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Thread: LSIG Breakers .

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    LSIG Breakers .

    I would just like to know more about LSIG breakers, firstly when (at what amperage) are they specified? What I have seen typically is that designers specify LSIG breakers once the service size/bus rating of the main swbd at and above 800A (@600V). However on smaller services (400A and below) - I dont usually see this being specified.

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    Quote Originally Posted by ElectDesigner View Post
    I would just like to know more about LSIG breakers, firstly when (at what amperage) are they specified? What I have seen typically is that designers specify LSIG breakers once the service size/bus rating of the main swbd at and above 800A (@600V). However on smaller services (400A and below) - I dont usually see this being specified.
    It's not directly related to the amperage rating, it's about system coordination. In a coordinated protection system, you want a fault to be cleared at the closest point to the fault, not at a larger system level unless absolutely necessary. Since that concept moves up the line, the higher you get in the distribution system, the longer you need the upstream devices to hold in without tripping. So as you get up there, the trip settings need more adjustability beyond what you see in an off-the-shelf thermal mag breaker. That's what an electronic trip breaker gives you, separately adjustable Long time, Short time, Instantaqneous and Ground fault tripping. Because they are a lot more expensive, you don't go there until you have to, so that explains why you usually don't see them used until you get to higher current ratings.
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    In most cases LSIG is an option that makes it posible to coordinate breakers.

    LSIG, Long time, Short time, Instantaneous, and ground fault are options that are available in breakers that electronic trip units. Breaker frames that have interchangeable trip units commonly start out with 250a frames, 400a, 600, 800, 1200a etc. A 250a frame can have a trip unit that is available down to 75a-250a, the 400a (70-400a), 600a (125-600), 800 (400-800), 1200 (400-1200).
    LIG is fairly common where you would have a rating plug which rated the breaker a specific amp rating, and adjustable magnetic (I), and ground fault pick-up (G) all which should be self explanatory. There is also an LSI option.
    The (s) for short time delay really doesn't bring much to the party unless there is a party, that is if it is related to with breakers. The adjustable short-time allows the breaker to be coordinated with another breaker or breakers such that the breaker closest to the even is given the opportunity to trip and clear first.

    The 'I', is for adjustable short time pick up and delay with a pick up of 200-800% as an example. After pick up then the delay can be set on some breakers up to 100ms while on larger breakers 100-500ms. Between the two adjustments it provides you with the capability to coordinate breakers.

    And likewise with ground fault, on smaller breakers the GF pick up range available can vary while on larger frames it can be from 25-100% of the frame rating not to exceed 1200a. Then you have the delay which often is inst. to 500ms on the smaller frame to 100-500ms with larger frames. Just like the (s) you are given the ability to coordinate.
    Please note that the time delay is limited to 500ms because of the breaker's withstand capablity

    These options are expensive and should not be specified unless you have a reason. Having these options doesn't make a breaker better.
    Last edited by templdl; 03-26-12 at 11:16 PM.

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    Quote Originally Posted by Jraef View Post
    It's not directly related to the amperage rating, it's about system coordination. In a coordinated protection system, you want a fault to be cleared at the closest point to the fault, not at a larger system level unless absolutely necessary. Since that concept moves up the line, the higher you get in the distribution system, the longer you need the upstream devices to hold in without tripping. So as you get up there, the trip settings need more adjustability beyond what you see in an off-the-shelf thermal mag breaker. That's what an electronic trip breaker gives you, separately adjustable Long time, Short time, Instantaqneous and Ground fault tripping. Because they are a lot more expensive, you don't go there until you have to, so that explains why you usually don't see them used until you get to higher current ratings.
    Thanks Jraef - well explained, now that think of it - makes more sense, because the available fault current varies depending on a number of factors (utility, transformer etc..) - you would always want to do a co-ordination study especially if its large distribution system, the results of this study would then dictate the interrupting capacity/response characteristics of downstream electrical equipment, an electronic breaker gives you more flexibility.

    Just out of curiosity do you know how much it would cost if you hire someone to do a co-ordination study of say a medium sized distribution system?

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    Quote Originally Posted by ElectDesigner View Post
    Thanks Jraef - well explained, now that think of it - makes more sense, because the available fault current varies depending on a number of factors (utility, transformer etc..) - you would always want to do a co-ordination study especially if its large distribution system, the results of this study would then dictate the interrupting capacity/response characteristics of downstream electrical equipment, an electronic breaker gives you more flexibility.

