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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
    mapj1:

    I agree it probably goes against the intention of that regulation.

    It's still out there, being driven across the dustier bits of the globe... 

    BS7671 is not written assuming that the supply  could be derived from anything including things as rough as a pair of clips onto a bare overhead line, with some uncertainty about which core is which.

    Power of opportunity brings a different set of risks and mitigations for which some of the standards assumptions are not so suitable, it does not mean you cant use the local electricity safely, you just have to plan for some odder cases - oh and never use any of the supply cables as earth.

    You probably won't like my torch bulb NE as tell-tale  indicator of lost neutral - if it lights up you have a bit of an NE offset problem if the filament has vaporised you have a big problem (or at least did have at some point since it was installed ).


    Mike.


    What is the torch bulb Voltage please Mike. I am asking for a friend.


    Z.


  • 0
    If I ran my supply through an old but serviceable Voltage operated earth leakage circuit breaker with the E terminal connected to an earth rod and the F terminal connected to the supply neutral will that disconnect my supply if the neutral conductor rises in Voltage above say 20 Volts to true earth? They are rated to trip at 50 Volts max. but in practice they trip off at 20 to 50 Volts.


    Z
  • 0
    Zoomup:
    AJJewsbury:

    I wouldn't expect SPD to offer any protection from longer-term overvoltage situations like broken neutrals - they operate by creating a low impedance L-PE or L-N and would have to dissipate huge amounts of (heat) energy if the overvoltage lasted for more than a small fraction of a second. The best we can hope for is that the SPD fails safe and disconnects itself before it starts a fire.


    As for other solutions there are plenty of voltage monitoring relays on the market that could be fitted (probably via a contactor) to disconnect the installation if the voltage went outside limits. (e.g.https://uk.rs-online.com/web/p/monitoring-relays/9143945/ )


       - Andy.


    Good morning Andy, how do we connect one of these to a single phase installation? Just use one phase terminal of the three available? Or are there single phase versions available?


    Z.




    Yes plenty of single phase versions available - e.g. https://uk.rs-online.com/web/p/monitoring-relays/1026131/ (this particular one complete with adjustable delay function too)

       - Andy.


  • 0
    But rather than adding an extra device to monitor the supply voltage (along with a contactor which would presumably have to draw power almost all the time), the obvious solution would be program an existing piece of equipment that already measures L-N voltage and has an in-built contact capable of disconnecting the installation - i.e. a smart meter. While it was at it, it could also directly inform the DNO/supplier of network problems which may require some intervention. It even sits rather well theologically as the supply industry is then responsible for ensuring that the supply is within safe limits, rather than the consumer.

       - Andy.
  • 0
    Andy,


    I understand this is being implemented in Australia, where incidents with their version of PME cause a lot more problems than here in the UK.


    I guess the red tape of the SMART Meter Data arrangements, along with GDPR, might well make UK implementation of this approach a minefield.



  • 0
    Making a smart meter do something useful? What crazy talk is this???
  • 0
    AJJewsbury:

    As for other solutions there are plenty of voltage monitoring relays on the market that could be fitted (probably via a contactor) to disconnect the installation if the voltage went outside limits. (e.g.https://uk.rs-online.com/web/p/monitoring-relays/9143945/ )


    I don't know whether I am being naïve here, but could one of these devices be used (along with a suitable contactor) for an EVCP? If not SP, perhaps TP?


  • 0
    Hi gkenyon


    Mapj1  Tells a solution which at first seems like it only has positives to it.


    I was thinking of single phase TNCS with the earth side of the Caps Linked to a separate earth rod. because

    1. the system earth rod voltage would rise to dangerous levels

    2  the capacitors would then make zero contribution to any earth leakage currents in system under normal or fault conditions.

    ,

    Can you think of significant negatives or do you know why...

     
    531.3.1.202 It is not permissible to introduce an external connection for the purpose of intentionally creating a residual current to trip an RCD.


    Was added in?  

    Perhaps they had more "Heath Robinson" contraptions in mind?

  • 0
    Out of interest, values of caps?
  • 0
    Nice idea Mike, and under the circumstances I can see your idea may have some merit, however being more worried about "incoming" myself, I should probably not bother. I am sure it is not compliant Graham because it probably is contrary to 531.3.1.202, but also increases the Earth current to a potentially higher level than necessary, therefore touch voltages are probably out of control. Under some circumstances it may make Mike safer, some others perhaps not.


    The loss of neutral event is very damaging because the N-Phase voltage can get well beyond the safe operating voltage of electronics for a significant period, (several cycles with mechanical disconnection). The "fastest fuses known to man" can be rated to withstand this kind of event in large and expensive equipment, but in cheaper domestic stuff, which is very price sensitive in component choice, it doesn't happen. I would expect a 100kVA UPS to withstand this kind of event, but probably not the TV or LED lamps. It is very different from the average transient because of the time of fault and the very large energy available. 400V on the 230V supply for a second probably wouldn't affect heating appliances or resistive loads but your iPhone charger might well fail. I suggest that the dictum of PME should be applied, in that it "never happens", except in quite a lot of occasions not reported to the HSE!


    Have you reported it Lyle?

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