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DNO Fuse breaking.

alanblaby

On the site I'm working at, one phase has blown its fuse twice in the last few days.

Its a 400Amp supply from the transformer (I havemt confirmed this, I was told it was that), which is fused down to 200A at the 3 ph cut out.

From the cut-out there are 2x 40+ way Lucy DB with individual 100A fuses for the 70+ flats and communal services.

I have noted here before about the poor design of the installation, but now it seems even dafter, firstly the flats will never pull anything like 100 amos (50A would be plenty), and why give a 400A supply, when it is fused down to 200A at the incoming supply?

Anyway, one of the phases has broken its fuse in the cut-out twice in 3 days. The Private DNO have been out, and replaced the fuse, but, strangely, have done no testing to see why it has broken. There is a minimal load on the full installation now, as it isnt occupied, just water pumps, some lighting, general safety services, and around 10 computers etc, and a few panel heaters in the site office in the building.I would estimate a peak of 30 amps per phase. There has been no breaking/blown individual fuses to the many DB's after the Lucy DB.

The DNO Employee wasnt saying much at all, I asked if he'd done any clamp testing on that phase, but he said he was only there to change the fuse!

I've been asked to have a look at it on Monday, firstly to check the consumption on each phase, and then inspect to see where the fault coudl lie. We arent going to be fixing it, we'll just be reporting to the Main Contractor,who will contact the DNO.

The main thing I think could be causing it will be a  high resistance joint on the LV supply between the Cut-out and the Lucy board. If the outgoing side of the Lucy board  was faulty, surely the 100A fuse upstream of that will break, rather than the cut-out 200A fuse. All of the testing after the Lucy DB has been done by our Guys, so I'm pretty confident it isnt that cabing that is at fault.

Any other ideas as to what to look for?

Parents
  • 0

    Could there be a secret cannabis farm in one of the flats?

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  • 0

    Could there be a secret cannabis farm in one of the flats?

Children
  • 0 in reply to Gel Jones

    Thanks for all the suggestions, it has been concluded now.

    No, there are no tenants, or hash farms there!

    It was a simple overload. Amongst its other issues are an imbalance on phase use. I clamp tested it today with no-one else in the building, L3 was pulling 60 amps, L1 at 12 amps and L2 couldnt be measured. The imbalance shouldnt really happen, as loads are spread across all phases, so we concluded some heaters must have been left on in every third flat, as clamp testing did not show any large loads other than typical heater use in a flat.

    Some more context after investigating. It was the 200A fuses in the ground floor Lucy fused DB that had gone. (another went on Tuesday). From the cut out, the cable goes to the Lucy DB, then another supply goes off to a further Lucy DB at the 3rd floor. I wasnt aware which building had gone, but this one has more flats than I expected - 94. Now, to the educated, there is no way 600 amps will be enough to supply this building.

    It was then pointed out that the Painters were turning on the heaters, and this was the cause of the fuse breakages. Simple really, but, what a terrible design to allow this to happen.

    We clamp tested one flat, and turned 2 heaters on. The usage immediately went to 12 amps. Half of the flats have 5 heaters, the other half 3. That's a lot of power usage when first turned on, and , quite clearly, the cause of the overload.

    Solutions - this has been taken seriously by the Management for once. The DNO have been out today, and will be replacing all of the fuses for larger ones, they are confident that the infrastructure will cope with that (the original design was for 400A/phase), even so, it works out as not much more than 12 amps per flat, and less if the communal heating is on.

    I can see call outs happening at peak usage times once they are occupied.

  • 0 in reply to alanblaby
    alanblaby said:

    It was then pointed out that the Painters were turning on the heaters, and this was the cause of the fuse breakages. Simple really, but, what a terrible design to allow this to happen.

    We clamp tested one flat, and turned 2 heaters on. The usage immediately went to 12 amps. Half of the flats have 5 heaters, the other half 3. That's a lot of power usage when first turned on, and , quite clearly, the cause of the overload.

    Glad that the cause has been established!

    Hopefully, once the premises have been occupied, there will be no circumstances when all the heaters will come on at once. However, should there be a power cut, what will happen once it is restored?

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