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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
  • 0

    HOWEVER has anyone known an MCB to intermittently trip due to a spur to a socket, the socket connected with incorrect polarity, off a ring final having correct polarity except for that one spurred socket. AND I MEAN intermittently trip with the same load plugged into that spurred socket, in this case a Router.



    If no RCDs present and the router was Class I I might suspect an intermittent N-PE fault in the router - elsewhere (again without RCDs) it would probably go unnoticed.


      - Andy.
  • 0
    Wi Fi Router not Woodworking router.   Installation had no problems until the face plates on three sockets were changed and a new spurred socket added, with reversed polarity.  On two of three face plates that were changed, either the line or the neutral terminals were loose.  The wi fi router was the only thing plugged into the reversed polarity spur and was left on constantly obviously.  Ever since MCB intermittently trips but not before any of the above changes.


    Loose connections rectified and socket polarity corrected, problem disappeared.  BUT which of the faults was the culprit causing intermittent tripping of MCB.  I have known about loose connections causing problems and have had my own theories and guess I wanted to ask the forum about the physics behind the phenomenon.  Just about the only challenge I find with Domestic Electrics is fault finding, it helps to relief the boredom of fitting ext fans and cookers and PIR sensors and switches and sockets and .....


    I used to be a Software Engineer and loved debugging until retraining at the ripe old age of 42 to be a Spark following redundancy.


    The Woodster
  • 0

    AJJewsbury:

    If MCBs could reliably trip on a loose connection they wouldn't be trying to flog us AFDDs.

       - Andy.




    AFDDs can't either.

  • 0
    Are you sure that the ring M.C.B. is not in fact an R.C.B.O? R.C.D.s can trip off if ring continuity is not perfect on live conductors, (L and N). Perhaps the ring had a fault elsewhere, or was overloaded.


    I have personally seen literally dozens of burnt out switches ranging from 30 to 50 Amps where a loose screw has caused a terminal to burn out due to it not being tightened fully and getting very hot. These switches mainly controlled cookers and electrical showers ranging from 6 to 11 kW approx. What happens in all cases is that a terminal runs hotter and hotter until it eventually can not conduct a current any more and the appliance stops working. I have never come across a tripped M.C.B. in this instance although arcing must have occurred.


    Z.

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