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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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  • 0
    why can it not express the value accurately

    I suspect the problem is not so much being able to express the value accurately - so much as measuring it accurately in the first place. Usually for a loop test they're trying to measure tiny changes in voltage when applying or removing a modest load - and somehow having to distinguish the intended variation from all the other changes that are happening in the local part of the network and indeed the grid generally, variability in the quality of the connection between the test probes and the installation, and probably a dozen other variables.

        - Andy.
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  • 0
    why can it not express the value accurately

    I suspect the problem is not so much being able to express the value accurately - so much as measuring it accurately in the first place. Usually for a loop test they're trying to measure tiny changes in voltage when applying or removing a modest load - and somehow having to distinguish the intended variation from all the other changes that are happening in the local part of the network and indeed the grid generally, variability in the quality of the connection between the test probes and the installation, and probably a dozen other variables.

        - Andy.
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