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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.

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  • 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.
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  • 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.
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