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Required. An explanation of the Causes of MCB Intermittent Tripping due to Loose Connections

The Woodster
Hello Everybody


I wish to gain a greater knowledge as to the causes of MCB intermittent tripping due to loose connections, so any help would be greatly appreciated and the more in-depth the better as that is what I want to understand, the physics behind it.

Thermal Tripping  I can understand that a loose connection can generate heat due to resistance, IR2, and I can appreciate that it could cause an MCB to thermal trip if the loose connection is at the actual MCB Terminal (OR even an MCB either side IF THIS IS POSSIBLE) but how can a loose connection further downstream, say 8 -9 metres away cause a thermal trip.  I find it hard to believe that enough heat could dissipate far enough to reach MCB and be hot enough to trip MCB.

Magnetic Tripping  How can a loose connection cause a magnetic trip.  Intuitively I guess it must be due to arcing and arc voltages but that suggests that air is a better conductor than copper (that would also suggest that air dielectric capacitors wouldn't work I think) to allow enough Amps to flow to trip MCB or there is a sudden increase in voltage over a very short period in time dv/dt and therefore perhaps capacitance plays a part.  Basically how does an MCB magnetic trip due to a loose connection downstream?


All contributions greatly appreciated


Regards The Woodster
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  • 0
    Two situations where this may be  true come to mind

    The currents in low loss inductors are very large at start up, and can be nearly doubled if you take a break for a half cycle, so instead of just building up a magnetisation from a neutral state you have to de-magnetise from being in the wrong direction.

    Or if you prefer imagine a star delta starter, being flicked, so it goes off and then comes on again as 'delta' without passing through star. Of course no-volt releases are usually there to catch that one.


    A great many circuits have many loads that normally get turned on sequentially (lights are a classic) and then there are several small inrush events. Interrupt and reconnect all together however and the inrush currents are all co-incident in time. In some installations you may struggle if  this is the situation where the MCB will not close on load, or at least it kicks back and it takes another go or two to get it started.


    It is not great design, but there will  be circuits like this that work perfectly well, except they trip  off in a power cut, or rather when power is restored, and  they are vulnerable to odd trips with scratchy contacts.


    RCDs tripping is something else, and I have seen an RCD tripped by a class 2 device with a poor connection, even thouhg there was no copper path to earth, and I attributed that to the generation of very fast sharp edges,  coupling in assymetric ways.
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  • 0
    Two situations where this may be  true come to mind

    The currents in low loss inductors are very large at start up, and can be nearly doubled if you take a break for a half cycle, so instead of just building up a magnetisation from a neutral state you have to de-magnetise from being in the wrong direction.

    Or if you prefer imagine a star delta starter, being flicked, so it goes off and then comes on again as 'delta' without passing through star. Of course no-volt releases are usually there to catch that one.


    A great many circuits have many loads that normally get turned on sequentially (lights are a classic) and then there are several small inrush events. Interrupt and reconnect all together however and the inrush currents are all co-incident in time. In some installations you may struggle if  this is the situation where the MCB will not close on load, or at least it kicks back and it takes another go or two to get it started.


    It is not great design, but there will  be circuits like this that work perfectly well, except they trip  off in a power cut, or rather when power is restored, and  they are vulnerable to odd trips with scratchy contacts.


    RCDs tripping is something else, and I have seen an RCD tripped by a class 2 device with a poor connection, even thouhg there was no copper path to earth, and I attributed that to the generation of very fast sharp edges,  coupling in assymetric ways.
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