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Thread: 700 Volt surges reported from a commercial site

  1. #11
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    Thanks Cracker

    We have a similar complaint after finishing a MV reticulation job.We have an Eskom 11Kv incoming along with a Genset feeding a step up transformer to 11Kv
    The Eskom and Genset 11Kv feeds onto an Automatic transfer switch which then feeds the ring with 4 x 200Kva minisubs fitted with Class 1&2 combined surge arrestors
    If Eskom falls away the genset automatically takes over the 11Kv feed.
    We are switching the MV 1 000 m away from the minisub and I would not expect any type of switching spike to have an effect on the LT supply

    The electrical contractor doing the LT works is having a problem with a Dali interface "blowing " when we switch between supplies.We have checked our side and don't find anything but when I looked at his panel I see all sorts of basic mistakes just waiting to go wrong including his class 2 surge arrestors being wired with 2,5sqmm wire - I am no expert in surge arrestors but it just seems wrong when you are 10m away from a 200Kva minisub with a 125Amp feeder CB.He does appear to have a 20Amp CB protecting the surge arrestors which to me is also incorrect.

  2. #12
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    I'm no SPD fundi at all but on our site, fed from a 320Kva minisub, I initially installed DehnGard? Class 2 SPDs. these are fed directly from the busbars with 16mm conductors for live, neutral and earth and are located as close as possible to the busbars. No o/load protection is fitted other than the 450amp main switch.

    Should this be different? I don't know yet, will wait and see what the data logging company suggests. This is their speciality besides the logging.

    But 20amp breakers protecting an SPD sounds wrong, I'd say there should be no protection at all. Don't quote me on this though, I'm not sure.

    Maybe your LT guy should fit class 3 protection directly on his Dali interface ..................

  3. #13
    New Member Jackt's Avatar
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    Cracker, I sent you a PM.

    If the voltage quality is costing the client money, I'd recommend installing a power quality meter on the supply to the problem area.
    Once again it must record events and some statistics of the event.
    (I normally set this at recording any event above or below 10% of nominal)

    I've had cases of recording so long with a portable instrument that the customer could have installed a permanent meter at that cost. and still not had any events occur.

  4. Thanks given for this post:

    Cracker (13-Feb-19)

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