Accessories 
  4.10 EFB300 external function box 
3VA molded case circuit breakers with IEC certificate 
Manual, 03/2019, A5E03603177010-03 
499 
Default output assignments 
The four outputs of the EFB300 are automatically assigned factory defaults in ETUs with 
firmware version 4.2 and higher. 
 
 
Zone selective interlocking (ZSI) 
Description 
Microprocessor-controlled, zone selective interlocking (ZSI) has been developed in order to 
control the total breaking time in low-voltage networks with multiple molded case circuit 
breakers connected in series. 
Regardless of the number of series-connected molded case circuit breakers, all short circuits 
in the network can be cleared within a maximum time period of 50 ms. Short-circuit 
clearance times should be minimized, particularly in the case of system short circuits of very 
large magnitude. 
 
 
Note 
Backward compatibility 
The ZSI function is backward-compatible with the ZSI function of the 3WL molded case 
circuit breakers.