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Thread: M C B Settings

  1. #1
    Bronze Member mikilianis's Avatar
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    M C B Settings

    Hi I wonder if any of the bright sparks (pun) could enlighten me on the settings of a Merlin Gerin NSX 400N Circuit breaker there are 3 settings Io, Ir and Isd the first one is the current setting not too sure what the other two are, could be tripping curves. I have tried to look it up on the net but there are a multitude of options except the one I want
    Thanks
    Mike
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    Diamond Member AndyD's Avatar
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    'In' is the current rating of the MCB which in your case is 400A I assume.
    'Io' is the course setting for the 'long time' protection threshold
    'Ir' is the fine setting for the long time protecton.

    You work out you overload protection using In x Io x Ir.

    Isd is the threshold of the short time pickup. Some breakers you can adjust the time delay on the Isd point (0-0.5 secs max usually) but I don't see any adjustment on yours. .

    Also some breakers have adjustment on the Ii threshold which is the point of Instantaneous short circuit protection, again yours doesn't appear to have this.
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  4. #3
    Bronze Member mikilianis's Avatar
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    Thanks AndyD
    Could it be that they are useing the incorrect breaker for the application,The breaker is supplying a Heat pump with two compressors eight fans and two pumps all the sums are correct, cable size, installation method, the breaker is correctly rated for the equipment what concerns me is that it is tripping open sometimes as low as 70 Amps not all the time though sometimes days in between having to be reset, the compressors start up with a delayed sequence so both dont start up at the same time the fans are also supplied via an invertor wich ramps them up so minimal inrush ?

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    Diamond Member AndyD's Avatar
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    The breaker would be specified according to the supply cable size, I wouldn't say it's incorrect for the application and Merlin Gerin are pretty reliable kit. Usually the fan motors and compressor motors would be protected within the machine using thermal overlaods/manual motor starters/DOL starter with thermal. Maybe the soft starters also incorperate protection for overlaod.

    Do the compressors and fans and pumps all have individual MCB's in the control panel? Are any of these tripping as well or just the main supply MCCB? Are any of the motors configured star delta?

    I would start by IR testing all components @ 1000v with the possible exception of the compressors at 500v if they're hermetic. If they're semi or open compressors then whack them with 1000v as well and see if they flinch

    How do you know it's tripping @ 70A sometimes? Were you monitoring all 3 phases? Was you ammeter measuring true RMS?
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    Bronze Member mikilianis's Avatar
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    Thank you AndyD the compressors and fans and pumps all have individual MCB's in the control panel and the fans and compressors are supplied by soft starters so the inrush problem is minimalised. I changed the settings on the Isd pot from 1,5 to 5 and the tripping problem seems to have subsidised all very well but what does that imply if In x Io x Ir = ? overload setting would be 400 x 1 x 5 = 2000 A is that correct or am I on the wrong path

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    Diamond Member AndyD's Avatar
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    The Isd changes the current threshold where the breaker changes from thermal protection (long time) to magnetic operation (short time). The puzzling thing is that no matter what the Isd setting it still shouldn't have tripped at 70A.

    If the nominal rating of your mccb is 400A (Io isset at 400) and the Ir is set at 1 then your overload protection would be 400A. The Isd doesn't come into the overload, it's more like altering the curve of the breaker.

    I have a pdf manual for these breakers if you would like it pls send me a pm with your gmail address. The filesize is 2.5megs.
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    Bronze Member mikilianis's Avatar
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    Thanks so the I0 remains at 400A and the Isd is the tripping curve. The reading of 70 Amps was from Major tech good enough to show that there is current flowing probably R.M.S.the breaker had just been reset I suspect it had not settled down from the last trip. It is now day 5 and neither of the 4 breakers have tripped, so the pot Ir that I changed from 1.5 to 5 must have done the trick, thanks I'll try to pm my gmail to you.

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    Bronze Member mikilianis's Avatar
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    Thanks AndyD I reachieved info, most informative.

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    Diamond Member AndyD's Avatar
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    You're welcome, I' m not sure if it covers your exact model of mccb, did it answer any of your problems?
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