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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
    The 100 m2 went a long time ago Mike, and 2 rings as described will have 1/2 the cable length, so 1/2 the voltage drop, each! Only one circuit through Zoomup, although 2 loops.


    I'm not that fussed about what is in or out of the regs per-se, area remains quite a sensible way to estimate total load if no better info is available. - as folk do not normally have more than a few kW of kit in a small room, as it heats up too much if you do. Not unlike the 250 watts per desk in some open plan offices.


    I agree the fact that  the original 1000 sq feet morphed into 100msq passing through 100sq yds on the way was a bit odd, but houses got gas heating and better fitting windows in the interim, which sort of justifies a lower heat density, and the whole point of a rule of thumb is just that, as a first estimate.


    A bit like estimating the size of bus bars, where the rule of thumb here is more like 1000A per square thumb, is never quite right, but at least it is not so far out you look totally stupid for asking about it..


    So two full area rings, in some large multi-room place, especially if lots of bedsits, serving perhaps 200msq or more between them, may be worth thinking about a bit more carefully, than the same butterfly arrangement doing one  2 up 2 down terraced house with a ring up and a ring down, as you could have probably had just one ring anyway.


    Mike.
  • 0
    mapj1:

    OK, now tear yourself away from sleeping lion songs, as I think Blondie has just the song lyrics for those wondering where they should  terminate that cable for the outside light just installed in the porch, or for tracing that elusive fault.

    All the way from the 1970s....

    One way,   Or another....



    Mike.


    The chap and chappess at 3 min 28 remind me about new shower curtains and that I soon need a haircut...


    Blinkin ek! Mike was a punk rocker. My favourite song by Blondie was......(I used to play Space Invaders to it in the local pub on a flat top glass coffee table thingy.)

    Blondie - Rapture - YouTube



    Z.


  • 0
    mapj1:

    If you have two haystacks  in separate fields and we use the telehandler to move the bales so you merge them into a new pile  in another place, is it too great a mental leap to see that we still have one haystack, just a rather larger one ?

    In that case  1+ 1 = 1 is not a problem, just the '1' symbols refer to different things, as this is not simple arithmetic..

     


    Haystacks of old existed when a circuit meant a circuit and not several connected to a single fuse.

    Haystacks - a common country sight in bygone times (1900s.org.uk)


    Z.


  • 0
    I reckon that the definition of a circuit or final circuit had to be redefined as originating from one protective device, as it would then be possible to connect together two circuits such as an upstairs lighting circuit and a downstairs lighting circuit, so as not to conflict with what is now Regulation 314.4 which requires that all final circuits to be separate from all others, and connected to a separate way in a distribution board. 


    Z.
  • 0
    Strangely, the definition in BS 7671 does indeed permit two (or more) independent circuits (as defined) to an mcb or RCBO.


    Whenever you have a final circuit, that has fused spurs, the spur is in fact (by definition) a circuit in itself ... and a final circuit at that.



    Another oddity that isn't always obvious ... if an EV charger has overcurrent protection built into it before the vehicle outlet / connecting cable, then the circuit that supplies the charging equipment is NOT a final circuit, but a distribution circuit. Take a look at the definitions for charging equipment and charging point, and then read carefully Regulation 722.531.101 in A1:2020 (or indeed BS 7671:2018, as the wording didn't change). Of course, if that's the case, 5 s (2 s 1 s TT) disconnection time permitted (unless you need an RCD at the supply end of the cable to the charging equipment for other reasons, 722 doesn't care if it's at or within the charging equipment, but it will need to be accessible for periodic checking via test button, especially domestic without the use of a tool).
  • 0
    If I reword my terminology as "A ring final circuit" (Bog Standard) and a Butterfly Circuit as a "two/three/seventeen looped Ring final Circuit" we might get a concensus then?.


    My one and only think of a two looped ring final came about because of drop fed sockets that I would normally put on one circuit for the main part of that particular  floor and the CU was sited approx mid position. I ended up making two seperate circuits to keep the ring lengths good rather than approaching  the higher end of reasonable and that`s what made me consider the two loop idea. I wanted to keep volt drop low too. It was discussing this that got me a few crazy comments like joining the two loop ends in the consumer unit to create a one looped ring which would almost double the loop length . Example two loops at 45m and 55m or one loop at 95m, which would you pick?
  • 0
    gkenyon:

    Strangely, the definition in BS 7671 does indeed permit two (or more) independent circuits (as defined) to an mcb or RCBO.


    Whenever you have a final circuit, that has fused spurs, the spur is in fact (by definition) a circuit in itself ... and a final circuit at that.



    Another oddity that isn't always obvious ... if an EV charger has overcurrent protection built into it before the vehicle outlet / connecting cable, then the circuit that supplies the charging equipment is NOT a final circuit, but a distribution circuit. Take a look at the definitions for charging equipment and charging point, and then read carefully Regulation 722.531.101 in A1:2020 (or indeed BS 7671:2018, as the wording didn't change). Of course, if that's the case, 5 s (2 s TT) disconnection time permitted (unless you need an RCD at the supply end of the cable to the charging equipment for other reasons, 722 doesn't care if it's at or within the charging equipment, but it will need to be accessible for periodic checking via test button, especially domestic without the use of a tool).


    All of this, two circuits can be one, and a new circuit starts at a protective device nonsense, started with the materialistic Harry Enfield type Yuppies of the 15th Edition era. Too much partying I reckon.

    Harry Enfield Loads of Money Top of the Pops 12/05/1988 - Bing video


    Z.


  • 0
    davezawadi (David Stone):

    Think of it this way Z, electrons start at one place (the CPD) and arrive back later. That is one circuit. How they went between the live and neutral is of no consequence, or if some go through one appliance and some through another. Logical really!


    Two circuits in parallel Dave. Here is some of my homework info. One supply source but two circuits in parallel.

    GCSE Science Revision Physics "Current in Parallel Circuits" - Bing video


    Z.


  • 0
    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!
  • 0
    davezawadi (David Stone):

    An electron (consider this as the carrier of the power) can take any route through any of the country


    If electrons actually have a net movement in AC circuits ... and even in DC circuits, charge carriers probably move far more slowly than the energy is transferred.


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