The setting values of circuit breakers and transformer low side circuit breaker setting
Today we mainly discuss about the low voltage circuit breaker setting value and the low voltage side circuit breaker setting. The reason of this discussion, is mainly a few days ago have a customer inquiry about a transformer low side circuit breaker selection problem. One of his 1000KVA transformer parameters : 10KV / 0.4KV. And then I hope we can provide a suitable for low side circuit breaker.
It is important to note: as long as no more than 30% at low voltage side of transformer transformer capacity load, and no special power equipment, can not consider the transformer overload. But selected circuit breaker rated current to exceed the rated current of 15%.
Through this example, we will take down about circuit breaker setting value. Here setting value is circuit breaker tripping device setting current, in general we choose is the rated current of the breaker tripping device, is rated current. Setting current refers to the instantaneous release for circuit breaker in setting current, general distribution is 10 times of the rated current for motor protection, with 12 times the rated current. Release current setting value certainly, because it must play a protective role, when the current short circuit current is large, to be instantaneous tripping ( also delayed type ). In particular we introduce two formula:
In 1, low voltage circuit breaker action sensitivity calibration: Ki = Ikmin / In
Type : Izd for circuit breaker instantaneous or short time delay trip setting action current;
Ikmin is protected at the end of the line of minimum short-circuit current, is generally taken as grounded single phase short-circuit current;
Ki breaker action reliability coefficient, the manual is generally recommended to take 1.3.
For low voltage circuit breaker reliably cut off the ground fault, usually according to the type check breaker action sensitivity:. Actually, Ki is also considered the breaker error, so it should be based on the specific circuit breaker tripping device error determination. For general electromagnetic tripping device, such as CM1, DZ20, its error is + 20%, in order to ensure the reliable operation of circuit breaker, the Ki should be greater than 1.2, so the Ki generally take 1.3. But for the use of electronic and other high-precision trip switch, such as the Schneider NS switch with STR release, the short circuit short-time delay and instantaneous tripping device error of + 15%, Ki only need is greater than 1.15, Ki 1.2 on the line.
In 2, long time-delay over-current release action current setting formula : In = Kzd1Ib
Type : Ib for line current calculation for low voltage circuit breaker; Kzd1 long delay tripping device reliability coefficient, are generally recommended as 1.1 manual. Actually, Kzd1 is mainly on account of the breaker error, it should be according to the long time delay overcurrent tripper error determination. For a general circuit breaker such as CM1, DZ20, long time-delay over-current trip device error is about 10%, so the Kzd1 of 1.1. But for companies such as Moller with digital release IZM switch emergency stop switch HW, the Schneider NS switch with STR53 release, its release error more + 5%, and the Kzd1 preferable 1.05. Kzd1 of small, can better protect the feeder cable.
The information above in electrical switchgear plant collection, www.jiandq.com 15957745520