explaining the purpose of neutral wire?

properleckie · 11 Oct 2009

Hellmooth, having taught 2330 level 2 for a while & spent lots of time argueing with college boss over lack of time to discuss issues not being understood by adult students quit and now give help privately to a couple of guys doing as you are (for an occassional beer)!

worked on military aircraft with 0-5v dc, 0-28v dc, & 110v 3 phase 400Hz.
all earth free supplies, try explaining that system to an 18year old 'luved-up' RAF erk!
Avionics lab totally insulated and earth free, fun.

Keep at it, most people are willing to help or they wouldn't be on here,

Spark123 · 11 Oct 2009

No earths on an aircraft

ban-all-sheds · 11 Oct 2009

Indeed not....

Goldberg · 11 Oct 2009

matt1e said:
Goldberg said:

Does it have a different purpose to the neutral in a 'mains' installation?

Click to expand...

no

Correct.

matt1e said:
i'm still waiting for your explanation

What on earth are you talking about?

Goldberg · 11 Oct 2009

ban-all-sheds said:
Goldberg said:

ban-all-sheds said:

(Assuming a 50Hz sine-wave output of at least 170V RMS).

Click to expand...

Erm, isn't 240 the RMS figure, i.e. isn't the wave +340 to -340 peak-to-peak?

Click to expand...

Which RMS figure?

The RMS figure for a sine wave that has peaks of +340V and -340V.

Goldberg said:
Goldberg said:

The neutral in a generator supply isn't at 240V wrt the live. If anything it's at -240V.

Click to expand...

So 240V can't be an RMS voltage.

I thought you were referring to a sine wave that reaches +240V 100 times per second and -240V 100 times per second.

An RMS voltage of 170 will give you peaks of ±240.4 volts, so 2 x +240V and 2 x -240V each cycle.

It would give 1 x 240V and 1 x -240V per cycle, and 2 x <every value between +240V and -240V> per cycle.

ban-all-sheds · 12 Oct 2009

Spark123 said:
Isn't that 1x 240v and 1x -240v per cycle? 50x each per second on a 50hz supply?

No - assuming a peak voltage >240V, then with the conventional way of drawing a sine wave, you'll see +240V on the way up in the 1st half-cycle, then +240V on the way down, then -240V on the way down in the 2nd half-cycle and then finally -240V on the way back up.

[ASIDE]

Exercise for the reader:

Assuming a 50Hz sinusoidal supply, what peak voltage is needed so that the ±240V points are crossed every 5ms?

[/ASIDE]

We use RMS as the average (mean) voltage of a sine wave over a cycle is zero.

Au contraire - we use RMS instead of the mean, because the mean is zero.

If The RMS voltage is 240v, the peak voltage is about +340v and -340v giving a peak to peak voltage of about 680v.

The peak voltage between which two conductors, or which two points on a circuit?

ban-all-sheds · 12 Oct 2009

Goldberg said:
The RMS figure for a sine wave that has peaks of +340V and -340V.

But "The neutral in a generator supply isn't at 240V wrt the live. If anything it's at -240V" cannot have meant any RMS voltage for any sine wave of any peak value.

So 240V can't be an RMS voltage.

Click to expand...

I thought you were referring to a sine wave that reaches +240V 100 times per second and -240V 100 times per second.

That can't be true, as I hadn't written anything on that subject when this sequence of posts appeared:

hellmooth said:
2. why does it have 0 volts?

ColJack said:
It has 0 volts because voltage is relative to something, in this case earth, and the neutral is tied to earth.. in a generator that is not earthed then it is relative to the live, so you could say that it is at 240v with respect to the live.. or that the live is at 240V with respect to it.

Goldberg said:
The neutral in a generator supply isn't at 240V wrt the live. If anything it's at -240V.

An RMS voltage of 170 will give you peaks of ±240.4 volts, so 2 x +240V and 2 x -240V each cycle.

Click to expand...

It would give 1 x 240V and 1 x -240V per cycle, and 2 x <every value between +240V and -240V> per cycle.

It would not.

"peaks of ±240.4 volts"

Space cat · 12 Oct 2009

BAS said:
Exercise for the reader:

Assuming a 50Hz sinusoidal supply, what peak voltage is needed so that the ±240V points are crossed every 5ms?

240 x sqrt(2) exactly.

matt1e · 12 Oct 2009

Goldberg said:
matt1e said:

i'm still waiting for your explanation

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What on earth are you talking about?

to explain how there is 0v on the neutral?
(without mentioning earth which has, according to you nothing whatsoever to do with it)

clue......... don't bother because you can't
good morning everyone by btw

hellmouth your right it did get a bit heated,but it is good that your asking questions it always makes fault finding etc easier if you understand the principles,I think we have all tried to explain best we can but as someone else said earlier not everyone makes a great teacher
I just noticed this in an earlier post

so when the current has been used by the load and returns through the neutral wire,finally being connected to earth back at substation,it is for want of a better word dissipated into the ground?

