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  1. Paul Taylor
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    Paul Taylor EF Member

    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
     
  2. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    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.
     
  3. spinlondon
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    spinlondon Trusted Advisor

    Location:
    Harlow Essex
    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.
     
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  4. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    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
     
  5. Paul Taylor
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    Paul Taylor EF Member

    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?
     
  6. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    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.
     
  7. Paul Taylor
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    Paul Taylor EF Member

    Thank you.
     
    • Agree Agree x 1
  8. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    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.
     
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  9. Pete999
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    Pete999 Trusted Advisor

    Top Poster Of Month

    Location:
    Northampton
    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
     
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  10. gazdkw82
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    gazdkw82 Trainee Trainee Access

    Location:
    leicester
    probably a simple answer but if this is the case, why during the safe isolation process would you measure N-E?
     
  11. Pete999
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    Pete999 Trusted Advisor

    Top Poster Of Month

    Location:
    Northampton
    Because you check everything you never know whats connected to what, check always, well I do anyway. sorry I did.
     
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  12. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    Just in case the polarity is incorrect or some other fault has made the neutral live.
     
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  13. gazdkw82
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    gazdkw82 Trainee Trainee Access

    Location:
    leicester
    ok but my point is, lets say your measuring N-E at a CU, technically your measuring N-N are you not?
     
  14. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    yes, so you should get close to 0V. if not, then there's reason to investigate why.
     
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  15. Risteard
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    Risteard Electrician's Arms

    Location:
    Derry, Ireland
    Business Name:
    Walsh Electrical Services
    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.
     
    • Agree Agree x 1
  16. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    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
     
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  17. gazdkw82
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    gazdkw82 Trainee Trainee Access

    Location:
    leicester
    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
     
  18. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    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
     
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