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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?

  • 0
    • I think i would first feel just how hot the troublesome  fuse is getting  secondly clamp it to see how much current is being drawn on that phase. I think if the fuse and holder were getting that hot  the DNO guy would if noticed it 
  • 0

    So is this a new build where none of the flats are yet occupied? And yet a 200A fuse is blowing?

  • 0

    A 200 amp supply does not sound sufficient for 70 flats. Presuming that the load is balanced fairly well, then one phase will be supplying 24 flats. Only 8.3 amps per flat.

    That however is not immediately relevant if the flats are indeed empty. Have you confirmed by PERSONAL INSPECTION that the flats are actually empty and not occupied by friends of the builders for example. Two portable heaters per flat could blow a 200 amp fuse.

    If the flats are ACTUALLY empty then some other factor must be involved. Perhaps an intermittant short circuit, or even unauthorized connections. 

  • 0 in reply to broadgage

    Yes, the flats are empty, and yes, I agree, the design is awful. However the flats do not have much of a load, 3x1kW panel heaters, 1x 3kW Immersion heater, a 13A plug in oven, and the biggest load, a std. electric hob, on a 32A breaker. Of course, if all loads are put on together, in each flat, it will definitely break the cut out fuses, but I presume the 'Designer' has taken this into account, but, somehow, I doubt it!

    Communal heat is from around 20 1kW panel heaters. Other communal facilities add up to less than 20kW when all working - water pumps, ventilation etc.

    I'm suspecting a loose connection somewhere, but would liek to hear others thoughts on it.

    Thanks.

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    A loose connection would cause local heating - so could reduce a fuse's effective rating somewhat if it heated the fuse itself - but I'd expect evidence of scorching in the DNO's fuseholder in that event. Otherwise a simple loose connection would tend to increase resistance and therefore reduce current flow - making a fuse blowing less rather than more likely I would have thought.

    A 200A fuse blowing while all the downstream 100A ones remains intact sounds more like overload than downstream fault to me. No chance a few of the flats have been taken over by some 'dodgy indoor monoculture gardeners'?

    A power logger might be useful - so you can see what's really happening to each phase when you're not there.

    Otherwise I might be thinking in terms of a bad batch of fuses (very very unlikely, but if all other options turn out to be impossible...)

        - Andy.

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    Agree that a data logger is the way forward. A steady current well in excess of 200 amps suggests that the power is being used perhaps illegally. A much lower current with an occasional peak of thousands of amps suggests a fault.

    Frequent observations with a clamp meter might help, but a continuous record from a data logger is more likely to reveal the problem.

  • 0

    Agree with earlier comment a high Z join will  get hot and may start a fire, but it will not blow a fuse, unless  that heating causes an insulation failure that turns into a short.

    Is there any metering on before the mains are split? I'd be tempted to look for a meter spinning like crazy somewhere.

    No chance that those 3kW  heaters in all the flats has been left on by mistake is there ?

    To pop a 200A fuse promptly takes several hundred amps, and on a cool day you may well get 300A passing for hours without it blowing.

    If nothing is in use you might pull the fuses from the Lucy boards and see if the main fuse survives the night.  I imagine that swapping phases over is not easy but if it was, then it might be interesting to see if the fault moves with one load or another.

    I agree it is an odd design - 25 flats a phase sharing a 200A fuse will not survive full occupation and a cold winter I suspect. I too would be looking for signs of unofficial occupation having started, or a connection for a large load, like an EV charger someone forgot to mention...

    Mike.

  • 0

    CCTV from a distance should reveal any signs of unofficial occupation, lights at night or comings and goings outside of working hours.

    Measuring the load will reveal any extra connections.

    What is the weather like in the area, if cold then heaters might be coming on via frost protection, or as already suggested might have been left on by mistake.

    All common uses of electricity turn it into heat, what about an external survey with an infrared camera? That should reveal occupation, needless heating, or farming.

  • 0

    Is there any frost protection kicking some heaters on somewhere ? 

    Gary

  • 0

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

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