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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?
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    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.
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    Andy is correct - SPD is not a solution for this - and it happens regardless of PEN failure or not, if the Neutral is lost say in a TN-S or TT system (say damaged overhead), the same thing can happen to loads.


    There are relay solutions for this (unvervoltage/overvoltage/phase loss/neutral loss), but the get the correct reliability of products is the challenge. Larger installations that have an intelligent breaker system in the primary switchboard can be better protected.


    OF course, whatever you choose, supplies to safety services should be considered for separate protection.
  • 0
    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.


  • 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.




    Single phase installation definitely have limitations in this respect:



    • PME and TT - all you can do to help prevent this is to use unvervoltage/overvoltage relay.

    • TN-S - overvoltage/undervoltage relay as above, but you could also measure N-PE voltage, and if this remains high, say over 50 V for a period of time, trip.


    To prevent nuisance tripping, these types of devices should be set not to operate for a number of seconds, especially in single-phase installations, as network voltage excursions can and do occur "normally" and this is permitted by ESQCR and DCode.
  • 0
    I take Malta work to BS7671, but with a different outcome?

    https://www.enemalta.com.mt/electricity-safety/basic-safety-consideration/
  • 0
    I have fitted capacitors N-E after a 4 pole RCD, so that if the NE voltage rises, the RCD sees the N-E current and fires and breaks everything but the CPC.

    In equipment in containers that may be  running from 'power of opportunity' with rather doubtful Zs, mains polarity and so on,  this also guards against the accidental swapping of one phase and the neutral as well, though on UK mains under and over-voltage drop out is a good thing to sense.

    Mike.
  • 0
    Yes Malta's regs are  BS7671, but local installation is influenced in style by both Italian and Arabic practices, as well as having that Mediterranean combination of TT earthnig  and a very dry soil... A whole building coming off-earth is not completely unknown.

    Mike.
  • 0
    If you read that link, did you note the advice to water your earth rod ?
  • 0
    mapj1:

    I have fitted capacitors N-E after a 4 pole RCD, so that if the NE voltage rises, the RCD sees the N-E current and fires and breaks everything but the CPC.

     


    Just interested on people's views as to whether this stacks up with Regulation 531.3.1.202 ?

     



    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.





    However, I am compelled to point out that I'm asking this question with the full knowledge that many appliances have a capacitor connected between N and PE - but we are talking about of orders of magnitude difference in capacitance (1-10 nF in appliances, vs perhaps 10-20 μF to trip a 30 mA RCD for an N-E voltage of 50 V)?

    Just edited to note that this Regulation was introduced in BS 7671:2018, and therefore I'm not necessarily saying that Mike's application was non-compliant at the time.


  • 0
    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.

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