1

In a miniature circuit breaker, as shown below, the distance between open contacts is much less than the distance between metal bars in the arc extinguisher part (the gap gets progressively longer from contacts to arc extinguisher.)

So why does the arc move from the contacts to the arc extinguisher?

i.e. Why doesn't it stay between the actual contacts, where the gap is minimum and has the least resistance/breakdown-voltage?

enter image description here

JRE
  • 67,678
  • 8
  • 104
  • 179
Microsoft Linux TM
  • 333
  • 3
  • 16
  • I'm not familiar with how exactly these work, but my guess would be a magnet somewhere in the housing to pull the arc aside. That is only a guess, though. – Hearth Dec 17 '19 at 17:38
  • And if the original image is upside-down, plasma is very hot and tends to rise immediately in air, helping to carry it to the chutes even without aid of magnetism. After the arc is drawn, it is very low resistance, so there is little difference between the red and blue lines. It is only the green and yellow which are far enough to begin quenching that arc. – rdtsc Dec 17 '19 at 18:04
  • @rdtsc no the picture is not upside down ... and it seems different manufacturers place the chutes differently ... some above the contacts and some below ... so it seems unlikely to be due to convection. – Microsoft Linux TM Dec 17 '19 at 18:39

2 Answers2

2

The resistance of the arc can be increased by splitting the arc into a number of smaller arcs in series. Each one of these arcs experiences the effect of lengthening and cooling. The arc may be split by introducing some conducting plates between the contacts. enter image description here

The bimetallic strip trips the breaker unless during short circuit condition, the sudden rising of electric current, causes electromagnetic plunger to strike the trip lever causing immediate release of latch mechanism consequently open the circuit breaker contacts much faster, while the bimetallic strip does it more accurately at lower currents.

This video on YouTube shows roughly how it should work. The current extinguishes at the next AC zero crossing but would not work for DC inductive loads.

According to the video as well as this link the arc moves from the contacts to the Arc extinguisher, due to magnetic field (e.g. magnetized chutes) or (depending on the breaker's design and how you mount it) due to convection and rise of the hot plasma.

Microsoft Linux TM
  • 333
  • 3
  • 16
Tony Stewart EE75
  • 1
  • 3
  • 54
  • 182
  • Could you trace the path of current flow through the breaker? There's so much going on its hard to tell without seeing how the parts move. So the current flows horizontally through the chutes when supressing the arc? – DKNguyen Dec 17 '19 at 17:58
  • 1
    Like going thru a cascade of capacitors thru a number of layers while each ionizing layer absorbs some energy – Tony Stewart EE75 Dec 17 '19 at 18:01
  • What direction does current flow through the chutes when the breaker is closed? Vertically? Or are the chutes compressed against each other when the breaker is closed? I can't tell if contact is made between the top/bottom of the chutes, or the left/right. – DKNguyen Dec 17 '19 at 18:02
  • The white arc-chute arrows show the possible paths of arcs.... here horizontal. Like a Honeywell furnace dust ionizer the actual spots are random between layers as a carbon build up means it wont be reused. The chutes divert the arc when Opened with inductive arcs – Tony Stewart EE75 Dec 17 '19 at 18:03
  • Let me re-phrase my last question: Do the chutes carry current when the breaker is closed? – DKNguyen Dec 17 '19 at 18:05
  • There are no (significant) closing arcs , even with start caps – Tony Stewart EE75 Dec 17 '19 at 18:06
  • Not asking about arcs. Just trying to figure out how current flows through the relay since there is no closed and open images and I can't tell if the chutes carry current while the breaker is closed (maybe they are compressed together or something when the breaker is close, I can't tell). – DKNguyen Dec 17 '19 at 18:30
  • @TonyStewartSunnyskyguyEE75 Thanks, but my main question is: when the contacts open, why does the arc (which is initiated between the contacts), move along the metal bars to the arc chutes? what doesn't it just stay between the contacts? i mean there should be a reason for it to move in a predicted manner to the chutes ... and i don't see any magnets in there ... are there any permanent magnets in there to force the arc in that direction? also it seems unlikely to be due to convection since if you look at my picture, in contrast to yours, the chutes are below the contacts ... not above ... – Microsoft Linux TM Dec 17 '19 at 18:33
  • @DKNguyen no the current doesn't flow through the chutes when the contacts are closed ... the chutes are electrically insulated from each other ... the current flows from input terminal and through the fixed contact in the picture above to dynamic contact to and through the solenoid then to and through the bimetal and then to the other terminal ... – Microsoft Linux TM Dec 17 '19 at 18:43
  • @Sudoer Ah, So the chutes are kind of like a detour. Interesting. – DKNguyen Dec 17 '19 at 18:46
  • @TonyStewartSunnyskyguyEE75 while the gap in between each two of them is smaller it doesn't seem the arc would move because of that ... since Insulated chutes being capacitors in series, the voltage across each one is a fraction of the total voltage ... in the video you linked to, it says it's due to magnetic fields ... and i found a link stating the same thing: http://www.cycloflow.com/2014/03/what-is-mcb-afci-and-gfci.html this link says it's due to a combination of magnetic effect and convection (and the occurrence of the latter depends on the design of the breaker and how you mount it) – Microsoft Linux TM Dec 17 '19 at 19:26
1

If you are asking about magnetic blowouts, then this is the reason: When a magnetic field and electrons are moving relative to each other, a side force is exerted on the electrons.

But that doesn't look like a magnetic blowout to me (but I am not really familiar with what they look like in real life either). Regardless, a magnetic blowout is one way to extinguish an arc.

enter image description here

From "A Course in Electrical Engineering, Volume I, Direct Currents, 1920, Chester L. Dawes

DKNguyen
  • 54,733
  • 4
  • 67
  • 153
  • Thanks, but looking at the pictures, i don't see any magnets in there ... are there any permanent magnets in there to force the arc in that direction? – Microsoft Linux TM Dec 17 '19 at 18:38
  • @Sudoer I'm actually not sure if magnetic blowouts use permanent magnets or electromagnets as I don't have a feel on how strong the field actually has to be. But as I said, I do not think your relay uses a magnetic blowout mainly because it doesn't look like how I would imagine, and you mention it's miniature. Magnetic blowouts tend to be used for big high power contacts. – DKNguyen Dec 17 '19 at 18:39