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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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    80% is a somewhat arbitrary rule of thumb that has a number of assumptions behind it. It's not so much for temperature rise under fault conditions as for the difference in temperature between when the impedance is measured (often when the system is unloaded) and normal operating conditions (i.e. when the conductors are much warmer). If you know the conditions you can likely calculate a more accurate factor - usually based on 1+0.004 per ºC (at 20ºC) - based on the actual temperature of the conductors when the test was carried out and their maximum operating temperature.

       - Andy.

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  • 0

    80% is a somewhat arbitrary rule of thumb that has a number of assumptions behind it. It's not so much for temperature rise under fault conditions as for the difference in temperature between when the impedance is measured (often when the system is unloaded) and normal operating conditions (i.e. when the conductors are much warmer). If you know the conditions you can likely calculate a more accurate factor - usually based on 1+0.004 per ºC (at 20ºC) - based on the actual temperature of the conductors when the test was carried out and their maximum operating temperature.

       - Andy.

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