neutral voltage

[morpheus83uk]

I know this is probably a basic question however I am curious. Should there be voltage over the neutral wire? I was testing with a multimeter last night when I had the garage fuse box off (the switch for the sockets anyway) And I got around 4 - 12 volts I am wondering if this is normal and if so why does it happen?

Thanks

James

 

[Nozzle]

It happens because the neutral wire is connected to the centre tap of a star-wound transformer, and if the phases are not equally balanced then the centre tap shows a voltage. The more imbalanced the phases, the greater the voltage on the neutral.

Nozzle

 

[Jackrae]

You measured your neutral voltage relative to what ; your earth wire.

What type of earthing arrangement do you have ?


The neutral point on a substation transformer is also the transformer earth point (they are physically connected) so any imbalance on phase loading should not raise this point above the earth reference point. If you have a TNS or TNCS system your earth reference is/should be the same as the transformer's. The imbalance simply shifts the neutral point in the vector such that each phase has a differing voltage to neutral/earth.

 

[StudWall]

were you touching the metal contacts on your multimeter?

been there before!

 

[morpheus83uk]

I was testing using the live wire (when it was switched off) and the neutral wire. and was seeing voltage. But I did then just connect the red probe to neutral and I was getting between 4 and 12V however whenever the earth and neutral wires came into contact it tripped the fuse... so I am unsure what this means or if its supposed to happen...

 

[ericmark]

Although at some point earth and neutral are connected the resistance of the supply cable means the neutral voltage will lift towards that of the line as there is current flowing but the earth will not as there should be no current flowing.

How much it raises above earth will depend on the earth system with a TN-C-S no more than 11.5 volt with a TN-S could reach 36.8 volts and with a TT is permitted to reach 50 volt but in real terms it would be lower than half of those figures as volt drop would be equal line and neutral with single phase or the neutral would have less of a volt drop with three phase or split phase.

So in real terms 5 volt is quite normal for TN-C-S and could be 15 volt with TN-S when a high load is connected. With TT the old ELCB-v had to disconnect at less than 45 volt so under normal conditions may reach 25 volt anything over that I would be looking for a fault.

A fuse does not trip it ruptures so I would assume you mean a RCD which should trip when 30 mA flows.

Since the voltage difference between neutral and earth depends on current drawn the resistance between neutral and earth which will cause the trip to activate depends on the current as well.

This does cause some problems if you consider a damp bit of bread stuck in a toaster between neutral and earth you could use the toaster many times with no problem as only 800W but switch the kettle on at 3000W and the RCD operates making one suspect the kettle is at fault when in fact it's the toaster.

Unplugging items not in use will stop this problem. But my toaster stays plugged in even when I know the problem. I blame the bara brith!

 

[westie101]

Possibly cased by net currents in the system

http://www.emfs.info/Sources+of+EMFs/distribution/UK/

The voltage found is typical of this

 

[Taylortwocities]

whenever the earth and neutral wires came into contact it tripped the fuse... so I am unsure what this means or if its supposed to happen...

No it did not trip the FUSE . It tripped the RCD

This often happens and is not a fault
Read the wIKI article for more info //www.diynot.com/wiki/Electric...ergised-circuits-with-Neutral-to-Earth-faults

Using a digital multimeter on mains circuits will often throw up odd readings due to induced voltages.

 

[JohnW2]

I was testing using the live wire (when it was switched off) and the neutral wire. and was seeing voltage. But I did then just connect the red probe to neutral and I was getting between 4 and 12V ...

This is different from what everyone else has been talking about (true potential differences between neutral and earth, due to various factors), and is simply due to induced (capacitive and/or inductive) 'pick up' of voltage, coupled with the sensitivity ('high input impedance') of your multimeter. It's normal, not dangerous, and nothing to be concerned about.

... however whenever the earth and neutral wires came into contact it tripped the fuse... so I am unsure what this means or if its supposed to happen...

I presume that the 'fuse' which tripped was an RCD. This is what usually happens when one joints neutral and earth in a circuit which has been 'switched off' only by switching the line/live .. and is essentially an indication that the RCD is doing what it's meant to do.

Kind Regards, John

 

[Taylortwocities]

Slow, slow

 

[JohnW2]

Slow, slow

True - but on this occasion, mainly because I chose to give some (hopefully helpful) 'explanations'

Kind Regards, John

 

[morpheus83uk]

Yes sorry the RCD tripped not the fuse Ii am used to everyone saying that it tripped the fuse! My mistake!

OK so when the circuit is switched off at the fuse box and I connect the live and neutral wires it caused more than 30 mA current and tripped the RCD which is normal.

And the voltage is normal too and it could be due to the multimeter picking up induced voltage?

So it sounds like my multimeter is very sensitive as when I turn it to AC voltage I get 0.0 volts and then some other random numbers like 14 or 20 so for example 0.014 as a reading without doing anything.

I hope I have understood that correctly?

 

[JohnW2]

Yes sorry the RCD tripped not the fuse Ii am used to everyone saying that it tripped the fuse! My mistake!

Indeed - you're certainly not the only one!

OK so when the circuit is switched off at the fuse box and I connect the live and neutral wires it caused more than 30 mA current and tripped the RCD which is normal

Yep - provided there is some electricity being used by something in your installation - which will usually be the case if just one circuit is switched off.

And the voltage is normal too and it could be due to the multimeter picking up induced voltage?

Yep.

So it sounds like my multimeter is very sensitive as when I turn it to AC voltage I get 0.0 volts and then some other random numbers like 14 or 20 so for example 0.014 as a reading without doing anything.

That's normal with modern digital multimeters. If you connected some sort of 'load' (e.g. a light bulb/lamp) across your meter, those readings would disappear.

Kind Regards, John

 

[morpheus83uk]

Brilliant thank you for that I just wanted to make sure that I had it correct in my own head!

Sorry I think I meant when the neutral and earth were touched it created the trip on the RCD not the live and neutral?

I thought it was fairly normal but now I know why it is so I will try that and hopefully see that everything returns to zero.

James

 

[JohnW2]

Brilliant thank you for that I just wanted to make sure that I had it correct in my own head! Sorry I think I meant when the neutral and earth were touched it created the trip on the RCD not the live and neutral?

You're welcome. I didn't even notice that typo - I knew what you meant on the basis of your having got it right the first time! With circuit (live) switched off, touching live and neutral would not trip the RCD.

I thought it was fairly normal but now I know why it is so I will try that and hopefully see that everything returns to zero.

It certainly should. Induced currents are capable of supporting very little in the way of current - the moment you connect any 'load' that will drag the voltage down to near-zero.

Kind Regards, John