Earthing and neutral help

[Paul Taylor]

Hi, just new here..

Just a quick question... as you have your live and neutral and earth, but both your neautral and earth are joined at your incoming and neutral is earthed at your transformer side.

Your neutral being your return feed, why does it have no voltage coming back or little, even tho it's a return? When you short out live to neutral? Why does it trip the mcb even though it's a return

 

[Wilko]

Hi Paul - the neutral and the installation earth are not always joined at the service head. TNCS is, TT and TNS are not. As you say the suppliers transformer has neutral connected to Earth. And this is the reason the neutral at the installation has very little voltage on it, relative to the general mass of the Earth. Hopefully.

 

[Deleted member 26818]

The Neutral does have voltage coming back, you are just not able to measure it using standard methods.
To measure the voltage, you would need to break the Neutral and insert a volt meter between the break.
Measuring between neutral and Earth would be the same as measuring between neutral and neutral.
Shorting out the Line and Neutral causes the MCB to operate because there is nothing in circuit to limit the current.

 

[Ian1981]

Only in a TNCS are the earth and neutral connected in the DNO cut out and then form the combined pen conductor back to the transformer.
A TNS has a separate earthing conductor all the way back to the source

 

[Paul Taylor]

Ah okay thanks guys. So when you short out the neutral live or live to earth the mcb will operate as it gets a surge of current, which trips the mcb?

 

[telectrix]

Ah okay thanks guys. So when you short out the neutral live or live to earth the mcb will operate as it gets a surge of current, which trips the mcb?

basically yes. the highewr the current L to N, the faster the MCB will trip. if you have a copy of the regs, the time/current graphs are in appx. 3.

 

[Paul Taylor]

Thank you.

 

[Ian1981]

If you had a ze on a TNCS of 0.10 ohms then the live to neutral loop will be the same if not very similar.
This gives a fault current at the origin of 2300amps or 2.3ka.
By the time you get to a circuit then the resistance will have increased and you may get say 400 amps of fault current live to neutral or live to earth at say a fault on a socket outlet circuit.
This amount of current will trip the mcb rather quickly.
The tables in appendix 3 only give the fault/time characteristics of a time of between 0.1-5 seconds.
Example would be a 32 amp type b mcb will need 160 amps to instantaneous trip at 0.1-5 seconds.

 

[Pete999]

Hi, just new here..

Just a quick question... as you have your live and neutral and earth, but both your neautral and earth are joined at your incoming and neutral is earthed at your transformer side.

Your neutral being your return feed, why does it have no voltage coming back or little, even tho it's a return? When you short out live to neutral? Why does it trip the mcb even though it's a return

Think the OP needs to understand Voltage doesn't flow, come back etc, the live and Neutral are at different potentialshttps://www.youtube.com/watch?v=1xPjES-sHwg

 

[gazdkw82]

The Neutral does have voltage coming back, you are just not able to measure it using standard methods.
To measure the voltage, you would need to break the Neutral and insert a volt meter between the break.
Measuring between neutral and Earth would be the same as measuring between neutral and neutral.
Shorting out the Line and Neutral causes the MCB to operate because there is nothing in circuit to limit the current.

probably a simple answer but if this is the case, why during the safe isolation process would you measure N-E?

 

[Pete999]

Because you check everything you never know whats connected to what, check always, well I do anyway. sorry I did.

 

[davesparks]

probably a simple answer but if this is the case, why during the safe isolation process would you measure N-E?

Just in case the polarity is incorrect or some other fault has made the neutral live.

 

[gazdkw82]

ok but my point is, lets say your measuring N-E at a CU, technically your measuring N-N are you not?

 

[telectrix]

ok but my point is, lets say your measuring N-E at a CU, technically your measuring N-N are you not?

yes, so you should get close to 0V. if not, then there's reason to investigate why.

 

[Risteard]

technically your measuring N-N are you not?

Not really - you're measuring an earthed conductor to earth. However neutral is a live conductor and you need to ensure that a dangerous potential (voltage) hasn't appeared on it.

 

[davesparks]

ok but my point is, lets say your measuring N-E at a CU, technically your measuring N-N are you not?

If the CU is at the origin of a TNCS installation then yes you pretty much are, assuming the tails are the right way round in the cutout.

The thing with safe isolation is that you test everything as its not worth risking your life on assumptions

 

[gazdkw82]

Absolutely. It just dawned on me that essential at a cu point on tncs N-E is essentially N-N. but like you say, always check everything

 

[Ian1981]

Absolutely. It just dawned on me that essential at a cu point on tncs N-E is essentially N-N. but like you say, always check everything

Think about when your doing an IR test as well.
If you don't isolate the neutral at the main switch you essentially will get 0.00 Mohms with a neutral to earth insulation resistance test