LV Power circuit Breaker / Air Circuit Breaker - Derating 2

[NickParker]

The maximum available type tested low voltage circuit breaker (ACB) is 6300A.
When we install this circuit breaker in the LV switchgear, what would be the derated capacity of this breaker?
The purpose is to determine the optimal size of the transformer (so that secondary full load current of this transformer does not exceed the derated capacity of the breaker).

 

[RRaghunath]

6300A is for ambient temperature not exceeding 40 degC (IEC).
In this case, the temperature inside the switchgear enclosure needs to be considered. If the same exceeds 40 degC, a derating factor (IEC) is applicable.

R Raghunath

 

[che12345]

NickParker (Electrical)(OP)25 Oct 23 13:06
" ....#1. The maximum available type tested low voltage circuit breaker (ACB) is 6300A......#2. When we install this circuit breaker in the LV switchgear, what would be the derated capacity of this breaker?....#3.The purpose is to determine the optimal size of the transformer (so that secondary full load current of this transformer does not exceed the derated capacity of the breaker)".
1. Most European IEC LV ACB stop with Iu at 40 C at 6.3 kA and Icu at 150 kA. For reason, see 3 below.
2. The ambient temperature derating is dependent on the design and the busbar deposition. Reference the OEM application manual. As an example, data taken from an OEM a breaker rated 6.3 kA is the same at 50 C, but 6.07 kA at 60 C, 5.6 kA at 70 C...etc. Attention: Force cooling is required when the cubicle internal temperature > 70 C.
3. The maximum LV transformer kVA size is not only limited by the current, but also the short-circuit kA of the busbar structure shall be considered. Attention: The short-circuit is NOT only from the power source but also add about 30-40% from the motor contribution. In theory, the busbar can be supported at say 200-300 mm apart with heavy structure, with other feeder breakers rated to the vey high kA, may not be economical or practical.
Che Kuan Yau (Singapore)