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MCCB Max ZS & Correction factor for temperature rise under fault.

Bees2699

Hello All,

I have recently completed an EICR which has MCCB's fitted most are Merlin Gerin NS with TM D or STR blocks all feeding large final or submains, allowing for a 5s dis connection time although the data tables the zs is the same for 0.4 or 5s. I have calculated the max Zs values for these taking into account the tolerance permitted by Schneider and the C min value. I have used these as the Max Zs recorded on the test sheet. However when coding I have taken 80% of this value in lieu of the possible temperature rise under fault conditions. The client has since queried this as it has caused some circuits to fail. When I spoke to Schneider they said as I have there maximum Zs values from there tables these can be used (which are the same as what I calculated on site) but there calculation are done at ambient of 30 degrees. Therefore I believe a derating factor would still need to be applied to allow for the possible temperature rise in the conductors under fault condition. And that where the measured Zs does not meet this corrected max Zs a C2 would be the correct coding.  

Look forward to hearing your opinions. 

Thank you

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  • 0

    Hmm 0.33 ohms is not good, fault of ~ 650A on a good day. The others are probably just a squeeze with bit of an eye-roll and some clucking.  No RCD /earth fault relay I suppose ?

    Does this stack with the cable type and length? - for copper use the rough and ready rule of 16 to check (a single copper core 1mm2 is 16 millioms, so take 16 milliohms, multiply by the length as you have XX metres, and then divide by the no of mm2, as you have lots of square mm in parallel - well on a 100A line I expect 25 or more of them !!.)
    This becomes a rule of  18/19 ish with hot cables, but allows the sort of walk around inspection where you  can look up and say  'clear pass/ clear fail/ well that needs more accurate verification' while less experienced types are still either running back to the van for the book of tables or re-booting the fondle-slab. (also allows a sanity check of figures.)


    Mike.

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  • 0

    Hmm 0.33 ohms is not good, fault of ~ 650A on a good day. The others are probably just a squeeze with bit of an eye-roll and some clucking.  No RCD /earth fault relay I suppose ?

    Does this stack with the cable type and length? - for copper use the rough and ready rule of 16 to check (a single copper core 1mm2 is 16 millioms, so take 16 milliohms, multiply by the length as you have XX metres, and then divide by the no of mm2, as you have lots of square mm in parallel - well on a 100A line I expect 25 or more of them !!.)
    This becomes a rule of  18/19 ish with hot cables, but allows the sort of walk around inspection where you  can look up and say  'clear pass/ clear fail/ well that needs more accurate verification' while less experienced types are still either running back to the van for the book of tables or re-booting the fondle-slab. (also allows a sanity check of figures.)


    Mike.

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