when should circuit breakers operate?
when should circuit breakers operate?
(OP)
Hi all,
If a fault occurs in the low voltage system, and if the LV circuit breakers operate at around 8th cycle (rough estimate), what happens before the 8th cycle...will the circuit breaker blow up?
Why shouldnt low voltage circuit breakers operate in earlier cycles...2 or 3cycle or even 1/2cycle (if technology permits)..(since the asymmetrical fault will be much greater)?
Thanks
If a fault occurs in the low voltage system, and if the LV circuit breakers operate at around 8th cycle (rough estimate), what happens before the 8th cycle...will the circuit breaker blow up?
Why shouldnt low voltage circuit breakers operate in earlier cycles...2 or 3cycle or even 1/2cycle (if technology permits)..(since the asymmetrical fault will be much greater)?
Thanks






RE: when should circuit breakers operate?
RE: when should circuit breakers operate?
If it has an interrupting rating of, say, 65kA, then it can safely carry and interrupt fault current at that level. If the fault current is higher than it's rating, it will probably 'blow up'.
RE: when should circuit breakers operate?
So...case scenario:
if a breaker is size to handle 100kA fault, when a fault occurs, the current is 110kA at 1/2cycle, but after 1 cycle, the fault is now 90kA...
1. is there any rule of thumb or standards which shows how much the mechanical features inside the breaker can handle faults greater than the rated size for a ? period of time.
RE: when should circuit breakers operate?
Yes, there are no rule of thumb but actual curves are available. Its called TCC (Time current curve) of a protective device such as circuit breaker and fuses. It will show how much current it can withstand for how long (RMS value). You have to draw a cutoff line at its rated short circuit current.
For excample, a TCC for a CB rated for 100A, and 10KAIC shall be read only for current up to 10,000A.
Besides breaker are built and tested to applicable ANSI/IEEE and UL standards which
allow for asymmetrical peak current withstand capabilites at 1/2 cycle as well as actual breaking capacity. Momentary duties without opening are higher (in amps) than acutal making or breaking capacities. Beacuse making and breaking capacities invariably occur after highest peak during a fault.
Heavy duty breakers usually used as mains or upstream in the system has to have capacities to withstand a fault current little longer than downstream devices to give the downstream breaker (closer to the fault) a chance to open before the upstream device opens. This is the very basic idea of device coordination.
RE: when should circuit breakers operate?
If you have a breaker rated less than the available fault current, you have a code violation. There is no way around it. You either need to reduce the fault current (increase the impedance in the system ahead of the breaker), or you need to replace the breaker with one rated for the conditions of use. Don't try to game the system.
RE: when should circuit breakers operate?
Although the circuit breakers are rated in symmetrical amps with an x/r value test values, is it true to say that the asymmetrical amps can be found by looking in to a asymmetrical/symmetrical vs x/r graph...therefore if the fault x/r is greater than the test x/r therefore you will have a bigger asymmetrical fault current than the circuit breakers asymmetrical values...Thanks
rbulsara:
in relation to TCC graph...can you expend a little more on it...Thanks
RE: when should circuit breakers operate?
The IEEE Blue Book would be a good reference to start with, followed by the IEEE Red Book.
RE: when should circuit breakers operate?
I am not sure what part you want expanded on, but here is an example of a TCC for a GE molded case breaker. Please review it. As you can see it will open in less than .02s at 10,000A (x- axis is in multiple of current rating of breaker) and somewhere around 20 seconds for 700A fora 100A breaker.
http://ww
Also follow what davidbeach said, you need to refer to those IEEE books for more information.
RE: when should circuit breakers operate?
The horizontal line at the top of the Instantaneous trip curve (at just under the 0.02sec line for the listed curve from rbulsara) is supposed to be the maximum time that that C/B will take to clear a fault at that current (from about 35x to just over 100x the C/B rating). This is not really true, since it typically will take 0.06sec (3-4 cycles) to clear the fault.
If a C/B is rated for 10kA interrupting, then that means it can handle the slightly higher fault current previous to the 10kA at 3-4 cycles.
For more info on this, check out IEEE 242, chapter 6, 'LV Circuit Breakers'
RE: when should circuit breakers operate?
When you say:
"If a C/B is rated for 10kA interrupting, then that means it can handle the slightly higher fault current previous to the 10kA at 3-4 cycles. "
But how much slightly are we talking about?
RE: when should circuit breakers operate?
I can try to reporduce a couple of them here.
The momentary MF is equal to: the square root of (1 + 2e^(-(2pi/(X/R))).
The interrupting MF for a HV C/B is: the square root of (1 + 2e^(-t(4pi/(X/R))), where t= the C/B contact parting time in cycles.
The fused and unfused LV C/Bs have different calculations, a little too complicated to write in here.
RE: when should circuit breakers operate?
IEEE red book says LV breakers or fuses(less than 1000V) are rated for symmetical currents at 1/2 cycle and incorporate asymmetrical capabiltiy as necessary for cirucuit X/R ratio of 6.6 or less. This will satisfy most installations fed by say 1500kVA transformers. For systems fed by larger or multiple transformers some derating will appply per IEEE C37.13.