The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 14 replies
  • Subscribers 31 subscribers
  • Views 4056 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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
Parents
  • 0
    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.
Reply
  • 0
    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.
Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines