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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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    I have now done what I should have done first and looked up the TM100D datasheet As far as I can tell the instant setting is 800A and fixed, as is 15 secs at 600A .  Only the slow trip current is adjustable from 70A to 100A. Of coures than may not actually be your model, but I think it is.

    So I assume your answers meet 800A if the supply is 240V  (so less than 0.30 ohms ), but not with enough slack to trip at a supply reduced to 80% of nominal and to allow for the resistance of the live cores pre-heated by full load. (so more like 0.22 ohms)

    Would it make the 600A 15 second trip ( everything is now 4/3 easier) - not ideal but gets you from 'risk of building fire' to 'risk of cable damage' and maybe from a C2 to a sternly worded C3. If it misses that boat  it is wrong as designed, if designed by experts with a computer or not.
    So was your test with the cable cold or hot? Are you looking for wiggle room that is more than  0.1 of an ohm - probably no chance of that without some new cable or extra earth bonding or is it off by a few tens of milliohms (might be a pass on a mates 'identical' meter.)

    I too caution that most hand held  meters with normal single contact probes are not that great at the 10s of milliohm level, and give you a misleadingly precise but untrustworthy answer - more of a fruit machine than a meter in the last digit.
    Mike

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  • 0

    I have now done what I should have done first and looked up the TM100D datasheet As far as I can tell the instant setting is 800A and fixed, as is 15 secs at 600A .  Only the slow trip current is adjustable from 70A to 100A. Of coures than may not actually be your model, but I think it is.

    So I assume your answers meet 800A if the supply is 240V  (so less than 0.30 ohms ), but not with enough slack to trip at a supply reduced to 80% of nominal and to allow for the resistance of the live cores pre-heated by full load. (so more like 0.22 ohms)

    Would it make the 600A 15 second trip ( everything is now 4/3 easier) - not ideal but gets you from 'risk of building fire' to 'risk of cable damage' and maybe from a C2 to a sternly worded C3. If it misses that boat  it is wrong as designed, if designed by experts with a computer or not.
    So was your test with the cable cold or hot? Are you looking for wiggle room that is more than  0.1 of an ohm - probably no chance of that without some new cable or extra earth bonding or is it off by a few tens of milliohms (might be a pass on a mates 'identical' meter.)

    I too caution that most hand held  meters with normal single contact probes are not that great at the 10s of milliohm level, and give you a misleadingly precise but untrustworthy answer - more of a fruit machine than a meter in the last digit.
    Mike

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