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

Heat generated at a high resistance terminal is I squared R in Watts.  "I2XR Watts.) This is unlikely to trip an M.C.B. with a loose terminal as the thermal bimetallic strip is inside the M.C.B. case. 


If the load side terminal is loose and subject to intermittent vibration or heat induced movement then possibly an inductive load may trip an M.C.B. due to frequent inrush currents when the circuit is made, and the load connected. The high inrush currents may operate the magnetic part of the M.C.B. The tripping is more likely if additional loads are connected through the M.C.B.


Z.

Hi.


I don't think I have ever heard of this happening but I like Zoomup's idea around loose-connections coupled with inrush. It'd be interesting to hear if that has been identified as a cause of tripping by anyone.


As to arcs at loose connections, they don't happen unless the connection is subjected to movement / vibration.  Even then, the arc would very slightly strangle the current, not increase it.  So no fault current there.


I suppose if your installation has loose connections, it may also have other nasty stuff that might trigger earth fault or over current detection.

M.C.B. operation.......

https://www.youtube.com/watch?v=T3NoJ_x8oiA


Z.

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

   - Andy.

I can appreciate that fact Andy, yet trolling through archive's of this very site, there are numerous posts about intermittent tripping of MCBs stating that it is caused by loose connections but absolutely no explanation as to WHY.  So either at least 10 -15 posters who state MCBs can trip due to loose connections are wrong or they are right and nobody can explain why.

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.

A loose connection to either of the MCB terminals will generate heat and in extreme circumstances enough of this heat could be conducted into the MCB to trip the thermal element.


A loose connection distant from the MCB wont usually trip the MCB, but can do so in certain circumstances. With a simple resistive load, a loose connection will reduce the thermally equivalent load, and cant trip the MCB.

In the case of say switched mode power supplies then tripping can result from loose connections for two reasons. Firstly if the supply is interrupted for perhaps one second, then on restoration, all the capacitors in the SMPSUs will charge up and possibly trip the magnetic element of the MCB. This wont be noticed in normal use if the items are turned on in small groups, but after a power failure the combined inrush is problematic. Even a very brief power failure caused by a loose connection.

Secondly, repeated very brief interruptions and the inrush currents afterward may heat up the thermal element to tripping point.

Finally, a poor connection may reduce the voltage at the load, and SMPSUs are constant wattage loads and will draw more amps at a lower voltage.

Many thanks to everybody who contributed.


So just to clarify

Thermal Intermittent Trip


Can be caused by loose connections directly at MCB cable/busbar connections due to high resistance creating joule heating for Bi-metallic strip and might be possible if MCBs either side have loose connections (I did say might be possible) for the same reason.


Also thermal trip POSSIBLY can activate if loose connections downstream if repeated disconnect and reconnect of loose cable terminal/connection if enough load (and even better if reactive load due to high dv/dt ideal for CAPS and Inductors) and therefore repeated high inrush current generating joule heating which would be present all along cable carrying the high current parts and therefore also at the MCB.

Magnetic Intermittent Trip


Could be caused by reactive loads causing repeated inrush currents which if timing of loose cable connection reconnecting at max or min point of AC cycle could be ideal for reactive loads to demand MAX current and cause MCB to trip.


I guess sudden vibration for whatever reason causing rapid connect and disconnect of a loose connection would be the perfect storm for any of the above.


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.


The Woodster

I'm guessing that this has nothing to do with loose connections, rather inrush or continuous overload causing protective device to operate. 


Regards Ts