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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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    You need 30mA from 230V + your margin of overvoltage so as to get reactance of about 1800 Ohms? However it would be much better to use some zener diodes and resistors, because you have much more control of the exact trip voltage. There is still the time to disconnection of your contactor though, so electronic switches are required, probably IGBT with a suitable working voltage, and a bit of gate drive stuff. In an inverter removing any gate drive from the switches should work, they are pretty resistant when turned hard off, probably the method used in a large UPS. It is the time which mainly kills semiconductors, they can resist spikes if short surprisingly well.
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    From memory capacitance was  ~ 1k j ohms so an uF or three- the sort of thing in a small lamp ballast. in normal use there is no NE voltage so no current flows in it - by the time VNE is > 20-30 volts something needs double checking urgently I think we'd have set to trip 30mA below 50V AC .  There was other stuff in the box to indicate phase rotation as well, and a few other credible fault conditions besides.

    torch bulbs as fuses either 12V or 6V in the MES style, both are higher than a well behaved NE offset. At a push a 24V vehicle lamp might do either. 

    I realise something far more sophisticated is possible, probably with a PIC or an arduino we could log to fractions of a volt and have it calling in its readings to a central computer system every few seconds.

    But I suspect that neutral faults are all or nothing, so it is either less than 6V, or a large fraction of 230 and a go /no-go indicator is good enough.


    M.
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    Thanks map1.  When it comes to safety I try to steer clear of sophistication, though it does well as a backup. I was thinking of single phase TNCS. With the arrangement I had in mind, the earth side of the caps would have to go to a separate earth rod for the tripping purpose, because the normal earth connections voltage would rise up,
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    I wonder how damaging short overvoltages (say a few seconds) are. Ordinary L-N (or L-PE on TN-C-S) faults on one phase will pull the N a long way towards the phase the fault is on, so consumers on other phases will see a much increased L-N voltage until the fault clears - which could easily be approaching 5s for faults withini consumers' installations, or even up to 60s for distribution network faults. I guess such problems must be relatively common - much more common than broken PENs etc - yet we don't have devices failing left right and centre.

       - Andy.
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    AJJewsbury:

    I wonder how damaging short overvoltages (say a few seconds) are. Ordinary L-N (or L-PE on TN-C-S) faults on one phase will pull the N a long way towards the phase the fault is on, so consumers on other phases will see a much increased L-N voltage until the fault clears - which could easily be approaching 5s for faults withini consumers' installations, or even up to 60s for distribution network faults. I guess such problems must be relatively common - much more common than broken PENs etc - yet we don't have devices failing left right and centre.

       - Andy.


    That would be my thoughts..... With electronic devices in particular I would have thought that the damage would be done in a few cycles - much faster than any monitoring device could detect & fire a contactor or shunt trip.


    I can see that such a device would reduce the risk from prolonged over-voltage from a loss of neutral but I'm not convinced it be be any help in saving electronic appliances.


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    I don't fully agree. The issue with extensive damage to equipment from a broken neutral event, particularly in three-phase installations, is the unpredictability. The neutral moves to a point depending on the balance of the phases. As overvoltage damages equipment, the loading changes, and the Neutral shifts. The cycle may go on for a number of minutes - and so potentially such a device could be used to limit damage.


    Perhaps less use in single-phase systems, but then again, if your device tripped out on undervoltage, it could very well save your TV and other expensive stuff when the Neutral shifts and puts you in overvoltage condition.
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    3079c6a05dd24b6316b2953e50cdb19b-original-dd3b8d59-5e89-4c87-a297-faf65378d9dc.jpg
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    This occupancy sensor is typical of the damage inflicted. It is widespread with damage sustained to lots of electronic equipment. Of course, there is no hard evidence of loss of neutral and I suppose there are other possible causes.
  • 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.




    Back to my comment about Malta. 


    Over voltage protection is compulsory, SPDs are optional. The installations are to comply with BS7671.


    Here is an example of a Maltese consumer unit with the over voltage device Andy has suggested linked to a contactor.
    https://www.enemalta.com.mt/electricity-safety/electricity-safety-regulations/


  • 0
    The Maltese consumer unit also has a couple pole C40 MCB as the main switch.

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