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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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    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.
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  • 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.
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