Ze

Discussion in 'Electrics UK' started by JohnW2, 20 Mar 2012.

  1. JohnW2

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    My apologies. Your response suggests that I must have completely misunderstood what westie meant by 'a fixed figure' (which takes into account the consumer's equipment). Could either he or you perhaps explain what was meant? Thanks.

    Kind Regards, John.
     
  2. RF Lighting

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    Give it a rest John. No one cares.

    I thought you were going to stop boring everyone on here to death on the 28th Jan 2012?
     
  3. westie101

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    No, fait=rs fair.

    John the rating of a domestic MCB is 6kA that is it's breaking capacity, (in theory if the current exceeds that it may suffer from a disruptive failure)

    The nominal voltage is 230V

    Ohms law should then tell you the minimum Z for the supply to that equipment.

    The same calculation applies to equipment we install though one of our fuses is rated at 33kA breaking capacity so the Z for that will be the lowest it could be as we do not use a higher rated fuse
     
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  4. westie101

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    Though of course you can always buy gear with a higher rating than 6kA or even, shock horror, use fuses
     
  5. JohnW2

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    Thanks. Needles to say, I understand all that, but maybe you haven't seen how today's onslaught on me started. Although it wasn't my intended meaning, something I wrote was taken to mean that that I was suggesting that the only thing that mattered about the Zs of a circuit was that it was low enough to achieve the required disconnection times. I was therefore challenged with "even if Zs <0.04&#937; ?" - presumably a reference to the fact that PSC rises to >6kA at just below 0.04&#937;. There was therefore an apparent expectation on the part of that electrician that a Ze low enough for PSC to exceed 6kA was not beyond credibility.

    I took your previous comment to mean that, since the great majority of domestic MCBs were rated at 6kA, that resulted in a 'fixed value' of minimum Ze (about 0.0383&#937;) that a DNO would ever provide. Was that not what you meant?

    Kind Regards, John.
     
  6. ban-all-sheds

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    I didn't see the how, either, but I did start to read the topic as I hadn't spotted who had started it.

    What I did see then was the who, and it was someone who all right-thinking people should have on their ignore list.
     
  7. ericmark

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    I did not read orignal post but there are two values we need to look at.
    PSC and Let through valve of the DNO fuse.
    Former is easy as we can measure but since we can't see the size of the DNO fuse we can't really read the let through value.
    However it's unlikely to exceed the 6kA of most items used in a domestic consumer unit and there are also 10kA variants on the market.
    The let through valve is talked about here
     
  8. JohnW2

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    Indeed. I'm pleased to see you calling it "let through value". It niggles me when I hear it called "let through energy", since it doesn't take much engineering or physics knowledge to tell one that, whatever else it may be, something with dimensions of current squared times time most certianly isn't 'energy' :)

    Kind Regards, John.
     
  9. westie101

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    I think I misread the question/intent.
    The problem as I see it is that measuring the loop impedance is only part of the story and doesn't really fit the correct measurement of PSC.

    The calculations we tend to make add the impedances from the source (the grid) to give us the "fault" PSC on our equipment.
    So for the impedance at a cut out the following should be taken into account based on a typical system of ours: -
    Source ( impedance at 132kV or more often at 33kV)
    11kV lines & cables, 11kV to LV transformer, LV main, LV service.

    By setting a figure solely based on the LV impedance it seems to me that there are assumptions about the rest of our system.

    But it's late at night so I'm possibly digging too deep, as I said earlier if the Z is low higher rated equipment is available
     
  10. flameport

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    Stuff:

    Attempting to measure Ze will only ever provide an approximate result, and the smaller it is, the greater the error becomes. Any measured value under 0.1 will be grossly inaccurate and next to worthless. If Ze was less than that and it was relevant, calculation would be used instead, and that certainly wouldn't be done for a domestic installation.

    Distance from the transformer is only part of the story, since no transformer has a zero impedance.

    All consumer units have a conditional rating of 16kA regardless of the rating of the individual devices within, BSEN 60439-3 Annex ZA if you can be bothered to read it.

    To get these 'too high' fault currents, the fault would have to be in the consumer unit, something which is highly unlikely in normal service. Even a few feet of cable on a final circuit will drastically reduce the fault current.

    Even if there was a fault in the consumer unit, the actual energy involved would be far less than you might imagine due to the way fuses fail. A fuse is not an on/off device, because immediately before failing, the element heats up to a very high temperature which significantly increases it's resistance.

    A fault of negligible impedance occurring within a consumer unit will probably cause serious damage to the entire CU anyway, so a single busted MCB would be of no relevance to anyone.

    Finally, many BS3036 rewireable fuses have a breaking capacity of only 1kA, yet many millions of them have been used everywhere and plenty are still in service.
     
  11. JohnW2

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    I understand all you're say but, yes, I think that you're probably getting more sophisticated than is necessary in relation to the fairly trivial issues that were being discussed (if that's the word!).

    On the downstream side of the meter, I think that, as far as the electricians are concerned, 'PSC' and loop impedence are simply two different ways of expressing exactly the same thing. When they talk of 'PSC' they are simply referring to supply voltage divided (by themselves or their meter) by loop impedance.

    Kind Regards, John.
     
  12. JohnW2

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    Thanks. It's amazing how this has arisen, across three threads (one already locked) in one day, out of what I thought was a simple statement I made about 'high Zs' and disconnection times!! I agree totally with most of what you say - just one comment and one question:
    Indeed - and that's why I asked westie about the 'lowest credible Ze' at the origin of a regular domestic installation. I've been talking of 'distance from transformer' only to provide a 'lower bound'.
    What does that mean? At face value, it doesn't really sound sensible!

    Kind Regards, John.
     
  13. ban-all-sheds

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    Overall I can recommend this:

    [​IMG]


    Costs less than the trade-in value of your BRB.
     
  14. 1john

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    Your saying I would have traded in the BRB for the BGB ??
     
  15. ban-all-sheds

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    No.


     
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