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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
    It is a problem that most  loop test instruments struggle to read low values of Zs really well, and the trend towards lower test currents in the interests of safety and not disturbing RCDs has made it worse (I have an old Edgecombe Peebles unit  from about 1970 that uses a 25 A test current, which I press into service if the newer one's reading is supect and you can sometimes hear the wiring twitch if it is delicate - the thinking back then was that any single strand hangers on deserved to fail during the test.)  Even so an instrumented test may well miss something that is electrically connected, but just hanging by one or two strands say.


    A fair approach would be to set the fail limits a little slacker for an inspection than for the original installation - there is not really a credible degradation mechanism that will make the copper cores just a bit thinner in the middle of a cable run over time, so you are looking for things like earths coming detached.

    I agree a 'FI' is the correct observation if the test is not a clear pass or fail within the limit of the instruments,  What form the further investigation takes is another matter.
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  • 0
    It is a problem that most  loop test instruments struggle to read low values of Zs really well, and the trend towards lower test currents in the interests of safety and not disturbing RCDs has made it worse (I have an old Edgecombe Peebles unit  from about 1970 that uses a 25 A test current, which I press into service if the newer one's reading is supect and you can sometimes hear the wiring twitch if it is delicate - the thinking back then was that any single strand hangers on deserved to fail during the test.)  Even so an instrumented test may well miss something that is electrically connected, but just hanging by one or two strands say.


    A fair approach would be to set the fail limits a little slacker for an inspection than for the original installation - there is not really a credible degradation mechanism that will make the copper cores just a bit thinner in the middle of a cable run over time, so you are looking for things like earths coming detached.

    I agree a 'FI' is the correct observation if the test is not a clear pass or fail within the limit of the instruments,  What form the further investigation takes is another matter.
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