Trippng MCB with style odd one

nickblake · Jun 10, 2011

Yes i bet you have all gone oh not again and normally i wouldnt even post one like this as normally easy faults short here short there problem solved but this one has got even my tiny mind baffled , new install all tested out 100% ok all R1R2 reading satisfactory all insulation reading over 200 meg lighting circuits with and without the switches on all Zs readings well within permitted levels then comes power up all worked then suddenly one circuit tripps with a nice pop and knocks out the RCD as well as the MCB , ting retest the circuit R1R2 0.86 insulation 200 200 200 plus Zs 1.21 ,all clear reset MCB RCD all ok , then for no reason bang MCB tripps re tested circuit all reading as previous its not water in the system as not going out side ,all connections are fine and sound , so i replaced the MCB and has not tripped since , why there is nothing in the MCB to cause a short how , i may be getting tiard but this ones got me , the circuit has been tested 5 times and all insulation tests are 100% ok no shorts and no blinking low energys in the install every thing GLS so im ruling them out all lights were on for over 4 hours after the ?MCB was replaced , it a head scratcher how could an MCB be faulty like that

Des 56 · Jun 10, 2011

why there is nothing in the MCB to cause a short how , i may be getting tiard but this ones got me ,

I think an mcb s tripping characteristic is determined by an internal Bi metal strip

Could it be that the Bi metal strip in the mcb was reacting instantaneously and you had a simple dud breaker

Deleted member 26818 · Jun 10, 2011

MCBs use both a Bi metal strip and a electromagnetic coil.
The Bi metal strip is the slow blow, and the coil is the fast blow.
There are some fake MCBs out there, and I have recently had problems with new genuine MK MCBs, which I think are now made in China.

MarkieSparkie · Jun 11, 2011

Just a thought, you may have unwittingly fallen prey of a scumbag trick which was rife a few years ago when MCBs were relatively more expensive. Said scumbag would return 'unused' stock to the wholesaler for a refund. Amongst the returned MCBs would be a number which having experienced a fault current in excess of the Ics rating would normally be considered unserviceable and be replaced, but the scumbag has returned them 'as new' only for somebody else to purchase in good faith and experience unexplained random tripping problems.