NO! it doesn't get dissipated into the ground the circuit consists of the transformer, the line and neutral cables and the "load"
have a read up on earthing systems it may help you understand better

so once again to try and recap
if you take a voltage measurement between neutral & earth the result will (or should be) close to 0v because they are connected together upstream
matt

ban-all-sheds · 12 Oct 2009

Space cat said:
BAS said:

Exercise for the reader:

Assuming a 50Hz sinusoidal supply, what peak voltage is needed so that the ±240V points are crossed every 5ms?

Click to expand...

240 x sqrt(2) exactly.

Are you sure about that?

holmslaw · 12 Oct 2009

ericmark · 12 Oct 2009

I can see how people sometimes find it hard to get their head around electric power as part of a big picture rather than on a local level.

As an apprentice I could not see the point of earthing as I felt if nothing was connected to earth then if one touched a live wire there would be no circuit and hence no shock.

In a way I was right and that is just how shaver sockets work. I was however looking too local and as one expands the system one has to look at multi-faults. If for example two shavers both with earth faults were both plugged into the same isolated supply and one had a fault to live 1 and the other to live 2 then one could get a shock.

Although you may get a twin outlet shaver socket the two outlets with be isolated from each other often one 110v and other 230v.

Working on an IT system in Hong Kong I soon realised the problems of no earth and the problems in tracing multi-faults. Although the red book says that IT systems are not permitted in UK I have seen them with DC supplies to cranes. Two bulbs were placed in series between the two live rails with centre tap to earth and any fault to earth would light one bulb bright and other would go out. When all was good both would glow dim.

The same idea was used on a German batching plant I worked on in Heathrow used in the building of terminal 5 in this case it was the AC control circuit 220vac and it used an electronic monitoring device. So in spite of IT systems not being permitted in UK they do exist. Again fault finding was a real pain especially since the problem was a faulty air solenoid to sound warning horn and on the plans this should have been from 24vac system.

So taking one leg of a supply to earth means fault finding became much easier and once one earths a supply leg we call it neutral as it should be at earth potential. Also it means any fault with the supply transformer which could put 3.3Kv on the 400v output would cause an overload and blow a fuse.

Although we always earth the Star point of the supply transformer or generator we have different systems in place as to if the neutral is earthed else where. There is some resistance to the idea of multiple earthing of the neutral (PME or TN-C), often it is earthed at multi points while supplied with heavy cable but as it enters the premises it is separated TN-C-S.

In some cases the multi earthing is not allowed by law for example petrol stations, caravans and boats because it does have some dangers.

The problem is where current flows breaking the cable will result in a voltage rise this will also happen where there is some resistance in the cable. For this reason neutral is regarded as a live wire.

You will only get a shock from a neutral when it’s resistance back to star point is high or open which is what of course happens as you disconnect a neutral wire while the line is still connected. Hence why borrowed neutrals are so bad.
However any body can become charged and the larger it is the more charge it can take and we have all charged up a balloon and stuck it on the ceiling. When working on high voltages one does have to equalise the charge there was a good video of men working on power lines and being enclosed in a Faraday cage. So yes you can get a shock without forming a circuit but very unlikely with low voltage (230v).

Goldberg · 12 Oct 2009

ban-all-sheds said:
Goldberg said:

The RMS figure for a sine wave that has peaks of +340V and -340V.

Click to expand...

But "The neutral in a generator supply isn't at 240V wrt the live. If anything it's at -240V" cannot have meant any RMS voltage for any sine wave of any peak value.

I see your point. Thank you.

So 240V can't be an RMS voltage.

Click to expand...

I thought you were referring to a sine wave that reaches +240V 100 times per second and -240V 100 times per second.

Click to expand...

That can't be true, as I hadn't written anything on that subject when this sequence of posts appeared:

<snip>

This was a case of me getting wires crossed with the wrong end of the stick and thereby not getting your drift. I had failed to notice that the peak value (for 170 RMS) is 240.4, and that you were merely being precise.

An RMS voltage of 170 will give you peaks of ±240.4 volts, so 2 x +240V and 2 x -240V each cycle.

Click to expand...

It would give 1 x 240V and 1 x -240V per cycle, and 2 x <every value between +240V and -240V> per cycle.

Click to expand...

It would not.

I concur, for the reason mentioned above.

Space cat · 12 Oct 2009

BAS said:
Are you sure about that?

Yes. The +240V points are symmetrically placed about the positive peak. If they are to be 5 msec apart, they must be at 45° either side of the peak where the amplitude of a perfect sine wave is peak / sqrt(2). From this it follows that the peak must be 240 x sqrt(2). QED!

ban-all-sheds · 12 Oct 2009

OK - just checking it wasn't a lucky guess...

Another open Q - do we think that it's a coincidence that the instantaneous voltages which occur at equal intervals is the RMS value?

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explaining the purpose of neutral wire?

Goldberg

Goldberg

holmslaw

Goldberg

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