6amp mcb tripping on non trip Zs test

Mike Me Mate · Jul 24, 2011

Hi I come across this once in while. When performing a zs non trip test on at 6amp mcb. It trips out. It's always on the cheaper boards. I've read on other sites that it is because of cheap Chinese mcbs. I havn't come across the topic on here and was wonder what you guys thought?

MarkieSparkie · Jul 24, 2011

Hi Mike, This can happen on any low current rated MCB, irrespective of manufacturer or origin. In this situation the easiest solution is to calculate Zs by adding your Ze and R1+R2 together. Alternatively you could temporarily replace the MCB with a higher value and measure Zs, but this is considered bad electrical practice because you would be testing with different fault protection in place.

spark 68 · Jul 24, 2011

Just a thought,

Can you use the method someone on here mentioned the other day, (sorry I forgot who posted it), where you open the MCB (first) and then you join L+CPC at the far end, and measure across the MCB ?

Apologies to whoever posted this originally

Mike Me Mate · Jul 24, 2011

MarkieSparkie said:
Hi Mike, This can happen on any low current rated MCB, irrespective of manufacturer or origin. In this situation the easiest solution is to calculate Zs by adding your Ze and R1+R2 together. Alternatively you could temporarily replace the MCB with a higher value and measure Zs, but this is considered bad electrical practice because you would be testing with different fault protection in place.

Yeah that's exactly what I did! Thanks. I thought it was just interesting and wondered why it was happening. Is it an indication of a faulty mcb? Or is it a bit sensitive.

ezzzekiel · Jul 24, 2011

megger 1552 used to trip alltime when testing 6a used to just temp move up to higher rated breaker, but switched to megger 1553 and problem seems to of gone away

Dunc · Jul 24, 2011

Just do the test on a 'no trip' test. The reason the MCB sometimes trips when you carry out the high current loop test is because you are putting 25A through the installation. I don't understand exactly how the whole test works but somehow they use a much lower applied current of 15mA to do the no trip test hence why the MCB doesn't even cough. Just have a look at the data sheet for the 1500 series Meggers to see what i mean (LINK).