Switching rails with circuit breakers and disconnectors

Discussion in 'Power Electronics' started by zuababa, May 1, 2017.

  1. zuababa

    Thread Starter New Member

    Aug 11, 2015
    Hello all.
    I don't have a good background in electronics, but for certain purposes I am trying to understand the way disconnectors and circuit breakers work. So I came across the explanation given here: http://electrical-engineering-portal.com/disconnectors-load-switches-switch-disconnectors-cbs#3 which explains what circuit breakers and what disconnectors are and the difference between them.
    In general, I get the difference: first open the CB, so the current is not flowing. Then you can open the disconnector. But with usage in context I am a bit more confused. I saw this part in switching between two rails: http://3.bp.blogspot.com/-je0x5PgZY.../g2LuPKUceKc/s1600/DoubleBusSingleBreaker.png . Let's consider only the utmost left part. We see 3 disconnectors and one circuit breaker (CB). If we want to switch the power line below from one rail to another, what would be the correct procedure? Open CB, open the disconnector on one rail, close the one on second rail and close the CB? Why is it okay then to completely disconnect the power line (with the CB) from either rail for a moment?
    Otherwise, if we can just close the disconnector on rail two, and then open the one on rail one, I would assume that arcs would occur.

    So, my questions are: (i) is the way this should work that I explain correct, and (ii) why is it okay to disconnect the power line below from either rail for a moment?
  2. fourtytwo


    May 2, 2017
    I am familiar with fault tolerant design and that is what you are looking at, the two horizontal lines are substation busbars typically fed from two different sources/transformers. The vertical lines are the feeds out from the substation to the service areas. As you have noted the circuit breaker is there to break the circuit when it may be carrying load or make the circuit when a load may be present i.e it is the first "switch" to be opened and the last to be closed.
    The dis connectors are there for safety isolation, they incorporate such features as mechanically forced open contacts and very wide contact separation as well as the ability to be padlocked open or closed. They ensure the safety of linesmen working on supposedly dead sections and also ensure that only an authorized person is able to configure the bus system (not inadvertently short them together), but as you have noted are not capable of making or breaking circuits under load.
    Hope this helps, electrical infrastructure is fun, usually more so when things go spectacularly wrong although its good to keep a distance as there is LOTS of energy available!
  3. Dyslexicbloke

    Distinguished Member

    Sep 4, 2010
    Whatever the circumstance, from white goods to utility level power any switch has a design function or functions.

    Its breaking capacity describes its ability to interrupt a loaded circuit but has little to do with its ability to guarantee the circuit cant be inadvertantly closed either mechanically or as a result of a voltage surge.
    Of course some switches, many in fact, are designed to perform both functions and would typically have a fail safe mechanical linkage, some method of locking off and an internal structure that provides a large contact gap along with a means of quenching an arc.

    Oviously as energy levels go up the it gets exponentally harder to build a switch with multiple capabilities and seperate units begin to make more sense both mechanically and from a cost perspective.

    Even in basic domestic installations a light switch is not an isolator, not because it couldnt be but because it would be more expensive to build it that way and it isnt necessarry most of the time.