Voltage in neutral?

[yoclick]

Hi all,

Just a quick question. If you have an earth which is 0v and a closed circuit, then connect it in a closed circuit between the positive end of a 240v cell and resistor i take it you will have get a P.D of 240v. However if you connect the same earth at the negative end of the cell, will there still be a P.D? Will it possibly be 240v/0v?

Quick diagram:

Or is it that the p.d across "- to E" fluctuates depending on what is turned on in the circuit? Basically all this stems from the fact that i have a neutral wire and i used two 100v-250v mains testers on it (screwdriver with neon light). One very dimly lit up and one didnt light up at all. I was a bit worried that it had lit one so i checked the voltage across this wire to the neutral of another point and there was a p.d of 50v? I really should have checked the p.d of that neutral to earth instead now thinking about it ! Im thinking this negative is connected at a junction box somwhere with shared neutral connections of other circuits; is this a likely/feasible reason?

Thanks for any help!

 

[tim west]

Putting AC across a polarity concious or DC device is something to avoid.

 

[ban-all-sheds]

1) In the abstract example of your drawing there would never be any PD between any parts of the circuit and earth because they are separated.

2) In your actual example you should throw those neon testers away and buy a proper instrument, then measure again and see what you get...

3) AC does not have + and - as your drawing shows - the voltage of your household supply (phase wo. neutral) varies from +325V to -325V.

 

[ColJack]

50V on a neutral can show up from coupling if it is open circuit ( DP switch ) ... it picks up some volts from the live it's next to
this is more usual in a 2 way light switch where the open switch wire picks up some volts from the live switch wires running next to it..

are you sure it's a neutral and not a live switch wire?

 

[yoclick]

tim west - Sorry, but im a tad lost! Obviously I wont be connecting a DC device into AC? Sorry if my question appeared that i was/have!

ban-all-sheds - Thanks for the info. Only used the neon testers as a quick test, not accurate. However i didnt realise they werent reliable? I used a digital clamp meter using leads to find the VAC.

col - 100% sure its a neutral however im thinking that the neutral may be shared with another circuit which does have two way lighting. I take it this is the most likely reason?

 

[333rocky333]

I think I get you
You are Thinking more dc than ac though
You understand you get 240 V between Live and earth ( Not + and E)
You thought you picked up voltage on the neutral return
So you measured between neutral return and a neutral of another circuit and got 50 volts between them

This PD as you call it should be near 0 regardless of the load
but in reality is not, as you know, cable has a resistance etc,

You possibly could work it out but
This voltage would be nearly 0 as
All the neutrals end up joined, eventually together in the supply cable , anyway.

NEUTRAL TO EARTH HOWEVER
We get called to lots of jobs where testers have noticed this voltage between Neutral and EARTH
And this apparently can be a problem if too high
I think this is classed as "STRAY VOLTAGE "
I think possibly caused sometimes by loose or bad terminations

Not heard of neutral to neutral voltage that high as both neutrals would be from the same bar in a house.
May be your meter unreliable

Google STRAY VOLTAGE and you may get an understanding
OR
NEUTRAL TO EARTH VOLTAGE