APPLICATION GUIDE
Overview
Power distribution cabinets usually require multi-level protection (selective protection) from the incoming line to the end load. A common question is "How should the upper-level and lower-level circuit breakers cooperate to ensure that the fault only trips the nearest level, rather than tripping the upper level together."
coordination principle
Current and trip settings should be reasonably stepped, but there is no fixed 2:1 general rule that applies to all systems; this should be confirmed with load calculations, expected short circuit current, and manufacturer selectivity/cascade tables.
Time-current characteristic coordination: The short-circuit instantaneous trip setting values need to be staggered, or an electronic trip unit with selective functions (such as the MicroLogic series) needs to be used.
Breaking capacity and cascading cooperation: Each circuit breaker should meet the expected short-circuit current requirements of its installation point; if a cascading/back-up protection combination that has been verified by the manufacturer is used, the lower-level equipment can be selected according to the corresponding cascading table. "The upper-level kA must be greater than or equal to the lower-level kA" as a general rule.
Typical classification path: main incoming line (frame circuit breaker) → branch power supply (molded case circuit breaker) → terminal circuit (miniature circuit breaker).
Typical circuit breaker grade matching examples for three-level power distribution
| distribution level | Common circuit breaker types | Current range (indicative) |
|---|---|---|
| main incomer | Frame circuit breaker (ACB)/large size molded case circuit breaker | 800A and above |
| branch distribution | Molded case circuit breaker (MCCB) | 100–630A |
| final circuits | Miniature circuit breaker (MCB) | 63A and below |
Note: The above table is an indication of common classification directions. The actual current classification and breaking capacity selection must be determined according to the specific load calculation book and short-circuit current calculation results, and shall be subject to official samples and local specifications.
Common Pitfalls
Only look at the rated current classification and ignore whether the tripping curves overlap, resulting in over-level tripping (only the last stage should be tripped, but the upper stage also trips).
The breaking capacity of the terminal circuit breaker is selected to be lower than the expected short-circuit current, which may damage the circuit breaker body during short circuit.
Ignore the withstand current (Icw) coordination between the circuit breaker and the upstream fuse/contactor.
Frequently Asked Questions
What is the difference between selective protection and non-selective protection?
Selective protection can ensure that only the circuit breaker closest to the fault will trip, and the upper level will not be affected; non-selective protection may trip the upper level together, with a wider range of influence.
Can MCCB and MCB be directly cascaded?
Yes, but you need to confirm whether the withstand short-circuit current meets the standard according to the cascade table provided by the manufacturer, and cannot be directly matched based on experience.
How much headroom should be generally selected for the main incoming circuit breaker?
There is no fixed "percentage increase" that applies to all projects. The main incoming circuit breaker should be combined with the design load, transformer/power supply capacity, conductor carrying capacity, protection setting, short-circuit current and upper and lower level selectivity, and be determined according to the applicable project specifications and the manufacturer's selectivity table.
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