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Loss of neutral

lyledunn
I have just been involved in a situation where temporary loss of neutral on a TNCS system caused thousands of pounds worth of damage. It seems this loss of neutral situation, either within or outside the installation, is occurring more frequently. SPDs are now commonly fitted but at DBS  and generally with a Up in the order of 860v, so giving no protection on loss of neutral in a three-phase and neutral system. Cost benefit analysis across the national spectrum might not support a compulsion but is it time designers should be raising the issue with clients and at least offering a solution. On the other hand, is there a packaged solution?
  • 0
    Andy,

    can you describe the settings on the OV device? I can’t make them out.
  • 0
    To me in that Maltese photo it looks like +7.5%. 0.8S. -6%. 


    It must be written down as a requirement in their local regs that supplement BS7671.


  • 0
    https://hydrolectric.com.mt/wp-content/uploads/2018/12/170420-OVR-SP-Settings.pdf
  • 0
    The over voltage setting seems reasonable, but the under voltage setting could let the voltage within the installation drop to around 190 volts before taking the installation off grid, which is 50 volts below the standard 240 volts.


    How important is the under voltage setting?
  • 0
    very few single phase loads will be damaged by 190V for a short period, and such a brown out may be quite common during the start up inrush of motors or other heavy loads on a system already close to the 207V or so that would be the long term permitted minimum of 230V +/-10%.

    If you set the limit too close, then all sorts of innocent events will trip it off.

    M.
  • 0
    So, taking this discussion in a slightly different direction....


    What would be the general consensus/opinion about using a 4 pole REC isolator to disconnect (for a single phase domestic installation) the phase, neutral & also disconnect the water & gas bonds away from the network earth/neutral conductor? I'm thinking perhaps use two of two of the poles to switch two 25mm^2 conductors connecting the MET to the network earth/neutral to give some redundancy against a failed switch contact.


    Over to the floor....
  • 0
    AdrianWint:

    So, taking this discussion in a slightly different direction....


    What would be the general consensus/opinion about using a 4 pole REC isolator to disconnect (for a single phase domestic installation) the phase, neutral & also disconnect the water & gas bonds away from the network earth/neutral conductor? I'm thinking perhaps use two of two of the poles to switch two 25mm^2 conductors connecting the MET to the network earth/neutral to give some redundancy against a failed switch contact.


    Over to the floor....


    Removing the main bonding  won't achieve anything at all in an installation with internal conductive service pipes. The pipes throughout the installation would still import PEN voltage from other installations.


    In addition, even if adjacent consumers had the same protective provision, metal service pipes "in the street" could still pick up risen PEN voltage from the street.


    I think it would be better for the SMART meter system to alert the DNO to the fact they may have a problem in a particular area so they can investigate. In a single-phase installation, the meter won't know whether it's a supply line conductor issue, supply transformer or tap issue, or a broken PEN conductor, but the DNO can build up a picture based on the voltages reported by meters from single phase installations on different phases, to determine which of these it might be.


  • 0
    gkenyon:

    Removing the main bonding  won't achieve anything at all in an installation with internal conductive service pipes. The pipes throughout the installation would still import PEN voltage from other installations.






    But would it not stop diverted neutral currents flowing in my bonding?


    There have been cases in the past whereby a secondary effect of a broken neutral has been hot bonding conductors/gas pipes/water pipes etc due to them carrying (possibly very large) diverted neutral currents. Wasn't that unfortunate series of explosions in a terrace of houses put down to overheated bonding connections to lead gas pipes? If the bonding had been (automaticaly?) disconnected from the neutral/earth together with the phase conductor would this unfortunate outcome have happened?


  • 0
    AdrianWint:
    gkenyon:

    Removing the main bonding  won't achieve anything at all in an installation with internal conductive service pipes. The pipes throughout the installation would still import PEN voltage from other installations.






    But would it not stop diverted neutral currents flowing in my bonding?


    There have been cases in the past whereby a secondary effect of a broken neutral has been hot bonding conductors/gas pipes/water pipes etc due to them carrying (possibly very large) diverted neutral currents. Wasn't that unfortunate series of explosions in a terrace of houses put down to overheated bonding connections to lead gas pipes? If the bonding had been (automaticaly?) disconnected from the neutral/earth together with the phase conductor would this unfortunate outcome have happened?




    You could just alter the pipe work in your installation, and ensure that it is all plastic,  so that main equipotential bonding is not required. The gas main up to the house would be plastic in most cases, and just internal gas pipes would be metal. If all metal parts are NOT extraneous parts then main bonding is not required. Keep all pipes off ground to comply. That way you are not inviting diverted neutral currents into your home. There is no incentive for them to come.


    Oil boiler pipes may also be metal and be extraneous conductive parts.


    Of course appliances' exposed conductive parts  connected to a P.M.E earth terminal can not be isolated. You would have to use all Class 2 (all insulated)  appliances to avoid a Voltage rise if N rose to a high level above true earth.


    Z.


  • 0
    But would it not stop diverted neutral currents flowing in my bonding?

    If I've pictured it correctly, L & N are disconnected, as are main bonds - but the main earthing conductor is still connected to the DNO's earth terminal - and hence the c.p.c.s to any equipment that happens to be in contact with metallic pipework (boilers, pumps and so on). So the diverted N currents will still flow, just now all through 1.0mm² or even 0.75mm² c.p.c.s rather than mostly through 10mm² bonding conductors. Given that diverted N currents (especially in a broken PEN situation) can be many tens possibly even hundreds of amps, I think I'd prefer to keep the 10mm² connected...

       - Andy.

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