Table 41.3 Max earth fault loop impedence (Zs)

[1capybara]

The heading of table 41.3 says it shows " max earth fault loop impedance Zs for Circuits Breakers..... giving compliance with 0.4s disconnnect time of 411.3.2.2 and 5s disconnnect time of 411.3.2.3."
411.3.2.2 is for final circuits not exceeding 32A. 411.3.2.3 is for distribution circuits and final circuits 40A and above. But something doesnt add up here.. There's a 12-fold difference between 0.4 seconds and 5 seconds. But the table (look at part a for type B circuit breakers) is linear and gradually decreasing between 32A and 40A. Somethings not right here, or I just dont understand. Can someone please explain ???

 

[Richard Burns]

Re: Tabvvle 41.3 Max earth fault loop impedence (Zs)

Circuit breakers reach a certain current and will then trip "instantaneously"; before a circuit breaker reaches that current the minimum disconnection time is about 12s for a type B MCB so does not meet the minimum disconnection time for either option, once the MCB reaches the instantaneous tripping point then the disconnection time complies with both 0.4s and 5s so the value of EFLI is the same for both disconnection times.
have a look at the time current graphs in appendix 3, e.g 3A4 for type B MCBs, and it should all become clear.

 

[1capybara]

followup:
if anyone compares the values in table 41.3 with the calculation results from the formula in 411.4.5 you will find that the table is much too conservative, and will give loop dimensions much smaller than that from the the formula. so the formula is much preferred. compare it yourself !!!