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ABB MCCB and 5 seconds disconnection.

Handyosprey
Hi all,

Looking for a bit of clarification on disconnection times here. I have information on an ABB Mccb that has been listed as failing on maz zs.

The max zs referenced for the unit is 0.07 ( 250a TMD TP set at maximum) the measured zs is 0.08, however, the max Zs is listed for 0.4 and 5 seconds.

Looking at the time current curve on the mccb once you hit 3.5.times 250a you are in the thermal tripping zone of the mccb. Using the curves software from ABB it indicates a 5 second disconnection can be achieved with 1.66ka.


How do I work this out? is the MaxZs listed as the same for both disconnection times because the only way to ensure a 5sec disconnection is to have an instantaneous trip?
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    I'm perhaps not reading the graphs etc. correctly, but it looks to me like it's saying for 5s disconnection you'd need something between 1.66kA and 3kA. There's always a band or envelope of acceptable time/current characteristics and any single device's performance may fall anywhere within that band. So to guarantee 5s disconnection you'd need 3kA - with equates to Zs of just over 0.07 (taking Cmin of 0.95 into account).


    The 1.66kA figure is more useful when you're trying to obtain discrimination (selectivity) with downstream devices or ensure switch-on surges don't cause nuisance tripping.


      -  Andy.
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  • 0
    I'm perhaps not reading the graphs etc. correctly, but it looks to me like it's saying for 5s disconnection you'd need something between 1.66kA and 3kA. There's always a band or envelope of acceptable time/current characteristics and any single device's performance may fall anywhere within that band. So to guarantee 5s disconnection you'd need 3kA - with equates to Zs of just over 0.07 (taking Cmin of 0.95 into account).


    The 1.66kA figure is more useful when you're trying to obtain discrimination (selectivity) with downstream devices or ensure switch-on surges don't cause nuisance tripping.


      -  Andy.
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