cause too much voltage sag on this distance upon startup. A better approach would be to relocate the transformer. I had decided this before I even called.
 
  • #40
sophiecentaur
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well, in series with earth
And where would that actually fit in the curcuit? try drawing it out and there isn't anywhere to put a meter / detector. The only way is to detect a difference in the current in the two legs. It would work (and the old earth leakage trip system used to work this way) for detecting current flowing in the actual earth wire but current through you to the wet bathroom floor wouldn't't necessarily go through the mains earth wire.
 
  • #41
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If the centre tap were not grounded a condition could arise where the centre to one line voltage was 130 , 140, 160 or whatever.
Can you please elaborate on how that could occur?
 
  • #42
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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.

IAL,

Take a transformer with a centre tap.
The voltages between each end and the centre are equal in magnitude (and of opposite phase)
Now short out one or more turns.

The voltage between one end of the transformer and the tap is now different from the voltage between the other end and the tap, as it is no longer exactly half way along the winding.

Or, as I said, the tap has shifted its voltage position from the centre.

Although the end to end voltage will have reduced it is now unevenly distributed betwen the two outputs so the bigger 'half' will be greater than 120.

go well
 
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  • #43
sophiecentaur
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Can you please elaborate on how that could occur?
1. If one leg became connected, by a a leakage path - or even a dead short - to earth then the other leg would assume the full 220V.
2. If the system were fully isolated from earth then a static charge could build up and raise the DC potential to any level. The capacity of the whole line, from transformer to consumer - plus all the connected devices - could be high enough to collect a lethal charge.
 
  • #44
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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.

IAL,

Take a transformer with a centre tap.
The voltages between each end and the centre are equal in magnitude (and of opposite phase)
Now short out one or more turns.

The voltage between one end of the transformer and the tap is now different from the voltage between the other end and the tap, as it is no longer exactly half way along the winding.

Or, as I said, the tap has shifted its voltage position from the centre.

Although the end to end voltage will have reduced it is now unevenly distributed betwen the two outputs so the bigger 'half' will be greater than 120.

go well
I am still unclear here. Suppose the transformer is 11KV/240V (with center taps).
So, suppose, the primary has 11000 turns and the secondary has 240 turns with tapping from 110 turns.e
Now, I think, the voltage induced per turn in the secondary is 1volt per turn.
So, no matter what happens to the other half of the turn, the voltage induced across the healthy half of 120 turns should still 120 Volts.
Because the voltage induced still equals E = N d(phi)/dt. So, still, the 120 turns can't have more than 120 volt induced in it.

Please tell me where I am wrong.

O.K. I will show you my math.
Suppose, out of 120 turns of the one half of the secondary, 80 turns gets shorted out. Now, there would be un-equal voltages on each half. But still Isn't it crystal clear that, the faulty half would have 120-80 = 40V induced and the healthy half would have still 120 Volt induced.
 
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  • #45
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Good call IAL, I think you are right.
 
  • #46
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Its a pleasure to hear that from you. :)
so, that leaves the only reason for grounding the central tap is to prevent exceptional voltage rise w.r.t. ground due to static charge build up as sophiecentaur pointed out.
 
  • #47
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At the risk of creating safety concern, I would Like to ask, In British System Or in American System, Is it safe to touch the neutral conductor standing on ground.
Since, you said, the neutral is grounded on the distribution transformer, it means we are already standing on neutral conductor. So, it appears that it must be safe to touch it because we are already touching it anyway.

I am not going to touch it, though, and please, Nobody do that, because you never know whats going on.

P.S. What is the best way to discuss in this topic. Last time I tried to be very careful, but missed somewhere and got the thread locked.
 
  • #48
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Is it safe to touch the neutral conductor standing on ground.
Usually you would be OK, particularly in the american system.

Usually there is some voltage between the neutral and earth in the british system, but not enough to hurt.

But why take a risk on there being a fault condition?

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.
 
  • #49
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O.K. Thanks.
That clears up much of my confusion.
When you are asserting that the neutral isn't grounded in post #6, you meant it isn't grounded on the consumer side. right?
Now, one more question. :)
In our house (Lets talk about British System for now), we usually have 3 wire, the line, the neutral and the ground. We connect power equipments between neutral and the line, and connect the body of the equipment to the ground wire. Obviously, the ground wire is connected to earth, locally at our house through some earthing arrangements (like a sheet of Copper Plate buried in Salt and Clay). So, the Ground Wire is in effect actually connected to the neutral wire through the earth, as the neutral is also connected to the earth (at the transformer).

The purpose of providing the ground wire and connecting it to the equipment body is for protection of personnel. i.e. If some-how the line comes in contact to equipment body then, since the body is already connected to ground wire-to-earth-to-neutral heavy current flows and trips the MCB.

All these makes sense. One thing which don't make much sense is "Why don't we connect the neutral itself to the equipment body and totally eliminate the use of ground wire?"

More questions may come. :)
 
  • #50
sophiecentaur
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AS,

Take a transformer with a centre tap.
The voltages between each end and the centre are equal in magnitude (and of opposite phase)
Now short out one or more turns.
I'm sure you don't mean "short out one or more turns". That would lead to a lot of current flowing in those shorted turns and would either burn out the transformer or get it very hot and waste a lot of energy. Far better to subtract some turns from the end of one secondary ('tap down', as they say).
 

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