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Dad, What's a Circuit?

Zoomup
Well son that a long story.


In my dictionary a circuit is: roughly a circular route, a track for motor racing in the U.K., a regular journey around a district by a judge to hear cases, a system of conductors and components forming a complete electrical path. It comes from the Latin word "circuitus." (No doubt related to the Latin word "circulus," which means circle.


I can see why ebee's friend considers two lighting circuits connected to just one M.C.B.to be two circuits connected together. Two individual circuits connected in parallel in fact.


When we are taught about electrical circuits at school we are told that the circuit starts at the source, which could be a battery terminal , continues to a load, which could be a light bulb, through the load, and then returns back the the battery's second terminal. Or the same arrangement could be made using a transformer's secondary winding.


But B.S. 7671 defines a circuit as: "An assembly of electrical equipment supplied from the same origin and protected by the same overcurrent protective device(s)."


So, a 1.0 mm2 T&E cable supplying lighting connected to a 6 Amp M.C.B. is a circuit. In B.S. 7671 land if a second 1.0 mm2  T&E lighting cable is added to the same M.C.B. there is still only one circuit even though one lighting circuit may supply upstairs in a house, and the other downstairs. Or perhaps originally the two originated from two separate M.C.B.s.


So in B.S. 7671 land, 1 circuit plus 1 circuit = 1 circuit. How could it be any different?


I personally, still though, prefer to call that situation two circuits connected in parallel.


Z.

  • 0
    Of course, if that's the case, 5 s (2 s TT) disconnection time

    Graham - you've mentioned 2s for TT a few times now - yet my copy (411.3.2.4) says 1s - is there a change in the pipeline?

       - Andy.
  • 0
    AJJewsbury:
    Of course, if that's the case, 5 s (2 s TT) disconnection time

    Graham - you've mentioned 2s for TT a few times now - yet my copy (411.3.2.4) says 1s - is there a change in the pipeline?

       - Andy.


    No ... it's me being silly .. an unconscious blunder. I've edited previous post.


    For clarity, with TT the default case with U0of 230 V is definitely 0.2 s (Table 41.1/Regulation 411.3.2) and 1 s (Where Regulation 411.3.2.4 applies).


  • 0
    davezawadi (David Stone):

    No Zoomup, unfortunately not. The video is subtly incorrect because at GCSE the definitions may be considered slightly "wooly". He means that the circuit (from a single power source) has two paths in parallel, with different resistances, so that the current splits between them as per Ohms law. You know that the next stage is to apply Kirchoffs law, which applies to a single circuit, and describes in more detail how to calculate the current in branched circuits which may be much more complex. There are then more steps, the superposition theorem, Thevenin's theorem, and Nortons theorem, which allow the analysis of more complex networks, all of which should be familiar to Electricians (but very rarely even remembered as names). Power electrics are extremely simple versions of much more complex circuits, which may have multiple sources of power (the Grid), many more component parts (motors and transformers), and components with more unusual properties (PFC capacitors) which equally respond to analysis with these "Electrical Network" theorems. Apart from the BS7671 definition (deliberately very simple) the entire country is really one huge Electric Circuit, because everything is connected together. An electron (consider this as the carrier of the power) can take any route through any of the country, its route does not define the path, the potentials along the route do. Perhaps, with a bit of luck, you can see why we use the BS7671 definition? Anything else is actually meaningless because we work only on a tiny part of one huge circuit!


    Don't you try to confuse me with all of the old dead geezers' complications Dave, I'm a very simple person. Muddying the water will not convince me or other sensible souls.


    Here is an example of a circuit. One circuit. The vehicles go around and around.

    https://www.racingcircuits.info/europe/united-kingdom/brands-hatch.html#.YL-i_KqSnIU


    Z.


  • 0
    An electron (consider this as the carrier of the power) can take any route through any of the country, its route does not define the path, the potentials along the route do. 


    But presumably only if it is pushed along by a D.C. Voltage, otherwise does it not just oscillate back and forth around its home?


    Z.


  • 0
    4th annual visit to self-build last week. I contemplated putting half-a-dozen cables into the CU, but it didn't seem very elegant. ?


    The CU is in the middle of the house and the lights (and extractor fans) radiate out in such a way that looping from one to another would not work. In the end, a short cable rose from the CU to a Wagobox with 6-way connectors in the ceiling void.


    So unequivocally one circuit, albeit rather like the Mississippi Delta.
  • 0
    Chris Pearson:

    4th annual visit to self-build last week. I contemplated putting half-a-dozen cables into the CU, but it didn't seem very elegant. ?


    The CU is in the middle of the house and the lights (and extractor fans) radiate out in such a way that looping from one to another would not work. In the end, a short cable rose from the CU to a Wagobox with 6-way connectors in the ceiling void.


    So unequivocally one circuit, albeit rather like the Mississippi Delta.


    Indeed, one B6 consumer unit way. But according to some, a new circuit starts at the fused connection unit that feeds an extractor fan, required to be fused at 3 Amps by the manufacturers Chris. A final circuit supplying a final final circuit??


    Z.


  • 0
    I am NOT trying to muddy the waters Z, I am pointing out the statement in BS7671 definitions of a CIRCUIT is, a collection of wiring which starts and ends at the CPD. However, this definition is not that of a true electrical circuit, which is a network of connections between a number (many in electronic items) including sources of power (possibly batteries) which may be almost beyond comprehension in the CPU of your computer. How the electricity actually works is, in reality, unknown, the velocity of charge carriers matches nothing physical to the velocity of propagation and therefore must be faulty, despite hand waving of the "Group versus phase velocity" and other nonsense. The only information we have comes from equations and data that cannot be verified, the very low figure of a few m/sec or less come from a string of assumptions as long as your arm, and the relativistic velocity calculation is not supported by the accelerating potential, the answer is, therefore, we don't know how electricity works. Inside your computer chip signals travel and cause actions at about half the speed of light, that cannot be electrons (insufficient accelerating potential) therefore must be something else, or quite possibly our understanding of electrons is completely incorrect, which is quite possible. However, this is not the place to discuss this further although I would be happy to do so elsewhere.


    What I am saying is that your idea of an electrical circuit is excessively simplified for Engineering as a whole, if it helps you then fine, I don't mind, but please don't try to tell everyone else this is as far as it goes, it is not by a million miles, the reality is infinitely more complex. Consider this, your brain and nervous system appear to operate by electricity and therefore are electrical circuits. Is your brain part of a complex system or a group of individual circuits (by your definition) which presumably means that they can be isolated and adjusted. Do you think that is the view of a neurosurgeon? Everyone who has tried has found that such results in massive damage to function.
  • 0
    A final circuit supplying a final final circuit?

    Absolutely!


    You might want to consider an events type setup where a flex supplying a DB has a plug on the (source) end, and is plugged into the outgoing socket of the generator.


    "Final" circuit was perhaps an unfortunate choice of words.


       - Andy.
  • 0
    As I`ve said before, a ring or radial can be a final circuit in its own right. If it has FCUs including SFCUs then as far as those are concerned it is the distribution circuit for them but still a final cct in its own right too.

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