    Just out of curiosity do you know how much it would cost if you hire someone to do a co-ordination study of say a medium sized distribution system?
    Interrupting capacity has to do with the available fault current and nothing to do with LSIG. Getting a system coordination study can be of a benefit first to assure that the breakers are suitable for the available fault in different parts of the distribution system. The only thing that would come into play as far a kaic ratings and available fault current is series rated devices which again LSIG is not a relevant factor.

    The second thing is to consider coordination of the devices in order to isolate event to as far down in the distribution system as practical such that there are less possibilities to interrupt power to a large portion of the distribution system. That's where LSIG comes into play.

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    Quote Originally Posted by templdl View Post
    Interrupting capacity has to do with the available fault current and nothing to do with LSIG. Getting a system coordination study can be of a benefit first to assure that the breakers are suitable for the available fault in different parts of the distribution system. The only thing that would come into play as far a kaic ratings and available fault current is series rated devices which again LSIG is not a relevant factor.

    The second thing is to consider coordination of the devices in order to isolate event to as far down in the distribution system as practical such that there are less possibilities to interrupt power to a large portion of the distribution system. That's where LSIG comes into play.
    Interesting. Wouldnt this mean that some downstream equipment would also require the LSIG option - making the system - overall more expensive? Just interested to how coordination was achieved prior to the invention of LSIG breakers?

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    Quote Originally Posted by ElectDesigner View Post
    Interesting. Wouldnt this mean that some downstream equipment would also require the LSIG option - making the system - overall more expensive? Just interested to how coordination was achieved prior to the invention of LSIG breakers?
    By nature of a higher ampacity breaker before a lower ampacity breaker there would be coordination if you look at the trip curves for thermal. It's the Inst. and GF that it could become a race as all breakers in series in that circuit would see the event. That's where the adjustable I and G are very helpful as one can not only assure that the pick-up of the upstream device is set max and adjust the delay at the lowest point which would give the down stream device a very brief opportunity to clear the event first. The time delay is a big plus for I and G. But the short time delay when it comes to the common down stream breakers may be splitting hairs for what the added cost would be. If I recall if you look at the short time pick up and delay it adjusts the thermal just before the instantaneous which in my judgment is not very helpful with an upstream device because the max PU setting would be equivalent to a standard breaker and can be adjusted lower which doesn't make sense for an upstream device. And when set at the max the delay isn't very useful either.

    As for older breaker with the common physical thermal magnetic elements the thermals coordinate by the time current curves. The instantaneous element looks good on paper but instantaneous is instantaneous and who knows how high and how fast fault current will rise to a point where the event will be to a point where the current will peak a where it can trip any of the breakers in series with the fault and then it becomes pot luck. Yes, I have been not very happy in a home where there is a fault in a branch circuit and the main breaker trips. You would have expected that the 15a branch breaker would have tripped. How do you explain that to the home owner? It’s similar to a student in a school who kicks an outlet in and it starts to arc internally in a circuit which at the time did not have GFI protection. That even often doesn’t draw enough current for a long enough period of time to trip the breaker thermally but yet not a high enough current to trip the breaker magnetically.

    All to aften electrical evens are unpredictable at best and we only hope that they would be within our coordination plans.

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    Quote Originally Posted by ElectDesigner View Post
    I would just like to know more about LSIG breakers, firstly when (at what amperage) are they specified? What I have seen typically is that designers specify LSIG breakers once the service size/bus rating of the main swbd at and above 800A (@600V). However on smaller services (400A and below) - I dont usually see this being specified.
    We have sites with 30 amp and larger circuit breakers with LSIG. Depens on the engineer and how much money the customer has.
    Brian John
    Leesburg, VA

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    Quote Originally Posted by brian john View Post
    We have sites with 30 amp and larger circuit breakers with LSIG. Depens on the engineer and how much money the customer has.
    Are they 100-150a frames? If so that is awesome that they can get the current sensors and electronice tucked into them. That gives more ability to coordinate with them as down stream devices.

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    Quote Originally Posted by ElectDesigner View Post
    ... Just interested to how coordination was achieved prior to the invention of LSIG breakers?
    Back in the olden days (wow, I'm sounding like my dad now...) we used trip curves printed to the same scale on trace paper (then later on clear plastic) and physically overlayed them to coordinate them. In a lot of cases you had to go to larger breakers than necessary just to make the curves work, which meant larger conductors etc. The advent of electronic trip breakers actually has probably saved a lot more money than they cost.
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